GANGED TUNING is the process used to tune two or more circuits with a single control.
HETERODYNING is the mixing of the incoming signal with the local oscillator frequency. This produces the two fundamentals and the sum and difference frequencies.
An IMAGE FREQUENCY is an undesired frequency capable of producing the desired frequency through heterodyning.
AUTOMATIC VOLUME/GAIN CONTROL is a circuit used to limit variations in the output signal strength of a receiver.
FADING is the variations in signal strength at the antenna of a receiver.
REVERSE AGC is when an amplifier is driven toward cutoff.
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FORWARD AGC is when an amplifier is driven toward saturation.
A BEAT-FREQUENCY OSCILLATOR is an additional oscillator used in a receiver when the receiver is receiving a cw signal and provides an audible tone.
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ANSWERS TO QUESTIONS Q1. THROUGH Q26. A1. Am, fm, cw, ssb. A2. It generates an rf carrier at a given frequency within required limits. A3. Power amplifier. A4. It converts audio (sound) into electrical energy. A5. When no modulation is present.
A6. It is an exact multiple of the basic or fundamental frequency. A7. 600 megahertz.
A8. To obtain higher carrier frequencies. A9. It saves power and frequency bandwidth. A10. For operator-to-operator service messages and frequency changes. A11. Reception, selection, detection, and reproduction.
A12. Sensitivity, noise, selectivity, and fidelity. A13. Heterodyning.
A14. To extract the modulating audio signal.
A15. Wide bandpass.
A16. A special type of detector and a carrier reinsertion oscillator. A17. Attenuates the strong and amplifies the weak. A18. To limit unwanted variations in the output.
A19. Weak signals produce bias, which could result in no usable receiver output.
A20. Dagc does not attenuate weak signals.
A21. It is heterodyned with the rf to produce an audio frequency. A22. It eliminates noise when no signal is being received.
A23. It controls the amount of bass and treble response. A24. It is used to achieve maximum selectivity.
A25. It is used to accurately control the frequency of the oscillator. A26. The process of selecting and/or heterodyning frequencies to produce a signal frequency.
3-1
Upon completion of this chapter you will be able to:
1. State the function of a radio communications handset, a radio set control, and a transfer switchboard.
2. Describe the functions and interrelationships of a radio transmitter.
3. Describe the functions of receive and transmit multicouplers.
4. Describe the differences between the codes used for manual telegraphy and teletypewriter transmissions.
5. Describe the two basic modes of teletypewriter operation.
6. Describe the two types of teletypewriter dc circuits.
7. State the two types of radio teletypewriter shift systems and describe their basic differences.
8. Describe the functions and interrelationships of radio-frequency-carrier shift send and receive systems.
9. Describe the signal flow in an audio-frequency-tone shift system.
10. State the function of the tone terminal set in an audio-frequency-tone shift system.
11. Describe the basic multiplexing process.
12. Describe the three operations performed by a facsimile system.
13. Describe the functions and interrelationships of facsimile equipment.
14. Describe the countermeasures that can be used to eliminate compromising emanations.
A communications system is a collection of equipment used together to do a specific job. You may see this equipment used to send or receive voice communications, or both, or to send, receive, or send and receive teletypewriter information.
Figure 3-1 is a basic block diagram of a voice system. You can see how this equipment is interconnected to form a basic communications system. We are going to look at several of the equipment blocks in detail.
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The handset converts acoustical energy (your voice) to electrical energy for use in modulating a radio transmitter. It also converts electrical energy to acoustical energy for reproduction of a received signal. When the push-to-talk button is depressed on the handset, the dc keying circuit to the transmitter is closed, placing the transmitter on the air.
Handsets are normally connected to a radio set control unit.
The radio set control unit shown in figure 3-2 provides a capability to remotely control some radiophone transmitter functions and the receiver output. Some of the controls are used for turning the transmitter on and off. Others are used for voice modulating the transmission (or keying when cw operation is desired). You can even control the audio output level of the receiver and silence the receiver when transmitting.
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Under standard operating conditions up to four of these units can be used in parallel with a single transmitter and receiver group to provide additional operating positions. This setup is often found aboard ship where a transmitter and/or receiver is controlled and operated from several locations such as the bridge or the combat information center.
A transmitter transfer switchboard provides the capability to transfer remote control station functions and signals to transmitters. Figure 3-3 is a representative transfer switchboard that provides the capability for selectively transferring any one, or all, of ten remote control station functions and signals to any one of six transmitters. The cabinet has ten rotary switches arranged in two vertical rows of five each. Each switch has eight positions. The circuitry is arranged so that you cannot parallel transmitter control circuits; that is, you cannot connect more than one transmitter to any remote control location.
Each switch operating knob corresponds to a remote control station. Each switch position (1 through 6) corresponds to a transmitter. One switch position, X, provides for transfer of all circuits to additional
3-4
transmitter transfer switchboards when more than six transmitters are installed in the system. When the rotary switch is placed in the OFF position the remote control station is removed from the system.
Let's look at an example of one transfer switchboard application. When remote control station number two is to have control of transmitter number three, the switch knob designated number two is rotated until its pointer indicates position three on its dial plate.
The receiver transfer switchboard permits the operator to transfer the audio output from a receiver to a remote control station audio circuit. A representative receiver transfer switchboard is shown in figure 3- 4. This switchboard contains ten seven-position switches. Each switch is connected to a remote control station, and each switch position (one through five) is connected to a receiver.
The X position on each switch allows transfer of circuits to additional switchboards just like with the transmitter transfer switchboard.
Q1. What are the basic functions of a handset?
Q2. What capability does a transmitter transfer switchboard provide?
Q3. What function does a receiver transfer switchboard perform?
You learned earlier that transmitters may be simple with low power (milliwatts) capabilities. These may be used to send voice messages a short distance. You may also use highly sophisticated units that use thousands (even millions) of watts of power to send many channels of data (for example voice, teletypewriter, television, telemetry) simultaneously over long distances. Let's look at a complete transmitter set.
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The applications, configurations, and components you will become familiar with here are typical of most general purpose transmitter systems used in the Navy. A specific transmitter is used only for ease of illustration and example.
We will be discussing a 1,000 watt, single-sideband radio transmitting set that is available to the Navy in any one of four setups. The normal configuration has a transmitter capable of voice, continuous wave, and radio teletypewriter transmissions in the 2- to 30-megahertz frequency range. Exact spacing and number of channels available within the frequency spectrum, modes of operation, and frequency range depend on the model of equipment and how it is configured for use. Stack or rack mounting is used in a ship or shore permanent installation with accessory equipment (for example an rf amplifier, coupler control unit, or power supply) to form a complete communications system. One of three different three- phase primary power sources can be used (depending on whether the transmitter is land, air, or shore based) to provide operating power to the set. Combinations available are 115 volts, 400 hertz; or 208/440 volts, 60 hertz.
Figure 3-5 shows the major units of this set. They are the radio transmitter, the radio frequency amplifier, the power supply, and the electrical equipment shock mount base. An antenna coupler group (consisting of a coupler and coupler control unit) is normally used to match the impedance of the system to a 50-ohm transmission line. If you want to operate with any 50-ohm antenna system, terminating connections are available.
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The transmitter unit provides an upper sideband (usb), lower sideband (lsb), independent sideband (isb), cw, fsk, or compatible AM signal. The output of the transmitter has enough power to drive the radio frequency amplifier.
Depending on the model, the transmitter tunes across the frequency range in 100- or 500-hertz increments. Digital circuitry is used to accomplish this process. Transmitter outputs are also applied to the rf amplifier to automatically tune it to the correct frequency. We will go through a detailed breakdown of the transmitter unit later in this chapter.
RADIO FREQUENCY AMPLIFIER.—The rf amplifier unit is a two-stage linear power amplifier that produces an output of 1,000 watts with a nominal input of 100 milliwatts. Nineteen frequency bands are used to cover the operating frequency range. The operating band is automatically selected by digital coding generated by the transmitter. The code controls two motor-driven band switch assemblies. Automatic control circuits protect the unit against overload and compensate for variations in system gain, mode of operation, and loading.
All low voltages required for operation (except two of the relay control voltages) are internally produced. The high voltages required in the amplifier stages are produced by the associated power supply (when using 60 hertz primary power) or the optional internally mounted power supply (when using 400 hertz primary power).
Let's take a look at figure 3-6 to see all the operating controls and indicators located on the front panel. Some controls are used only for initial setup and are protected by a hinged access cover. All connections are made at the rear of the case. The amplifiers and the associated interstage broadband transformer assemblies are cooled by forced ventilation. Cooling air is drawn through a filter on the front panel and exhausted through a port on the rear of the case. You should always take particular care to clean or replace any filter in electronic equipment as a regular part of your preventive maintenance program.
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POWER SUPPLIES.—One power supply produces operating voltages for the amplifier when operating from a 60-hertz power source. All components of the power supply, except the power transformers, are mounted on a chassis and panel assembly that is hinge-mounted to a metal case. The power transformers are constructed as part of the case and there are no operating controls.
The other power supply produces operating voltages for the rf amplifier when a 400-hertz, three- phase, 115-volt primary power source is used.
ANTENNA COUPLER GROUP.—The antenna coupler group is an automatic antenna tuning system. However, the equipment design includes provisions for manual or semiautomatic tuning. This makes the system adaptable for use with other radio transmitters. The manual tuning capability is useful when a failure occurs in the automatic tuning circuitry. Tuning can also be accomplished without the use of rf power (SILENT TUNING). This method is useful in installations where radio silence must be maintained except for brief transmission periods.
The antenna coupler matches the impedance of a 15-, 25-, 28-, or 35-foot whip antenna to a 50-ohm transmission line at any frequency in the 2- to 30-megahertz range. Control signals from the associated antenna coupler control unit automatically tune the matching network in less than five seconds. During manual and silent operation, tuning is accomplished by the operator with the controls mounted on the antenna coupler control unit. A low power (not to exceed 250 watts) cw signal is required for tuning. Once tuned, the coupler is capable of handling 1,000 watts peak envelope power (pep).
The coupler is enclosed in an aluminum, airtight, pressurized case. Six mounting feet enable the unit to be attached to the mast of a ship at the base of a whip antenna. The coupler is pressurized with dry nitrogen to aid internal heat transfer and to prevent corona and arcing. All components of the coupler are secured to a chassis that is mounted to the case so that an air duct exists between the chassis plate and the
3-8
case. An internal fan circulates the nitrogen over and through the heat-producing elements and then through the air duct. While passing through the air duct, the nitrogen loses its heat to the bottom of the case. This heat is then transferred by convection through fins on the bottom of the case and by conduction through the mounting feet.
Figure 3-7 shows the antenna coupler control unit. This unit provides the power and control signals required to tune the coupler. Control signals are either automatically produced by the coupler control when a tune cycle is initiated or manually produced with the front panel controls.
All dc operating voltages are produced from a 115-volt, 48- to 63- or 350- to 450-hertz, single-phase primary power source. Meter and protection circuits are used to give you complete control of the coupler from the remotely positioned coupler control unit.
Q4. If the rf amplifier discussed has an 80 milliwatt input, what would be the maximum output?
Q5. What are the tuning modes for the coupler group discussed?
Q6. What is the purpose of an antenna coupler?
Q7. Why is the coupler pressurized with nitrogen?
RADIO TRANSMITTER.—Figure 3-8 shows the front panel of the radio transmitter unit. The radio transmitter accepts audio or coded intelligence and uses it to modulate one of 280,000 possible operating radio frequencies in the 2.0- to 29.999-megahertz frequency range. Tuning is accomplished digitally by means of five control knobs and a switch located on the front panel. The transmitter has a normal rf output level of at least 100 milliwatts and is designed to be used with an associated rf power amplifier.
3-9
When the AM and ssb transmit modes of operation are used, the output from a handset is applied to the transmitter. The voice signals are amplified and used to modulate a 500-kilohertz local carrier that produces a 500-kilohertz IF. The resulting double sideband signal is filtered in the AM mode, amplified, and converted by a triple-conversion process to the desired rf operating frequency. The rf signal is amplified to a nominal 100 milliwatt level. In cw operation, the 500-kilohertz local carrier is inserted directly into the IF amplifiers. The signal is further processed in the same manner as the voice signals in the AM or ssb modes of operation. In fsk operation, the loop current is converted to audio frequencies representing marks and spaces. These audio signals are applied to the audio circuits of the transmitter. Thereafter, these signals are processed in the same manner as the voice signals in AM or ssb modes of operation. A typical radio transmitting set block diagram is shown in figure 3-9.
