Satellite communication lecture9
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The document discusses the key components and subsystems of satellite communication systems. It is broken down into two main segments: 1) The space segment, which includes the satellites in orbit that provide the communication relay services, and the ground control stations that monitor and control the satellites. 2) The ground segment, which consists of the terminals on the Earth's surface that utilize the satellite communication capabilities. These include fixed, transportable, and mobile terminals. The space segment has two main components - the satellite bus, which provides basic support functions like structure, power, thermal control etc., and the payload which provides the actual communication relay equipment like transponders and antennas.
Satellite communication lecture9
- 3. SATELLITE COMMUNICATION SEGMENTS The satellite communications portion is broken down into two areas or segments: the space segment and the ground (or earth) segment. The space segment includes the satellite (or satellites) in orbit in the system, and the ground station that provides the operational control of the satellite(s) in orbit. The ground station is variously referred to as the Tracking, Telemetry, Command (TT&C) or the Tracking, Telemetry, Command and Monitoring (TTC&M) station.
- 5. GROUND SEGMENT • The ground segment of the communications satellite system consists of the earth surface area based terminals that utilize the communications capabilities of the Space Segment. • The ground segment terminals consist of three basic types: • fixed (in-place) terminals; • transportable terminals; • mobile terminals.
- 6. BASIC ELEMENTS OF SPACE SEGMENT • The space segment of the satellite system consists of the orbiting satellite (or satellites) and the ground satellite control facilities necessary to keep the satellites operational. • The space segment equipment carried aboard the satellite can be classified under two functional areas: the bus and the payload.
- 7. BUS The bus refers to the basic satellite structure itself and the subsystems that support the satellite. The bus subsystems are: • The physical structure • Power subsystem • Attitude and orbital control subsystem • Thermal control subsystem • Command and telemetry subsystem.
- 8. PAYLOAD • The payload on a satellite is the equipment that provides the service or services intended for the satellite. • A communications satellite payload consists of the communications equipment that provides the relay link between the up- and downlinks from the ground. • The communications payload can be further divided into: • Transponder • Antenna subsystems.
- 9. SATELLITE BUS: PHYSICAL STRUCTURE • The physical structure of the satellite provides a ‘home’ for all the components of the satellite. • The basic shape of the structure depends of the method of stabilization employed to keep the satellite stable and pointing in the desired direction, usually to keep the antennas properly oriented toward earth. • Two methods are commonly employed: • Spin stabilization • Three-axis or body stabilization. • Both methods are used for GSO and NGSO satellites.
- 10. SPIN STABILIZATION • A spin stabilized satellite is usually cylindrical in shape, because the satellite is required to be mechanically balanced about an axis, so that it can be maintained in orbit by spinning on its axis. For example: GSO satellites. • The spinning satellite will maintain its correct attitude without additional effort, unless disturbance torques are introduced. • External force such as solar radiation, gravitational gradients, and meteorite impacts can generate undesired torques. • Internal effects such as motor bearing friction and antenna subsystem movement can also produce unwanted torque in the system. • Impulse type thrusters, or jets, are used to maintain spin rate and correct any wobbling or nutation to the satellite spin axis.
- 11. THREE AXIS STABILIZATION • A three-axis stabilized satellite is maintained in space with stabilizing elements for each of the three axes, referred to as roll, pitch, and yaw, in conformance with the definitions first used in the aircraft industry. • The entire body of the spacecraft remains fixed in space, relative to the earth, which is why the three-axis stabilized satellite is also referred to as a body-stabilized satellite. • Active attitude control is required with three-axis stabilization. • Control jets or reaction wheels are used, either separately or in combination, to provide correction and control for each of the three axes. • A reaction wheel is basically a flywheel that absorbs the undesired torques that would shift spacecraft orientation. • Fuel is expended for both the control jets and for the reaction wheels, which must periodically be ‘unloaded’ of momentum energy that builds up in the wheel.
- 12. POWER SUBSYSTEM • The electrical power for operating equipment on a communications satellite is obtained primarily from solar cells, which convert incident sunlight into electrical energy. • The radiation on a satellite from the sun has an intensity averaging about 1.4 kW/𝑚2 . Solar cells operate at an efficiency of 20–25 % at beginning of life (BOL), and can degrade to 5–10 % at end of life (EOL), usually considered to be 15 years. • All spacecraft must also carry storage batteries to provide power during launch and during eclipse periods when sun blockage occurs.
- 13. ATTITUDE CONTROL • The attitude of a satellite refers to its orientation in space with respect to earth. • Attitude control is necessary so that the antennas, which usually have narrow directional beams, are pointed correctly towards earth. • Several forces can interact to affect the attitude of the spacecraft, including gravitational forces from the sun, moon, and planets; solar pressures acting on the spacecraft body, antennas or solar panels; and earth’s magnetic field. • Orientation is monitored on the spacecraft by infrared horizon detectors, which detect the rim of earth against the background of space. Four detectors are used to establish a reference point, usually the center of the earth, and any shift in orientation is detected by one or more of the sensors. A control signal is generated that activates attitude control devices to restore proper orientation. Gas jets, ion thrusters, or momentum wheels are used to provide active attitude control on communications satellites.
