Fundamentals Of Satellite Communication
Satellite Communication
1.Satellite systems:
There are three different types of satellite systems.
- International satellite communication system INTELSAT.
- Domestic satellite system DOMSAT.
- Search and rescue system SARSAT.
INTELSAT:
The INTELSAT Organization was established in 1964 to handle the myriad of technical and administrative problems associated with a world wide telecommunication system. The international regions served by INTELSAT are divided in to the Atlantic Ocean region (AOR), the Pacific Ocean Region (POR), and the Indian Ocean region (IOR). For each region , satellites are positioned in geo-stationary orbit above the particular Ocean, where they provide a transoceanic telecommunication route. In addition to providing trans oceanic routes, the INTELSAT satellites are used for domestic services within any given country and regional services between countries. Two such services are vista for telephony and Intelnet for data exchange.
DOMSAT
Domestic satellites are used to provide various telecommunication services, such as voice, data, and video transmission (T.V channels), with in a country. Satellite cell phones allow global travelers and those in remote areas to avoid landlines and terrestrial cell phone services entirely. Satellite cell phones relay your call to a satellite and down through a hub to the end user. This means that most of the earth's geographical area is now accessible by a satellite cell phone! Third party providers of satellite cell include Satcom Global, Roadpost Satcom, Online Satellite Communications, and others.
SARSAT
SARSAT is one type of Polar orbiting satellites.
Polar-orbiting satellites orbit the earth in such a way as to cover the north and south polar-regions. Infinite number of polar polar satellite orbits are possible
Polar satellites are used to provide environmental data, and to help locate ships and aircrafts in distress .This service known as SARSAT, for search and rescue satellite.
This figure shows polar satellite path and earth rotation
2.Kepler's Laws:
Artificial satellites which orbit the earth follow the same laws that govern the motion of the planets around the sun. Johannes Kepler (1571 -1630) was derived empirically three laws describing planetary motion . In 1665, Newton was able to derive Kepler's laws from his own laws of mechanics and theory of gravitation.
Kepler's laws:
Kepler's first law: Kepler's first law states that the path followed by the satellite around the ptimary will be an ellipse
Kepler's second law: Kepler's second law states that for equal time intervals, the satellite will sweep areas in its orbital plane.
Kepler's third law: Kepler's third law states that the square of periodic time of orbit is proportional to the cube of the mean distance between the two bodies.
3.Definitions And Related Terms Of Earth-Orbiting Satellites
Apogee. The point farthest from earth.
Perigee. The point of closest approach to earth.
Line of apsides. The line joining the perigee and apogee through the center of the earth.
Ascending node. The point where the orbit crosses the equatorial plane going from south to north.
Descending node. The point where the orbit crosses the equatorial plane going from north to south.
Line of nodes. The line line joining the ascending and descending nodes through the center of the earth.
Inclination. The angle between the orbital plane and the earth's equatorial plane.
Prograde orbit. An orbit in which the satellite moves in the same direction as the earths rotation.
Retrograte orbit. An orbit in which the satellite moves in a direction counter to the earth's rotation.
Argument of perigee. The angle from ascending node to perigee, measured in the orbital plane at the earth's center in direction of satellite motion.
Mean anomaly. Mean anomaly M gives an average value of the angular position of the satellite with reference to the perigee
True anomaly. The true anomaly is the angle from perigee to the satellite position, measured at the earth's center. this gives the true angular position of the satellite in the orbit as a function of time.
4.Satellite system
A satellite communication system can be broadly divided into two segments, a ground segment and a space-segment. The space system includes Satellite.
Satellite system consist of the following systems.
Power supply:
The primary electrical power for operating electronic equipment is obtained from solar cells. Individual cells can generate small amounts of power, and therefore array of cells in series-parallel connection are required .
Cylindrical solar arrays are used with spinning satellites, (The gyroscopic effect of the spin is used for mechanical orientational stability) Thus the array are only partially in sunshine at any given time.
