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UNIT VII LOW EARTH ORBIT AND GEO-STATIONARY SATELLITE SYSTEM

INTRODUCTION There are some specialized applications that require non-geostationary satellites. Surveillance of the earth's surface, for both military data gathering and earth resources applications, requires satellites in low earth orbit that cover the entire surface of the earth. Satellites providing global navigation, such as the Global Positioning System (GPS) constellation, must utilize orbits that place the satellites in widely spaced positions in the sky, as seen by the receiver. Some of the satellites can be in GEO, but most must be in inclined orbits with an even distribution over the earth's surface. GPS uses 24 satellites in orbits with an altitude of 20,000 km and an inclination of 55°. Mobile satellite communication systems demand an earth station with a low gain antenna that has a near omnidirectional pattern. A GEO satellite used for communication with a satellite telephone that is handheld, like a cellular telephone, requires a very large antenna with hundreds of beams to achieve a very high gain. The high gain satellite antenna is needed to compensate for the low gain of the antenna employed by the user's telephone handset. An alternative to a GEO satellite with a high gain antenna is a LEO or ME0 satellite constellation with a smaller multibeam antenna. Because the satellite is not geostationary, a large number of satellites are required to maintain continuous coverage. The Iridium system used 66 satellites in LEO, for example, to provide continuous global coverage.

Geostationary Augmentation of Global Satellite Navigation–1991 Update

Journal of Navigation, 1992

This paper and the following five papers were presented at the NAV91 Conference on Satellite Navigation held at Church House, London, 6–8 November 1991. Copies of the conference proceedings are available from the Director, price £85 (non-members) and £65 (members).In the last two years, the idea of a civil geostationary overlay (augmentation) to GPS and GLONASS has moved from the discussion/concept stage to something approaching reality. Many details of implementation have been or are in the process of being determined. Major developments that have taken place in this period of time include: (i) INMARSAT'S adoption of a four ocean region satellite deployment (three had been employed previously), now including one region (AOR-West) that covers the conterminous United States, (ii) Inclusion in the specifications, and contracting for, navigation repeater payloads in the four INMARSAT-3 spacecraft, now under construction, (iii) Major improvements in the Test Bed, bringing the test s...

A Comparative Study of Satellite Orbits as Low Earth Orbit (LEO) and Geostationary Earth Orbit (GEO)

SAMRIDDHI : A Journal of Physical Sciences, Engineering and Technology, 2015

It is known facts that satellites are used to receive the signal at geostationary orbit by remaining stationary above a particular point on the Earth. The orbit that is chosen for a satellite depends upon its application. Those used for direct broadcast television use geostationary orbit. Many communication satellites similarly use geostationary orbit. Other satellite systems used for satellite phones use Low Earth orbiting systems. Similarly, satellite systems used for navigation like Nav-star or Global Positioning (GPS) system occupy a relatively Low Earth Orbit. There are also many other types of satellites : Weather satellites Research satellites and many others. Each will have its own type of orbit depending upon its application. The actual satellite orbit that is chosen will depend on factors including its function, and the area of serving. At some instances, the satellite orbit may be as low as 100 miles (160 km) for a Low Earth Orbit (LEO), whereas others may be over 22 000 ...