3-10
The receiver we will discuss is a triple-conversion superheterodyne, tunable from 2 to 30 megahertz. Triple conversion uses three IF frequencies to give better adjacent-channel selectivity and greater image- frequency suppression. Figure 3-10 shows the front panel of this receiver where tuning is done digitally by five controls and a switch. A display window directly above each control provides a digital readout of the frequency setting. The displayed frequency can be changed in 1-kilohertz increments. The front panel switch allows the operating frequency to be changed in 100- or 500-hertz increments depending on the model. This will provide you with 280,000 discrete frequencies locked to a very accurate frequency standard. You can continuously tune each 1,000-hertz increment by selecting the VERNIER position of the hertz switch. When using the vernier, the full accuracy of the frequency standard is sacrificed. The receiver demodulates and provides audio outputs for the lsb, usb, isb, AM, cw, and fsk types of received signals.
3-11
Q8. What are the transmitter operating modes?
Q9. What type of tuning does the receiver use?
Receiving antenna distribution systems operate at low power levels and are built to fit a standard 19- inch rack. Each piece of distribution equipment is fitted with termination or patch fittings designed for ease of connecting and disconnecting. A basic patch panel is shown in figure 3-11. Even a fundamental distribution system has several antenna transmission lines and several receivers. Normally a patch panel consists of two basic patch panels. One panel is used to terminate the antenna transmission lines and the other the lines leading to the receivers. Any antenna can be patched to any receiver through the use of patch cords.
Many distribution systems are more complex. A complex distribution system to cover most situations is illustrated in figure 3-12. In this system you can patch four antennas to four receivers, or you can patch one antenna to more than one receiver via the multicouplers (multicouplers are covered later in
3-12
this chapter). You can also patch rf and audio from one compartment to another. A frequency standard is connected (through a distribution amplifier not shown) to the receivers.
Transmitting antenna distribution systems perform the same functions as receiving systems. However, because of the higher power levels, design and fabrication problems are more difficult. The ideal design would be to have all the transmission lines designed for the highest power level. But because high-power patch cords are expensive, large, and difficult to handle, this approach is seldom followed.
In practice, the basic patch panel we just looked at in figure 3-11 is practical for low power levels. Another type of transmitter patch panel is shown in figure 3-13.
3-13
This type of transmitting antenna patch panel is interlocked with the transmitter so that no open jack connection can be energized and no energized patch cord can be removed. This provides you with a greater degree of personnel and equipment safety.
Figure 3-14 is a filter assembly multicoupler that provides seven radio frequency channels in the 14- kilohertz to 32-megahertz range. Any or all of these channels may be used independently of any of the other channels, or they may operate simultaneously. You can make connections to the receiver by means of coaxial patch cords, which are short lengths of cable with plugs attached to each end.
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A set of nine plug-in type filter assemblies is furnished with the equipment and covers the entire vlf, lf, mf, and hf bands. Only seven of the assemblies may be installed at one time, and you have the option of selecting those you need to cover the most used frequency bands.
Figure 3-12 illustrates how the filter assembly is used in combination with other units to pass an rf signal from an antenna to one or more receivers.
Most multicouplers for the hf range are designed for use with either transmitters or receivers, although some are used with both. There are a large number of channels in a multicoupler so that many transmitters can be used at the same time on one antenna. This is especially true in the 2- to 12-megahertz range.
Figure 3-15 shows you an antenna coupler group designed primarily for shipboard use. Each coupler group permits several transmitters to operate simultaneously into a single, associated, broadband antenna. You can see this reduces the total number of antennas required in the limited space aboard ship.
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These antenna coupler groups provide a coupling path of prescribed efficiency between each transmitter and its associated antenna. They also provide isolation between transmitters, tunable bandpass filters, and matching networks.
In previous areas we have discussed different methods of voice communications. At times, however, the message is too long for practical transmission by voice. To get information or an idea across to another person far away, you may also need a chart, map, or photograph. Teletypewriter (tty) and facsimile equipment allow us to do just that, with ease. Let's see how this is done.
To give you an idea of how intelligence is sent via teletypewriter, let's take a look at the manual telegraph circuit. This circuit, shown in figure 3-16, includes a telegraph key, a source of power (battery), a sounder, and a movable sounder armature. If the key is closed, current flows through the circuit and the armature is attracted to the sounder by magnetism. When the key is opened, the armature is retracted by a spring. With these two electrical conditions of the circuit, intelligence can be transmitted by means of a teletypewriter code. These two conditions of the circuit are referred to as MARKING and SPACING. The marking condition occurs when the circuit is closed and a current flows; the spacing condition occurs when it is open and no current flows.
If the key at station A is replaced by a transmitting teletypewriter and the sounder arrangement at station B is replaced by a receiving teletypewriter, the basic teletypewriter circuit (loop) shown in figure 3-17 is formed.
3-16
If a teletypewriter signal could be drawn on paper, it would resemble figure 3-18. This is the code combination for the letter R. Shaded areas show intervals during which the circuit is closed, and the blank areas show the intervals during which the circuit is open. The signal has a total of seven units. Five of these are numbered and are called INTELLIGENCE units. The first and last units of the signal are labeled START and STOP. They are named after their functions: the first starts the signal, and the last stops it. These are a part of every teletypewriter code signal: the START unit is always spacing, and the STOP unit is always marking.
The teletypewriter signal is theoretically a perfect signal. The time between each unit remains the same during transmission of the signal. The shift from mark to space (and vice versa) is called a TRANSITION. A transition occurs at the beginning and end of each unit when it shifts from mark to space or space to mark; a character may have two, four, or six transitions.
When figuring the time duration of a signal character, no allowance for transition time is made since the transition is instantaneous and is considered to have zero time duration. The time duration for each unit is measured in milliseconds.
Q10. What is the function of an antenna patch panel?
Q11. What are the functions of a multicoupler?
Q12. What are the terms used to describe an open or closed telegraph circuit?
Q13. How many units are in a tty signal and what are they?
3-17
Two of the codes the Navy uses are found in manual telegraphy and in teletypewriter operation. One is very easy to understand while the other is more complex. Let's look at these two types and how they work.
MANUAL TELEGRAPHY.—In manual telegraphy, the most widely used code is the Morse code. In this code, two distinctive signal elements are employed-the dot and the dash. The difference between a dot and a dash is its duration, a dash being three times as long as a dot. Each character is made up of a number of dots and/or dashes. The dot and dash elements making up any character are separated from each other by a time interval equal to the duration of one dot. The time interval between the characters for each word is equal to the duration of three dots. The interval between words is equal to seven dots. (A signal-man uses the Morse code to send visual flashing-light messages. The radioman uses the Morse code to send messages electrically.)
TELETYPEWRITER MESSAGE TRANSMISSION.—In teletypewriter operation, the code group for each character is of uniform length. Since the Morse code is an uneven length code, it cannot be used in teletypewriter operation without additional code converters.
The FIVE-UNIT (five-level) CODE has been the most commonly used in modern printing telegraphy and is universally used in teletypewriter operation. This is also known as the Baudot code. The mechanical sending device in the teletypewriter divides the sending time for each character into five short code elements (impulses) of equal duration. The five-unit code is an example of what is called an even length or constant length code (one in which the number of signal elements for a character is the same for every character and the duration of each element is constant). In the five-unit code, each character consists of a combination of five signal elements; each element may be either a mark or a space. A total of thirty-two combinations of signal elements are possible with this arrangement.
The thirty-two possible combinations available from the five-unit code are insufficient to handle the alphabet and numbers since twenty-six combinations are required for the letters of the English alphabet alone. This leaves only six combinations for numerals, symbols, or nonprinting functions. This number of combinations is obviously inadequate; therefore, two of the thirty-two combinations are used as shift signals. The shift signals are often referred to as case-shift signals (one case is a letter shift, and the other a figure shift.) These two shift signals permit the remaining code combination to be used as letter-shift signals for letters and as figure-shift signals for numerals, function signs, and so forth. When a letter shift is transmitted, it sets the receiving instrument in a condition to recognize any letter signal combination. It will recognize letter combinations until a figure shift is received. Then the receiving instrument sets itself in a condition to recognize any figure signal combination received. The interpretation of a signal combination is determined by the previous shift signal. This plan enables 30 of the 32 available combinations to have two meanings.
Q14. There are not enough combinations of the five-unit code to handle the alphabet, symbols and so forth. What is used to increase the number of available code combinations?
The two basic modes of teletypewriter operation are ASYNCHRONOUS (start-stop) and SYNCHRONOUS. The most common mode used in teletypewriter operation is the start-stop mode. Synchronous operation is used more in high-speed data systems. Let's examine their differences.
ASYNCHRONOUS.—In the start-stop mode of operation, the receiving device is allowed to run for only one character. It is then stopped to await the reception of a start signal indicating the next character is
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about to start. In this manner any difference in speed between the transmitting and receiving devices can accumulate only during the duration of one character. However, you should note that a penalty must be paid for this advantage. The length of each character must be increased to include a unit (element) to start the receiving device and another to stop it.
The start unit precedes the first intelligence unit and is always a space signal. Its purpose is to start the receiving machine. The stop unit follows the last code unit and is always a mark signal. Its purpose is to stop the receiving machine in preparation for receiving the next character. The start unit must be equal to at least one unit of the code. The standard mode uses a stop unit that is 1.42 times the length of one intelligence unit. It is common practice to refer to a code unit as an element and to use the terms interchangeably. You will also hear duration of a unit referred to as the unit interval.
The length of time required to transmit the entire character is called the CHARACTER INTERVAL. Character interval becomes very important in some transmissions because certain items of equipment are character length conscious or code conscious. Stop unit intervals of various lengths are used or produced by various equipment (1.0, 1.27, 1.5, 1.96, 2.0, and so forth). Basically, the only difference between them is the length of time required to transmit one character.
SYNCHRONOUS.—Synchronous teletypewriter operation does not in all cases have to rely upon elements of the transmitted character to maintain proper position in relation to the receiving device. External timing signals may be used that allow the start and stop elements to be discarded. You will then see only the elements necessary to convey a character.
Synchronous systems have certain advantages over asynchronous systems. The amount of time taken to transmit stop and start elements is made available for information transmission rather than for synchronizing purposes. Only the intelligence elements are transmitted. In start-stop signaling, the ability of the receiving device to select the proper line signal condition is dependent upon signal quality. For example, suppose the stop-to-start transition arrives before it should; then, because of atmospherics, all subsequent selection positions in that character will appear earlier in time in each code element. A synchronous system has a higher capability for accepting distorted signals because it does not depend on a start-stop system for synchronization.
Several terms are used to refer to teletypewriter modulation rates or signaling speeds. These include BAUD RATE, BITS PER SECOND, and WORDS PER MINUTE. Baud is the only term that is technically accurate. The other terms are either approximations or require explanation.
The word baud by definition is a unit of modulation rate. You will sometimes see it used to refer to a signal element, but this reference is technically incorrect. Baud rate is the reciprocal of the time in seconds of the shortest signal element. To find the modulation rate of a signal in bauds, you must divide the number 1 by the time duration of the shortest unit interval present in the signal. For example, 22 milliseconds (.022 seconds) is the time interval of the shortest unit in the five-unit code at 60 words per minute. To find the number of bauds corresponding to 60 words per minute, divide 1 by .022. Rounding off the result of the division gives us the number 45.5, which is the baud equivalent of 60 words per minute. Each increase in words per minute will correspondingly decrease the signal unit time interval. (The defense communications system standard speed for teletypewriter operation is 100 words per minute or 75 baud.)
Words per minute is used only when speaking in general terms for an approximation of speed. The term 100 words per minute means 100 five letter words with a space between them can be transmitted in a 60-second period. However, you can obtain this nominal words-per-minute rate in several systems by
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varying either modulation rate or the individual character interval (length). For this reason, the modulation rate (baud) method of reference rather than words per minute is used.
BIT is an acronym for the words binary digit. In binary signals, a bit is equivalent to a signal element. Because of the influence of computer and data processing upon our language, modulation rate is sometimes expressed in bits per second. When you understand all signal elements being transmitted are of equal length, then the modulation rate expressed in bits per second is the same as the modulation rate expressed in baud.
