Space segment
- 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)
- 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).
- 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.
- 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