Satellite: The ability to launch a satellite and keep it in orbit depends upon following well-known physical and mathematical laws that are referred to collectively as orbital dynamics. In this section, we introduce these principles before discussing the physical components of a satellite and how it is used in various communication applications.
Principles of Satellite Orbits and Positioning
If a satellite were launched vertically from the earth and then released, it would fall back to earth because of gravity. For the satellite to go into orbit around the earth, it must have some forward motion. For that reason, when the satellite is launched, it is given both vertical and forward motion. The forward motion produces inertia, which tends to keep the satellite moving in a straight line. However, gravity tends to pull the satellite toward the earth. The inertia of the satellite is equalized by the earth’s gravitational pull. The satellite constantly changes its direction from a straight line to a curved line to rotate about the earth.
If a satellite’s velocity is too high, the satellite will overcome the earth’s pull and go out into space. It takes an escape velocity of approximately 25,000 mi/h to cause a spacecraft to break the gravitational pull of the earth. At lower speeds, gravity constantly pulls the satellite toward the earth. The goal is to give the satellite acceleration and speed that will exactly balance the gravitational pull.
The closer the satellite is to earth, the stronger the effect of the earth’s gravitational pull. So in low orbits, the satellite must travel faster to avoid falling back to earth. The lowest practical earth orbit is approximately 100 mi. At this height, the satellite’s speed must be about 17,500 mi/h to keep the satellite in orbit. At this speed, the satellite orbits the earth in approximately 11⁄2 h. Communication satellites are usually much farther from the earth. A typical distance is 22,300 mi. A satellite need travel only about 6800 mi/h to stay in orbit at that distance. At this speed, the satellite rotates about the earth in approximately 24 h, the earth’s own rotational time.
There is more to a satellite orbit than just the velocity and gravitational pull. The satellite is also affected by the gravitational pull of the moon and sun.
A satellite rotates about the earth in either a circular or an elliptical path, as shown in Fig. 17-1. Circles and ellipses are geometric figures that can be accurately described mathematically. Because the orbit is either circular or elliptical, it is possible to calculate the position of a satellite at any given time.
A satellite rotates in an orbit that forms a plane passing through the center of gravity of the earth called the geocenter (Fig. 17-2). In addition, the direction of satellite rotation may be either in the same direction as the earth’s rotation or against the direction of the earth’s rotation. In the former case, the orbit is said to be posigrade, and in the latter case, retrograde. Most orbits are posigrade. In a circular orbit, the speed of rotation is constant. However, in an elliptical orbit, the speed changes depending upon the height of the satellite above the earth. Naturally the speed of the satellite is greater when it is close to the earth than when it is far away.
In a circular orbit, the height is simply the distance of the satellite from the earth. However, in geometric calculations, the height is really the distance between the center of the earth and the satellite. In other words, that distance includes the radius of the earth, which is generally considered to be about 3960 mi (or 6373 km). A satellite that is 5000 mi above the earth in circular orbit is 3960 1 5000, or 8960, mi from the center of the earth (see Fig. 17-3).
When the satellite is in an elliptical orbit, the center of the earth is one of the focal points of the ellipse (refer to Fig. 17-4). In this case, the distance of the satellite from the earth varies according to its position. Typically the two points of greatest interest are the highest point above the earth—the apogee—and the lowest point—the perigee. The apogee and perigee distances typically are measured from the geocenter of the earth.
As indicated earlier, the speed varies according to the distance of the satellite from the earth. For a circular orbit the speed is constant, but for an elliptical orbit, the speed varies according to the height. Low earth satellites of about 100 mi in height have a speed in the neighborhood of 17,500 mi/h. Very high satellites such as communication satellites, which are approximately 22,300 mi out, rotate much more slowly, a typical speed of such a satellite being in the neighborhood of 6800 mi/h.
The period is the time it takes for a satellite to complete one orbit. It is also called the sidereal period. A sidereal orbit uses some external fixed or apparently motionless object such as the sun or star for reference in determining a sidereal period. The reason for using a fixed reference point is that while the satellite is rotating about the earth, the earth itself is rotating.