You were told the two conditions mark and space may be represented by any convenient means. The two most common are NEUTRAL and POLAR operation. In neutral, current flow represents the mark, and no current flow represents the space; in polar operation, current impulses of one polarity represent mark, and impulses of the opposite polarity of equal magnitude represent the space.
NEUTRAL.—Neutral circuits make use of the presence or absence of current flow to convey information. A neutral teletypewriter circuit is composed of a transmitting device, a battery source to supply current, a variable resistor to control the amount of current, a receiving device, and a line for the transmission medium.
POLAR.—Polar operation differs from neutral operation in two ways. Current is always present in the polar system, and it is either positive or negative. A polar teletypewriter circuit contains the same items as a neutral circuit plus an additional "battery" source. The battery referred to here is not an actual battery but is a solid-state dc power supply. It provides variable current to the teletypewriters. The reason for having an extra battery source is because polar circuits use positive battery for marks and negative battery for spaces.
You will find in polar operation that the distortion of a signal is almost impossible through low line currents, high reactance, or random patching of signal circuits or equipment. In polar signaling when you experience a complete loss of current (a reading of zero on a milliammeter), you know you have line or equipment trouble; whereas the same condition with neutral signaling may indicate a steady space is being transmitted. This gives us a condition called RUNNING OPEN. Under this condition, the teletypewriter appears to be running because the machines is decoding the constant space as the Baudot character blank and the type hammer continually strikes the type box but there is no printing or type box movement across the page.
Q15. What are the two teletypewriter modes of operation?
Q16. Define baud.
Q17. Define bit.
Q18. What are the two types of dc operations used to represent mark and space conditions?
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When two ttys are connected by communications wire or cable (over short or long distances), the exchange of information between them is direct. When the teletypewriters are not physically joined, exchange of information is more involved. Direct-current mark and space intervals cannot be sent through the air. The gap between the machines must be bridged by radio using a radio transmitter and receiver. The transmitter produces a radio frequency carrier wave to carry the mark and space intelligence. A KEYER is needed to change the dc pulses from the tty into corresponding mark and space modulation for the carrier wave in the transmitter. The radio receiver and a CONVERTER are required to change the radio frequency signal back to dc pulses.
The Navy uses two basic radio teletypewriter (ratt) systems. These are the TONE-MODULATED SYSTEM, referred to as audio-frequency tone shift (afts), and the CARRIER-FREQUENCY SHIFT SYSTEM, referred to as radio-frequency-carrier shift (rfcs). The rfcs system is also called frequency-shift keying (fsk).
Figure 3-19 shows a modulated carrier wave with audio tone impulses impressed on the radio- frequency carrier wave. These correspond to dc mark and space signals.
We can best explain the rfcs signal by comparing it to the on-off cw signal. Cw signals are essentially a constant frequency with no variations along the frequency axis. Figure 3-20, view A, is an example. The complete intelligence is carried as variations in the signal amplitude. Figure 3-20, view B, shows the same signal as a shift in frequency between the mark and space.
3-21
AUDIO FREQUENCY TONE SHIFT.—Tone-modulated (afts) systems use amplitude modulation to change dc mark and space impulses into audio electrical impulses.
A basic tone-modulated system is shown in figure 3-21. Conversion to audio tones is accomplished by an audio oscillator in the tone converter. Rapid varying of the tone, according to the characters transmitted from the teletypewriter equipment, amplitude modulates the carrier wave in the transmitter. The receiver receives the modulated signal and separates the audio signal from the carrier. This process of separating the modulated signal is known as detection or demodulation.
3-22
RADIO-FREQUENCY-CARRIER SHIFT.—For frequency-shift (fsk) systems, the transmitter provides a source of radio-frequency excitation. Figure 3-22 illustrates a basic frequency-shift keyed system. In modern systems, the keyer is built into the transmitter. The keyer shifts the signal box below or above the assigned frequency to correspond with the mark or space required to transmit tty characters. Normally the keyer is adjusted for an 850-hertz spread, 425 hertz above and 425 hertz below the assigned frequency. A spacing impulse will be 425 hertz above the operating frequency, and a marking impulse will appear 425 hertz below.
3-23
In both the tone-modulated system and the carrier-frequency shift system, all tty signals pass through the tty panel that controls the looping current in all the circuits. Looping current is the current supplied by the tty battery. The tty panel integrates the tone-modulated and the carrier-frequency shift systems. It provides every possible interconnection of available tty equipment. With this configuration maximum operational flexibility is achieved with the least amount of circuitry and equipment.
Q19. What is the function of a keyer?
Q20. What is the function of a converter?
Q21. Basically describe an afts system.
Q22. Basically describe an rfcs system.
Figure 3-23 shows an rfcs teletypewriter transmit communications system. You should refer to this figure frequently while reading the functional descriptions of the equipment shown.
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TELETYPEWRITER SETS.—Most of the teletypewriter sets used by the Navy belong to one family of tty equipment. This equipment features various weights and sizes, quiet operation, and high operating speeds. They present relatively few maintenance problems. Because of this they are well suited for severe shipboard conditions of roll, vibration, and shock.
These teletypewriters operate at various speeds. Conversion from one speed to another is usually only a matter of changing the gears that are located within the equipment.
Teletypewriters may be send/receive units or receive only units. They may be designed as floor models, table models, or rack and wall-mounted sets. The teletypewriter shown is a send/receive floor model.
The teletypewriter receives messages and prints them on page-size copy paper. In addition, it can receive and record messages on perforated tape. You can use the keyboard or perforated tape to send messages. Page print monitoring is available with both methods. The set shown can prepare perforated and printed tape for separate transmission. It does this with or without simultaneous transmission and page-print monitoring. The combinations of services available are extensive.
The tty set may include a CABINET, KEYBOARD, PAGE PRINTER, TYPING PERFORATOR, TRANSMITTER DISTRIBUTOR, TYPING REPERFORATOR, power distribution panels, and a power supply.
In operation, the components are linked by electrical or mechanical connections. You are given a wide range of possibilities for sending, receiving, or storing tty messages. All equipment components are housed within the cabinet. Transmission signals are initiated through the keyboard (kybd) or through the
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transmitter distributor (td). Signals received or local transmissions can be monitored on the page printer. The typing perforator and typing reperforator are devices for preparing tapes on which locally initiated or incoming tty messages can be stored for future transmission through the td.
COMMUNICATION PATCHING PANELS.—Ttys are provided flexibility by jacks that are used to terminate all ttys and associated equipment. The jacks are wired in communications patching panels, usually referred to as tty patch panels. You are able to connect any combination of equipment electrically by means of patch cords.
The plugs on the patch cords are inserted into the jacks at the front of the panel. These plugs have three different parts. They are the tip, ring, and sleeve. The tip carries the intelligence signal while the ring carries the synchronizing (step) or timing signals. The sleeve carries an alarm signal that indicates (both visually and audibly) a problem to the operator. The problem may be equipment failure, loss of loop current, or improper patching. Commonly used combinations of equipment are often wired together within the panel (called normal-through). Individual pieces of equipment are wired on jacks to allow you to use them alone or in combination.
Tty patch panels also furnish a central point for connecting the dc voltage supply into the tty circuits. One source of supply can be used for all circuits passing through a particular panel.
RED and BLACK are used on patch panels to identify whether that panel is used for passing secure or nonsecure information. Red indicates that secure (encrypted) information is being passed through the panel. Black indicates that nonsecure (unencrypted) information is being passed. Patch panels through which secure information is passed are indicated by a red sign on the front that has inch high white block letters that say "RED PATCH PANEL." Panels through which nonsecure information is passed are indicated by two black signs on the front with inch high white block letters. One sign says "BLACK PATCH PANEL" and the other "UNCLAS ONLY."
Each panel contains six channels. Each channel has its own series circuit of looping jacks, set jacks, and a rheostat for adjusting line current. The number of looping and set jacks in each channel varies with the panel model. Each panel includes a meter and rotary selector switch for measuring the line current in any channel. There are six miscellaneous jacks. Any tty equipment not regularly assigned to a channel, may be connected to one of these jacks.
If the desired tty equipment is wired in the same looping channel as the radio adapter used, no patching is required. But, if the desired tty is not wired in the same looping channel as the keyer or converter, it must be patched. For example, let's put a tty on channel 1 and a converter on channel 3. If you want to receive, you must insert one end of the patch cord in the set jack for channel 1 and the other end in either one of the two looping jacks of channel 3.
In any switching operation between the plugs and jacks of a tty panel, the cord plug must be pulled from the looping jack before you remove the other plug from the set (machine) jack. Pulling the plug from the set jack first opens the circuits to the channel, causing all tty messages in the channel to be interrupted.
Removing the set (machine) jack before the looping jack exposes a dangerous dc voltage on the exposed plug.
Q23. Most Navy tty sets operate at what speeds?
Q24. A receive tty set provides outputs in what formats?
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Q25. What does the color red indicate on a tty patch panel?
CRYPTOGRAPHIC EQUIPMENT.—Cryptographic equipment is used to ENCRYPT and DECRYPT tty messages that require security handling. (Encrypting is the method used to code a transmitted message; decrypting is used to decode a received message.) To code or decode any message, the send and receive cryptographic equipment must be compatible.
REMOTE TRANSMITTER CONTROL UNIT.—The remote transmitter-control unit is mounted close to the kybd and permits remote control of the transmitter (xmtr). It has a transmitter power on-off switch, a power-on indicator lamp, a carrier-on indicator lamp, and a three-position rotary selector switch. For rfcs operation you set the switch to CFS SEND to transmit and to CFS REC to receive. Use the TONE S/R position for both transmitting and receiving afts signals.
An audio frequency tone-shift system will be discussed later in this chapter.
TRANSMITTER TRANSFER SWITCHBOARD.—The transmitter transfer switchboard is used in this system to connect the remote transmitter control unit to the radio transmitter.
RADIO TRANSMITTER.—The radio transmitter transmits the tty signal. You should be careful when tuning the transmitter for rfcs operation. The carrier frequency setting is critical and must be properly set to ensure a correct output from the transmitter.
Q26. What are the functions of cryptographic equipment?
Figure 3-24 shows the rfcs receive system used to receive the transmitted signal and translate it back to a usable output. You should look at this figure while studying the units in this section.
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ANTENNA FILTER ASSEMBLY.—The antenna filter assembly is connected to the antenna and receives the rf signal from the antenna. It filters out any unwanted rf signals and allows the desired band of frequencies to pass.
RADIO RECEIVER.—The radio receiver takes the rf signal passed on by the antenna filter and translates it to an audio signal.
RECEIVER TRANSFER SWITCHBOARD.—The receiver transfer switchboard is used to tie the receiver to any converter unit connected to it. This allows you a wide selection of equipment for connection to the same receiver.
CONVERTER-COMPARATOR GROUP.—The converter-comparator group is used with receivers in either space or frequency diversity operation. When diversity operation is not required, each converter can be used separately with a single receiver.
Each converter has its own COMPARATOR circuitry. This built-in design feature results in a considerable reduction in size from older units. The comparator was located in a separate chassis in the older units. Size has been further reduced through the use of microelectronics.
Figure 3-25 shows the basic method we use to convert a frequency-shift rf signal into a signal that controls the dc loop of a tty. The frequency shifts of the af output from the receiver are converted into dc pulses by the af discriminator. The dc pulses are then fed into the keyer. The keyer opens and closes the dc loop of the tty according to the mark and space characters received.
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In diversity operation the comparator section of the converter-comparator group (shown in figure 3- 24) compares the strength of the signals from two receivers. Signals from each converter are fed into a comparator circuit that compares the signals. This comparison is displayed on a crt on the front of the equipment. The comparison is in the form of LISSAJOUS PATTERNS. A lissajous pattern is a combined, simultaneous display of the amplitude and phase relationships of two input signals. One signal is applied to the vertical and the other to the horizontal deflection circuits. Lissajous patterns have many applications in electronics. They have operational uses as well as uses in corrective and preventive maintenance. Further coverage on lissajous patterns can be found in NEETS, Module 19, The Technician's Handbook. Figure 3-26 shows several typical lissajous monitoring patterns for the converter- comparator group. Once we have a correctly tuned signal, the comparator feeds it to the communication patching panel for patching to the tty. Now let's refer back to figure 3-24 while we discuss the rest of the units in the system.
COMMUNICATION PATCH PANEL.—The communication patch panel serves the same functions on the receive side of the rfcs system as it did on the transmit side. It routes the dc signal to the
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proper cryptographic equipment. It also routes the decoded teletypewriter signal from the cryptographic equipment to the selected tty.
