ORBITS FOR COMMUNICATION SATELLITE
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The path a Satellite or a planet follows around a planet or a star is defined as an orbit. In general the shape of an orbit of a satellite is an ellipse with the planet located at one of the two foci of the ellipse. The circular orbit may also be considered as an ellipse where the two foci of the ellipse coincide at the center of the circle. Satellite Orbits are classified in two broad categories i.e.
• Non-Geostationary Orbit (NGSO)
• Geo Stationary Orbit (GSO)
Non-Geostationary Orbit (NGSO)
Early ventures with satellite communications used satellites in Non-geostationary low earth orbits due to the technical limitations of the launch vehicles in placing satellites in higher orbits. With the advancement of launch vehicles and satellite technologies, once the Geo Stationary Orbit (GSO) was achieved, majority of the satellites for telecommunications started using GSO due to its many advantages. During 1990s the interests in NGSOs were rekindled due to several advantages of NGSO in providing global personal communications in spite of its many disadvantages.
Advantages of NGSO are:
• Less booster power required to put a satellite in lower orbit
• Less space loss for signal propagation at lower altitudes (<10,000 km) leading to lower on board power requirement
• Less delay in transmission path – reduced problem of echo in voice communications
• Suitability for providing service at higher latitude
• Lower cost to build and launch satellites at NGSO
• Use of VHF and UHF frequency bands at NGSO permits low cost antennas for hand-held terminals
Disadvantages of NGSO are:
• Requirement of a large number of orbiting satellites for global coverage as each low earth orbit satellite covers a small portion of the earth’s surface for a short time.
• Complex hand over problem of transferring signal from one satellite to another
• Less expected life of satellites at NGSO requires more frequent replacement of satellites compared to satellite in GSO
• Compensation of Doppler shift is necessary
• Satellites at NGSO undergoes eclipse several times a day necessitates the requirement of robust on board battery system for the satellite for operations without solar power during eclipse
• Complex network management for a constellation of satellites and corresponding ground system
• Problem of increasing space debris in the outer space
Geostationary Orbit (GSO)
There is only one geostationary orbit possible around the earth, lying on the earth’s equatorial plane and the satellite orbiting at the same speed as the rotational speed of the earth on its axis. For a Satellite to have an orbital period equal to that of earth’s rotation i.e. a sidereal day (23 Hrs 56 min. 4.09 sec.) an altitude of 35,786 km is required. Such a satellite orbiting at a velocity of 3.075 km/sec remains fixed relative to any point on earth or geostationary. With the idealized assumptions that the geostationary satellite is at rest relative to the earth the conditions required to be satisfied for geostationary orbit are:
• The orbit shall be circular
• The period of the orbit shall be equal to the period of rotation of the earth about itself
• The plane of the orbit shall be the same as the equatorial plane but the sub-satellite longitude, i.e. the longitude of the projection of the satellite on the Earth’s surface can be selected arbitrarily.
The principle of satellite communications based on this concept of geostationary orbit was originated by Arthur C Clarke. Main advantage of geostationary satellite being the permanent contact between the ground segment and the satellite with fixed directional antennas at both the earth station and the satellite.