Another method of expressing the time for one orbit is the revolution or synodic period. One revolution (1 r) is the period of time that elapses between the successive passes of the satellite over a given meridian of earth longitude. Naturally, the synodic and sidereal periods differ from each other because of the earth’s rotation. The amount of time difference is determined by the height of the orbit, the angle of the plane of the orbit, and whether the satellite is in a posigrade or retrograde orbit. The period is generally expressed in hours. Typical rotational periods range from about 11⁄2 h for a 100-mi height to 24 h for a 22,300-mi height.
Angle of Inclination
The angle of inclination of a satellite orbit is the angle formed between the line that passes through the center of the earth and the north pole and a line that passes through the center of the earth but that is also perpendicular to the orbital plane. This angle is illustrated in Fig. 17-5(a). Satellite orbits can have inclination angles of 0° through 90°.
Another definition of inclination is the angle between the equatorial plane and the satellite orbital plane as the satellite enters the northern hemisphere. This definition may be a little bit easier to understand, but it means the same thing as the previous definition.
When the angle of inclination is 0°, the satellite is directly above the equator. When the angle of inclination is 90°, the satellite passes over both the north and the south poles once for each orbit. Orbits with 0° inclination are generally called equatorial orbits, and orbits with inclinations of 90° are referred to as polar orbits.
When the satellite has an angle of inclination, the orbit is said to be either ascending or descending. As the satellite moves from south to north and crosses the equator, the orbit is ascending. When the satellite goes from north to south across the equator, the orbit is descending [see Fig. 17-5(b)].
Angle of Elevation
The angle of elevation of a satellite is the angle that appears between the line from the earth station’s antenna to the satellite and the line between the earth station’s antenna and the earth’s horizon (see Fig. 17-6). If the angle of elevation is too small, the signals between the earth station and the satellite have to pass through much more of the earth’s atmosphere. Because of the very low powers used and the high absorption of the earth’s atmosphere, it is desirable to minimize the amount of time that the signals spend in the atmosphere. Noise in the atmosphere also contributes to poor performance. The lower the angle of radiation, the more time that this signal spends in the atmosphere. The minimum practical angle of elevation for good satellite performance is generally 5°. The higher the angle of elevation, the better.
To use a satellite for communication relay or repeater purposes, the ground station antenna must be able to follow or track the satellite as it passes overhead. Depending upon the height and speed of the satellite, the earth station is able to use it only for communication purposes for that short period when it is visible. The earth station antenna tracks the satellite from horizon to horizon. But at some point, the satellite disappears around the other side of the earth. At this time, it can no longer support communication.
One solution to this problem is to launch a satellite with a very long elliptical orbit so that the earth station can “see” the apogee. In this way, the satellite stays in view of the earth station for most of its orbit and is useful for communication for a longer time. It is only during that short time when the satellite disappears on the other side of the earth (perigee) that it cannot be used.
The intermittent communication caused by these orbital characteristics is highly undesirable in many communication applications. One way to reduce interruptions is to use more than one satellite. Typically three satellites, if properly spaced in the correct orbits, can provide continuous communication at all times. However, multiple tracking stations and complex signal switching or “hand-off” systems between stations are required. Maintaining these stations is expensive and inconvenient.
Despite the cost and complexity of multiple-satellite systems, they are widely deployed in global telecommunication applications. These systems use anywhere from 24 to more than 100 satellites. At any given time, multiple satellites are in view anywhere on earth, making continuous communication possible.
Multiple satellite systems are usually located in two ranges above the earth. Low earth-orbiting satellites, commonly referred to as LEOs, are placed in the range of 400 to 1000 mi above the earth. Medium earth-orbiting satellites or MEOs occupy the range of 1000 to 6000 mi above the earth.
The greater the height above the earth, the better the view and the greater the radio area coverage on the earth’s surface. When the goal is broader coverage per satellite, the MEO is obviously preferred over the LEO. However, the higher the satellite, the higher the power required for reliable communication and the longer the delay. Even though radio waves travel at 186,400 mi/s, there is a noticeable delay in any voice signal from the uplink to the downlink. For MEOs, the round-trip delay averages about 100 ms. The delay in LEOs averages 10 ms.