CRYPTOGRAPHIC EQUIPMENT.—The cryptographic equipment converts the transmitted coded signal to a decoded signal that can be printed out in its original state.
TELETYPEWRITER.—The tty equipment is used to convert the dc signal received from the communication patch panel to a printed copy of the original transmitted message. The tty shown is used only for receive and does not have the ability to transmit.
Q27. What are the functions of a converter-comparator group?
Figure 3-27 is a simplified block diagram of a HALF-DUPLEX (send or receive) uhf, audio- frequency-tone shift system. A half-duplex communications circuit permits two-way communications between stations. Communications can be in either direction but not simultaneously. The term half-duplex is qualified by adding send only, receive only, or send or receive. Let's use the block diagram to trace a signal through the system.
SIGNAL FLOW.—On the transmit side, dc signals from the tty set are fed to the communication patching panel. From the panel they are patched to the tone terminal set. The tone terminal set converts
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the dc signals into audio tone-shift signals. These signals are then patched to the transmitter section of the transceiver through the transmitter transfer switchboard. The audio tone-shift signals modulate the rf carrier generated by the transmitter (xmtr). The rf tone-modulated signals are then radiated by the antenna.
On the receive side, the rf tone-modulated signals are received at the antenna. You then patch the signal via the multicoupler to the receiver section of the transceiver. Demodulation takes place at this point. The resulting audio tone-shift signals are then patched through the receiver transfer switchboard. The signals now go from the switchboard to the tone terminal set, where they are converted back to dc signals. The dc signals are then patched through the communication patching panel to the tty for printing.
TONE TERMINAL SET.—In tone modulation transmission, the tty pulses are converted into corresponding audio tones. These tones amplitude modulate the rf carrier in the transmitter. Conversion to audio tones is accomplished by an audio oscillator in the tone converter.
An internal relay in the tone converter closes the control line to the transmitter. This keys the transmitter on the air when the operator begins typing a message. The transmitter remains keyed until after the message has been transmitted.
On the receive side, the tone converter accepts the mark and space tones coming in from a receiver and converts them into signals suitable to operate a relay in the converter. The make and break contacts of the relay are connected in the local tty dc loop circuit. This causes the teletypewriter to print in unison with the mark and space signals from the distant tty.
The number of communications networks in operation throughout any given area is increasing. As a result, all areas of the rf spectrum have become highly congested.
The maximum number of intelligible transmissions taking place in the radio spectrum is being increased through the use of MULTIPLEXING. Multiplexing is the simultaneous transmission of a number of intelligible signals (messages) in either or both directions using only a single rf carrier. You may use two methods of multiplexing. These are TIME-DIVISION and FREQUENCY-DIVISION.
TIME-DIVISION.—With AM voice and tone communications, we want to transmit and receive for 360 degrees of each sine wave. However, an audio signal may be transmitted and received satisfactorily by periodically sampling the signal. The sampling process yields a received signal like the one shown in figure 3-28. There is no limit to the maximum number of samples that may be made, but you must sample at least twice per cycle of audio to get satisfactory results. In practical systems, 2.4 samples per cycle are usually taken. This concept of sampling forms the basis for time-division multiplex (tdm) operation.
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Figure 3-29, view A, illustrates, the fundamental principle of tdm. Let's look at an example. Assume that a 3,000-hertz tone is applied to each of the six channels in the transmitter. Assume also that the rotating switch turns fast enough to sample, in turn, each of the six channels 2.4 times during each cycle of the 3,000-hertz tone. The speed of rotation of the switch must then be 2.4 × 3,000 or 7,200 rotations per second. This is the optimum sampling for a practical system.
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When the transmitter and receiver switches are synchronized, the signals will be fed in the proper sequence to the receiver channels. The samples from transmitter channel one will be fed to receiver channel one. In this way, many channels of audio are combined to form a single output (multiplexed) chain. Time spacing occurs between the components of the separate channels. The chain is transmitted (via wire or radio path) to distant demultiplexing receivers. Each receiving channel functions to select and reconstruct only the information included in the originally transmitted channel.
In most present day applications, electronic switching is used as the sampling component. The main advantage to electronic sampling is the longer life of an electronic switch when compared to an electromechanical switch. We use a mechanical system in our example to make this concept easier for you to see.
Now let's look at figure 3-29, view B, where channel one is shown sampled four times. (This is the output of channel one in our transmitter.) Figure 3-29, view C, shows all six channels being sampled four times during each cycle. (This is the output of the rotating switch in our transmitter.) What you see here is a continuous, time-sharing waveform.
More than six channels (perhaps 24 or more) may be used. As we increase the number of channels, the width of each sample segment must be reduced. The problem with reducing the width of the pulse is that the bandwidth (bw) necessary for transmission is greatly increased. Decreasing the pulse width decreases the minimum required rise time of the sampling pulse and increases the required bandwidth. When you increase the number of channels, you increase the bw. The bw is also affected by the shape of the sampling pulse and the method used to vary the pulse.
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Common methods of time-division multiplexing include PULSE AMPLITUDE MODULATION (pam), PULSE WIDTH or PULSE DURATION MODULATION (pwm or pdm), PULSE POSITION MODULATION (ppm), and PULSE CODE MODULATION (pcm). We have been studying an example of pulse amplitude modulation. (These methods of tdm were discussed in NEETS, Module 12, Modulation Principles.)
FREQUENCY DIVISION.—Frequency division multiplexing (fdm), unlike tdm, transmits and receives for the full 360 degrees of a sine wave. Fdm used presently by the Navy may be divided into two categories. One category is used for voice communications and the other for tty communications.
The normal voice speaking range is from 100 to 3,500 hertz. During single channel AM voice communications, the audio frequency amplitude modulates a single rf (carrier frequency). However, in voice fdm, each voice frequency modulates a separate frequency lower than the carrier frequency (subcarrier frequency). If these subcarrier frequencies are separated by 3,500 hertz or more, they may be combined in a composite signal. This signal modulates the carrier frequency without causing excessive interference.
In figure 3-30, the output of channel one is the voice frequency range of 100 to 3,500 hertz. The output of channel two is the combination of a different voice frequency with a subcarrier frequency of 4,000 hertz. The output of channel three is another voice frequency. This voice frequency combined with a subcarrier frequency of 8,000 hertz gives you an output frequency range of 8,100 to 11,500 hertz. The overall bw for the composite modulation package shown is 100 to 15,500 hertz. Each separate channel occupies its own band of frequencies. The composite signal is used to modulate the carrier frequency of the transmitter.
Multichannel broadcast and ship/shore terminations use tty fdm. With this system, each channel of the composite tone package of the broadcast is assigned an audio frequency. By multiplexing tty circuits, up to 16 circuits may be carried in any one of the 3,000 hertz multiplexed channels described above. Don't confuse the two types of multiplexing. In the first case, 3,000 hertz audio channels have been combined. In the second case, a number of dc tty circuits are converted to tone keying and combined in a single 3,000-hertz audio channel. Figure 3-31 illustrates a 16-channel, tty-multiplexing system. The output of the dc pulsed circuits is converted to audio keying. Each channel has a separate audio center frequency. Channel frequencies range from 425 hertz for the lowest channel to 2,975 hertz for the highest
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channel. A mark in an individual tty loop keys an audio tone 42.5 hertz below the center frequency. A space in the input signal keys an audio tone 42.5 hertz above the center frequency. Let's look at an example. The mark and space frequencies for channel one are calculated as 382.5 hertz and 467.5 hertz, respectively (425 ± 42.5). Combining these keyed tones into a composite signal results in a tone package within a standard 3,000-hertz bandwidth. By occupying no more than 3,000 hertz of the audio spectrum, the output signal is suitable for transmission via radio or landline.
Q28. What is the function of a tone terminal set?
Q29. What are the two types of multiplexing?
Q30. What is the purpose of multiplexing?
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FACSIMILE (fax) is a method of transmitting still images over an electrical communications system. The images, called "pictures" or "copy" in fax terminology, may be weather maps, photographs, sketches, typewritten or printed text, or handwriting. Figure 3-32 shows a facsimile transceiver. You must realize that the still image serving as the fax copy or picture cannot be transmitted instantly in its entirety. Three distinct operations are performed. These are (1) scanning, (2) transmitting, and (3) recording or receiving.
Scanning consists of subdividing the picture in an orderly manner into a large number of segments. This process is accomplished in the fax transmitter by a scanning drum and phototube arrangement.
The picture you want to transmit is mounted on a cylindrical scanning drum. This drum rotates at a constant speed and at the same time moves longitudinally along a shaft. Light from an exciter lamp illuminates a small segment of the moving picture and is reflected by the picture through an aperture to a phototube. During picture transmission, the light crosses every segment of the picture as the drum slowly spirals past the fixed lighted area.
The amount of light reflected back to the phototube is a measure of the lightness or darkness of the segment of the picture being scanned. The phototube changes the varying amounts of light into electrical signals. These are used to amplitude modulate the constant frequency output of a local oscillator. The modulated signal is then amplified and sent to the radio circuits.
Signals received by the fax receiver are amplified and actuate a recording mechanism. This recorder makes a permanent recording (segment by segment) on paper. The paper is attached to a receiver drum similar to the one in the fax transmitter. The receiver drum rotates synchronously with the transmitter drum. Synchronization of the receiver and transmitter is done to reduce distortion. Synchronization is obtained by driving both receiver and transmitter drums with synchronous motors operating at the same speed. Drum rotation continues until the original picture is reproduced. The recording mechanism may reproduce the picture photographically by using a modulated light source shining on photographic paper or film. It may also reproduce directly by burning a white protective coating from specially prepared black recording paper.
The receiver drum is FRAMED with respect to the transmitter drum by a series of phasing pulses that are transmitted just before transmission. The pulses operate a clutch mechanism that starts the
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scanning drum in the receiver. This ensures proper phasing with respect to the starting position of the scanning drum in the transmitter.
Figure 3-33 is a block diagram of the equipment necessary for radio facsimile operation. View A shows the receiving system. This system consists of a standard radio receiver, a frequency-shift converter, and a facsimile recorder. View B shows two systems for transmitting TIF signals. The upper row of blocks is for carrier-frequency shift transmission. This system consists of a facsimile transceiver, a keyer adapter, a frequency shift keyer and a transmitter capable of fsk emission. The lower row of blocks is for audio-frequency shift transmission and uses a fax transceiver, a radio modulator, and an AM transmitter.
Security, quality monitoring, and safety are important areas that you must be aware of. If the fundamentals are followed, you will see higher quality communications. You will also help meet the communications goals of the Navy. Let's find out what these fundamentals are and what they will do for you.
Compromising emanations (ce) are, generally referred to as TEMPEST. These signals may be unintentional, data-related, or intelligence-bearing. If intercepted or analyzed, these signals could disclose classified information. TEMPEST problems are associated with material transmitted, received, handled, or otherwise processed by electrical information processing equipment or systems. Any electrical information processing device may cause problems. Even your electric typewriter or a large, complex data processor may emit interceptable compromising emanations. Some countermeasures taken to ensure against TEMPEST problems are listed below:
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Transmission security includes all measures designed to protect transmission from interception, traffic analysis, and imitative deception. Every means of transmission is subject to interception. In radio transmission, it should be assumed that all transmissions are intercepted.
Three fundamental requirements of a military communications system are reliability, security, and speed. Reliability is always first. Security and speed are next in importance and, depending on the stage of an operation, are interchangeable. During the planning phase, security is more important than speed. During the execution phase, speed sometimes passes security in importance.
When a message is transmitted by radio, the originator may know some of those who are receiving it, but will never know all of those who are receiving the message. You must assume that an enemy receives every transmission. Property prepared messages using modern cryptographic systems may prevent an enemy from understanding a message. However, they can still learn a lot. For example, as time for a planned operation approaches, the number of messages transmitted increases. An enemy then knows that something will occur soon, and their forces are alerted. Strict radio silence is the main defense against radio intelligence.
The amount of radio traffic is not the only indicator used by an enemy. Statistical studies of message headings, receipts, acknowledgments, relays, routing instructions, and services are also used by an enemy. Communications experts can often learn much about an opponent from these studies. Direction finders are another aid the enemy can use to determine where messages originate.
Radiotelephone networks are operated so frequently that many operators tend to be careless. There are too many instances of interception of vhf and uhf transmissions at distances of many thousands of miles. You may have occasion to work on or around this type of equipment. If you are ever required to bring any transmitter on the air for any purpose, you must be familiar with and use all the correct procedures.
