The ground stations, or earth station, is the terrestrial base of the system. The ground station communicates with the satellite to carry out the designated mission. The earth station may be located at the end user’s facilities or may be located remotely with ground-based intercommunication links between the earth station and the end-user. In the early days of satellite systems, earth stations were typically placed in remote country locations. Because of their enormous antennas and other critical requirements, it was not practical to locate them in downtown or suburban areas. Today, earth stations are still complex, but their antennas are smaller. Many earth stations are now located on top of tall buildings or in other urban areas directly where the end-user resides. This offers the advantage of eliminating complex intercommunication systems between the earth station and the end-user.
Like the satellite, the earth station is made up of a number of different subsystems. The subsystems, in fact, generally correspond to those onboard the satellite but are larger and much more complex. Furthermore, several additional subsystems exist at earth stations that would not be appropriate in a satellite.
An earth station consists of five major subsystems: the antenna subsystem, the receive subsystem, the transmit subsystem, the ground control equipment (GCE) subsystem, and the power subsystem (see Fig. 17-17). Not shown here are the telemetry, control, and instrumentation subsystems.
The power subsystem furnishes all the power to the other equipment. The primary source of power is the standard ac power lines. This subsystem operates power supplies that distribute a variety of dc voltages to the other equipment. The power subsystem also consists of emergency power sources such as diesel generators, batteries, and inverters to ensure continuous operation during power failures.
As shown in Fig. 17-17, the antenna subsystem usually includes a diplexer, i.e., a waveguide assembly that permits both the transmitter and the receiver to use the same antenna. The diplexer feeds a bandpass filter (BPF) in the receiver section that ensures that only the received frequencies pass through to the sensitive receiving circuits. This bandpass filter blocks the high-power transmit a signal that can occur simultaneously with reception. This prevents overload and damage to the receiver.
The output of the bandpass filter feeds a low-noise amplifier that drives a power divider. This is a waveguide-like assembly that splits the received signal into smaller but equal power signals. The power divider feeds several down converters. These are standard mixers fed by local oscillators (LOs) that translate the received signals down to an intermediate frequency, usually 70 MHz. A bandpass filter ensures the selection of the proper sidebands out of the down converter.
The IF signal containing the data is then sent to the GCE receive equipment, where it is demodulated and fed to a demultiplexer, where the original signals are finally obtained. The demultiplexer outputs are usually the baseband or original communication signals. In actual systems, several levels of demodulation and demultiplexing may have to take place to obtain the original signals. In the GCE transmit subsystem, the baseband signals such as telephone conversations are applied to a multiplexer that permits multiple signals to be carried on a single channel.
All ground stations have a relatively large parabolic dish antenna that is used for sending and receiving signals to and from the satellite. Early satellites had very low power transmitters, and so the signals received on earth were extremely small. Huge high-gain antennas were required to pick up minute signals from the satellite. The ground stations dishes were 80 to 100 ft or more in diameter. Antennas of this size are still used in some satellite systems today, and even larger antennas have been used for deep-space probes.
Modern satellites now transmit with much more power. Advances have also been made in receiver components and circuitry. For that reason, smaller earth station antennas are now practical. In some applications, antennas having as small as 18-in diameter typically, the same antenna is used for both transmitting and receiving. A diplexer is used to permit a single antenna to be used for multiple transmitters and/or receivers. In some applications, a separate antenna is used for telemetry and control functions.
The antenna in an ground stations must also be steerable. That is, it must be possible to adjust its azimuth and elevation so that the antenna can be properly aligned with the satellite. ground stations supporting geosynchronous satellites can generally be fixed in position, however. Azimuth and elevation adjustments are necessary to initially pinpoint the satellite and to permit minor adjustments over the satellite’s life.
The downlink is the receive subsystem of the ground stations. It usually consists of very low-noise preamplifiers that take the small signal received from the satellite and amplify it to a level suitable for further processing. The signal is then demodulated and sent on to other parts of the communication system.
The receiving subsystem consists of the LNA, down converters, and related components. The purpose of the receive subsystem is to amplify the downlink satellite signal and translate it to a suitable intermediate frequency. From that point, the IF signal is demodulated and demultiplexed as necessary to generate the original baseband signals. Refer to the general block diagram of the receive subsystem shown in Fig. 17-18.
Fig. 17-18(a) shows a typical dual-conversion down converter. The input bandpass filter passes the entire 500-MHz satellite signal. This is fed to a mixer along with a local oscillator. The output of the mixer is an IF signal, usually 770 MHz. This is passed through a bandpass filter at that frequency with a bandwidth of 36 MHz.
The signal is then applied to another mixer. When combined with the local- oscillator frequency, the mixer output is the standard 70-MHz IF value. An IF of 140 MHz is used in some systems. A 36-MHz-wide bandpass filter positioned after the mixer passes the desired channel.
In dual-conversion down converters, two different tuning or channel selection arrangements are used. One is referred to as RF tuning, and the other is referred to as IF tuning. In RF tuning, shown in Fig. 17-18(a), the first local oscillator is made adjustable. Typically, a frequency synthesizer is used in this application. The frequency synthesizer generates a highly stable signal at selected frequency increments. The frequency synthesizer is set to a frequency that will select the desired channel. In RF tuning, the second local oscillator has a fixed frequency to achieve the final conversion.