The best solution is to launch asynchronous or geostationary satellites. In a geosynchronous earth orbit (GEO), the satellite orbits the earth about the equator at a distance of 22,300 mi (or 35,888 km). A satellite at that distance rotates about the earth in exactly 24 h. In other words, the satellite rotates in exact synchronism with the earth. For that reason, it appears to be fixed or stationary, thus the terms synchronous, geosynchronous, or geostationary orbit. Since the satellite remains
apparently fixed, no special earth station tracking antennas are required. The antenna is simply pointed at the satellite and remains in a fixed position. With this arrangement, continuous communication is possible. Most communication satellites in use today are of the geosynchronous variety. Approximately 40 percent of the earth’s surface can be “seen” or accessed from such a satellite. Users inside that area can use the satellite for communication. Higher power is required for such a great distance, and the round-trip delay is about 260 ms, which is very noticeable in voice communication.
Position Coordinates in Latitude and Longitude
To use a satellite, you must be able to locate its position in space. That position is usually predetermined by the design of the satellite and is achieved during the initial launch and subsequent position adjustments. Once the position is known, the earth station antenna can be pointed at the satellite for optimum transmission and reception. For geosynchronous satellites, the earth station antenna can be adjusted once, and it will remain in that position except for occasional minor adjustments. The positions of other satellites above the earth vary according to their orbital characteristics. To use these satellites, special tracking systems must be employed. A tracking system is essentially an antenna whose position can be changed to follow the satellite across the sky. To maintain optimum transmission and reception, the antenna must be continually pointed at the satellite as it rotates. In this section, we discuss methods of locating and tracking satellites.
The location of a satellite is generally specified in terms of latitude and longitude, just as a point on earth would be described. The satellite location is specified by a point on the surface of the earth directly below the satellite. This point is known as the subsatellite point (SSP). The subsatellite point is then located by using conventional latitude and longitude designations.
Latitude and longitude form a system for locating any given point on the surface of the earth. This system is widely used for navigational purposes. If you have ever studied a globe of the earth, you have seen the lines of latitude and longitude. The lines of longitude, or meridians, are drawn on the surface of the earth between the north and south poles. Lines of latitude are drawn on the surface of the earth from east to west, parallel to the equator. The centerline of latitude is the equator, which separates the earth into the northern and southern hemispheres.
Latitude is defi ned as the angle between the line drawn from a given point on the surface of the earth to the point at the center of the earth called the geocenter and the line between the geocenter and the equator (see Fig. 17-7). The 0° latitude is at the equator, and 90° latitude is at either the north or south pole. Usually, an N or an S is added to the latitude angle to designate whether the point is in the northern or southern hemisphere.
A line drawn on the surface of the earth between the north and south poles is generally referred to as a meridian. A special meridian called the prime meridian is used as a reference point for measuring longitude. It is the line on the surface of the earth, drawn between the north and south poles, that passes through Greenwich, England. The longitude of a given point is the angle between the line connected to the geocenter of the earth to the point where the prime meridian and equator and the meridian containing the given point of interest intersect (see Fig. 17-7). The designation east or west is usually added to the longitude angle to indicate whether the angle is being measured to the east or west of the prime meridian. As an example, the location of Washington, D.C., is given by the latitude and longitude of 39° north and 77° west.
To show how latitude and longitude are used to locate a satellite, refer to Fig. 17-8. This figure shows some of the many geosynchronous communication satellites serving the United States and other parts of North America. Because geosynchronous satellites rotate about the equator, their subsatellite point is on the equator. For that reason, all geosynchronous satellites have a latitude of 0°.
Azimuth and Elevation
Knowing the location of the satellite is insufficient information for most earth stations that must communicate with the satellite. The earth station really needs to know the azimuth and elevation settings of its antenna to intercept the satellite. Most earth station satellite antennas are highly directional and must be accurately positioned to “hit” the satellite. The azimuth and elevation designations in degrees tell where to point the antenna (see Fig. 17-9). Azimuth refers to the direction where north is equal to 0°. The azimuth angle is measured clockwise with respect to north. The angle of elevation is the angle between the horizontal plane and the pointing direction of the antenna.