Q31. The transmission of still images over an electrical communications system is known as what?
Q32. The term TEMPEST refers to what?
Q33. What are the three fundamental requirements of a military communications system?
Q34. Which of the above requirements is most important?
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In recent years the volume of shipboard communications has increased greatly. This expansion has led to the shipboard installation of sophisticated equipment. Factors such as frequency accuracy and dc signal distortion are critical to the operation of communications systems. These systems demand precise initial lineup and monitoring to ensure satisfactory operations are maintained. System degradation is often caused by many small contributing factors. When these factors are added together, the system becomes unusable.
When you perform scheduled, logical checks that ensure continuous, optimum performance of shipboard communications systems, you are doing SCHEDULED MAINTENANCE. In many cases this maintenance prevents outages before they occur. Some of the scheduled checks will include the following:
Many complex electronic systems are installed aboard Navy ships. In modern ships, complex systems with higher power and greater sensitivity are being crowded into a restricted and corrosive area. Figure 3-34 is a Spruance class destroyer with its crowded (compact) communications environment. The ability of these systems to perform their individual functions without interference is known as ELECTROMAGNETIC COMPATIBILITY (emc). Emc is concerned with the structure of the ship and its electrical and electronic system. Compact environment is a major limitation to the effectiveness of a total ship system concept.
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Operation of a total ship system in this unique shipboard environment presents a challenge to all concerned. You must always consider the effects that motion, temperature variations, and exposure to adverse elements will have on the performance of the total ship system. This is particularly true on those system components that are mounted topside.
On board ship, you will find much attention is given to keeping the topside cosmetically and mechanically shipshape. It is equally important to keep it electronically shipshape. Minor mechanical problems, such as loose connections, broken bond straps, or rusty junctions can cause serious communications problems. These sources of electromagnetic radiations reduce receiver performance and are known as ELECTROMAGNETIC INTERFERENCE (emi). Sources of emi can be divided into the following broad categories:
• Functional. Emi can originate from any source designed to generate electromagnetic energy and which may create interference as a normal part of its operation. The interference may be unintentional or caused by other on board or adjacent platform systems. This interference also may be intentional or caused by electronic countermeasures (ECM).
• Incidental. Emi can originate from man-made sources. These are sources not designed specifically to generate electromagnetic energy but which do in fact cause interference. Examples of incidental emi sources include power lines, motors, and switches.
• Natural. Emi can be caused by natural phenomena, such as electrical storms, rain particles, and solar and interstellar radiation. It is recognized by the following audible noises:
• Hull-generated. Emi can be caused by the interaction of radiated signals with elements of the hull and rigging of a ship. (The functional signals themselves do not cause interference.)
The following are two general methods by which emi is transmitted:
Conduction. Undesired energy from one equipment is coupled to interconnecting cables or components of another equipment. This energy is conducted via the wiring in the shielded enclosure that protects sensitive circuits. You will find proper design, adequate isolation, and shielding of cables and equipment can control this problem.
Radiation. Energy is beamed directly from the transmitting antenna, or source, to the victim receiving antenna. When this interference is picked up by a receiver, you have two solutions. Interfering energy can be eliminated at the source or you can filter, or blank it out at the victim equipment. Filtering is far less desirable. Interference may be on the same frequency as the desired signal and will not be eliminated without affecting the reception of all desired signals.
Most unprotected shipboard receivers are susceptible to emi over a frequency range much wider than their normal bandpass. Off-frequency rejection rarely excludes strong, adjacent-channel signals. These signals enter the receiver and degrade receiver performance by being processed along with the desired, tuned signal. Usually, the presence of emi will be apparent to you. It has a bad effect. Upon the desired signal quality, such as that in CROSS-MODULATION where a spurious response occurs when the carrier
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of a desired signal intermodulates with the carrier of an undesired signal. Extremely strong, off-frequency signals may even burn out the sensitive front-end stages of a receiver. Emi also can degrade overall receiver performance in a less noticeable way. It does this by desensitizing the receiver front end. The noise level is raised and effectively lowers the signal to noise ratio and thus the sensitivity. This causes a decrease in desired signal amplification. For these reasons, shipboard receive systems are designed to include protective circuitry between the antenna and receiver to filter out off-frequency signals. This prevents or limits interference, desensitization, or burnout. Depending upon the system, these protective devices may include filters, multicouplers, preselectors, and so forth. These devices can minimize interference caused by inadequate frequency separation or poor physical isolation between transmit and receive antennas.
Q35. What is the purpose of QMCS?
Q36. What is emi?
Q37. What are the two emi transmission methods?
Radio-frequency (rf) transmitting systems with high-power transmitting tubes and high-gain antennas have increased the possibility of injury to personnel working in the vicinity.
An electromagnetic radiation hazard exists when electronic equipment generates a strong enough electromagnetic field to fall in a category listed below:
These hazardous situations can be caused by a transmitter or antenna installation. These generate electromagnetic radiation in the vicinity of personnel, ordnance, or fueling operations in excess of established safe levels. Sometimes the existing electromagnetic radiation levels increase to a hazardous level. When personnel, ordnance, or fueling evolutions are located in an area that can be illuminated by electromagnetic radiation, hazardous situations may occur.
Electromagnetic radiation is hazardous to personnel in two ways. It can cause rf burns; and it can cause biological, thermal, and neurological effects to personnel (RADHAZ). Because of the differences in characteristics and safety precautions required for each of the two types, they will be discussed separately.
An rf burn hazard is a hazardous condition caused by the existence of radio frequency (rf) voltages in places where they are not intended to be. Any ship with high-power hf transmitters is susceptible. Potentially hazardous voltages have been found in many areas. Some of these areas are lifelines, vertical ladders, ASROC launchers, gun mounts, rigging for underway replenishment, and boat davits. Another of these areas is on aircraft tied down on carrier and helicopter flight decks.
Whether or not an induced voltage creates an rf burn hazard depends on whether personnel will come into contact with the object being energized. Generally, only the voltage between an object and the deck is important. The rf burn occurs when a person comes into contact with a source of rf voltage in a
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manner that allows rf current to flow through the area of contact. Resistance of the skin to the current flow at the areas of contact causes heat. The effect of the heat on a person at the point of contact ranges from noticeable warmth to a painful burn.
The most useful and widespread technique in the reduction of rf burn hazards is the proper bonding and grounding of all metallic objects in the rf radiation field.
In some cases, the rf burn hazard can be eliminated only through the use of restrictive operating procedures. These procedures govern the simultaneous use of transmitting and cargo equipment. Techniques such as operation of transmitters at reduced power and the prohibition of simultaneous use of certain combinations of antennas, frequencies, and cargo handling equipment are used.
Figure 3-35 shows typical rf radiation hazard warning signs.
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Most studies on the subject of radiation hazards (RADHAZ) have emphasized the impact of electromagnetic radiation on man. Man is singled out because of the biological, thermal, and neurological effects that occur in human organs and other biological tissues. Certain organs of the body are considered to be more susceptible than others to the effects of electromagnetic radiation. Presently available information and experience indicate that the eyes and testes are the most vulnerable body organs. The overwhelming danger to date appears to be the hazard from thermal effects, which are a function of intensity of radiation and frequency. This is particularly true in the range of 1 to 3 gigahertz. Thermal effects appear to taper off in severity outside this range.
When the body is irradiated by energy from a point source, the total body surface is usually not exposed. The larger the area exposed and the larger the radiation power density, the higher the body temperature rise and the greater the hazard. Microwave radiation from a radar source will "cook" you internally, just as a microwave oven cooks a chicken.
An injury of great concern is that to the lens of the eye. Exposure of the lens to high-intensity microwaves may cause cataracts. Current medical evidence indicates that a significant temperature elevation of the lens is required for cataract formation. If exposure is limited to 10 milliwatts per centimeter squared, the lens temperature is not elevated to levels at which cataracts occur.
In addition to thermal effects, nonionizing radiation is known to produce nonthermal effects. An association of a biological hazard with the nonthermal effects has not been demonstrated.
A peculiar effect experienced by some personnel is the sensation of sound when they are exposed to pulsed microwave fields. This occurs at levels below stated hazard limits and is not, by itself, considered dangerous.
Q38. Electromagnetic radiation is hazardous to personnel in what two ways?
Q39.
Now that you have completed this chapter, a short review of what you have learned is in order. The following summary will refresh your memory of basic systems equipment, its principles, terms, and typical circuitry required for you to understand this concept.
A RADIO SET CONTROL UNIT is used to remotely control certain transmitter and receiver functions.
TRANSMITTER TRANSFER SWITCHBOARDS selectively transfer remote control station functions and signals to transmitters.
RECEIVER TRANSFER SWITCHBOARDS transfer receiver audio outputs to remote control station audio circuits.
A TRANSMITTER generates an rf carrier, modulates it with intelligence, amplifies it, and applies it to an antenna.
An ANTENNA COUPLER is a device used for impedance matching between an antenna and a transmitter or receiver.
A RECEIVER receives electromagnetic energy (rf) and may convert it to a visible or audible form.
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MULTICOUPLERS patch several receivers or transmitters to one antenna. They also filter out harmonics and spurious responses, and provide impedance matching.
MARKING is when a circuit is closed and current flows in teletypewriter operation.
SPACING is when a circuit is open and no current flows in teletypewriter operation.
INTELLIGENCE is any signal that conveys information (voice, teletypewriter, facsimile).
A START unit is the first unit of a teletypewriter signal. It is always a space.
A STOP unit is the last unit of a teletypewriter signal. It is always a mark.
A TRANSITION is the time it takes to shift from a mark to a space condition or from a space to a mark condition.
A CODE in teletypewriter operation is a combination of mark and space conditions representing symbols, figures, or letters.
NONSYNCHRONOUS teletypewriter operation is when both transmitter and receiver do not operate continuously.
SYNCHRONOUS teletypewriter operation is when both transmitter and receiver operate continuously.
WORDS-PER-MINUTE is an approximate rate of speed. It means the number of five letter words with a space between them that can be transmitted or received in a one-minute period.
BAUD is a measurement of speed based on the number of code elements or units per second.
BITS-PER-SECOND is an acronym of the words binary digit. One bit is equal to one signal unit or element.
NEUTRAL teletypewriter operation is where current flow represents a mark and no flow represents a space.
POLAR teletypewriter operation is where current flow of one polarity represents a mark and current of the opposite polarity is a space.
RUNNING OPEN is the teletypewriter condition where the type hammer constantly strikes the type box but does not print or move across the page.
A KEYER is a device that changes dc pulses to mark and space modulation for teletypewriter transmissions.
A CONVERTER changes an audio signal back to dc pulses during teletypewriter reception.
AUDIO FREQUENCY TONE SHIFT systems use amplitude modulation to change dc mark and space impulses into audio impulses.
RADIO FREQUENCY CARRIER SHIFT systems use a keyer to shift a radio frequency signal above or below an assigned frequency. These shifts correspond to marks and spaces.
A TELETYPEWRITER is a machine that can transmit and or receive letters, numbers, or symbols. It may have a keyboard similar to a typewriter.
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A PERFORATOR is a device that stores a teletypewriter message on a paper tape by punching Baudot coded messages into it.
TRANSMITTER DISTRIBUTOR is a device that reads Baudot code from paper tape and allows a message to be sent or a message to be printed on a page printer.
A REPERFORATOR stores an incoming tty signal on paper tape.
A PAGE PRINTER prints teletypewriter characters one at a time in a full-page format. This is usually a high-speed printer.
RED is the reference color of equipment that passes classified information. It normally refers to patch panels.
BLACK is the reference color of equipment that passes unclassified information. It normally refers to patch panels.
A PATCH PANEL is used to tie a receiver or transmitter to its associated equipment.
A COMPARATOR compares incoming signals and selects the strongest to be fed to a teletypewriter through a patch panel. This is used in diversity operation
A LISSAJOUS PATTERN is a combined, simultaneous display of the amplitude and phase relationships of two input signals on a crt.
A TONE-TERMINAL set converts tty dc pulses into audio tones for modulation of a transmitter in audio-frequency tone shift transmissions.
MULTIPLEXING is the process of transmitting a number of intelligence signals simultaneously over a single rf carrier.
TIME-DIVISION multiplexing is the process that periodically samples several intelligence signals. This can be a received signal or one to be transmitted.
FREQUENCY-DIVISION multiplexing transmits and receives the full 360 degrees of each sine wave.
