UNIT-2

1). Elements of Communication Satellite design

Ans:

Figure 2.1 Elements of Communication Subsystem

 .Low Noise Amplifier(LNA):The first amplifier provides about 20 dB gain to the very
weak signal and adds low noise that is called LNA
 More gain of the 30dB or more are provided in in the subsequent an amplifying stages to
the input requirement of down converter.
 Important specification of RF amplifier are
Noise Figure
Receiver sensitivity or minimum detectable signal level
Gain
Dynamic Range
 Down converter: It is nonlinear device which mixes input signal with locally generated
signal to produce required downlink frequency.
 To reduce unwanted harmonics a BPF is put after the Mixer.
 In some cases down conversion to IF at lower frequency is also done and then the signal is
up converted, this is called double conversion type repeater.
 Local Oscillator (LO): The local oscillator base frequency is of the order of 10 to 100
MHz.
 This is multiplied and amplified to generate the required LO frequency for mixing.
Stability of LO is typically 1 PPM.
 Important parameters for LO are,
  LO stability
 Oscillator phase noise (short term random fluctuations in frequency of phase.
 Input Multiplexer: Input sub-band formation is done through a set of BPF called input de
multiplexer.
 These filters should have high adjacent channel rejection and low amplitude and phase
ripple over the pass band.
 Power Amplifier: When power requirement of more than 20 W Travelling wave tube
TWTA is used as HPA. It introduces nonlinearity. Linearizers are used but that increases
the complexity, weight and cost.
 SSPA are used when lower than 20 W is required. SSPA is less efficient compared to
TWTA but it needs less space, weight and lower voltage operation.
 Output DE Multiplexer: All HPA outputs are combined through another bank of BPF,
this is called output DE multiplexer. This is high power and low loss filter. Output of
OMUX is connected to transmit antenna.
2) Transponders:
 The transponder is essentially a receiver which receives the signal transmitted from the
earth by the uplink, amplifies it and retransmits it with the downlink, with a different
frequency. Thus the word “Transponder” is formed by combining the two words-
TRANSmitter and resPONDER.
 Most satellites have anything between 10 to 30 transponders of different bandwidth on
board. Basic Transponder elements are shown in figure 2.11.
 Transponders can be either active or passive. A passive transponder allows a computer or
robot to identify an object. Magnetic labels, such as those on credit cards and store items,
are common examples. A passive transponder must be used with an active sensor that
decodes and transcribes the data the transponder contains.
 It consists of input Mux, Power Amplifier, and output Mux.
 The overall specification of transponder is
Channel bandwidth
Adjacent channel rejection
Group delay
 Figure 2.2:Basic Transponder elements

Classification of Transponders:

Figure 2.3: Simplified single conversion transponder (bent type) for 6/4 GHz band

