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Posted by 2018 article


Low-Earth-orbit (LEO) satellites fly at different heights above Earth and can vary in the amount of time they are visible to a ground station (GS) to transferring data to and from the GS. For a typical LEO satellite in orbit 600 km above Earth, visibility to a GS may be less than 20 minutes per orbit, limiting the amount of data that can be exchanged. By recruiting a geosynchronous-Earth-orbit (GEO) satellite as part of the overall data link, however, it can be possible to increase the available LEO satellite data-transmission time (and amount of data).

Since the percentage of a LEO satellite’s visibility to a GS is less than 20% of the satellite’s orbital period, the time to download data in a satellite-communications (satcom) system is limited. To increase the imaging data capability and capacity of a remote-sensing LEO satellite, the imagery data must be stored onboard the satellite and downloaded at a higher data rate during such a limited visibility period. Furthermore, a greater number of networked GS sites around the Earth must be used.

Most of Earth’s surface is covered by water, so accomplishing this task would require that a large number of “ground” terminals be located on ships—at prohibitive cost. However, a GEO satellite is visible to a LEO spacecraft for more than one-half of the LEO satellite’s orbit, as a possible component in a data relay system. In theory, two widely spaced GEO satellites can provide continuous visibility/coverage for a LEO satellite.

LEO os called Low earth orbit, MEO is called Medium Earth Orbit and GEO is called Geostationary orbit. LEO are about 500 Km to 1500 Km above the earth, so the delay is very small and the losses is small too. MEO are installed at 5000 to 12000 km above the earth and generally used for navigation communications like GPS. GEO is about 35800 Km above the equator, the delay and losses are greater, but the advantages is more coverage (it covers 40% of the earth) and there no need to track the satellite, so the earth terminal is cheaper.

I would like to add few additional information as follows:
1. Since LEO satellite is nearer to planet compare to MEO and GEO, it has small cell (or foot print).
2. Due to small cell coverage, more number of LEO satellite need to install to cover the planet(for instance earth).
3. LEO moves very fast compare to MEO, so very frequent hand-off is required between two LEO satellites to avoid interrupt for ongoing calls between two users.
4. GPS satellite uses medium earth orbit.
5. Satellites which use geostationary orbit known as GEO. GEO satellite rotates with speed as earth and take 365 days to complete one rotation.

Satellites in Low Earth Orbit are within 1000 miles of the surface, and routinely go around about 18 times every day.
Satellites in Mid Earth Orbit are up around 6000 to 20,000 miles, and orbit generally twice per day (some are more).
Satellites in geosynchronous orbit are placed there so they remain fixed over the Earth's surface, and go around exactly once per day.

Low-Earth-orbit (LEO) satellites fly at different heights above Earth and can vary in the amount of time they are visible to a ground station (GS) to transferring data to and from the GS. For a typical LEO satellite in orbit 600 km above Earth, visibility to a GS may be less than 20 minutes per orbit, limiting the amount of data that can be exchanged. By recruiting a geosynchronous-Earth-orbit (GEO) satellite as part of the overall data link, however, it can be possible to increase the available LEO satellite data-transmission time (and amount of data).

Since the percentage of a LEO satellite’s visibility to a GS is less than 20% of the satellite’s orbital period, the time to download data in a satellite-communications (satcom) system is limited. To increase the imaging data capability and capacity of a remote-sensing LEO satellite, the imagery data must be stored onboard the satellite and downloaded at a higher data rate during such a limited visibility period. Furthermore, a greater number of networked GS sites around the Earth must be used.

Most of Earth’s surface is covered by water, so accomplishing this task would require that a large number of “ground” terminals be located on ships—at prohibitive cost. However, a GEO satellite is visible to a LEO spacecraft for more than one-half of the LEO satellite’s orbit, as a possible component in a data relay system. In theory, two widely spaced GEO satellites can provide continuous visibility/coverage for a LEO satellite.



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