Once the azimuth and elevation are known, the earth station antenna can be pointed in that direction. For a geosynchronous satellite, the antenna will simply remain in that position. For any other satellite, the antenna must be moved as the satellite passes overhead.
For geosynchronous satellites, the angles of azimuth and elevation are relatively easy to determine. Because geosynchronous satellites are fixed in position over the equator, special formulas and techniques have been developed to permit easy determination of azimuth and elevation for any geosynchronous satellite for any point on earth. Example 17-1 shows how calculations are made.
A satellite earth station is to be located at longitude 95° west, latitude 30° north. The satellite is located at 121° west longitude in geosynchronous orbit. Determine the approximate azimuth and elevation settings for the antenna.
Satellite Communication Systems
Communication satellites are not originators of information to be transmitted. Although some other types of satellites generate the information to be transmitted, communication satellites do not. Instead, these satellites are relay stations for earth sources. If a transmitting station cannot communicate directly with one or more receiving stations because of line-of-sight restrictions, a satellite can be used. The transmitting station sends the information to the satellite, which in turn retransmits it to the receiving stations. The satellite in this application is what is generally known as a repeater.
Repeaters and Transponders
Fig. 17-10 shows the basic operation of a communication satellite. An earth station transmits information to the satellite. The satellite contains a receiver that picks up the transmitted signal, amplifies it, and translates it on another frequency. The signal on the new frequency is then retransmitted to the receiving stations on earth. The original signal being transmitted from the earth station to the satellite is called the uplink, and the retransmitted signal from the satellite to the receiving stations is called the downlink. Usually, the downlink frequency is lower than the uplink frequency. A typical uplink frequency is 6 GHz, and a common downlink frequency is 4 GHz.
The transmitter-receiver combination in the satellite is known as a transponder. The basic functions of a transponder are amplification and frequency translation (see Fig. 17-11). The reason for frequency translation is that the transponder cannot transmit and receive on the same frequency. The transmitter’s strong signal would overload, or “desensitize,” the receiver and block out the very small uplink signal, thereby prohibiting any communication. Widely spaced transmit and receive frequencies prevent interference.
Transponders are also wide-bandwidth units so that they can receive and retransmit more than one signal. Any earth station signal within the receiver’s bandwidth will be amplified, translated, and retransmitted on a different frequency. Although the typical transponder has a wide bandwidth, it is used with only one uplink or downlink signal to minimize interference and improve communication reliability. To be economically feasible, a satellite must be capable of handling several channels. As a result, most satellites contain multiple transponders, each operating at a different frequency. For example, a communication satellite may have 24 channels, 12 vertically polarized and 12 horizontally polarized. Each transponder represents an individual communication channel. Various multiple-access schemes are used so that each channel can carry multiple information transmissions.
Most communication satellites operate in the microwave frequency spectrum. However, there are some exceptions. For example, many military satellites operate in the 200- to 400-VHF/UHF range. Also, the amateur radio OSCAR satellites operate in the VHF/UHF range. VHF, UHF, and microwave signals penetrate the ionosphere with little or no attenuation and are not refracted to earth, as are lower-frequency signals in the 3- to the 30-MHz range.
The microwave spectrum is divided up into frequency bands that have been allocated to satellites as well as other communication services such as radar. These frequency bands are generally designated by a letter of the alphabet. Fig. 17-12 shows the various frequency bands used in satellite communication.
One of the most widely used satellite communication bands is the C band. The uplink frequencies are 5.925 to 6.425 GHz. In any general discussion of the C band, the uplink is generally said to be 6 GHz. The downlink is in the 3.7- to the 4.2-GHz range. But again, in any general discussion of the C band, the downlink is nominally said to be 4 GHz. Occasionally, the C band is referred to by the designation 6/4 GHz, where the uplink frequency is given first.
Over the past several years, there has been a steady move toward the higher frequencies. Currently, the Ku band is receiving the most attention. The uplinks are in the 14- to the 14.5-GHz range, and the downlinks are from 11.7 to 12.2 GHz. You will see the Ku band designated as 14/12 GHz. The use of the Ka-band is also increasing.