FACSIMILE is the method for transmitting and receiving still images. These images can be maps, photographs, and handwritten or printed text.
SCANNING is the process of subdividing a picture in an orderly manner into segments. This is used in facsimile transmission.
FRAMING is the process of synchronizing a facsimile receiver to a transmitter. This allows proper picture reproduction.
TEMPEST is a term normally used to describe compromising emanations. These emanations are unintentionally radiated signals that could disclose classified information.
ELECTROMAGNETIC INTERFERENCE is a term used to describe the degradation of a receiver or system by externally produced rf energy.
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ANSWERS TO QUESTIONS Q1. THROUGH Q39.
A1. To convert energy electrical/acoustic to acoustic/electrical and to key/unkey a transmitter. Also it mutes a receiver when transmitting.
A2. Transferring remote control functions and signals to transmitters.
A3. Transfers receiver audio outputs to remote control stations.
A4. 800 watts.
A5. Automatic, semiautomatic, and manual.
A6. It matches the impedance of an antenna to that of a transmission line at any desired frequency.
A7. To aid in heat transfer and prevent corona and arcing.
A8. Lsb, usb, isb, AM, cw, fsk.
A9. Digital.
A10. To connect an antenna/transmission line to a receiver/transmitter.
A11. Patching and filtering and permits the multiple use of receivers and/or transmitters on a single antenna.
A12. Space and mark.
A13. Intelligence (5), start (1), stop (1).
A14. Shift signals.
A15. Synchronous and nonsynchronous.
A16. A unit of modulation rate.
A17. Binary digit.
A18. Neutral and polar.
A19. Converts dc to corresponding mark and space modulation.
A20. Converts the audio signal to dc pulses.
A21. Uses AM to change dc to audio.
A22. A keyer provides rf excitation, which can be shifted above or below the assigned frequency.
A23. 60, 75, or 100 wpm.
A24. Page-size copy paper and perforated tape.
A25. It handles classified information.
A26. To code or decode messages.
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A27. The comparator compares the signal strengths from the receivers and the converter converts the frequency-shift rf signal into a tty set dc loop control signal.
A28. It converts dc to audio or vice versa.
A29. Time-division and frequency-division.
A30. It allows simultaneous transmission of multiple signals on a single transmission path.
A31. Facsimile.
A32. Compromising emanations.
A33. Reliability, security, and speed.
A34. Reliability.
A35. To ensure continuous, optimum performance of communications systems.
A36. Electromagnetic interference.
A37. Conduction and radiation.
A38. Rf burns and biological, thermal, and neurological effects.
A39. Proper bonding and grounding.
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Upon completion of this chapter you will be able to:
1. Describe the basic operation of the two types of satellites.
2. Describe the basic components of an operational satellite system.
3. Describe the function of earth terminal equipment.
4. Describe the basic signal flow of a typical shipboard receive-only system.
5. Describe the basic signal flow of a typical shipboard transceiver system.
6. Describe the advantages of satellite communications in terms of capacity, reliability, vulnerability, and flexibility.
7. Describe the limitations of satellites in terms of power, receiver sensitivity, and availability.
The first artificial satellite was placed in orbit by the Russians in 1957. That satellite, called Sputnik, signaled the beginning of an era.
The United States, who was behind the Russians, made an all-out effort to catch up, and launched Score in 1958. That was the first satellite with the primary purpose of communications.
The first regular satellite communications service was used by the Navy in 1960. The moon was used to bounce teletypewriter signals between Hawaii and Washington, D.C. During the early 1960s, the Navy used the moon as a medium for passing messages between ships at sea and shore stations. This method of communications proved reliable when other methods failed.
Military satellite communications technology was at a low level until 1965. At that time high quality voice transmissions were conducted between a satellite and two earth stations. That was the stepping stone to the Initial Defense Communications Satellite Program (IDCSP), which will be covered later in this chapter.
Experience with satellite communications has demonstrated that satellite systems can satisfy many military requirements. They are reliable, survivable, secure, and a cost effective method of telecommunications. You can easily see that satellites are the ideal, if not often the only, solution to problems of communicating with highly mobile forces. Satellites, if properly used, provide much needed options to large, fixed-ground installations.
For the past fifty years, the Navy has used high-frequency (hf) transmissions as the principal method of sending messages. In the 1970s, the hf spectrum was overcrowded and "free" frequencies were at a
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premium. Hf jamming and electronic countermeasures (ECM) techniques became highly sophisticated during that period. As a result the need for new and advanced long-range transmission methods became apparent.
Communications via satellite is a natural outgrowth of modern technology and of the continuing demand for greater capacity and higher quality in communications.
In the past, the various military branches have had the resources to support their communications needs. Predicted usage indicates that large-scale improvements will have to be made to satisfy future needs of the Department of Defense. These needs will require greater capacity for long-haul communications to previously inaccessible areas. Satellite communications has the most promise for satisfying these future requirements.
The Defense Communications Satellite Program (DCSP) was initiated by the Secretary of Defense in 1962. Phase I of the program was given the title Initial Defense Communications Satellite Program (IDCSP). The first satellite launch occurred in June 1966 when seven experimental satellites were placed into orbit. The final launch of this program consisted of eight satellites and occurred in June 1968.
The Phase II Defense Satellite Communications System (DSCP Phase II) has changed from an all- analog communications system to an all-digital communications system. The performance capability provided by the Phase II DSCS is limited by equipment availability. Extensive digital traffic capability has become common. You can credit this to the availability of digital modems (modulator/demodulator) and broadband equipment. Overall performance of the Phase II DSCS is a great improvement over the capabilities provided by Phase I DSCS. The Phase II satellites provide a great increase in effective radiated power and rf bandwidths. You will find these satellite configurations use wide coverage and narrow beam antennas. They provide an extensive range of communications services and capabilities. (This will be further discussed later, in this chapter.)
A satellite communications system uses satellites to relay radio transmissions between earth terminals. The two types of communications satellites you will study are ACTIVE and PASSIVE. A passive satellite only reflects received radio signals back to earth. An active satellite acts as a REPEATER; it amplifies signals received and then retransmits them back to earth. This increases signal strength at the receiving terminal to a higher level than would be available from a passive satellite.
A typical operational link involves an active satellite and two or more earth terminals. One station transmits to the satellite on a frequency called the UP-LINK frequency. The satellite then amplifies the signal, converts it to the DOWN-LINK frequency, and transmits it back to earth. The signal is next picked up by the receiving terminal. Figure 4-1 shows a satellite handling several combinations of links simultaneously.
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The basic design of a satellite communications system depends to a great degree upon the characteristics of the orbit of the satellite. In general terms, an orbit is either elliptical or circular in shape. A special type of orbit is a SYNCHRONOUS ORBIT. In this type you will find the period (time required for one revolution) of the orbit the same as that of the earth. An orbit that is not synchronous is called ASYNCHRONOUS. A period of orbit that approaches that of the earth is called NEAR SYNCHRONOUS (subsynchronous). Orbits are discussed in more detail later in this chapter.
In addition to the fundamental components shown in figure 4-1, the design of the overall system determines the complexity of the various components and the manner in which the system operates. Current satellites are capable of handling many teletypewriter (tty) and voice circuits at the same time.
Orbits generally are described according to the physical shape of the orbit and the angle of inclination of the plane of the orbit. These terms aye discussed in the following paragraphs:
PHYSICAL SHAPE.—All satellites orbit the earth in elliptical orbits. (A circle is a special case of an ellipse.) The shape of the orbit is determined by the initial launch parameters and the later deployment techniques used.
PERIGEE and APOGEE are two, of the three parameters used to describe orbital data of a satellite. These are shown on figure 4-2. Perigee is the point in the orbit nearest to the center of the earth. Apogee is the point in the orbit the greatest distance from the center of the earth. Both distances are expressed in nautical miles.
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ANGLE OF INCLINATION.—The ANGLE OF INCLINATION (angle between the equatorial plane of the earth and the orbital plane of the satellite) is the third parameter used to describe the orbit data of a satellite. Figure 4-3 depicts the angle of inclination between the equatorial plane and the orbital plane. Most satellites orbit the earth in orbital planes that do not coincide with the equatorial plane of the earth. A satellite orbiting in any plane not identical with the equatorial plane is in an INCLINED ORBIT.
The inclination of the orbit determines the area covered by the path of the satellite. As shown in figure 4-4, the greater the inclination, the greater the amount of surface area covered by the satellite.
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SPECIAL TYPES OF INCLINED ORBITS.—A satellite orbiting in a plane that coincides with the equatorial plane of the earth is in an EQUATORIAL ORBIT. A satellite orbiting in an inclined orbit with an angle of inclination of 90 degrees or near 90 degrees is in a POLAR ORBIT.
SPECIAL TYPES OF CIRCULAR ORBITS.—We stated previously that a circular orbit is a special type of elliptical orbit. You should realize a circular orbit is one in which the major and minor axis distances are equal or approximately equal. Mean height above earth, instead of perigee and apogee, is used in describing a circular orbit. While we are discussing circular orbits, you should look at some of the terms mentioned earlier in this chapter. A satellite in a circular orbit at a height of approximately 19,300 nautical miles above the earth is in a synchronous orbit. At this altitude the period of rotation of the satellite is 24 hours, the same as the rotation period of the earth. In other words, the orbit of the satellite is in sync with the rotational motion of the earth. Although inclined and polar synchronous orbits are possible, the term synchronous usually refers to a synchronous equatorial orbit. In this type of orbit, satellites appear to hover motionlessly in the sky. Figure 4-5 shows how one of these satellites can provide coverage to almost half the surface of the earth.
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Three of these satellites can provide coverage over most of the earth (except for the extreme north and south polar regions). A polar projection of the global coverage of a three-satellite system is shown in figure 4-6.
A satellite in a circular orbit at other than 19,300 nautical miles above the earth is in a near- synchronous orbit. If the orbit is lower than 19,300 nautical miles, the period of orbit of the satellite is less than the period of orbit of the earth. The satellite then appears to be moving slowly around the earth from west to east. (This type of orbit is also called subsynchronous.) If the orbit is higher than 19,300 nautical miles, the period of orbit of the satellite is greater than the period of orbit of the earth. The satellite then appears to be moving slowly around the earth from east to west. Although inclined and polar near-synchronous orbits are possible, near synchronous implies an equatorial orbit.
A satellite in a circular orbit from approximately 2,000 miles to 12,000 miles above the earth is considered to be in a MEDIUM ALTITUDE ORBIT. The period of a medium altitude satellite is considerably less than that of the earth. When you look at this altitude satellite, it appears to move rather quickly across the sky from west to east.
Q1. What are the two types of communications satellites?
Q2. A typical satellite communications operational link consists of a satellite and what two other components?
Q3. A satellite in a synchronous orbit can cover how much of the surface of the earth?
Q4. What areas of the earth are not normally covered by satellites?
Early communications satellites were limited in size to the diameter of the final stage of the rocket that was used for launching. Weight was determined by the thrust of the rocket motors and the maximum weight the rocket could lift into orbit.
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As early as June 1960, two satellites were successfully placed in orbit by the same launch vehicle. With the development of multilaunch capability, added flexibility became available. We then had choices as to the size, weight, and number of satellites to be included in each launch.
Using our multilaunch capabilities, the Defense Satellite Communications System (DSCS) has placed larger and heavier satellites in synchronous equatorial orbits. Figure 4-7 is a drawing of a DSCS satellite. It shows each pair of transmit and receive dish antennas. As you can see, a large area of the earth can be covered using only one satellite.
Early communications satellites were severely limited by the lack of suitable power sources. This severely limited the output power of the satellite transmitter. The only source of power available within early weight restrictions was a very inefficient panel of solar cells without battery backup. A major disadvantage of this type of power source is that the satellite has no power when it is in ECLIPSE (not in view of the sun). For continuous communications, this outage is unacceptable.
A combination of solar cells and storage batteries is a better prime power source. This is a practical choice, even though the result is far from an ideal power source. About ten percent of the energy of the sunlight that strikes the solar cells is converted to electrical power. This low rate is sometimes decreased even further. You find this when the solar cells are bombarded by high-energy particles that are sometimes found in space.
Early satellites had over 8,500 solar cells mounted on the surface of the satellite, which supplied about 42 watts of power. No battery backup was provided in these satellites.
Newer communications satellites have about 32,000 solar cells mounted on the surface of the satellite, and they supply about 520 watts. A nickel cadmium battery is used for backup power during eclipses.
