Saturday, 21 December 2013

Satellite communication system

“Satellite is an object which revolves around the plants in a particular orbit”.

The are two types of satellite
1. Natural satellite
2. Artificial (non-made) satellite.

• Natural satellite of earth is moon, due to its large distance from earth, it cant be used for any application.
• Artificial satellite have been designed and launched various application including communication.
• The concept of using objects located in space for same applications was first mentioned by “Arthus C. Clark in 1945.
• The satellite is placed at a distance of 42,000 km from earth centre.
• It will rotate at the some speed as the earth rotation.

History development of satellites
• The first artificial satellite was launched by Russian in October 1957 and it was names as Sputnik-I.
• USA has launched its first satellite in Dec. 1958 and it is norms as “Score.
• Later in August 1960 a satellite by name “ECHO-I” was launched by “AT & T” company of USA.
• The “ECHO-I” satellite is large balloon like structure of 100 feet diameters.
• Later in Jan 1964 a satellite by name ‘ECHO-II’ in May 1962, a satellite by name “TELSTAR-I’ and in May 1963, satellite by name “TELSTAR-II’ is launched by ‘A & T’ company of USA.
• Communication satellite corporation [COMSAT] was established in 1963 in USA. They launched a series of satellite’s by name ‘SYNCOM’.

The 5 points GPS:
1. GPS operation is based on triangulation of satellite signal.
2. triangulation GPS measuring distances using radio signal travel time.
3. Each situate sends it location and it is precise time of it is transmission.
4. GPS user Equipment receiver the signal from each satellite and records the position and signal arrival time.
5. The GPS Receiver position completes from the calculated distance. The user 3d co-ordinates and precision calculation using signal from 4 satellite.

1. World wide 24 hours coverage.
2. Grater accuracy.
3. Power required is less
4. time required is less.
GPS has the potential to expose the important military illustrations, Hence it can expose security thread to the country.
1. It is used in military commercial, civil scientific application.
2. It is used for studies of the earth quake.
3. GPS is used as navigation & positioning tools in airplane & boots. Cars and for almost all outdoor recreational activities.
The heart of communication satellite is the communication sub system this is set of transponder that receiver the up link signal and re-transmit them to earth. The main sub system that constitutes the satellite are as follows.
1. Power sub system
2. Communication sub system
3. Antenna sub system
4. Telemetry tracking & command sub system.
5. Propulsion & attitude stabilization sub system.

Power sub system: The solar panel supply the electric power to for the space craft. They drive regulators that distribute de power to all other sub system. They charge batteries that operates the satellite daring eclipse periodic. Both Dc-to-DC converters and Dc-t—Dc inverter are used to supply special voltage to some subsystem.
Comm. Sub system: These receive the uplink signals, amplify them translate in frequency and amplify them again for retransmission down link signals.
Antenna subsystem: The transponder share a common antenna sub system for both reception and transmission parameters of importance are coverage gain.
Telemetry tracking and command subsystem: The telemetry tracking, and command subsystem (TIC) monitors on-board conditions such as temperature and battery voltage and transmits this data back to a ground station for analysis.
The ground station may then issue order to the satellite by transmit a signal to the command sub system it is used to control many space craft function such as firing the jet thruster.
Attitude control sub system: It consist of small rocket thruster that keep the satellite I current orbital position and keep antenna in direction.
Propulsion and attitude stabilization subsystem:
The jet thruster and the apogee kick motor (AKM) are port of the propulsion sub system, they are controlled by command from the ground the attitude control sub system provides stabilization in orbit & senses changes in orientation It fires the jet thrusters to perform attitude adjustment and station keeping maneuvers that keep the satellite in its assigned orbital position parameter of importance are accuracy and mass of propellant.

Geo-stationary or Geo-synchronous satellites
Geo-synchronous satellites are placed in synchronous orbits. The attitude of these satellite from earth surface is 36,000 km. These satellites are revolving in a circular orbit around the earth. The phone of the orbit is parallel to earths equator i.e. equatorial orbit. The synchronous satellite in this orbit requires 24 hours to complete one revolution as same as that of earth speed. It will move in the same direction as that of earth i.e. from west to east. Such satellite appear stationary from a point on the earth surface. So the synchronous satellites are called as geo-stationary satellites. They are continuously available at a place on the earth surface. It will contain various electronic equipments with them. The antenna present in it is forced towards the earth it will also contain huge solar cells to get the required power supply. Each geo-stationary satellite can cover only 38% of earth surface in the equatorial plane. Such 3 satellites or placed in equatorial orbit with angular distance of 120 so can cover entire globe.

Station keeping
After satellite is launched and placed in orbit it may drift. This drift in undesirable in geo-synchronous satellite whose position is fixed for reliable continuous communication. Orbit drift is caused by variety of force’s such as gravitational pull of sun, moon and earth because of this drift the orbit of the satellite must be periodical objected.

The above undesired effects have to be overcome. So that the satellite always remarks in its prepare place. This is done by on earth station. The process of doing this operation is called “station keeping”.

The station keeping process is done by passing certain commands from earth station to be the satellite when the satellite receives these commands, they fire appropriate “Booster rocket” or thrusters in the opposite direction of satellite movement. Thus it comes back with same force to its normal position the station keeping so uses Newton’s III law”. The station keeping processes is repeated at regular time intervals. When the stored fuel in the satellite and the “thrusts” are completed, no more station keeping is possible and the life of the satellite is said to be over.

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