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Satellite Communication Segments

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					     MODULE III

Satellite Communication
        Segments
                    Space Segment




UPLINKS                                   DOWNLINKS


                        CONTROL
                         STATION
                         (TT & C)




     TRANSMITTERS                     RECEIVERS
                     Ground Segment


Architecture of Satellite Communication System
             Satellite communication –
                     SEGMENTS


   Satellite communication systems can be broadly
    divided into two segments :
    Space segment : It includes the satellites, ground
    facilities needed to keep satellites operational,
    referred to as Tracking, Telemetry & Command
    (TT&C )

    Earth segment : It consists of transmit & received
    earth stations.
        Satellite Communication
                 Systems
   Ground Segment
       Tracking
       Telemetry
        Control

   Space Segment
       Payload
       Bus
       Transponder
                      Space segment

   Equipment carried aboard the Satellite can be classified as :
    Payload : refers to the equipment used to provide the service
    for which satellite has been launched.
    Bus : it not only refers to the vehicle which carries the payload
    but also, to the various sub - systems which provide :
    Power
    Attitude control
    Orbital control
    Thermal control
    Command & telemetry functions
Payload & Bus Elements
                  Power supply
Primary electrical power for operating the electronic
equipment is obtained from solar cells.
Individual cells can generate only small amounts of power,
therefore arrays of cells in series – parallel connection are
required.
Higher powers can be achieved with solar panels arranged in
the form rectangular “Solar sails”.
They must be folded during launched phase & extended when
in GEO.
During Eclipse, power is provided by two nickel-cadmium
long life batteries, which will deliver 830 Watts of power.
Nickel-Hydrogen batteries are being used in Hughes HS 601
& in Intelsat IV.
                Attitude control

Attitude of satellite refers to its orientation in space.
Much of equipment carried aboard a satellite is for purpose of
controlling the attitude.
It is necessary to ensure that directional antennas point in
proper direction.
In case of earth environmental satellites, earth sensing
instruments must cover the required regions of the earth:
requires attitude control.
Number of forces, disturbance torques can alter the attitude
e.g. being solar radiations, meteorite impacts etc.
          Attitude control contd……..

   Infrared sensors refer to as Horizon detectors are used to
    detect the rim of the earth against the background of space.
   With the use of four such sensors, one for each quadrant, the
    center of the earth can be readily established as a reference
    point.
   Any shift in orientation is detected by one or other of the
    sensors & a corresponding control signal is generated which
    activates a restoring torque.
   An attitude maneuver is executed where a shift in attitude is
    desired.
   The control signals needed to achieve this maneuver may be
    transmitted from earth stations.
            Types of attitude control

   Passive attitude control : it refers to the use of
    mechanism which stabilize the satellite without
    putting a drain on the satellite energy supply. E.g.
    spin stabilization & gravity gradient stabilization.
   Active attitude control : in this case there is no
    overall stabilizing torque present to resist the
    disturbance torque.
     Instead, corrective torques are applied as required in
    response to disturbance torque. Methods used to
    generate active control talks include em coils, gas jets
    etc.
        How satellite attitude is defined                                    ?
   It is defined by three axes : roll,   To                                     Roll axis
                                          earth’s
    pitch & yaw                           center    Yaw

   All three axes pass through the                                          Roll rotation
    center of gravity of satellite.
   Movement of satellite about the
    roll axis moves the antenna
    footprint north & south.
   Movement about the pitch axis                                       Pitch rotation
    moves the footprint east & west.
   Movement of satellite about the
    yaw axis rotates the antenna                      Pitch axis (normal to orbital
    footprint.                                        plane)
                       Spin Stabilization
   Spin Stabilization used with
    cylindrical satellites.
   Satellite constructed so that it is                           Satellite
    mechanically balanced about one                               spin
    particular axis & is then set                   N
    spinning around the axis.
   For GEO spin axis is adjusted
    parallel to the N-S axis of the
    earth.
   In absence of disturbing torques,
    the spinning satellite would                    S
    maintain its correct attitude
    relative to the earth.
   In presence of disturbing torques,      Geostationary Orbit
    spin rate decrease & direction of
    angular spin axis will change.
   Jets can be used to increase the
    spin rate & to shift axis back to its
    correct N-S orientation.
          Three Axis Stabilization
   There are stabilizing elements for each of the three axes,
    roll, pitch & yaw.
   Because the body of the satellite remain fixed relative to
    earth, this stabilization is also known as body
    stabilization.
   Active Attitude control is used with three axis
    stabilization.
   This may take the form of control jets fired to correct
    the attitude of satellite.
   Reaction wheels can also be used.
       Orbital control (Station keeping)
   Station keeping : used for maintaining a satellite in its correct
    orbital position.
   Equatorial ellipticity of the earth causes GEO to drift slowly
    along the orbit, to one of two stable points, at 75 deg east
    &105 deg west.
   To counter this drift, an oppositely directed velocity
    component is imparted to the satellite by means of jets.
   This results in satellite drifting back through its nominal
    station position, coming to a stop & recommencing the drift
    along the orbit until the jets are pulsed once again these are
    called east-west station keeping maneuvers.
         Orbital Control Contd….
   Satellites in the 6/4-GHz band must be kept within +-
    1deg of designated longitude, & in the 14/12 GHz band,
    within +-.05deg.
   Satellite which is in GEO will also drift in latitude, the
    main perturbing forces being the gravitational pull of the
    sun & the moon.
   These forces causes the inclination to change at a rate
    of .85deg / year.
   Counteracting jets must be pulsed to correct the
    inclination.
   These maneuvers are termed as north-south station
    keeping manuvers.
                     Thermal control