Most new communication satellites will operate in the Ku band. This upward shift in frequency is happening because the C band is overcrowded. Many communication satellites are in orbit now, most of them operating in the C band. However, there is some difficulty with interference because of the heavy usage. The only way this interference will be minimized is to shift all future satellite communication to higher frequencies. Naturally, the electronic equipment that can achieve these higher frequencies is more complex and expensive. Yet, the crowding and interference problems cannot be solved in any other way. Furthermore, for a given antenna size, the gain is higher in the Ku band than in the C band. This can improve communication reliability while decreasing antenna size and cost. Two other bands of interest are the X and L bands. The military uses the X band for its satellite and radar. The L band is used for navigation as well as marine and aeronautical communication and radar.
Recall the frequencies designated for the C band uplink and downlink. These are 5925 to 6425 and 3700 to 4200 MHz, respectively. You can see that the bandwidth between the upper and lower limits is 500 MHz. This is an incredibly wide band, capable of carrying an enormous number of signals. In fact, 500 MHz covers all the radio spectrum so well known from VLF through VHF and beyond. Most communication satellites are designed to take advantage of this full bandwidth. This allows them to carry the maximum possible number of communication channels. Of course, this extremely wide bandwidth is one of the major reasons why microwave frequencies are so useful
in communication. Not only can many communication channels be supported, but also
very high-speed digital data requiring a wide bandwidth is supported.
The transponder receiver “looks at” the entire 500-MHz bandwidth and picks up any transmission there. However, the input is “channelized” because the earth stations operate on selected frequencies or channels. The 500-MHz bandwidth is typically divided into 12 separate transmit channels, each 36 MHz wide. There are 4-MHz guard bands between channels that are used to minimize adjacent channel interference (see Fig. 17-13).
Note the center frequency for each channel. Remember that the uplink frequencies are translated by frequency conversion to the downlink channel. In both cases, the total bandwidth (500 MHz) and channel bandwidth (36 MHz) are the same. Onboard the satellite, a separate transponder is allocated to each of the 12 channels.
Although 36 MHz seems narrow compared to 500 MHz, each transponder bandwidth is capable of carrying an enormous amount of information. For example, one typical transponder can handle up to 1000 one-way analog telephone conversations as well as one full-color TV channel. Each transponder channel can also carry high-speed digital data. Using certain types of modulation, a standard 36-MHz bandwidth transponder can handle digital data at rates of up to 60 Mbps.
A satellite transponder operates in the C band (see Fig. 17-13). Assume a local oscillator frequency of 2 GHz.
a. What is the uplink receiver frequency if the downlink transmitter is on channel 4? The downlink frequency of channel 4 is 3840 MHz (Fig. 17-13). The downlink frequency is the difference between the uplink frequency fu and the local-oscillator frequency fLO:
b. What is the maximum theoretical data rate if one transponder is used for binary transmission?
The bandwidth of one transponder channel is 36 MHz. For binary transmission, the maximum theoretical data rate or channel capacity C for a given bandwidth B is
C = 2B
=2(36) = 72 Mbps
Although the transponders are quite capable, they nevertheless rapidly become overloaded with traffic. Furthermore, at times there is more traffic than there are transponders to handle it. For that reason, numerous techniques have been developed to effectively increase the bandwidth and signal-carrying capacity of the satellite. Two of these techniques are known as frequency reuse and spatial isolation.
One system for effectively doubling the bandwidth and information-carrying capacity of a satellite is known as frequency reuse. In this system, a communication satellite is provided with two identical sets of 12 transponders. The first channel in one transponder operates on the same channel as the first transponder in the other set, and so on. With this arrangement, the two sets of transponders transmit in the same frequency spectrum and, therefore, appear to interfere with each other. However, this is not the case. The two systems, although operating on exactly the same frequencies, are isolated from each other by the use of special antenna techniques.
One technique for keeping transmissions separate is to use different antenna polarizations. For example, a vertically polarized antenna will not respond to a horizontally polarized signal and vice versa. Or a left-hand circularly polarized (LHCP) antenna will not respond to a right-hand circularly polarized (RHCP) signal and vice versa. Another technique is to use spatial isolation. By using narrow beam or spot beam antennas, the area on the earth covered by the satellite can be divided up into smaller segments. Earth stations in each segment may actually use the same frequency, but
because of the very narrow beam widths of the antennas, there is no interference between adjacent segments. This technique is referred to spatial-division multiple access (SDMA) in that access to the satellite depends on location and not frequency.