Another type of solar panel is the rectangular array or solar sail. solar sail must be folded during the launch phase and extended when in geo-stationary orbit. Since the full component of solar cells are exposed to sun light ,and since the Sail rotate to track, the sun , they capable of greater power output than cylindrical arrays having a comparable number of cells.
To maintain service during an eclipse, storage batteries must be provided .
Attitude control:
The attitude of a satellite refers to its Orientation in space. Much of equipment carried abroad a satellite is there for the purpose of controlling its attitude. Attitude control is necessary, for example, to ensure that directional antennas point in the proper directions. In the case of earth environmental satellites the earth-sensing instrument must cover the required regions of the earth, which also requires attitude control. A number of forces, referred to as disturbance forces can alter attitude, some examples being the gravitational forces of earth and moon, solar radiation, and meteorite impacts.
Station keeping:
A satellite that is normally in geo-stationary will also drift in latitude, the main perturbing forces being the gravitational pull of the sun and the moon . the force cause the inclination to change at the rate of about 0.85 deg./year. if left uncorrected, the drift would result in a cycle change in the inclination going 0 to 14.67deg in 26.6 years and back to zero , when the cycle is repeated. To prevent the shift in inclination from exceeding specified limits, jets may be pulled at the appropriate time to return the inclination to zero. Counteracting jets must be pulsed when the inclination is at zero to halt that change in inclination.
Thermal control:
Satellites are subject to large thermal gradients, receiving the sun radiation on one side while the other side faces into space. In addition, thermal radiation from the earth, and the earth's abedo, which is the fraction on the radiation falling on the earth which is reflected can be sight for low altitude, earth-orbiting satellites, although it is negligible for geo-stationary satellites. Equipment in the satellite also generates heat which has to be removed. the most important consideration is that the satellite's equipment should operate as near as possible in a stable temperature environment. various steps are taken to achieve this. Thermal blankets and shields may be used to provide insulation. radiation mirrors are often used to remove heat from communication payload. These mirrored drums surrounded the communication equipment shelves in each case and provide good radiation paths for the generated heat to escape in to surround space.
To maintain constant-temperature conditions, heaters may be switched on to make up for the hearts may be switched on to make reduction that occurs when transponders are switched off.
TT&C subsystem
Telemetry system
The telemetry, tracking, and command (TT&C) subsystem performs several routine functions abroad a spacecraft. the telemetry or "telemetering" function could be interpreted as "measurement at a distance". specifically, it refers to the over all operation of generating an electrical signal proportional to the quantity being measured, and encoding and transmitting this to a distant station, which for satellite is one of the earth stations, which for the satellite is one of the earth stations. Data that are transmitted as telemetry signals include attribute information such as obtained from sun earth sensors; environmental information such as magnetic field intensity and direction; the frequency of meteorite impact and so on ;and spacecraft information such as temperatures and power supply voltages, and stored fuel pressure.
Command systems
Command system receives instructions from ground system of satellite and decodes the instruction and sends commends to other systems as per the instruction.
Tracking
Tracking of the satellite is accomplished by having the satellite is accomplished by having the satellite transmit beacon signals which are received at the TT&C earth stations. Tracking is obviously important during the transmitter and drift orbital phases of the satellite launch. When on-station, a geo-stationary satellite will tend to shifted as a result of the various distributing forces, as described previously . Therefore it is necessary to be able 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.
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. Thus, although reference may be made to specific transponder, this must be thought of as an equipment channel rather than single item of equipment.
Transponder consist of wideband receivers, input de-multiplexer, power amplifier components.
Antenna sub system
The Antennas carried abroad a satellite provide the dual functions of receiving the up link and transmitting the down link signals. They range from dipole-type antennas, where omni directional characteristics are required, to the highly directional antennas required for telecommunications purposes and TV relay and broadcasting.