Figure 2.12 shows a typical single conversion bent pipe transponder of the type used on
many satellites for the 6/4 GHz band.
 The output power amplifier is usually a solid state power amplifier (SSPA) unless
a very high output power (>50 W) is required, when a traveling wave tube amplifier
(TWTA) would be used.
 The local oscillator is at 2225 MHz to provide the appropriate shift in frequency from
the 6-GHz uplink frequency to the 4-GHz downlink frequency, and the band-pass filter
after the mixer removes unwanted frequencies resulting from the down-conversion
operation.
 The attenuator can be controlled via the uplink command system to set the gain of the
transponder. Redundancy is provided for the high-power amplifiers (HPA) in each
transponder by including a spare
 TWT or solid-state amplifier (SSPA) that can be switched into circuit if the primary
power amplifier fails. The lifetime of HPAs is limited, and they represent the least
reliable component in most transponders.
  Providing a spare HPA in each transponder greatly increases the probability that the
satellite will reach the end of its working life with all its transponders still
operational. Transponders can also be arranged so that there are spare transponders
available in the event of a total failure. The arrangement is known as M or N
redundancy.
3) Explain about Spacecraft (Satellite) Subsystem (or) Integration
Ans:
Attitude and Orbit Control System (AOCS)
This subsystem consists of rocket motors that are used to move the satellite back to
the correct orbit when external forces cause it to drift off station and gas jets or inertial
devices that control the attitude of the satellite.
Telemetry, Tracking, Command, and Monitoring (TTC&M)
 These systems are partly on the satellite and partly at the controlling earth station..
The telemetry system sends data derived from many sensors on the satellite, which
monitor the satellite's health, via a telemetry link to the controlling earth station.
 The tracking system is located at this earth station and provides information on the
range and the elevation and azimuth angles of the satellite.
 Repeated measurement of these three parameters permits computation of orbital
elements, from which changes in the orbit of the satellite can be detected.
 Based on telemetry data received from the satellite and orbital data obtained from the
tracking system, the control system is used to correct the position and attitude of the
satellite.
 It is also used to control the antenna pointing and communication' system configuration
to suit current traffic requirements, and to operate switches on the satellite.
Power System
 All communications satellites derive their electrical power from solar cells. The
power is used by the communications system, mainly in its transmitters, and also by
all other electrical systems on the satellite.
Communications Subsystems
 It is usually composed of one or more antennas, which receive and transmit over wide
bandwidths at microwave frequencies, and a set of receivers and transmitters that
amplify and retransmit the incoming signals.
 The receiver-transmitter units are known as transponders.
 There are two types of transponder in use on satellites: the linear or bent pipe transponder
that amplifies the received signal and retransmits it at a different, usually lower,
frequency.Baseband processing transponder which is used only with digital signals,
that converts the received signal to baseband, processes it, and then retransmits a digital
signal.
Satellite Antennas
  Most satellite antennas are designed to operate in a single frequency band, for
example, C band or Ku band. A satellite which uses multiple frequency bands usually has
four or more antennas.
 Four main types of antennas used on satellites:
Wire antennas: monopoles and dipoles
Horn antennas
Reflector antennas
Array antennas.
4) TELEMETRY, TRACKING, COMMAND, AND MONITORING
Ans)
The TT&C system is essential to the successful operation of a communication satellite.
 The main functions of satellite management are to control the orbit and attitude of the
satellite, monitor the status of all sensors and subsystems on the satellite, and switch on or
off sections of the communication system.
 On large geostationary satellites, some re-pointing of individual antennas is also
possible, under the command of the TT&C system. Tracking is performed primarily by
the earth station.
Telemetry and Monitoring System
 The monitoring system collects data from, many sensors within the satellite and sends these
data to the controlling earth station.
 There may be several hundred sensors located on the satellite to monitor pressure in the fuel
tanks voltage and current in the power conditioning unit, current drawn by each subsystem,
and critical voltages and currents in the communications electronics.
 The sensor data, the status of each subsystem, and the positions of switches in the
communication system are reported back to the earth by the telemetry system.
 The sighting devices used to maintain attitude are also monitored via the telemetry link: this
is essential in case one should fail and cause the satellite to point in the wrong direction.
The faulty unit must then be disconnected and a spare brought in, via the command
system, or some other means of controlling attitude devised.
 Figure 2.8: Typical TTC&M system
 Telemetry data are usually digitized and transmitted as phase shift keying (PSK) of a low-
power telemetry carrier using time division techniques. A low data rate is normally used to
allow the receiver at the earth station to have a narrow bandwidth and thus maintain a
high carrier to noise ratio. Typical TTC&M system is shown in figure 2.8
Tracking
 The tracking system at the control earth station provides information about the range,
elevation, and azimuth for a satellite.
 A number of techniques can be used to determine the current orbit of a satellite.
Velocity and acceleration sensors on the satellite can be used to establish change in
orbit from the last known position, by integration of the data.
 The earth station controlling the satellite can observe the Doppler shift of the
telemetry carrier or beacon transmitter carrier to determine the rate at which range is
changing.
 Together with accurate angular measurements from the earth station antenna, range is
used to determine the orbital elements.
 Active determination of range can be achieved by transmitting a pulse, or sequence of
pulses, to the satellite and observing the time delay before the pulse is received
again.
 The propagation delay in the satellite transponder must be accurately known, and
more than one earth station may make range measurements.
 If a sufficient number of earth stations with an adequate separation observing the
satellite, its position can be established by triangulation from the earth station by
simultaneous range measurements.
 3.4 POWER SYSTEMS 71

