Space segment satellite Communication

1. The Space Segment

2. Satellite System

3. Satellite System
• Space Segment
• Satellite
• TT&C- Tracking, Telemetry and Control
• Ground Segment
• Earth Station
TRANSPONDER

4. Major Components of Space Segment
• The power Supply
• Attitude Control: The attitude of a satellite refers to its orientation in
space
• Spinning satellite stabilization
• Momentum wheel stabilization
• Station Keeping
• Thermal Control
• TT & C Control
• Antenna Subsystem

5. The Power Supply
• solar sails

6. The Power Supply-solar sails
• solar sails

7. Attitude Determination and
Control System

8. SATELLITE’S ATTITUDE
• Orientation of satellite as perceived in a certain
frame of Reference

9. CHANGE IN ATTITUDE
 Satellite tends to change its orientation because of environmental
torques
• Drag of residual atmosphere
• Solar radiation pressure
• Gravity gradient
• Interaction of Satellite electronics with earth’s magnetic field

10. Need for ATTITUDE CONTROL
Needed because-
• Payload requirements
• Eg. Focusing the satellite camera to a particular location on earth
• Communication requirements
• Pointing the antenna towards ground
• Power system requirements
• Tracking the sun to achieve maximum power generation

11. Components of ADCS
 Sensors- To determine the orientation and position of the satellite
Algorithms-To calculate the deviation from the desired orientation
and to generate actuation command to counter the deviation
 Actuators-To act upon the signals given by the control algorithms
and to produce the necessary torqes

12. SENSORS
 Measure the attitude of the satellite
Types:
• Gyroscopes:
• Sense rotation in 3-D space without reliance on observation of external objects
• Consists of a spinning mass, also includes laser gyros utilizing coherent light reflected
around a closed path
• Gyros require initialization by some other means as they can only measure “changes” in
orientation

13. SENSORS contd…
• All gyro instruments are subject to drift and can maintain orientation for limited times only
(typically tens of hours or less)
• Horizon indicators
• Optical instrument that detects light at the horizon
• Can be a scanning or staring instrument
• Infrared is often used which can function even on the dark side of earth
• Tends to be less precise than sensors based on stellar observation

14. SENSORS contd…
• Orbital gyro compassing
• Uses a horizon sensor to sense the direction of earth’s centre
• Uses a gyro to sense rotation about an axis normal to orbital plane
• Hence it provides pitch and roll measurements
• Sun Sensor
• Senses the direction of Sun
• Can be simple as solar cells and shades or complex as a steerable telescope

15. SENSORS contd…
• Star Trackers
• Optical device measuring the direction to one or more stars (using a photocell or solid
state camera to observe the star)
• Require high sensitivity ,may become confused by sunlight reflected from the exhaust
gases emitted by thrusters
• Global Positioning System(GPS)
• Required for position measurements
• Determines position and speed of the satellite in space

16. CONTROL ALGORITHMS
Control Algorithms are computer programs that receive input data
from vehicle sensors and derive the appropriate torque commands
to the actuators to rotate the vehicle to the desired attitude

17. Details of Control Algorithms
 “actuator and sensor processing”
• Establishes the interfaces to the sensors and the actuators
needed for attitude control
• Determines the necessary commanding for the actuators
from the torques computed by the layer estimation
prediction control
• Performs the time critical communications with actuators
and determinates the state of actuators

19. ACTUATORS
• Apply the torques needed to re-orient the vehicle to the desired
attitude
Types:
• Thrusters (often mono propellant rockets)
• limitation: fuel
• Spin -stabilization
• Momentum wheels
• Electric motor driven rotors made to spin in the direction opposite to that required to re-
orient the vehicle

20. ACTUATORS
• Make up a small fraction of the spacecraft’s body, are computer
controlled to give precise control
• Momentum wheels are generally suspended on ‘magnetic bearings’
to avoid bearing friction and breakdown problem
• To maintain orientation in 3D space , minimum of 2 must be used
,additional units provide single failure protection

