Satellite Systems can be classified based upon their orbits as low earth orbit, medium earth orbit & geostationary earth orbit systems. Geostationary is also the highest earth orbit and hence, also provides the greatest visibility using only a few satellites. The coverage region of a satellite is called its footprint. This is the region from which the satellite is visible. Three geostationary satellite footprints ensure complete coverage of the earth as shown:
Hence, there is permanent or 24 hour visibility of geostationary satellites without the need of handoffs. While LEO & MEO satellites do not have 24 hour visibility as the satellites have smaller footprints since they are closer to the earth (low satellite height). Hence, a larger number of satellites are needed to cover the earth. Also, since each satellite has a small footprint, handoffs are also required between satellites.
Major differences between LEO, MEO & GEO satellite systems:
|Satellite Height||500-1500 km||5000-12000 km||35,800 km|
|Orbital Period||10-40 minutes||2-8 hours||24 hours|
|Number of Satellites||40-80||8-20||3|
|Number of Handoffs||High||Low||Least(none)|
|Gateway Cost||Very Expensive||Expensive||Cheap|
- HEO refers to highly elliptical orbits which have a visibility of about 12 hours.
- ICO or intermediate circular orbit is an example of MEO.
- GPS satellites are not in Geostationary orbits but instead, they orbit twice for every rotation of the earth at a height of 20, 000 km.
- Handheld terminals have low sending power are hence use LEO for mobile communication. LEO are also closest to the earth, have better signal strength and less time delay.