In IF tuning, the first local oscillator is fixed in frequency and the second is made tunable. Again, a frequency synthesizer is normally used for the tunable local oscillator [see Fig. 17-18(b)].
Refer to Fig. 17-18(b). If the earth station downlink signal received is at fs 5 4.08 GHz, what local-oscillator frequencies fLO are needed to achieve IFs of 770 and 140 MHz?
Receiver Ground Control Equipment
The receiver ground control equipment (GCE) consists of one or more racks of equipment used for demodulating and demultiplexing the received signals. The down converters provide initial channelization by the transponder, and the demodulators and demultiplexing equipment process the 70-MHz IF signal into the original baseband signals. Other intermediate signals may be developed as required by the application.
The outputs from the down converters are usually made available on a patch panel of coaxial cable connectors. These are interconnected via coaxial cables to the demodulators. The demodulators are typically packaged in a thin, narrow vertical module that plugs into a chassis in a rack. Many of these demodulators are provided. They are all identical in that they demodulate the IF signal. In FDM systems, each demodulator is an FM detector. The most commonly used type is the phase-locked loop discriminator. Equalization and deemphasis are also taken care of in the demodulator.
In systems using TDM, the demodulators are typically used to detect four-phase, or quadrature, PSK at 60 or 120 Mbps. The IF is usually at 140 MHz. Again, a patch panel between the down converters and the demodulators permits flexible interconnection to provide any desired configuration. When the video is transmitted, the output of the FM demodulator is the baseband video signal, which can then be transmitted by cable or used on the premises as required.
If the received signals are telephone calls, the demodulator outputs are sent to demultiplexing circuits. Again, in many systems, a patch panel between the demodulator outputs and the demultiplexer inputs is provided to make various connections as required. In FDM systems, standard frequency-division multiplexing is used. This consists of additional single-sideband (SSB) demodulators and filters. Depending upon the number of signals multiplexed, several levels of channel filters and SSB demodulators may be required to generate the original baseband voice signals. Once this is done, the signal may be transmitted over the standard telephone system network as required.
In TDM systems, time-division demultiplexing equipment is used to reassemble the originally transmitted data. The original baseband digital signals may be developed in some cases, or in others, these signals are used with modems as required for interconnecting the earth station with the computer that will process the data.
The uplink is the transmitting subsystem of the earth station. It consists of all the electronic equipment that takes the signal to be transmitted, amplifies it, and sends it to the antenna. In a communication system, the signals to be sent to the satellite might be TV programs, multiple telephone calls, or digital data from a computer. These signals are used to modulate the carrier, which is then amplified by a large traveling-wave tube or klystron amplifier. Such amplifiers usually generate many hundreds of watts of output power. This is sent to the antenna by way of microwave waveguides, combiners, and diplexers.
The transmit subsystem consists of two basic parts, the upconverters, and the power amplifiers. The upconverters translate the baseband signals modulated onto carriers up to the final uplink microwave frequencies. The power amplifiers generate the high-power signals that are applied to the antenna. The modulated carriers are created in the transmit GCE.
Transmit Ground Control Equipment
The transmit subsystem begins with the baseband signals. These are first fed to a multiplexer if multiple signals are to be carried by a single transponder. Telephone calls are a good example. Frequency- or time-division multiplexers are used to assemble the composite signal. The multiplexer output is then fed to a modulator. In analog systems, a wideband frequency modulator
is normally used. It operates at a carrier frequency of 70 MHz with a maximum deviation of;18 MHz. Video signals are fed directly to the modulator; they are not multiplexed.
In digital systems, analog signals are first digitized with PCM converters. The resulting serial digital output is then used to modulate a QPSK modulator. Transmitter Circuits. Once the modulated IF signals have been generated, upconversion and amplification will take place prior to transmission. Individual upconverters are connected to each modulator output. Each up converter is driven by a frequency synthesizer that allows the selection of the final transmitting frequency. The frequency synthesizer selects the transponder it will use in the satellite. The synthesizers are ordinarily adjustable in 1-kHz increments so that any upconverter can be set to any channel frequency or transponder.
As in down converters, most modern up converters use dual conversion. Both RF tuning and IF tuning are used (Fig. 17-19). With IF tuning, a tunable carrier from a frequency synthesizer is applied to the mixer to convert the modulated signal to an IF level, usually 700 MHz. Another mixer fed by a fi xed-frequency local oscillator (LO) then performs the fi nal up conversion to the transmitted frequency.
In RF tuning, a mixer fed by a fixed-frequency local oscillator performs an initial upconversion to 700 MHz. Then a sophisticated RF frequency synthesizer applied to a second mixer provides upconversion to the final microwave frequency.
In some systems, all the IF signals at 70 or 140 MHz are combined prior to the upconversion. In this system, different carrier frequencies are used on each of the modulators to provide the desired channelization. When translated by the upconverter, the carrier frequencies will translate to the individual transponder center frequencies. A special IF combiner circuit mixes all the signals linearly and applies them to a single-up converter. This upconverter creates the final microwave signal.
In most systems, however, individual upconverters are used on each modulated channel. At the output of the upconverters, all the signals are combined in a microwave combiner, which produces a single output signal that is fed to the final amplifiers. This arrangement is illustrated in Fig. 17-20.