To maximize the use of the available spectrum in satellite transponders and to ensure access for as many users as possible, all satellites use some form of multiplexing. Frequency-division multiplexing (FDM), more commonly called frequencydivision multiple access (FDMA), was widely used in early satellites. Today, time-division multiplexing (TDM), also known as time-division multiple access (TDMA), is more prevalent. This digital technique assigns each user a time slot on the full bandwidth of the transponder channel.
Modulation methods are BPSK and QPSK, although multilevel QAM (16 QAM, 32 QAM, and 256 QAM) is also used to increase digital transmission speeds in a given bandwidth. Spread spectrum is used in some of the newer satellites. Also known as code-division multiple access (CDMA), this digital method spreads the signals of multiple users over the full transponder channel bandwidth and sorts them by use of pseudorandom codes. CDMA also provides the security so important in today’s wireless systems and today, more and more satellites use SDMA to give multiple access while conserving spectrum.
All satellite communication systems consist of two basic parts, the satellite or spacecraft, and two or more earth stations. The satellite performs the function of a radio repeater or relay station. Two or more earth stations may communicate with one another through the satellite rather than directly point-to-point on the earth.
Satellites vary in size from about 1 ft3 for a small LEO satellite to more than 20 ft long. The largest satellites are roughly the size of the trailer on an 18-wheeler. Weight ranges from about 100 lb for the smaller satellites to nearly 10,000 lb for the largest.
The heart of a communication satellite is the communication subsystem. This is a set of transponders that receive the uplink signals and retransmit them to earth. A transponder is a repeater that implements a wideband communication channel that can carry many simultaneous communication transmissions.
The transponders are supported by a variety of additional “housekeeping” subsystems. These include the power subsystem, the telemetry tracking and command subsystems, the antennas, and the propulsion and attitude stabilization subsystems. These are essential to the self-sustaining nature of the satellite.
Fig. 17-14 is a general block diagram of a satellite. All the major subsystems are illustrated. The solar panels supply the electric power for the spacecraft. They drive regulators that distribute dc power to all other subsystems. And they charge the batteries that operate the satellite during eclipse periods. And ac-to-dc converters and dc-to-ac inverters are used to supply special voltages to some subsystems. Total power capability runs from a few hundred watts in the smaller units to several kilowatts in the largest systems.
The communication subsystem consists of multiple transponders. These receive the uplink signals, amplify them, translate them in frequency, and amplify them again for retransmission as downlink signals. The transponders share an antenna subsystem for both reception and transmission.
The telemetry, tracking, and command (TT&C) subsystem monitors onboard conditions such as temperature and battery voltage and transmits this data back to a ground station for analysis. The ground station may then issue orders to the satellite by transmitting a signal to the command subsystem, which then is used to control many spacecraft functions such as firing the jet thrusters.
The jet thrusters and the apogee kick motor (AKM) are part of the propulsion subsystem. They are controlled by commands from the ground.
The attitude control subsystem provides stabilization in orbit and senses changes in orientation. It fires the jet thrusters to perform attitude adjustment and station-keeping maneuvers that keep the satellite in its assigned orbital position.
The main payload on a communication satellite, of course, is the communication subsystem that performs the function of a repeater or relay station. An earth station takes the signals to be transmitted, known as baseband signals, and modulates a microwave carrier. The three most common baseband signals are voice, video, and computer data. These uplink signals are then amplified, translated in frequency, and retransmitted on the downlink to one or more earth stations. The component that performs this function is known as a transponder. Most modern communication satellites contain at least 12 transponders. More advanced satellites contain many more. These transponders operate in the microwave frequency range.
The basic purpose of a transponder is simply to rejuvenate the uplink signal and retransmit it over the downlink. In this role, the transponder performs the function of an amplifier. By the time the uplink’s signal reaches the satellite, it is extremely weak. Therefore, it must be amplified before it can be retransmitted to the receiving earth station.
