Satellite Communication

Voice and data communication can be realised by the use of communication satellites. Satellite communication can be classified according to whether the satellites are positioned in a geostationary orbit (GEO) or low-Earth orbit (LEO).

GEO satellites orbit in an altitude of 35785 kilometres above the Earth. In this height they move at a speed which is synchronous with the circulation of the Earth. Thus, they are stationary relative to a point on the Earth's surface. Due to the large distance to the Earth each geostationary satellite can cover a huge area. Assuming a minimum ground antenna elevation angle of 10 degrees, a single satellite in geostationary orbit can cover about 34 percent of the Earth's surface.

Geostationary satellite communication systems are provided by Inmarsat and Qualcomm. Inmarsat operates nine geostationary satellites which provide global coverage. Only four of the satellites are active and five are for emergency back-up.

Figure 1: Global coverage with four geostationary satellites

Source: Inmarsat

Communications via the Inmarsat-C system are data or message-based. Messages are transferred to and from an Inmarsat-C terminal at an bitrate of 600 bps. Frequencies are 1530.0-1545.0 MHz (downlink) and 1626.5-1645.5 MHz (uplink). The EutelTRACS service provided by Qualcomm is realised by two satellites covering Europe, the Mediterranean, an the Middle East. The satellites operate on the frequency bands 10.70-11.70 GHz and 12.50-12.75 GHz (downlink) and 14.00-14.25 GHz (uplink) providing low bitrate data communications. The downlink bitrate is between 5 kbps and 15 kbps while the uplink is between 55 bps and 165 bps.

Low-Earth orbit satellite communication systems use satellites which are in much lower orbits than geostationary satellites. Due to the low orbits, those satellites are not geostationary and orbit the Earth in 1.5 to 10 hours depending on the height. They provide a small geographic coverage and thus, more satellites are required if continuous coverage is desired. Due to the lower distance to the Earth, less intense and smaller transmitters are required - for both the satellites and the ground side systems. ORBCOMM provides LEO communication systems with 35 satellites orbiting in a height of about 775 kilometres.

Figure 2: Global coverage is only achieved with latency [1]

As the above figure illustrates, global coverage is not continuously. Short gaps in the coverage are closed by one of the passing satellites in a few minutes, providing global coverage with latency. The satellites operate at frequencies of 137.00-138.00 MHz (downlink) and 148.00-150.05 MHz (uplink). The downlink bitrate is at 4.8 kbps while the uplink is at 2.4 kbps.

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