The final combined signal to be transmitted to the satellite appears at the output of the RF combiner. But it must first be amplified considerably before being sent to the antenna. This is done by the power amplifier. The power amplifier usually begins with an initial stage called the intermediate-power amplifier (IPA). This provides sufficient drive to the fi nal high-power amplifier (HPA). Note also in Fig. 17-20 that redundant amplifiers are sed. The main IPA and HPA are used until a failure occurs. Switches (SW) automatically disconnect the defective main amplifier and connect the spare to ensure continuous operation. The amplified signal is then sent to the antenna via a waveguide, the diplexer, and a filter.
Three types of power amplifiers are used in earth stations: transistor, traveling-wave tube (TWT), and klystron. Transistor power amplifiers are used in small and medium earth stations with low power. Powers up to 50 W are common. Normally, power gallium arsenide field-effect transistors (GaAs MESFETs) or silicon MOSFETs are used in this application. The newer GaAs MESFETs and MOSFET power devices, when operated in parallel, can achieve powers of up to approximately 100 W. Newer GaN transistors are now being deployed and can achieve power levels of 100 watts or more when parallel configurations are used.
Most medium- and high-power earth stations use either TWTs or klystrons for the power amplifiers. There are two typical power ranges, one in the 200- to the 400-W range and the other in the 2- to the 3-kW range. The amount of power used depends upon the location of the station and its antenna size. Satellite transponder characteristics also infl uence the power required by the earth station. As improvements have been made in low-noise amplifiers, and satellites have been able to carry higher-power transponders, earth station transmitter power requirements have greatly decreased. This is also true for antenna sizes.
Most earth stations receive their power from the normal ac mains. Standard power supplies convert the ac power to the dc voltages required to operate all subsystems. However, most earth stations have backup power systems. Satellite systems, particularly those used for reliable communication of telephone conversations, TV programs, computer data, and so on, must not go down. The backup power system takes over if an ac power failure occurs. The backup power system may consist of a diesel engine driving an ac generator. When ac power fails, an automatic system starts the diesel engine. The generator creates the equivalent ac power, which is automatically switched to the system. Smaller systems may use uninterruptible power supplies (UPS), which derive their main power from batteries. Large battery arrays drive dc-to-ac inverters that produce the ac voltages for the system. Uninterruptible power supplies are not suitable for long power failures and
interruptions because the batteries quickly become exhausted. However, for short interruptions of power, i.e., less than an hour, they are adequate.
Telemetry and Control Subsystems
The telemetry equipment consists of a receiver and the recorders and indicators that display the telemetry signals. The signal may be received by the main antenna or a separate telemetry antenna. A separate receiver on a frequency different from that of the communication channels is used for telemetry purposes. The telemetry signals from the various sensors and transducers in the satellite are multiplexed onto a single carrier and are sent to the earth station. The earth station receiver demodulates and demultiplexed the telemetry signals into the individual outputs. These are then recorded and sent to various indicators, such as strip chart recorders, meters, and digital displays. Signals may be in digital form or converted to digital. They can be sent to a computer where they can be further processed and stored.
The control subsystem permits the ground stations to control the satellite. This system usually contains a computer for entering the commands that modulate a carrier that is amplified and fed to the main antenna. The command signals can make adjustments in the satellite attitude, turn transponders off and on, actuate redundant circuits if the circuits they are backing up fail, and so on.
In some satellite systems where communication is not the main function, some instrumentation may be a part of the ground stations. Instrumentation is a general term for all the electronic equipment used to deal with the information transmitted back to the earth station. It may consist of demodulators and demultiplexers, amplifiers, filters, A/D converters, or signal processors. The instrumentation subsystem is in effect an extension of the telemetry system. Besides relaying information about the satellite itself, the telemetry system may be used to send back information related to various scientific experiments being conducted on the satellite. In satellites used for surveillance, the instrumentation may be such that it can deal with digital still photographs or TV signals sent back from an onboard camera. The possibilities are extensive depending upon the actual satellite mission.
Very Small-Aperture Terminal
A very small aperture terminal (VSAT) is a miniature low-cost satellite ground stations. In the past, most ground stations were large and expensive—large because of the huge dish antennas that were often needed, and expensive because of the equipment costs. But over the years, semiconductor and other technological breakthroughs have greatly reduced the size and cost of ground stations. The VSAT is one result. These units are extremely small and mount on the top or side of a building and in some versions even fit into a suitcase. Costs range from a few thousand dollars to no more than about $6000 today.
They can be installed very quickly by plugging them in and pointing the antenna. A VSAT is a full receive-transmit earth station. Most of those used in the United States and Europe operate in the Ku band, so the antennas are very small, typically 0.6 to 2.4 m (about 2 to 8 ft) in diameter. Receive-only (RO) VSATs are also available for special applications such as digital video broadcasts (DVDs). Most of the transmit-receive electronics in a VSAT are contained in a housing mounted on the dish. A box containing the feed horn antenna at the focal point of the dish contains the LNA, down converters, and demodulators in the receiver and the transmit power amplifier. In some systems, these circuits may be contained in housing near the base of the antenna to keep the coaxial connection between the feed horn antenna and transmitter-receiver unit short. A cable then connects the baseband inputs and outputs (voice, video, or data) to a computer that is connected to the system.
The most common application of VSATs today is in connecting many remote company or organization sites to the main computer system. For example, most gas stations and retail stores use VSATs as point-of-sale (PoS) terminals to transmit sales transaction Information to the home office, check customer credit cards and relay inventory data.
