The document discusses the subsystems of an Earth station. It describes the major subsystems as the transmitter, receiver, antenna, tracking, and power subsystems. The transmitter subsystem takes signals to transmit, amplifies them, and sends them to the antenna. The receiver subsystem amplifies and processes signals received from the satellite. The antenna subsystem includes the feed system and reflector to radiate or receive electromagnetic waves. The tracking subsystem keeps the antenna beam aligned on the satellite. The power subsystem provides power from the electric grid and has backup generators or batteries.

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