However, transponders are more than just amplifiers. An amplifier is a circuit that takes a signal and increases the voltage or power level of that signal without changing its frequency or content. Such a transponder, then, literally consists of a receiver and a transmitter that operate on the same frequency. Because of the close proximity of the transmitter and the receiver in the satellite, the high transmitter output power for the downlink is picked up by that satellite receiver. Naturally, the uplink signal is totally obliterated. Furthermore, the transmitter output fed back into the receiver input causes oscillation.
To avoid this problem, the receiver and transmitter in the satellite transponder are designed to operate at separate frequencies. In this way, they will not interfere with each other. The frequency spacing is made as wide as practical to minimize the effect of the transmitter desensitizing the receiver. In many repeaters, even though the receive and transmit frequencies are different, the high output power of the transmitter can still affect the sensitive receiver input circuits and, in effect, desensitize them, making them less sensitive in receiving the weak uplink signals. The wider the frequency spacing between transmitter and receiver, the less of desensitizing problem this is.
In typical satellites, the input and output frequencies are separated by huge amounts. At C band frequencies, the uplink signal is in the 6-GHz range and the downlink signal is in the 4-GHz range. This 2-GHz spacing is sufficient to eliminate most problems. However, to ensure maximum sensitivity and minimum interference between uplink and downlink signals, the transponder contains numerous filters that not only provide channelization but also help to eliminate interference from external signals regardless of their source.
Three basic transponder configurations are used in communication satellites. They are all essentially minor variations of one another, but each has its advantages and disadvantages. These are the single-conversion, double-conversion, and regenerative transponders.
A single-conversion transponder uses a single mixer to translate the uplink signal to the downlink frequency. A dual-conversion transponder makes the frequency translation in two steps with two mixers. No demodulation occurs. A regenerative repeater demodulates the uplink signal after the frequency is translated to some lower intermediate frequency. The recovered baseband signal is then used to modulate the downlink signal.
Virtually all modern communication satellites contain multiple transponders. This permits many more signals to be received and transmitted.
A typical commercial communication satellite contains 12 transponders, 24 if frequency reuse is incorporated. Military satellites often contain fewer transponders, whereas the newer, larger commercial satellites have provisions for up to 50 channels. Each transponder operates on a separate frequency, but its bandwidth is wide enough to carry multiple channels of voice, video, and digital information.
There are two basic multichannel architectures in use in communication satellites. One is a broadband system, and the other is a fully channelized system.
As indicated earlier, a typical communication satellite spectrum is 500 MHz wide. This is typically divided into 12 separate channels, each with a bandwidth of 36 MHz. The center frequency spacing between adjacent channels is 40 MHz, thereby providing a 4-MHz spacing between channels to minimize adjacent channel interference. Refer to Fig. 17-13 for details. A wideband repeater (Fig. 17-15) is designed to receive any signal transmitted within the 500-MHz total bandwidth.
The receive antenna is connected to a low-noise amplifier (LNA) as in every transponder. Very wideband tuned circuits are used so that the entire 500-MHz bandwidth is received and amplified. A low-noise amplifier, usually a GaAs FET, provides gain. A mixer translates all incoming signals to their equivalent lower downlink frequencies. In a C-band communication satellite, the incoming signals are located between 5.925 and 6.425 MHz. A local oscillator operating at the frequency of 2.225 GHz is used to translate the inputs to the 3.7- to the 4.2-GHz range. A wideband amplifier following the mixer amplifi es this entire spectrum.
The channelization process occurs in the remainder of the transponder. For example, in a 12-channel satellite, 12 bandpass filters, each centered on one of the 12 channels, are used to separate all the various received signals. Fig. 17-13 shows the 12 basic channels with their center frequencies, each having a bandwidth of 36 MHz. The bandpass filters separate out the unwanted mixer output signals and retain only the difference signals. Then individual high-power amplifiers (HPAs) are used to increase the signal level. These are usually traveling-wave tubes (TWTs). The output of each TWT amplifier is again filtered to minimize harmonic and intermodulation distortion problems. These filters are usually part of a larger assembly known as a multiplexer, or combiner. This is a waveguide–cavity resonator assembly that filters and combines all the signals for application to a single antenna.