Companies such as Shell, Wal-Mart, and Barnes & Noble all use VSATs. Most stores that sell lottery tickets use a VSAT as do most tollbooths using SpeedPass and othe radio-frequency identifi cation (RFID) of vehicles for tolls. Broadcast companies like Fox, CNN, ABC, CBS, NBC, and others use VSATs for remote news gathering and reporting.
A good example of a RO VSAT is the set top box receiver used by consumers for Direct Broadcast Satellite (DBS) TV reception.
Every satellite is designed to perform some specific task. Its predetermined application specifies the kind of equipment it must have onboard and its orbit, satellites are useful for observation purposes.
The main application for satellites today is in communication. Satellites used for this purpose act as relay stations in the sky. They permit reliable long-distance communication worldwide. They solve many of the growing communication needs of industry and government. Communication applications will continue to dominate this industry.
The primary use of communication satellites is in long-distance telephone service. Satellites greatly simplify long-distance calls not only within but also outside the United States.
Another major communication application is TV. For years, TV signals have been transmitted through satellites for redistribution. Because of the very high-frequency signals, involved in TV transmission, other long-distance transmission methods are not technically or economically feasible. Special coaxial cables and fiber-optic cables, as well as microwave relay links, have been used to transmit TV signals from one place to another. However, with today’s communication satellites, TV signals can be transmitted easily from one place to another. All the major TV networks and cable TV companies rely on communication satellites for TV signal distribution.
Direct Broadcast Satellite (DBS)
A more recent satellite TV service is the Direct Broadcast Satellite (DBS) that uses special broad U.S. coverage satellites with high power to transmit cable-TV-like services direct to homes equipped with the special DBS receivers. The direct broadcast satellite (DBS) system is an all-digital system. Data compression techniques are used to reduce the data rate in order to produce high-quality pictures and sound.
To receive the digital video from the satellite, a consumer must subscribe to one of the two U.S. satellite TV companies, DirecTV or DISH, and acquire a satellite TV receiver and antenna. These satellite receivers operate in the Ku band. By using high frequencies as well as higher-power satellite transponders, the dish antenna can be extremely small. The antennas are only 18 inches in diameter. Several DirecTV and DISH satellites are in orbit. These multichannel systems provide full coverage of the major broadcast and cable networks and the premium channels usually distributed to homes by cable TV.
The video to be transmitted is fi rst put into digital form if it is not already digital. The digital bit stream for high-defi nition TV today can be as high as 270 Mbps. To lower the data rate and improve the reliability of transmission, the DBS system uses compressed digital video. Older systems use MPEG2 compression, but newer systems use the more efficient MPEG4.
(MPEG means Motion Picture Experts Group, which is a standard organization that establishes technical standards for movies and video.) Digital compression greatly reduces the actual transmitting speed to somewhere in the 20- to the 30-Mbps range. This allows the satellite to transmit up to about 200 channels using a TDM format.
The compressed serial digital signal is then encrypted to prevent nonsubscribers from decoding the signals. Next, the signals are subjected to forward error correction (FEC). Both Reed-Solomon and
Viterbi FEC is used to ensure reliable reception and a low BER. The final signal modulates the uplink carrier using QPSK.
The DBS satellite uses the Ku band with a frequency range of 11 to 14 GHz. Uplink signals are usually in the 14- to 14.5-GHz range, and the downlink usually covers the range of 10.95 to 12.75 GHz.
Finally, the digital signal is transmitted from the satellite to the receivers using circular polarization. The DBS satellites have right-hand and left-hand circularly polarized (RHCP and LHCP) helical antennas. By transmitting both polarities of signal, frequency reuse can be incorporated to double the channel capacity.
A generic block diagram of a representative DBS digital receiver is shown in Fig. 17-21. The receiver subsystem begins with the antenna and its low-noise block (LNB) down-converter. The LNB includes an LNA, a mixer, and a local oscillator that form the remote front-end of the receiver. An integrated unit including the feedhorn is designated LNBF. The horn antenna picks up the Ku band signal reflected from the parabolic dish, and the LNB translates a wide portion of the band down to the 950- to the 2150-MHz range. This approach eliminates the need to send the Ku band signals over a coax cable, which would have enormous attenuation. Control signals from the receiver to the antenna select between RHCP and LHCP. A control signal from the receiver can also switch the local oscillator frequency for some channel-change operations. The RF signal from the antenna and LNB is sent by coaxial cable to the receiver. The type of cable used is usually RG-6/U.
A typical DBS downlink signal occurs in the 12.2- to a 12.7-GHz portion of the Ku band. Each transponder has a bandwidth of approximately 24 MHz. The digital signal usually occurs at a rate of approximately 27 Mbps. The signal is transmitted as a series of packets, each with both video and audio segments, channel identification, encryption type, and FEC.
The signal received over the coax is passed through another mixer with a variable frequency local oscillator to provide channel selection. The digital signal at the second IF at 70 or 480 MHz is then demodulated to recover the originally transmitted digital signal. The signal is first decrypted and passed through a forward error correction (FEC) circuit. This circuit is designed to detect bit errors in the transmission and to correct them on the fly. Any bits lost or obscured by noise during the transmission process are usually caught and corrected to ensure a near-perfect signal.