 With precision equipment at the earth stations, the position of the satellite can be
determined within 10 m.
 Ranging tones are also used for range measurement.
Command
 A secure and effective command structure is vital to the successful launch and operation
of any communication satellite.
 The command system makes changes the position and attitude of the satellite, controls
antenna positioning and communication system configuration, and operates switches at the
satellite.
 During launch it is used to control the firing of the apogee kick motor (AKM) and to spin
up a spinner or extend the solar sails of a three-axis stabilization satellite.
 The command structure must have safeguards against inadvertent operation of a control
due to error. The control code is converted into a command word which is sent in a
TDM frame. After checking for validity in the satellite, the command word is sent back to
the control earth station via the telemetry link where it is checked again.
 If it is received correctly, then an execute instruction is sent to the satellite so that the
command is executed.
Power Systems
 All communications satellites obtain their electric power from solar cells which convert
incident sunlight into electrical energy.
 Three types of power systems
Solar – the most frequently used in commercial satellites
Chemical – used for backup to power satellite during solar eclipses
Nuclear – used for satellites leaving the Earth orbit (deeper space exploration)
 But because of the danger to people on the earth in case of launch fail and
consequent nuclear spread, communications satellites have not used nuclear generators.
 Block diagram of solar power generation system is shown in figure 2.9 . Solar radiation falling
on a satellite at geostationary orbit has an intensity of 1.39 kW/ m2.
5) What is Equipment Reliability and Space Qualification
Ans:
Communication satellites built already have provided operational lifetimes of up to
15years. Once a satellite is in geo stationary orbit, there is little possibility of repairing
components that fail or adding more fuel for station keeping. The components that make
up the satellite must therefore have very high reliability in the hostile environment of outer
space, and a strategy must be devised that allows some components to fail without causing
the entire communication capacity of the satellite to be lost. Two separate approaches are
used: space qualification of every part of the satellite to ensure that it has a long life
expectancy in orbit and redundancy of the most critical components to provide continued
operation when one component fails.
Space Qualification:

Outer space, at geostationary orbit distances is a harsh environment. There is a total

vacuum and the sun irradiates the satellite with 1.4kw of heat and light on each square
meter of exposed surface. Electronic equipment cannot operate at such extremes of
temperature and must be housed within the satellite and heated or cooled so that its
temperature stays within
the range 0 to 750 C. This requires a thermal control system that manages heat flow
throughout a GEO satellite as the sun moves around once every 24hr.

When a satellite is designed, three prototype models are often built and tested. The
mechanical model contains all the structural and mechanical parts that will be included in
the satellite and is tested to ensure that all moving parts operate correctly in a vacuum,
over a wide temperature range. The thermal model contains all the electronics packages
and other components that must be maintained at correct temperature. The electrical
model contains all electronic parts of the satellite and is tested for correct electrical
performance under total vacuum and a wide range of temperatures.

Many of the electronic and mechanical components that are used in satellite are known to
have limited life times, or a finite probability of failure. If failure of one of these
components will jeopardize the mission or reduce the communication capacity of the
satellite, a backup, or redundant, unit will provided. The design of the system must be
such that when one unit fails, the backup can automatically take over or be switched into
operation by a command from the ground.

Reliability
Reliability is counted by considering the proper working of satellites critical components.
Reliability could be improved by making the critical components redundant. Components
with a limited lifetime such as travelling wave tube amplifier etc should be made
redundant.
Example: consider a system having i parallel components in which reliability of each
element is independent of others.

If Qi is the unreliability of the ith parallel element, then the probability that all units will
fail is the product of the individual un-reliabilities:
Qs = Q1 Q2 Q3 …. Qi
When the un-reliability of all elements is equal, then Qs = Qi where Q is the un-reliability
of each element.

By doing a complete failure analysis, one could find out which failure occurs more than
the rest and such analysis help in finding out the manufacturing defects in the product of a
given batch of components or probably a design defect.

This analysis is done to reduce the overall reliability to a value less than that predicted by
the above analysis.

Co-related failures could also be reduced by using units from different manufacturers. The
design defects are generic to all satellite produced in a series. Generally these defects are
detected and corrected to minimize their impact. This is done when a complete design
change cannot be implemented.

Even through the reliability can be improved by adding redundant devices and
components, the weight of the satellite increases which again becomes a problem.
Redundant component also increase the cost of the satellite.

The two major cost components are:

o Cost of equipment together with the switching and failure sensing mechanism used.
o The associated increase in weight of the satellite resulting in an increased launch cost.

Optimization techniques are performed for cost minimization purpose.

Bathtub curve for probability of failure