21. ACTUATORS
 Control Moment Gyros
• Include rotors spun at constant speed mounted on Gimbals
• Provides control about the two axes orthogonal to the gyro spin axes, triaxial
control still requires 2 units
• CMG is a bit more expensive in cost and mass since , gimbals and their drive
motors must be provided
• Max. torque exerted by CMG is greater than than for a momentum wheel
(suitable for larger spacecraft)

22. ACTUATORS
• Drawback: additional complexity increases failure points
 Solar Sails
• Produce thrust as a reaction force induced by reflecting incident light
• Used to make small attitude control and velocity adjustments
• Saves larger amounts of fuel by producing control moments

23. ACTUATORS
 Pure passive Attitude Control
• Gravity gradient Stabilization
• Magnetic Field
Main advantage is that no power or fuel is required to
achieve attitude control

24. REFERENCE SYSTEM
• The three critical flight dynamics parameters are rotations in three
dimensions around the vehicle’s coordinate system origin ,the centre
of mass. These angles are pitch, roll and yaw.
• Pitch: rotation around the lateral or transverse axis. Ex. Nose pitches
up and the tail down or vice versa.
• Roll: rotation around longitudinal axis.
• Yaw: rotation about the vertical axis.

25. REFERENCE
SYSTEM
PITCH , YAW AND ROLL
AXES
Pitch

26. ADCS of PRATHAM
 Sensors finalised :
• SunSensors
• Magnetometer
• GPS
• Gyros
 Control Law
 Actuator finalised:
• Magnetorquer
(as on 18 sept 2008)

27. Spinning satellite stabilization

28. Spinning satellite stabilization
• Spin stabilization may be achieved with cylindrical satellites. The
satellite is constructed so that it is mechanically balanced about one
partic- ular axis and is then set spinning around this axis. For
geostationary satellites, the spin axis is adjusted to be parallel to the
N-S axis of the earth
• In the absence of disturbance torques, the spinning satellite would
maintain its correct attitude relative to the earth.
• Disturbance torques are generated in a number of ways, both
external and internal to the satellite.
• Solar radiation, gravitational gradients, and meteorite impacts are all
examples of external forces which can give rise to disturbance torques

29. Spinning satellite stabilization- How to Achieve
Reference Attitude
• Nutation, which is a form of wobbling, can occur as a result of the
disturbance torques and/or from misalignment or unbalance of the
control jets. This nutation must be damped out by means of energy
absorbers known as nutation dampers.
• The antenna feeds can therefore be connected directly to the
transponders without the need for radiofrequency (rf) rotary joints,
while the complete platform is despun. Of course, control signals and
power must be transferred to the despun section, and a mechanical
bearing must be provided.
• The complete assembly for this is known as the bearing and power
transfer assembly (BAPTA).

30. Momentum wheel stabilization

31. Thermal Control
• Satellites are subject to large thermal gradients:
1) One side toward sun, the other into space.
2) Heat from equipment.
3) Heat from ground.
Mirrors and isolators are used

32. TT&C- Tracking, Telemetry and Control

33. Antenna Sub-system
• The antennas carried aboard provide the dual functions of
Uplink/Downlink RX/TX operations
• Antennas range from omni-directional (dipole type) to highly
directional antennas required for telecom and TV purposes.
• Directional beams are usually produced by means of reflector-type
antennas, the paraboidal reflector being the most common.

34. Antenna Sub-system
• Wide beams for global coverage are produced by simple horn
antenna at 6/4 GHz.
• The same feed horn can be used to TX and RX carriers with the
same frequency (Diplexer), also XPD can be used.
• Simple bi-conical dipole antenna is used for tracking and control
signals.

35. Antenna Sub-system-Example

36. Transponder: A transponder is the series of interconnected units which
forms a single communications channel between the receive and
transmit antennas in a communications satellite.

37. The Wide band Receiver

38. The input De-multiplexer

39. The Power Amplifier