- 14. ORBITAL CONTROL • Orbital control, often called station keeping, is the process required to maintain a satellite in its proper orbit location. It is similar to, although not functionally the same as, attitude control, discussed in the previous section. • Orbital control is usually maintained with the same thruster system as is attitude control. • The non-spherical (oblate) properties of the earth, primarily exhibited as an equatorial bulge, cause the satellite to drift slowly in longitude along the equatorial plane. • Control jets are pulsed to impart an opposite velocity component to the satellite, which causes the satellite to drift back to its nominal position. These corrections are referred to as east-west station keeping maneuvers, which are accomplished periodically every two to three weeks.
- 15. ORBITAL CONTROL • Latitude drift will be induced primarily by gravitational forces from the sun and the moon. • These forces cause the satellite inclination to change about 0.075◦ per month if left uncorrected. • Periodic pulsing to compensate for these forces, called north-south station keeping maneuvers, must also be accomplished periodically to maintain the nominal satellite orbit location.
- 16. THERMAL CONTROL • Thermal radiation from the sun heats one side of the spacecraft, while the side facing outer space is exposed to the extremely low temperatures of space. • Much of the equipment in the satellite itself generates heat, which must be controlled. Low orbiting satellites can also be affected by thermal radiation reflected from the earth itself. • The satellite thermal control system is designed to control the large thermal gradients generated in the satellite by removing or relocating the heat to provide an as stable as possible temperature environment for the satellite. • Thermal blankets and thermal shields are placed at critical locations to provide insulation. • Radiation mirrors are placed around electronic subsystems, particularly for spin-stabilized satellites, to protect critical equipment. • Heat pumps are used to relocate heat from power devices such as traveling wave power amplifiers to outer walls or heat sinks to provide a more effective thermal path for heat to escape. • Thermal heaters may also be used to maintain adequate temperature conditions for some components, such as propulsion lines or thrusters, where low temperatures would cause severe problems.
- 17. TRACKING, TELEMETRY, COMMAND, AND MONITORING • The tracking, telemetry, command, and monitoring (TTC&M) subsystem provides essential spacecraft management and control functions to keep the satellite operating safely in orbit. • The TTC&M links between the spacecraft and the ground are usually separate from the communications system links. • The satellite TTC&M subsystems comprise the antenna, command receiver, tracking and telemetry transmitter, and possibly tracking sensors. • Telemetry data are received from the other subsystems of the spacecraft, such as the payload, power, attitude control, and thermal control.
- 19. TRACKING • Tracking refers to the determination of the current orbit, position, and movement of the spacecraft. The tracking function is accomplished by a number of techniques, usually involving satellite beacon signals, which are received at the satellite TTC&M earth station. • The Doppler shift of the beacon (or the telemetry carrier) is monitored to determine the rate at which the range is changing (the range rate). • Angular measurements from one or more earth terminals can be used to determine spacecraft location. • The range can be determined by observing the time delay of a pulse or sequence of pulses transmitted from the satellite. Acceleration and velocity sensors on the satellite can be used to monitor orbital location and changes in orbital location.
- 20. TELEMETRY • The telemetry function involves the collection of data from sensors on-board the spacecraft and the relay of this information to the ground. • The telemetered data include such parameters as voltage and current conditions in the power subsystem, temperature of critical subsystems, status of switches and relays in the communications and antenna subsystems, fuel tank pressures, and attitude control sensor status.
- 21. COMMAND • Command is the complementary function to telemetry. The command system relays specific control and operations information from the ground to the spacecraft, often in response to telemetry information received from the spacecraft. Parameters involved in typical command links include: • changes and corrections in attitude control and orbital control; • antenna pointing and control; • transponder mode of operation; • battery voltage control.
- 22. SATELLITE PAYLOAD: TRANSPONDER • The transponder in a communications satellite is the series of components that provides the communications channel, or link, between the uplink signal received at the uplink antenna, and the downlink signal transmitted by the downlink antenna. • A typical communications satellite will contain several transponders, and some of the equipment may be common to more than one transponder. • Each transponder generally operates in a different frequency band, with the allocated frequency spectrum band divided into slots, with a specified center frequency and operating bandwidth. • The number of transponders can be doubled by the use of polarization frequency reuse, where two carriers at the same frequency, but with orthogonal polarization, are used. Both linear polarization (horizontal and vertical sense) and circular polarization (right-hand and left-hand sense) have been used.
- 23. ANTENNAS • The antenna systems on the spacecraft are used for transmitting and receiving the RF signals that comprise the space links of the communications channels. • The antenna system is a critical part of the satellite communications system, because it is the essential element in increasing the strength of the transmitted or received signal to allow amplification, processing, and eventual retransmission. • The most important parameters that define the performance of an antenna are antenna gain, antenna beam width, and antenna sidelobes. • The common types of antennas used in satellite systems are the linear dipole, the horn antenna, the parabolic reflector, and the array antenna.