Earth Station Subsystem
- 1. Earth Station Subsystems By Ms. Madhuri N. Sachane
- 2. Earth Station • Introduction • The role of an Earth station • Types of Earth station • Earth station subsystems • Earth station figure-of-merit • Services offered by Earth stations
- 3. Earth Station • The earth segment of satellite communication system mainly consists of three earth stations. • There are three categories of earth stations • Transmit and receive type: The earth station can transmit signal for satellite and receive signal from satellite. • These stations are used for two way communication like telephony, data transfer. • Receive only type: These earth stations can only receive the signal from a satellite. • Mostly used in CATV systems. • Transmit only type: here the earth stations can transmit signal only towards satellite. • Mostly used in data collection systems.
- 4. Earth Station • Block Diagram of Earth Station • Designing of an Earth station depends not only on the location of earth station but also on some other factors. • The depending factors are type of service providing, frequency bands utilization, transmitter, receiver and antenna characteristics.
- 5. Earth Station • Block Diagram of Earth Station • Digital information in the form of binary digits from terrestrial networks enters earth station and is then processed (filtered, multiplexed, formatted etc.) by the base band equipment. • The encoder performs error correction coding to reduce the error rate, by introducing extra digits into digital stream generated by the base band equipment. • The extra digits carry information. The presence of noise and non-ideal nature of any communication channel produces error rate is established above which the received information is not stable. • The function of the modulator is to accept the symbol stream from the encoder and use it to modulate an intermediate frequency (I.F) carrier.
- 6. Earth Station • Block Diagram of Earth Station • In satellite communication, I.F carrier frequency is chosen at 70 MHz for communication using a 36 MHz transponder bandwidth and at 140 MHz for a transponder bandwidth of 54 or 72 MHz. • The I.F is needed because it is difficult to design a modulator that works at the uplink frequency of 6 GHz (or 14GHz) directly. • The modulated I.F carrier is fed to the up-converter and frequency-translated to the uplink r-f frequency. • This modulated R.F carrier is then amplified by the high power amplifier (HPA) to a suitable level for transmission and radiation by the antenna to the satellite.
- 7. Earth Station • Block Diagram of Earth Station • On the receive side, the earth station antenna receives the low-level modulated R.F carrier in the downlink frequency spectrum. • The low noise amplifier (LNA) is used to amplify the weak received signals and improve the signal to Noise ratio (SNR). The error r ate requirements can be met more easily. • R.F is to be reconverted to I.F at 70 or 140 MHz because it is easier design a demodulation to work at these frequencies than 4 or 12 GHz. • The demodulator estimate which of the possible symbols was transmitted based on observation of the received if carrier.
- 8. Earth Station • Block Diagram of Earth Station • The decoder performs a function opposite that of the encoder. Because the sequence of symbols recovered by the demodulator may contain errors. • The decoder must use the uniqueness of the redundant digits introduced by the encoder to correct the errors and recover information-bearing digits. • The information stream is fed to the base-band equipment for processing for delivery to the terrestrial network. • The tracking equipments track the satellite and align the beam towards it to facilitate communication.
- 9. Earth Station Subsystems • There are four major subsystems that are present in any earth station. Those are transmitter, receiver, antenna and tracking subsystem. • The ground station, 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 consists of major subsystems: –The transmit subsystem –The receive subsystem –The antenna subsystem –The tracking subsystem –Power subsystem
- 10. Earth Station Subsystems • Transmitter Subsystems • 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. • Signals modulate a carrier, are amplified, and sent to an antenna via waveguides, combiners, and diplexers.
- 11. Earth Station Subsystems • Transmit Ground Control Equipment • The transmit subsystem begins with the baseband signals, which are first fed to a multiplexer, if multiple signals are to be carried by a single transponder. • The multiplexer output is then fed to a modulator. • In analog systems, a wideband frequency modulator is normally used. • In digital systems, analog signals are first digitized with PCM converters. • The resulting serial digital output is then used to modulate a QPSK modulator
- 12. Earth Station Subsystems • Receive Subsystems • The downlink is the receive subsystem of the earth station. • 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 receive 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. • The IF signal is then demodulated and de-multiplexed as necessary to generate the original baseband signals.
- 13. Earth Station Subsystems • Receiver Ground Control Equipment • The receiver ground control equipment (GCE) consists of one or more racks of equipment used for demodulating and de- multiplexing the received signals. • The down converters provide initial channelization by transponder, and the demodulators and de-multiplexing equipment process the 70-MHz IF signal into the original baseband signals. • Other intermediate signals may be developed as required by the application.
- 14. Earth Station Subsystems • Antenna subsystem • The major parts of Earth station Antenna are feed system and Antenna reflector. • These two parts combined together radiates or receives electromagnetic waves. • Since the feed system obeys reciprocity theorem, the earth station antennas are suitable for both transmitting and receiving electromagnetic waves. • Parabolic reflectors are used as the main antenna in earth stations. • The gain of these reflectors is high. • They have the ability of focusing a parallel beam into a point at the focus, where the feed system is located.
- 15. Earth Station Subsystems • Tracking Subsystem • The Tracking subsystem keeps track with the satellite and make sure that the beam comes towards it in order to establish the communication. • The Tracking system present in the earth station performs mainly two functions. • Those are satellite acquisition and tracking of satellite. • This tracking can be done in one of the following ways. • Those are automatic tracking, manual tracking & program tracking.
- 16. Earth Station Subsystems • a)Satellite Acquisition • Before communication can be established it is necessary to acquire a satellite. • One method is to program the antenna to perform a scan around the predicted position of the satellite. • The automatic tacking is switched on when the receiver signal strength is sufficient to lock the tracking receiver to the beacon. • b)Automatic Tracking • After acquisition a satellite needs to be tracked continuously. • This function is performed by the automatic tracking system. Auto-tack systems are closed-loop control systems and are therefore highly accurate. • This tracking mode is the preferred configuration when accuracy is the dominant criterion.
- 17. Earth Station Subsystems • c)Manual track: • To avoid a total loss of communication due to a failure in the tracking system, earth stations generally also have manual mode. • In this mode an antenna is moved through manual commands. • d)Program Track: • In this tracking mode the antenna is driven to the predicted satellite position by a computer. • The satellite position predictions are usually supplied by the satellite operators. • It may be noted that since a program track system is an open- loop control system, its accuracy is mainly governed by the accuracy of the prediction data.
- 18. Earth Station Subsystems • Communication satellites transmit a beacon which is used by earth stations for tracking. The received beacon signal is fed into the auto-track receiver where tracking corrections or, in some auto-track systems estimated positions of the satellite are derived. MAIN ELEMENTS OF A SATELLITE TRACKING SYSTEM
- 19. Earth Station Subsystems • Tracking Subsystem • In other auto-track techniques the feed system provides the required components of error signals. • The outputs of the auto-track receivers are processed and used to drive each axis of the antenna to the estimated satellite position. • In the manual mode, an operator sets the desired angles for each axis on a control console. • This position is compared with the actual antenna position obtained through shaft encoders, and the difference signal is used to drive the antenna. • In the program track mode the desired antenna position is obtained from a computer. • The difference in the desired antenna positions constitutes the error and is used to drive the antenna.
- 20. Earth Station Subsystems • 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 electronic equipment used to deal with the information transmitted back to the earth station.
- 21. Earth Station Subsystems • Power Subsystems • 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. • Most earth stations have backup power systems that take over if an ac power failure occurs. • The backup power system may consist of a diesel engine driving an ac generator, which automatically starts when ac power fails. • Smaller systems may use uninterruptible power supplies (UPS), which derive their main power from batteries.
- 22. Thank you