The resulting error-corrected signals are then sent to the audio and video decompression circuits. Then they are stored in random access memory (RAM), after which the signal is decoded to separate it into the video and the audio portions. The formatted digital signals are then sent to the TV receiver over a high-definition multimedia interface (HDMI) cable. Older systems sent the digital signals to D/A converters, which modulate a channel 3 or 4 RF modulator that sends the older analog format signals to the TV set antenna terminals.
Most newer DBS receivers also include a digital video recorder (DVR) a feature that lets the consumer record the program being viewed or a program on another channel. The DVR uses a computer hard drive for storage. The system is programmable so that a consumer can record future shows on any channel at any time. Most systems permit a single antenna to serve multiple receivers with splitters. Also, some receivers permit multiple recordings from several channels. Receivers may also communicate with the satellite provider over a standard telephone line or an Internet connection.
Satellite Cell Phones
A common satellite application today is satellite-based cellular telephone service. Current cellular telephone systems rely on many low-power, ground-based cells to act as intermediaries between the standard telephone system and the millions of roving cellular telephones. The satellite systems use low-earth-orbit satellites to perform the relay services to the main telephone system or to make connections directly between any two cellular telephones using the system.
One of the oldest and most widely used is the Iridium system. It uses a constellation of 66 satellites in six polar orbits with 11 satellites per orbit 420 mi above the earth (refer to Fig. 17-22). The satellites operate in the L band over the frequency range of 1.61 to 1.6265 GHz. The satellites communicate with ground stations called gateways that connect the system to the public switched telephone network. The satellites also communicate among themselves. Both gateway and intersatellite communication take place over Ka band frequencies. The system provides truly global coverage between any two handheld cellular telephones or between one of the cellular telephones and any other telephone on earth.
Each satellite in the system transmits back to earth, creating 48 spot beams that in effect form moving cell areas of coverage. The spot beams provide spatial separation to permit more than one satellite to use the same frequencies for simultaneous communication. Frequency reuse makes efficient use of a small number of frequencies.
Transmission is affected by digital techniques. A special form of QPSK is used for modulation. Time-division multiple access (TDMA), a special form of time-division multiplexing, is used to provide multiple voice channels per satellite.
In addition to voice communication, Iridium will be able to provide a whole spectrum of other communication services including
- Data communication E-mail and other computer communication.
- Fax Two-way facsimile.
- Paging Global paging to receivers with a two-line alphanumeric display.
- Radio Determination Services (RDSs) A subsystem that permits satellites to locate
transceivers on earth. Accuracy is expected to be within 3 mi.
Next to Iridium, Globalstar is the most widely used. This LEO system uses 44 satellites in orbits inclined 52° and CDMA for voice and data communication. Another worldwide satellite system is INMARSAT. INMARSAT, one of the oldest satellite services companies, has served the marine industry for decades. They use 11 geostationary satellites to provide worldwide coverage. There are several smaller companies offering satellite phone service using geosynchronous satellites.
Satellite phones are expensive because they require higher power to reach the satellites hundreds or thousands of miles away. Higher power also calls for larger batteries. All satellite phones also have a large antenna that must be fully extended. And all calls must be made outside so the phone can “see” the satellite. Indoor calls do not work.
Digital Satellite Radio
One of the most popular options in new cars and trucks is digital satellite radio. This service provides hundreds of channels of music, news, sports, and talks radio primarily to car portable and home radios. Conventional AM and FM radio stations cover only short distances and are subject to local and even national radio propagation effects. As you are traveling by car or truck, radio stations come and go every 40 mi or so. And if you are driving in the rural areas of the United States, you may not get any station. This is not so with satellite radio, which provides full continuous coverage of the station you select wherever you are in the United States. The system uses digital transmission techniques that ensure high-quality stereo sound that is immune to noise. Furthermore, the digital format allows the satellites to transmit other information such as song title and artist, type of music, ads, and other data, which are displayed on an LCD screen.
Two such systems in the United States are XM Satellite Radio and Sirius Satellite Radio. These were two separate companies initially, but they have now merged to form Sirius XM Radio. The two satellite systems are still separate and different. XM Satellite Radio uses three geosynchronous satellites, named Rock, Roll, and Rhythm, positioned to cover the continental United States. The Sirius Satellite Radio system uses three elliptical orbit satellites. Each satellite appears over the United States for approximately 16 h/aday.
One satellite is always available for coverage anywhere in the United States. Both the XM and Sirius systems use digital audio compression and time-division multiplexing to put hundreds of different “channels” on the air. Both systems operate in the 2.3-GHz S-band. XM and Sirius generate their programming as do other radio stations, and then they up-link their digitized signals to the satellites. The satellites provide continuous coverage to vehicles on the road and homes in the United States.
The microwave receivers are mounted in cars as are other car radios, but they require a special outside antenna for optimum reception. Typically, the unit also contains standard AM and FM receiver capability. These satellite digital radios are available as an option in most new cars, and several manufacturers are offering aftermarket add-on radios for existing vehicles. Home and portable units are also available.
Another application of satellites is in surveillance or observation. From their vantage point high in the sky, satellites can look at the earth and transmit what they see to ground stations for a wide variety of purposes. For example, military satellites are used to perform recon Naissance. Onboard cameras take photographs that can later be ejected from the satellite and brought back to earth for recovery. TV cameras can take pictures and send them back to earth as electric signals. Infrared sensors detect heat sources. Small radars can profile earth features.