Nuclear power sources have been used in space for special purposes, but their use stops there. Technology has not progressed sufficiently for nuclear power sources to be used as a power source.
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Satellite orientation in space is important for continuous solar cell and antenna orientation. Since the primary source of power in most satellites is from solar cells, a maximum number of the solar cells must be exposed to the sun at all times. The satellite antenna must also be pointed at the appropriate earth terminals. Our communications satellites use what is termed spin stabilization to meet these important requirements.
Spin stabilization operates on the principle that direction of the spin axis of a rotating body tends to remain fixed in space. An example of spin stabilization is the effect of the rotation of the earth in keeping its axis fixed in space. A satellite that has a spin axis parallel to the axis of the earth will maintain this position since both axes are fixed in space. Figure 4-8 illustrates the use of this principle. It depicts an equatorial orbit satellite used to keep a doughnut-shaped antenna pattern pointing toward the earth.
Once the system is in motion, spin stabilization requires virtually no additional energy. A spin- stabilized satellite is usually constructed like a flywheel. Its heavier equipment is mounted in the same plane and as close to the outside surface as possible.
After reaching its orbit, the radial jets are pulsed to start the satellite spinning. The satellite spin axis is orientated to the axis of the earth by means of small axial jets. Velocity jets are used to place the satellite in orbit position and provide velocity correction. Figure 4-9 is an example of spin stabilization.
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Solar cells are installed around the outside surface of a spin-stabilized satellite. This gives you a large number of solar cells exposed to the sun at all times (except when the satellite is in eclipse). The use of omnidirectional antennas causes a small part of the total radiated energy to be directed toward the earth at all times.
Ominidirectional antennas radiate only a small amount of energy toward the earth. Many techniques have been tried to achieve an earth-oriented antenna system. One system developed uses spin stabilization for orientation of the satellite. It uses a stationary inner platform for mounting remote controlled antennas. The satellite is constructed in two parts with both parts having radial jets. The inner platform contains the communications antennas and the communications package. After the satellite is stabilized in space, inner radial jets spin the inner platform. The inner platform is stationary with respect to earth and is oriented to such a position that the communications antennas point continuously toward the earth. This arrangement allows the use of high-gain directional antennas that concentrate the majority of the radiated energy in the direction of the earth.
The latest versions of communications satellites use a stationary platform with four high-gain antennas. Two steerable narrow beam antennas are used for communications between and within regions of high traffic density. Two horn antennas provide for earth communications between facilities outside the narrow beam coverage. The antenna arrangement for these types of communications satellites is shown in figure 4-7.
Q5. What was the major operational limitation of early communications satellites?
Q6. Satellite orientation in space is important for what two reasons?
Communications satellite earth terminals are usually located in areas remote from the actual users of these communications. This is necessary to minimize rf interference to the satellite.
Locating the terminals in these remote locations requires interconnecting communications links. Links permit communications flow to and from the users of the satellite systems. Interconnect links are usually connected via telephone cables or microwave radio links with normal terminal equipment.
Earth satellite communications terminals generally have a single, large antenna; a highly sensitive receiver; a powerful transmitter; multiplex equipment; modulating-demodulating equipment; and telemetry equipment. Each of these elements will be discussed later in this chapter.
Earth terminal antennas are highly directional, high-gain antennas capable of transmitting and receiving signals simultaneously. Generally, large, high-gain, parabolic antennas are used.
Generally speaking, three sizes of parabolic-type antennas are currently in use at earth terminal sites. These are a parabolic antenna sixty feet in diameter, a parabolic antenna forty feet in diameter, and a cluster of four parabolic antennas, each ten feet in diameter. These four in combination are equal to a parabolic antenna eighteen feet in diameter. They are shown in figures 4-10, 4-11, and 4-12, respectively.
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All satellite communications earth terminals are equipped with specially designed, highly sensitive receivers. These receivers are designed to overcome down-link power losses and to permit extraction of the desired communications information from the weak received signal. The terminals currently in use have specially designed preamplifiers mounted directly behind the antennas.
All earth terminal transmitters generate high-power signals for transmission to the communications satellites. High-powered transmitters and highly directional, high-gain antennas are combined in this configuration. This is necessary to overcome up-link limitations and to ensure that the signals received by the satellite are strong enough to be detected by the satellite. Each transmitter has an exciter/modulator and a power amplifier. The modulator accepts the input signal from the terminal equipment and modulates an IF carrier. The exciter translates the IF signal to the up-link frequency and amplifies it to the level required by the power amplifier.
Transmitters used in earth terminals have output power capabilities that vary from 10 watts to 20 kilowatts, depending on the type used and the operational requirements.
Telemetry equipment is included in all communications satellite systems. This permits monitoring of the operating conditions within the satellite. Telemetry can be used also for remote control of satellite operations, such as energizing axial jets for changing the spin axis of the satellite.
Q7. What type of antennas are generally used at earth terminals?
Q8. Why do earth terminals require highly sensitive receivers?
Q9. What is the range of earth terminal transmitter output power?
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The purpose of a shipboard receive-only system is to receive fleet multichannel teletypewriter broadcasts, which, as you recall from chapter 1, require no receipt. These are transmitted from a ground station and relayed to naval vessels by satellite.
Figure 4-13 is a typical shipboard receive-only system. In this system the transmitted carrier may be frequency modulated (fm) or phase-shift-key (psk) modulated for tty operation. The receiving antennas for this system are positioned about the ship. They are arranged in a manner (normally one in each quadrant of the ship) that at no time allows the line-of-sight to be blocked between the relay satellite and one or more of the antennas. Incoming signals pass from the antennas to an amplifier-converter. Each amplifier-converter routes an IF signal on one line of a twin axial cable that connects it to the combiner- demodulator. An operating power and local-oscillator signal are coupled from the combiner-demodulator to each amplifier-converter on the other line of the cable used for the IF signal. Because of signal path variations, shading, and reflections, the incoming signals are subject to random phase and amplitude variations. The combiner operation performed within the combiner-demodulator removes the phase variations from each input signal. It then measures the amplitudes of the signals for optimum combining and sums the signals. After being combined, the signal is demodulated and coupled from a receiver transfer switchboard to a telegraph demultiplex terminal.
Q10. What is the function of shipboard receive-only equipment?
Q11. What types of modulation are shipboard receive-only equipment designed to receive?
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An essential operation in communicating by satellite is the acquisition (locating) of the satellite by the earth terminal antenna and the subsequent tracking of the satellite. Initial acquisition depends upon an exact knowledge of the position of the satellite. In combination with the geographic location of the earth terminal, knowing the position of the satellite enables you to compute accurate antenna pointing information. The degree of difficulty in locating and tracking a satellite is determined largely by what type orbit the satellite is in.
The locating and tracking of a synchronous satellite is relatively simple. This is because the satellite appears to be stationary. Locating a near-synchronous satellite is also relatively simple because of the slow relative motion of the satellite However, the movement of a near-synchronous satellite is enough that accurate tracking is required to keep the narrow beam antenna pointed toward the satellite. Satellites in medium altitude circular orbits or in elliptical orbits are more difficult to acquire and to track because of the rapid changes in position.
To acquire and track a satellite in space, the earth terminal antennas must be provided with very accurate pointing information. Antenna pointing information is based upon the orbital prediction of the satellite. This information is derived from an EPHEMERIS table. This table provides the coordinates of a satellite or a celestial body at specific times during a given period. After you know the ephemeris data of a satellite, you can predict for any given location the apparent track of the satellite as viewed from that location.
The constants defining an orbit are initially obtained by the process of tracking. At the time of launch, the rocket is tracked by radar from lift-off to orbit and then until it passes out of sight. Tracking data obtained in this way is sufficient for making rough predictions of the orbit.  These other tracking stations watch for the satellite during its first trip and record additional data. During the first week of orbiting, tracking stations all around the world are obtaining progressively more accurate data concerning the Satellite. This data is put into a computer where corrections of earlier estimates of the orbit are made.
Once the initial predictions are complete and the satellite link becomes operational, very little change in these calculations is made. The orbits of a satellite will change slightly over a period of time; however, these changes are so gradual that predictions will be accurate enough to be used for weeks or even months without further corrections. When the orbits are known precisely, an ephemeris can be calculated for each satellite of the system.
Antenna pointing instructions for each satellite must be computed separately for each ground station location. A satellite that bears due south of station A at an elevation of 25 degrees may simultaneously bear due southeast of station B at an elevation of 30 degrees. Antenna pointing instructions are determined by taking into consideration the orbital prediction and the latitude and longitude of each ground station.
To establish radio contact with a satellite, the ground station needs to know the bearing and elevation of a satellite. This allows the antenna to be properly pointed.
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The acquisition of satellite signals by a ground station equipped with large antennas and operated at microwave frequencies places severe requirements on the system. Several factors must be considered. These factors are discussed below:
SPATIAL-TIME FACTOR.—Very accurate antenna pointing information is available to earth terminals from the satellite control facility located in Sunnyvale, California. Because of equipment limitations, a small search about the predicted location of the satellite must often be conducted to make initial contact. Either a manual or automatic scan is made around a small area close to the point where the satellite appearance is predicted.
FREQUENCY CONTROL.—The frequency of a radio signal received from a satellite is not generally the exact assigned down-link frequency. This variation depends upon the type of orbit of the satellite. The greatest frequency variations in signals from satellites occur in medium altitude circular or elliptical orbits. The smallest frequency variations occur in signals from satellites in near-synchronous or synchronous orbits.
When a particular satellite has been acquired, the earth terminal antenna will track that satellite for as long as it is used as a communications relay. Several methods of tracking are in actual use; however, we will explain PROGRAMMED TRACKING and AUTOMATIC TRACKING.
PROGRAMMED TRACKING.—In programmed tracking the known orbital parameters of the satellite are fed into computation equipment to generate antenna pointing angles. The antenna pointing angles are fed as commands to the antenna positioning servomechanisms. (You may want to review servos in NEETS, Module 15, Principles of Synchros, Servos, and Gyros.) These point the antenna in the required direction. The amount of data and computations involved in using programmed tracking is extensive. These are a result of the antenna mount flexing and atmospheric and ionospheric bending of radio waves. Because of these uncertainties, programmed tracking is not used extensively.
AUTOMATIC TRACKING.—In automatic tracking, the equipment generates antenna pointing information by comparing the direction of the antenna axis with the direction from which an actual satellite signal is received. Automatic tracking systems track the apparent position of a satellite. The direction of arrival of the radio signal and the real position of the satellite is not required. The automatic tracking system uses a servomechanism to move the antenna. Once the satellite has been located, the servomechanism generates its own pointing data. This eliminates the requirement for continuous data input and computation.
SATELLITE OUTAGE TIME.—The satellite outage time specifications allow for stewing (moving) the earth terminal antennas, acquiring the satellite signal, and checking for circuit continuity at HAND OVER. (Hand over is the period of time for one earth terminal to yield control to another as a satellite moves out of its area of coverage.) This hand over period represents an outage time. If the control terminal is unable to hand over to another terminal within a specified time, other arrangements are made. For example, control may be retained or transferred to another terminal within the coverage area. There are several reasons why a terminal may be unable to assume control on time; these reasons may combine to increase the outage time. The difference of drift velocities of the satellites leads to bunching within a coverage area. This causes gaps in coverage and increases outage times. When two or more satellites simultaneously occupy the same space of the terminal antennas, they will interfere with each other. This prevents reliable communications. Other factors leading to increased outage times are SATELLITE-SUN CONJUNCTION (increased noise while the satellite passes near the sun), SATELLITE ECLIPSE
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(absence of power from solar cells), and satellite failures. The distribution of outage times is a complicated function of time and earth-station locations. With careful coverage coordination, maximum communications effectiveness is obtained.
Q12. Why is satellite acquisition and tracking important?
In the context of a worldwide military communications network, satellite communications systems are very important. Satellite communications links add capacity to existing communications capabilities and provide additional alternate routings for communications traffic. Satellite links, as one of several kinds of long-distance links, interconnect switching centers located strategically around the world. They are part of the defense communication systems (DCS) network. One important aspect of the satellite communications network is that it continues in operation under conditions that sometimes render other methods of communications inoperable. Because of this, satellites make a significant contribution to improved reliability of Navy communications.