   Satellites are subject to large thermal gradients, receiving the
    sun’s radiation on one side while the other side faces into
    space.
   Thermal radiation from the earth & the earth’s albedo, which
    is the fraction of radiation falling on earth which is reflected :
    significant for low altitude earth orbiting satellites.
   Negligible for GEO.
   Equipment in the satellite also generates heat which has to be
    removed.
   Thermal blankets & shields which may be provided for
    insulations.
   Radiation mirrors are often used to remove heat from payload.
                   TT&C Subsystem

   Telemetry, tracking, & command subsystem:
   Telemetry : means measurement at a distance. It refers to
    generating an electrical signal proportional to quantity being
    measured, & encoding & transmitting this to a distant station,
    which for the satellite is one of the earth station.
   Data which are transmitted as telemetry signals include
    attitude information, environment information etc.
   This subsystem transmits information about the satellite to the
    earth station while command subsystem receives command
    signals from earth station, in response to telemetred
    information.
          TT&C Subsystem contd…….
   The command subsystem demodulate & decodes the command
    signals & routes these to appropriate equipment needed to
    execute the necessary action.
   Command signals are often encrypted.
   Tracking : it is accomplished by having the satellite transmit
    signals which are received at TT&C earth stations.
   It is important during the transfer & drift orbital phases of the
    satellite launch.
   A GEO satellite will tend to shift as a result of the various
    distributing forces.
   Therefore it is necessary to track the satellites movements and
    send correction signals as required.
   Satellite range is also required for time to time.
    This can be determined by measurement of propagation delay of
    signals specially transmitted for ranging purposes.
    Satellite Control System
                                                             Telemetry Tracking &
                                                             command facilities
                             Ranging
Ranging                      commands &
                             Telemetry


                                        TTAC
                                                               TTAC Station
                  Ranging
                                        Station                eastern
                  data
                                                               hemisphere
Other                            Voice & Data Links
Telesat
Stations          Data Communication
                           &
                    reduction facility


                            Satellite
Analysis center                                   Computer
                            Control
                             Center                Center
                    Transponders


   A transponder is the series of interconnected units
    which forms a single communication channel
    between the receive and transmit antennas in a
    communication satellite.
   Some of the units utilized by a transponder in a given
    channel may be common to a number of
    transponders.
   Transponder consist of wideband receivers, input de-
    multiplexer, power amplifier components.
    Frequency arrangement of C band
                Satellite
 Transmission Band: C Band
  4 GHz (Down Link), 6 GHz (Uplink).
Bandwidth allocated for C-band service: 00 MHz.
This is divided into sub bands, one for each
  transponder.
A typical Transponder BW is 36 MHz allowing for a
  4 MHz guard band between transponders.
12 such transponders can be accommodated in
  500 MHz BW.
    Frequency arrangement of C band
            Satellite contd…
   Polarization isolation can be used to double the no. of
    transponders.
   Polarization isolation refers to the fact that carriers,
    which may be on the same frequency but with opposite
    senses of polarization, can be isolated from one another
    by receiving antennas matched to the incoming
    polarization.
   With linear polarization, vertically & Horizontally
    polarized carriers can be separated in this way.
   Because carriers with opposite senses of polarization
    may over lap in frequency , this technique is referred to
    as Frequency Reuse.
               Polarization plan

                             500

      4          36
                                Vertical Polarization

                             6105     6145




Horizontal            6085         6125
polarization
      Satellite Transformer Channels
                                 O/P MUX             Power gain
                                           I/P MUX   blocks
                                                        3.720
                                                       3.760
                                                       3.800
                                                       3.840
                                                       3.880