Intelligence satellites collect information about enemies and potential enemies. They permit monitoring for the purpose of proving other countries’ compliance with nuclear test ban and missile stockpile treaties.
There are many different kinds of observation satellites. One special type is the meteorological, or weather, satellite. These satellites photograph cloud cover and send back-to-earth pictures that are used for determining and predicting the weather. Geodetic satellites photograph the earth for the purpose of creating more accurate and more detailed maps.
Global Navigation Satellite Systems
Global Navigation Satellite System (GNSS) refers to the multiple satellite systems used for worldwide navigation. The original GNSS was the U.S.’s Global Positioning System (GPS) and still is the most widely used across the globe. Other countries now have separate systems; these will be discussed later in this section. The Global Positioning System (GPS), also known as Navstar, is a satellite-based navigation system that can be used by anyone with an appropriate receiver to pinpoint her or his location on earth. The array of GPS satellites transmits highly accurate, time-coded information that permits a receiver to calculate its exact location in terms of the latitude and longitude on earth as well as the altitude above sea level.
GPS was developed by the U.S. Air Force for the Department of Defense as a continuous global radio navigation system that all elements of the military services would use for precision navigation. Development was started in 1973, and by 1994, the system was fully operational.
The GPS Navstar system is an open navigation system; i.e., anyone with a GPS receiver can use it. The system is designed, however, to provide a base navigation system with a horizontal accuracy to within 3 m. This precision is available to any GPS user. As a result, GPS is gradually replacing older military systems and civilian land-based navigation systems.
The GPS is an excellent example of a modern satellite-based system and the high technology communication techniques used to implement it. Learning about GPS is an excellent way to bring together and illustrate the complex concepts presented.
The GPS consists of three major segments: the space segment, the control segment, and the user segment.
The space segment is the constellation of satellites orbiting above the earth with transmitters that send highly accurate timing information to GPS receivers on earth. The receivers in the user segment themselves may be used on land, sea, or air.
The fully implemented GPS consists of 24 main operational satellites plus multiple active spare satellites (see Fig. 17-23). The satellites are arranged in six orbits, each orbit containing three or four satellites. The orbital planes form a 55° angle with the equator. The satellites orbit at a height of 10,898 nautical miles above the earth (20,200 km). The orbital period for each satellite is approximately 12h (11 h 58 min).
Each of the orbiting satellites contains four highly accurate atomic clocks. They provide precision timing pulses used to generate a unique binary code, i.e., a pseudorandom code identifying the specific satellite in the constellations that are transmitted to earth. The satellite also transmits a set of digitally coded ephemeris data that completely define its precise orbit. The term ephemeris is normally associated with specifying the location of a celestial body. Tables have been computed so that it is possible to pinpoint the location of planets and other astronomical bodies at precise locations and times. Ephemeris data can also be computed for any orbiting body such as a satellite. This data tells where the satellite is at any given time, and its location can be specified in terms of the satellite ground track in precise latitude and longitude measurements. Ephemeris information is coded and transmitted from the satellite, providing an accurate indication of the exact position of the satellite above the earth at any given time. The satellite’s ephemeris data is updated once a day by the ground control station to ensure accuracy.
A GPS receiver on earth is designed to pick up signals from three, four, or more satellites simultaneously. The receiver decodes the information and, using the time and ephemeris data calculates the exact position of the receiver. The receiver contains a high-speed, floating-point microcomputer that performs the necessary calculations. The output of the receiver is a decimal display of latitude and longitude as well as altitude.
Readings from only three satellites are necessary for latitude and longitude information only. A fourth satellite reading is required to compute altitude. Most GPS receivers now include a detailed map display on a color LCD.
Each GPS satellite carries two transmitters that together transmit the timing and location signals to the earth receivers. One of the transmitters sends a signal called L1 on 1575.42 MHz. The signal transmitted is a pseudorandom code (PRC) called the coarse acquis on (C/A) code. It is transmitted at a 1-Mbps rate using BPSK. It repeats every 1023 bits. The C/A code is like the pseudorandom codes used in the spread spectrum in that they are used to distinguish between transmitted signals at the receiver. All 24 GPS satellites transmit on the same frequency, but the PRC is unique to each satellite so that the receiver can tell them apart.
Each satellite also contains another transmitter on a frequency of 1227.6 MHz. This is called the L2 signal. It contains another PRC known as the P-code transmitted at a 10-Mbps rate. The P-code can also be encrypted, and in this form, it was called the Y code. The P and Y codes were originally designed for use by only the military, so that military GPS receivers would be more accurate than the civilian (or enemy) receivers, which receive only L1 signal. This feature was called selective availability (SA). The SA feature was abandoned on May 1, 2000. Today all receivers pick up both signals, enabling all receivers to have an accuracy that identifies the location to within 7.8 m at a 95 percent confidence level.
The basic information contained in the L1 signal consists of almanac data, ephemeris data, and the current date and time. The almanac data effectively notifies each receiver of where each satellite is during the day. The almanac data helps the receiver to initially lock onto a signal. The ephemeris data contains the exact position and timing of each satellite. It is this data that the receiver uses in the calculations to pinpoint its location. The time and date signal come from the atomic clocks carried by each satellite.