Satellite communications have unique advantages over conventional long distance transmissions. Satellite links are unaffected by the propagation variations that interfere with hf radio. They are also free from the high attenuation of wire or cable facilities and are capable of spanning long distances. The numerous repeater stations required for line-of-sight or troposcatter links are no longer needed. They furnish the reliability and flexibility of service that is needed to support a military operation.
The present military communications satellite system is capable of communications between backpack, airborne, and shipboard terminals. The system is capable of handling thousands of communications channels.
Communications satellite frequencies are not dependent upon reflection or refraction and are affected only slightly by atmospheric phenomena. The reliability of satellite communications systems is limited only by the equipment reliability and the skill of operating and maintenance personnel.
Destruction of an orbiting vehicle by an enemy is possible. However, destruction of a single communications satellite would be quite difficult and expensive. The cost would be excessive compared to the tactical advantage gained. It would be particularly difficult to destroy an entire multiple-satellite system such as the twenty-six random-orbit satellite system currently in use. The earth terminals offer a more attractive target for physical destruction. These can be protected by the same measures that are taken to protect other vital installations.
A high degree of freedom from jamming damage is provided by the highly directional antennas at the earth terminals. The wide bandwidth system that can accommodate sophisticated anti-jam modulation techniques also lessens vulnerability.
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Most operational military satellite earth terminals are housed in transportable vans. These can be loaded into cargo planes and flown to remote areas. With trained crews these terminals can be put into operation in a matter of hours. Worldwide communications can be established quickly to remote areas nearly anywhere in the free world.
Limitations of a satellite communications system are determined by the technical characteristics of the satellite and its orbital parameters. Active communications satellite systems are limited by two things. Satellite transmitter power on the down links and receiver sensitivity on the up links. Some early communications satellites have been limited by low-gain antennas.
The amount of power available in an active satellite is limited by the weight restrictions imposed on the satellite. Early communications satellites were limited to a few hundred pounds because of launch- vehicle payload restraints. The only feasible power source is the inefficient solar cell. (Total power generation in the earlier satellites was less than 50 watts.) As you can see, the rf power output is severely limited; therefore, a relatively weak signal is transmitted by the satellite on the down link. The weak transmitted signal is often reduced by propagation losses. This results in a very weak signal being available at the earth terminals. The level of signals received from a satellite is comparable to the combination of external atmospheric noise and internal noise of standard receivers. Special techniques must be used to extract the desired information from the received signal. Large, high-gain antennas and special types of preamplifiers solve this problem but add complexity and size to the earth terminal. (The smallest terminal in the defense communication systems network has effectively an 18-foot antenna and weighs 19,500 pounds.) Development of more efficient power sources and relaxation of weight restrictions have permitted improved satellite performance and increased capacity.
Powerful transmitters with highly directional antennas are used at earth stations. Even with these large transmitters, a lot of signal loss occurs at the satellite. The satellite antenna receives only a small amount of the transmitted signal power. A relatively weak signal is received at the satellite receiver. This presents little problem as the strength of the signal received on the up link is not as critical as that received on the down link. The down-link signal is critical because the signal transmitted from the satellite is very low in power. Development of high-gain antennas and highly sensitive receivers have helped to solve the down-link problem.
The availability of a satellite to act as a relay station between two earth terminals depends on the locations of the earth terminals and the orbit of the satellite. All satellites, except those in a synchronous orbit, will be in view of any given pair of earth stations only part of the time. The length of time that a nonsynchronous satellite in a circular orbit will be in the ZONE OF MUTUAL VISIBILITY (the satellite can be seen from both terminals) depends upon the height at which the satellite is circling. Elliptical orbits cause the satellite zone of mutual visibility between any two earth terminals to vary from orbit to orbit. These times of mutual visibility are predictable. Figure 4-14 illustrates the zone of mutual visibility.
4-17
Q13. What are the two limitations to an active satellite communications system?
Satellite communications are becoming well established in the Navy. In October 1983 the Department of the Navy established the Naval Space Command, which assumed operational responsibility for Navy space systems plus coordination responsibility with other operational activities. Most ships have satellite communications capability. New systems have been installed on ships and are fully compatible with other electronic systems and equipment. Communications via satellite has increased existing Navy communications capabilities for the command and control of naval forces. Satellite communications has not replaced all existing means of radio communications. However, it is a major step in modernizing Navy communications. It has relieved the Navy of its total dependence on hf radio transmission and reduced the need for many hf ground stations overseas.
A recent step in the advancement of satellite communications was the start of the DSCS Phase III. The first Phase III satellite was launched into orbit by the space shuttle in the summer of 1984. Seven satellites will be placed in space during this phase. Figure 4-15 shows a Phase III satellite being tested in a simulated space environment, Figure 4-16 shows the Phase III satellite as it appears in space. Phase III will develop the use of 40-watt, solid-state amplifiers to replace the currently used traveling-wave tube (twt). It will also be used to develop new type filters. These filters will provide increased channel bandwidth, which will provide additional communications capacity.
4-18
The survivability of reliable communications for the command and control of our strategic nuclear forces is paramount. Space systems perform many missions more effectively than earthbound systems.
4-19
Spaceborne communications increase the effectiveness of military operations. The Department of Defense is engaged in the development of new communications techniques and systems, including some that are space based. As the use of space continues its march forward, vital new opportunities for national defense will continue to materialize. This will improve the survivability of our strategic communications against nuclear and electronic attack.
More information on satellite communications can be found in Navy publication NTP 2, Navy Satellite Operations. This publication was written to concisely explain the role of the Navy in the Defense Communications Satellite Program. It also issues procedures for effective, coordinated use of available satellite resources.
Now that you have completed this chapter, a short review of what you have learned will be helpful. The following review will refresh your memory of satellite communications, equipment, and theory.
A PASSIVE SATELLITE is one that reflects radio signals back to earth.
An ACTIVE SATELLITE is one that amplifies the received signal and retransmits it back to earth.
REPEATER is another name for an active satellite.
The UP LINK is the frequency used to transmit a signal from earth to a satellite.
The DOWN LINK is the frequency used to transmit an amplified signal from the satellite back to earth.
A SYNCHRONOUS ORBIT is one in which the satellite moves or rotates at the same speed as the earth.
An ASYNCHRONOUS ORBIT is one where the satellite does not rotate or move at the same speed as the earth.
A NEAR SYNCHRONOUS ORBIT is one in which the satellite rotates close to but not exactly at the same speed as the earth.
PERIGEE is the point in the orbit of a satellite closest to the earth.
APOGEE is the point in the orbit of a satellite the greatest distance from the earth.
The ANGLE OF INCLINATION is the angular difference between the equatorial plane of the earth and the plane of orbit of the satellite.
INCLINED ORBITS are orbits where there is some amount of inclination. These include equatorial and polar orbits.
An EQUATORIAL ORBIT is an orbit that occurs when the plane of a satellite coincides with the plane of the earth at the equator.
A POLAR ORBIT is an orbit that has an angle of inclination of or near 90 degrees.
A MEDIUM ALTITUDE ORBIT is an orbit from 2,000 to 12,000 miles above the earth. The rotation rate of the earth and satellite are quite different, and the satellite moves quickly across the sky.
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An ECLIPSE is when the satellite is not in view or in direct line of sight with the sun. This happens when the earth is between them.
An EPHEMERIS is a table showing the precalculated position of a satellite at any given time.
PROGRAMMED TRACKING uses known satellite orbital parameters to generate antenna pointing angles.
AUTOMATIC TRACKING is done by the equipment comparing the direction of the antenna axis and the direction of the received signal.
HAND OVER is the period of time for one earth terminal to yield control to another as a satellite moves out of its area of coverage.
SATELLITE-SUN CONJUNCTION is when the satellite and sun are close together and the noise from the sun prevents or hampers communications.
A SATELLITE ECLIPSE is an eclipse where the rays of the sun don't reach the satellite. This prevents recharging of the solar cells of the satellite and decreases the power to the transmitter.
The ZONE OF MUTUAL VISIBILITY is where the satellite can be seen by both the up- and down-link earth terminals.
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ANSWERS TO QUESTIONS Q1. THROUGH Q13.
A1. Passive and active.
A2. Earth terminals.
A3. Approximately one-half.
A4. The extreme polar regions.
A5. The lack of suitable power sources.
A6. To allow maximum solar cell exposure to the sun and satellite antenna exposure to earth terminals.
A7. Large, high-gain parabolic antennas.
A8. To overcome satellite transmitter low power and permit extraction of the desired information from the received signal.
A9. Up to 20 kilowatts.
A10. To receive fleet multichannel tty broadcasts.
A11. Fm or psk.
A12. To ensure earth terminal antennas are always pointed towards the satellite.
A13. Satellite down-link transmitter power and up-link receiver sensitivity.
5-1
Upon completion of this chapter you will be able to:
1. Describe the basic operation of communications systems that operate at medium frequencies and below.
2. Describe the basic microwave line-of-sight communications system.
3. Describe the basic tropospheric scatter communications system.
4. Describe the objective/purpose of the naval tactical data system (NTDS).
5. Describe the naval tactical data system (NTDS) data transmission subsystems in terms of links.
6. Explain the various applications of portable communications equipment.
7. Define the term laser.
8. Describe the basic theory of operation of lasers
9. Describe the possible applications of lasers in communications.
In the previous four chapters we've looked at communications equipment and systems that were used in several frequency ranges. Some have had many applications. In this chapter you will look at systems used in some portions of the rf spectrum that have not been covered in detail. We will also discuss the naval tactical data system (NTDS), which operates in the high-frequency and ultrahigh-frequency regions. Various portable communications equipments used in the military and an introduction to the laser and its uses in communications are included. Some of the applications presented are fairly new to the military community.
As discussed in chapter 1, the frequency range from elf to shf is from below 300 hertz up to 30 gigahertz. The first area we will cover is the lower frequency bands (medium frequency [mf] and below). You will then get a look at the microwave region and the high-frequency and ultrahigh-frequency range as it pertains to the naval tactical data system (NTDS).
5-2
Most of the receivers and transmitters that you will see used in the mf portions of the rf spectrum and below are very similar in design. In chapter 1 we discussed the operational uses of the equipment; now let's look at the equipment itself.
Equipment items covered in this and other chapters are meant to be merely representative of equipment that may be encountered in naval communications. No attempt will be made to include all of the possible equipment or equipment configurations.
You should realize the transmitters used in bands of medium frequency and below are similar to those you studied in chapter 2. In other words, a transmitter used in one frequency range is basically the same as one used in another range. However, there are some differences. Two of the differences are component size and the use of a technique called DOUBLING UP.
The components used in bands of medium frequency and below are much larger physically than the ones previously discussed. This is because of the higher operating voltage and current levels required to produce the very high-powered rf outputs needed for the uses covered in chapter 1. A given resistor used in an hf application may be rated at 1/2 watt, whereas the same resistor used in a lower frequency application would probably be rated in tens or even hundreds of watts.
A block diagram of a doubled-up transmitter is shown in figure 5-1. Remember, bands of medium frequencies and below are used almost exclusively for broadcast and are on the air continuously. Doubling up increases reliability. As you can see, two transmitters are located in the same equipment cabinet. This allows you to quickly transfer circuits if one should fail. This dual installation also allows both amplifiers to be used together to double the output power. When you use this application, you sacrifice the doubling-up capability of only the power amplifier. All the other components are still available as backups. Let's go through figure 5-1 and describe the block functions.
5-3
The frequency generator part of the frequency generator and fsk block is an oscillator. It provides the carrier frequencies for the cw mode. The fsk part is a FREQUENCY SYNTHESIZER (a frequency source of high accuracy). It makes both the mark and space frequencies from a very stable clock oscillator. The keying pulses determine which fsk frequency the keyer chooses to transmit. This signal is then sent to the transmitter control console where it is distributed to the first rf amplifier. This amplifier is referred to as the preliminary intermediate-power amplifier (pre-ipa). The pre-ipa uses linear, untuned, push-pull, rf amplifiers to provide amplified rf to drive other rf amplifiers. The pre-ipa output goes to the intermediate power amplifier (ipa).
The ipa receives the pre-ipa output, amplifies the signal, and drives other selected power amplifiers. The ipa is a single-stage, untuned, linear, push-pull, rf circuit that uses water and forced-air cooled tubes.
Signals are then sent through the amplifier control, where they are used for signal monitoring purposes before being applied to the final rf amplifier (pa). The pa amplifies the signal to the final desired power level. The pa also contains variometers (variable inductors) for coupling. This coupled output is fed to the rf tuning unit.