                         Wide Band                     3.920
          BPF             Receiver                     3.960
From                                 3.7                           To
receive                                                4
                                     GHZ                           Tx
                5.925-
antenn                                                 4.040
                                                                   Ant
                6.425
as              GHz                                    4.080       3.7-
                                                                   4.2
                                                       4.120
                                                                   GHz
                                                       4.160


                                                     Center
                                                     frequencies
                                                     GHz
     Satellite Wideband Receiver
   A duplicate receiver is provided so that , if one
    fails, the other is automatically switched in.
   The combination is referred to as redundant
    receiver, meaning although two are provide one
    is active at a time.
   The first stage is LNA.
   Then is the mixer stage followed by amplifiers
    (may utilize BJTs or FETs).
         Satellite Wideband Receiver
                      Redundant Receiver


              LNA



From
I/P                                                  To I/P
Filter      Pre amp                                  DEMUX
                             Mixer




                                               Amp
               LNA




                            Oscillator 2.225
                            GHz
          Input DeMultiplexer
   The I/P DEMUX separates the broad band
    input, covering the frequency range 3.7-
    4.2 GHz, into the transponder frequency
    channels.
   The channels are arranged in even
    numbered & odd numbered groups.
   This provides greater frequency separation
    b/w adjacent channels in a group, which
    reduces ADI.
             Input Demultiplexer
   The channels are usually arranged in even numbered &
    odd numbered groups.
   This provides greater frequency separation b/w adjacent
    channels in a group, reducing ADI.
   The O/P from Rx is fed to a power splitter which in turn
    feeds to separates circulators.
   Broadband signal is transmitted along each chain,
    channelizing is achieved by means of channel filters
    connected to each circulator.
   Each filter has BW of 36 MHz & is tuned to appropriate
    center frequency.

             Power Amplifier for each
    Power amplifier provides the output power
    transponder channel.
   Each power amplifier is preceded by an attenuator.
   Attenuator has a fixed or a variable section.
   Fixed attenuator is needed to balance out variations in
    the input attenuation so that each transponder channel
    has the same nominal attenuation.
   The variable attenuator is needed to set the level as
    required for different type of services (for e.g.
    requirement for input power back – off ). The variable
    attenuator adjustment is an operational requirement, it
    must be under the control of the ground TT&C station.
         Power Amplifier contd…
   Traveling wave tube amplifier (TWTAs) are widely used in
    transponders to provide the final output required to the transponder
    channel.
          Block Diagram showing relative
              levels in a Transponder
                                                 5




                                                     6
                             2                              7

                                         3
              1                              4

                             2
                                                 5   6
1. Input filter   6. Amplifier
2. Wide band Rx   7. Mux         60 dB

3. 3 dB coupler
4. DeMux                                                 102.5 dB

5. Attenuator
              Antenna Subsystem
   The antenna carried abroad a satellite provide the dual functions of
    receiving the uplink and transmitting the downlink signals.
   Ranges from dipole type antenna where omni – directional
    characteristics are required to the highly directional antennas
    required for telecommunications, TV relays and
    broadcast.directional beams are produced by means of reflector
    type antennas, the paraboloidal reflector being the most common.
     The gain of the reflector is given as
                                      G=     n(i) [3.14 *D]^2 / L^2
              n(i) = aperture efficiency = 0.55 (typically)
              D = reflector diameter
              L = wavelength of the signal
    Antenna Subsystem contd….
   The beam width is given by
                       th (3 dB) = 70 L/D degree
      As the gain increases the beam width becomes narrower, by
    increasing the reflector size or decreasing the wavelength.
   Wide beams for global communication are produced by simple horn
    antennas at 6/4 GHz as these horns beam the signal directly to the
    earth without the use of the reflectors.
   A simple biconical dipole antenna is used for the tracking and
    control signals.
   The same feed horn may be used to transmit and receive carriers
    with the same polarization.Transmit & received signals are
    separated by a device called diplexer.
Communications Satellite Components
           Satellite system link model

   Satellite system consists of
    three basic sections :
    Uplink
    Satellite transponder
    Down link
                         Uplink model
                                                              To
                                                              transponder


                          Up converter

Baseband
                   BPF
                                             RF
       Modulator         IF     Mixer             BPF   HPA




                              MW generator
                               6-14GHz
                       Satellite Transponder
                                   Frequency Translator



                                                      Rf
                              RF
                        LNA              MIXER             BPF          LPA
          BPF




                                    MW shift oscillator
                                         2GHz


                                                                 To Earth station 4 or
From Earth station 6                 To other                    12 GHz
or 14 GHz                            transponders
              Down Link Model
From
Transponder
                                      IF
                    RF



   BPF        LNA         MIXER            BPF     Demodulator



                                RF

                         MW Generator            Baseband Out
                          4 or 12 GHz



                     Down Converter

				
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posted:12/1/2011
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