Like all other satellites, GPS satellites contain a TT&C unit that is used by the ground stations to transmit updated ephemeris data and to make sure the satellite is in its exact position. Small thrusters fired from the ground allow the ground stations to correct minor drift that introduces errors into the measurements.
Control Segment and Atomic Clocks
The control segment of the GPS refers to the various ground stations that monitor the satellites and provide control and update information. The master control station is operated by the U.S. Air Force in Colorado Springs. Additional monitoring and control stations are located in Hawaii, Kwajalein, Diego Garcia, and the Ascension Islands. These four monitoring stations are not staffed. They constantly monitor the satellites and collect range information from each. The positions of these monitoring stations are accurately known. The information is sent back to the master control station in Colorado, where all the information is collected and position data on each satellite calculated. The master control station then transmits new ephemeris and clock data to each satellite on the S-band uplink once per day. This data updates the NAV-msg or navigation message, a 50 bps signal that modulates the L1 carrier and contains bits that describe the satellite orbits, clock corrections, and other system characteristics.
The telemetry data transmitted as part of the NAV-msg is also received by the ground control station to keep track of the health and status of each receiver. The uplink S-band control system allows the ground stations to do some station keeping to correct the satellite position as needed. Positioning is accomplished with the hydrazene thrusters.
The precision timing signals are derived from atomic clocks. Most digital systems derive their timing information from a precision crystal oscillator called a clock. Crystal oscillators, although precise and stable, do not have the necessary precision and stability for the GPS. Timing data must be extremely precise to provide accurate navigation information.
Atomic clocks are electronic oscillators that use the oscillating energy of gas to provide a stable operating frequency. Certain chemicals have atoms that can oscillate between low and high energy levels. This frequency of oscillation is extremely precise and stable.
Cesium and rubidium are used in atomic clocks. In gaseous form, they are irradiated by electromagnetic energy at a frequency near their oscillating point. For cesium, this is 9,192,631,770 Hz; for rubidium, it is 6,834,682,613 Hz. This signal is generated by a quartz crystal-controlled oscillator whose frequency can be adjusted by the application of a control voltage to a varactor. The oscillation signal from the cesium (or rubidium) gas is detected and converted to a control signal that operates the oscillator. An error detector determines the difference between the crystal frequency and the cesium oscillations and generates the control voltage to set the crystal oscillator precisely. The output of the crystal oscillator is then used as the accurate timing signal. Frequency dividers and phase-locked loops are provided to generate the lower-frequency signals to be used. These signals operate the C/A and P-code generators.
The GPS satellite contains two cesium clocks and two rubidium clocks. Only one is used at any given time. The clocks are kept fully operational so that if one fails, another can be switched in immediately.
A GPS receiver is a complex superheterodyne microwave receiver designed to pick up the GPS signals, decode them, and then compute the location of the receiver. The output is usually an LCD display giving latitude, longitude, and altitude information and/or a map of the area.
There are many different types of GPS receivers. More than 40 manufacturers now make some form of GPS receiver. The larger and more sophisticated units are used in military vehicles. There are also sophisticated civilian receivers for use in various kinds of precision applications such as surveying and mapmaking. Different models are available for use in aircraft, ships, and trucks. Handheld units are also available. The most widely used GPS receiver is the popular handheld portable type, not much larger than an oversized handheld calculator. Most of the circuitry used in making a GPS receiver has been reduced to integrated-circuit form, thereby permitting an entire receiver to be contained in an extremely small, portable battery-operated unit. Keep in mind that all GPS receivers are not only superheterodyne communication receivers but also sophisticated computers. A considerable amount of high-level mathematics must be carried out to compute the receiver position from the received data.
Fig. 17-24 is a general block diagram of a GPS receiver, typical of the simpler, low-cost handheld units on the market. The receiver consists of the antenna, the RF/IF section, thee frequency standard clock oscillator, and a frequency synthesizer that provides local-oscillator signal s as well as clock and timing signals for the rest of the receiver.
Other sections of the receiver include a digital signal processor and a control microcomputer along with its related RAM and ROM. Interface circuits provide a connection to the LCD or other type of display.
The antenna system is a type of patch antenna made on a printed circuit board. It is designed to receive right-hand circularly polarized (RHCP) signals from the GPS satellites. In the handheld units, the antenna is part of the single physical structure and is connected directly to the receiver front end. In some larger and more complex GPS receivers, the antenna is a separate unit and may be mounted at a high clear point and connected to the receiver with coaxial cable.
The receiver can determine its exact position only by computing the position information obtained from four satellites. The receiver picks up signals from four satellites simultaneously (see Fig. 17-25). R1 through R4 are the ranges to the satellites from the yacht. Because spread spectrum is used, the receiver ignores all the signals except the one whose pseudocode has been entered and used to obtain lock.
Once the receiver locks on the one satellite, all the information is extracted from the satellite. Then the C/A code is switched to another satellite within view, and the process is repeated. In other words, the receiver performs a time-multiplexing operation on th four satellites within view of the receiver. The data is extracted from each of the four satellites and stored in the receiver’s memory. Data from three satellites are needed to fix the receiver’s position. If data from a fourth satellite is available, altitude can be calculated.
Once all the data has been accumulated, the high-speed control and data microprocessor in the receiver performs the final calculations. A microprocessor is typically a 16-bit unit with floating-point capability. Floating-point numbers must be used to provide the precision of calculation for accurate location.