It is logical to assume that if the receive function can be accomplished by wideband amplifier and mixer circuits, then it must be possible to provide the transmit function in the same way. However, it is generally not possible to generate very high output power over such wide bandwidth. The fact is that no components and circuits can do this well. The high-power amplifi ers in most transponders are traveling-wave tubes that inherently have limited bandwidth. They operate well over a small range but cannot deal with the entire 500-MHz bandwidth allocated to a satellite. Therefore, to achieve high power levels, the channelization process is used.
Today virtually every satellite uses solar panels for its basic power source. Solar panels are large arrays of photocells connected in various series and parallel circuits to create a powerful source of direct current. Early solar panels could generate hundreds of watts of power. Today huge solar panels are capable of generating many kilowatts. A key requirement is that the solar panels always be pointed toward the sun. There are two basic satellite configurations. In cylindrical satellites, the solar cells surround the entire unit, and therefore some portion of them is always exposed to sunlight. In body-stabilized, or three-axis, satellites, individual solar panels are manipulated with various controls to ensure that they are correctly oriented with respect to the sun.
Solar panels generate a direct current that is used to operate the various components of the satellite. However, the dc power is typically used to charge secondary batteries that act as a buffer. When a satellite goes into an eclipse or when the solar panels are not properly positioned, the batteries take over temporarily and keep the satellite operating. The batteries are not large enough to power the satellite for a long time; they are used as a backup system for eclipses, initial satellite orientation, and stabilization, or emergency conditions.
The basic dc voltage from the solar panels is conditioned in various ways. For example, it is typically passed through voltage regulator circuits before being used to power individual electronic circuits. Occasionally, voltages higher than those produced by the solar panels must also be generated. For example, the TWT amplifiers in most communication transponders require thousands of volts for proper operation. Special dc-to-dc converters are used to translate the lower dc voltage of the solar panels to the higher dc voltage required by the TWTs.
Telemetry, Command, and Control Subsystems
All satellites have a telemetry, command, and control (TC&C) subsystem that allows a ground station to monitor and control conditions in the satellite. The telemetry system is used to report the status of the onboard subsystems to the ground station (see Fig. 17-16). The telemetry system typically consists of various electronic sensors for measuring temperatures, radiation levels, power supply voltages, and other key operating characteristics. Both analog and digital sensors may be used. The sensors are selected by a multiplexer and then converted to a digital signal, which then modulates an internal transmitter. This transmitter sends the telemetry information back to the earth station, where it is recorded and monitored. With this information, the ground station then determines the operational status of the satellite at all times.
A command and control system permits the ground station to control the satellite. Typically, the satellite contains a command receiver that receives control signals from an earth station transmitter. The control signals are made up of various digital codes that tell the satellite what to do. Various commands may initiate a telemetry sequence, activate thrusters for attitude correction, reorient an antenna, or perform other operations as required by the special equipment specific to the mission. Usually, the control signals are processed by an onboard computer.
Most satellites contain a small digital computer, usually microprocessor-based, that acts as a central control unit for the entire satellite. The computer contains a built-in ROM with a master control program that operates the computer and causes all other subsystems to be operated as required. The command and control receiver typically takes the command codes that it receives from the ground station and passes them on to the computer, which then carries out the desired action.
The computer may also be used to make necessary computations and decisions. Information collected from the telemetry system may be first processed by the computer before it is sent to the ground station. The memory of the computer may also be used to store data temporarily prior to processing or prior to its being transmitted back to earth. The computer may also serve as an event timer or clock. Thus the computer is a versatile control element that can be reprogrammed via the command and control system to carry out any additional functions that may be required, particularly those that were not properly anticipated by those designing the mission.
The application subsystem is made up of the special components that enable the satellite to fulfill its intended purpose. For a communication satellite, this subsystem is made up of transponders.
An observation satellite such as those used for intelligence gathering or weather monitoring may use TV cameras or infrared sensors to pick up various conditions on earth and in the atmosphere. This information is then transmitted back to earth by a special transmitter designed for this purpose. There are many variations of this subsystem depending upon the use. The Global Positioning System (GPS) for satellites is an example of a subsystem, the application payload for which is used for navigation. This system is discussed in detail.
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