NEETS Module 17 — Radio-Frequency Communications Principles
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1-11 to 1-20
, 2-1 to 2-10
2-11 to 2-20
,2-21 to 2-30
2-31 to 2-37
, 3-1 to 3-10
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Figure 4-12.—Parabolic antenna cluster.
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?
SHIPBOARD RECEIVE-ONLY EQUIPMENT SYSTEMS
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.
Figure 4-13.—Typical shipboard receive only system.
Q10. What is the function of shipboard receive-only equipment?
Q11. What types of modulation are shipboard receive-only equipment designed to receive?
SATELLITE ACQUISITION AND TRACKING
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 predictions are made rapidly with a computer and sent to tracking stations all over the world. 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.
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
(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?
ROLE OF SATELLITE COMMUNICATIONS
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.
ADVANTAGES OF SATELLITE 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.
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.
Figure 4-14.—Zone of mutual visibility.
Q13. What are the two limitations to an active satellite communications system?
FUTURE SATELLITE COMMUNICATIONS
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.
Figure 4-15.—DSCS Phase III satellite being tested.
Figure 4-16.—DSCS Phase III satellite as it appears in space.
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.
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.
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.
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.
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