The computations performed by the receiver are given below. First the receiver calculates the ranges R1 through R4 to each of the four satellites. These are obtained by measuring the time shift between the received pulses, which is the delay in transmission between the satellite and the receiver. These are the T1 through T4 times given below.
Multiplying these by the speed of light c gives the range in meters between the satellite and the receiver. These four range values are used in the final calculations:
The basic ranking calculation is the solution to four simultaneous equations, as indicated in Fig. 17-27. The X, Y, and Z values are derived from the NAV-msg data transmitted by each of the satellites. The CB is the clock bias. Because there is a difference between the clock frequency in the receiver and the clock in the satellites, there will be some difference called bias. However, by factoring the clock error into the equations, it will be canceled out.
The goal of the microprocessor is to solve for the user position, designated UX, UY, and UZ in the equations. Once the calculation has been made, the microprocessor converts that information to the latitude, longitude, and altitude data, which is displayed on the LCD screen. The receiver display also shows the time of day, which is highly precise because it is derived from the atomic clocks in the satellites.
Over the years, a number of services have been created to improve on the accuracy of the GPS. That inherent accuracy is less than 10 m. Yet for many, this accuracy is insufficient. Errors caused by signal propagation speed differences in the ionosphere and troposphere, minor variations in satellite position, multipath signals, and even timing differences caused by clock drift add up to the loss of position accuracy.
One of the enhanced services is differential GPS (DGPS). This service is implemented by the U.S. Coast Guard and is available only in the United States, mostly on the coasts and along major waterways. DGPS uses a fixed station whose precise location is known. This station then monitors all satellites and compares location data from the satellite to its known position. It determines any errors in a position that they create and Transmits these errors to GPS receivers, where the error data updates the received data to give a more accurate position. The error signals are transmitted on a separate radio, so a GPS receiver must also have a receiver for the DGPS signal to provide the error correction information. DGPS-enabled GPS radios are widely available but cost a bit more than a standard unit. If you want the most accurate position information available with an error of less than 5 m, this is the service to use, if it is available locally.
Another enhanced GPS is called the Wide-Area Augmentation System (WAAS). It was developed by the Federal Aviation Administration (FAA) and the Department of Transportation (DoT) so that aircraft could use GPS for blind instrument control landings. Currently, the error even with DGPS is just too great to assume that a plane will be able to precisely identify the end of the runway and its extremities.
The WAAS consists of about 25 ground stations around the United States with precisely known locations and two coastal stations that collect all the data from the other stations. The collected data is used to determine all errors, and then differential correction signals are transmitted up to one of two geosynchronous satellites that in turn transmit the correction signals to GPS receivers. As with DGPS, the receiver must be WAAS-enabled to receive the corrective data. The use of WAAS improves the accuracy with an error of less than 3 ft.
The primary application of GPS is military and related navigation. GPS is used by all services for ships, aircraft of all sorts, and ground troops. Civilian uses have also increased dramatically because of the availability of many low-cost portable receivers. In fact, it is now possible to purchase a handheld receiver for less than $200. Most civilian applications involve navigation, which is usually marine-or aviation-related. Hikers and campers and other outdoor sports enthusiasts also use GPS.
Commercial applications include surveying, mapmaking, and construction. Vehicle location is a growing application for trucking and delivery companies, taxi, bus, and train transportation. Police, fire, ambulance, and forest services also use GPS. GPS-based navigation systems are now widely available as accessories in cars to provide a continuous readout of current vehicle location.
GPS is finding new applications every day. For instance, it is used to keep track of fleets of trucks. A GPS receiver in each truck transmits its position data by way of a wireless connection, such as a wireless local-area network or cell phone. Most smartphones contain a GPS receiver that automatically reports the location of the user if he or she makes a 911 call. Called Enhanced 911 service (E911), this feature is one that all cell phone companies must provide. While most location-based services will be used for 911 calls, eventually other location services may be developed for cell phones. Not all cell phones use GPS. Some use a unique triangulation method based on the cell phone being able to be in touch with at least three cell sites. Finally, GPS receivers are so inexpensive and accurate that they have led to a new hobby called geocaching. In this sport, one team hides an item or “treasure” and then gives the other team coordinates to follow to find the treasure within a given time.
The success of GPS encouraged other countries to build their own GNSS. Over the years, several similar systems have been deployed. The largest is Russia’s GLONASS. It was initially functional in 1995 and was updated and restored in 2011. It too uses 24 satellites at a height of 11,890 miles. Operational frequencies are 1.602 GHz and 1.246 GHz. China is building its own system, called Compass. It is an expansion of its regional system called Beidou. It is made up of 30 satellites at an altitude of 13,140 miles and 5 geostationary satellites. It also uses L-band frequencies of 1.561098, 1.589742, 1.20714, and 1.26852 GHz.
The European Union (EU) is building its own system, called Galileo. Currently, only a few satellites are operational, but plans call for a total of 30 satellites at a height of 14,430 miles. The system is expected to be more accurate than GPS. The plan is for Galileo to be fully operational by 2020. It uses L-band frequencies of 1.164–1.215 GHz, 1.26–1.3 GHz, and 1.559–1.592 GHz.
Because all the GNSS use the same basic CDMA signals in the L band, they can complement one another to provide increased accuracy. Many GNSS receivers are capable of receiving both GPS and one or more other GNSS signals and combining them for improved precision of location.
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