There are three basic orbital patterns used by communications satellites:
- Low earth orbit (LEO)
- Medium earth orbit (MEO)
- Geostationary earth orbit (GEO)
See the figure below for an illustration of the basic arrangement
LEO and MEO satellites orbit the earth in less than 24 hours so they appear to move relative to a fixed point on the ground.
At about 1000 km altitude, a LEO satellite circles the earth in just 90 minutes. Each ground location will only see a particular LEO satellite for about ten minutes, as at rises from one point on the horizon and sets at the opposite point. To provide continuous communications service, there must be at least 48 satellites in coordinated operation - this is called a constellation.
MEO satellites orbit at an altitude of about 10,000 km so they have a much longer orbit period than LEO satellites. A MEO satellite moves slowly across the sky, and only 10 are needed to assure continual service.
Finally, a GEO satellite appears to remain fixed with respect to the ground. As a result antennas can be fixed as well if desired. Because it is not moving, one satellite can serve an entire region. Multiple satellites can be used to provide added capacity and an alternate path to achieve the highest availability possible.
GEO satellites, provide the only means currently available to provide broadband communications.
Basic orbit arrangement of a single satellite, in GEO, MEO and LEO.
GEO satellites are the proven means of consistent and reliable communications in the inhabited portions of the Earth. Over 250 GEO communications satellites are currently in operation.
Only the North and South Poles are left uncovered, because of the lack of line-of-sight to latitudes greater than 81.3 degrees (North or South) from geostationary orbit. Demand for services in these areas, however, is extremely small.
The links between the ground and GEO are very stable because it is easy to obtain a line of sight path.
Requirements for the antennas and electronics to be used with a GEO satellite can be determined well ahead of time because the satellite characteristics are known as the properties of the RF link itself. Unlike terrestrial wireless services, you will know ahead of time what it will take to communicate.
- Most GEO applications are to fixed dishes and employ digital transmission at data rates of 1.5 Mbps and greater (as high as 155 Mbps, where requirements dictate).
- Mobile applications using GEO satellites have grown because of demand for rapid deployment and mobile broadband communications.
- RVs equipped with satellite dishes.
- Aircraft and ships equipped for high-speed Internet access.
- Vehicles with digital audio radio service (DARS) receivers to receive XM and Sirius.
- Portable units for voice, Internet and video uplink.
To understand how a satellite network operates, get to know it piece by piece.