Communication satellites have interesting properties that make them interesting for many applications. In its simplest form, a communication satellite can be seen as a large microwave transponder in the sky. It contains several transponders, each listening to one part of the spectrum, amplifying the incoming signal and then transmitting it to another frequency to avoid interference with the input signal. This mode of operation is called a bent pipe.
Digital processing can be added to separately control or redirect data streams in a common range, or digital information can even be received by satellite and retransmitted. Thus, the regeneration of the signal improves the performance concerning a bent pipe, since the satellite does not amplify the noise in the upstream signal. The descending rays can be wide, covering a large part of the Earth’s surface, or narrow, covering an area with a diameter of several hundred kilometers.
Geostationary satellites, including orbits, solar panels, radio frequencies, and launching procedures. Geostationary satellites are spaced 360 degrees much closer to 2 degrees in the equatorial plane to avoid interference. With a 2-degree space in the sky, there can only be 360/2 = 180 satellites at a time. However, each repeater can use multiple frequencies and polarizations to increase the available bandwidth.
To avoid chaos in the sky, the orbital space distribution is done by the ITU. This process is highly political in those countries that have barely emerged from the Stone Age need “their” orbit slot (to be able to lease them the highest price).
The slots of the orbit are not the only point of contention. Frequencies are also a problem because downlink transmissions interfere with existing microwave users. As a result, the ITU has allocated certain frequency bands to satellite users.
Medium-Earth Orbit Satellites
At much lower altitudes, between the two Van Allen belts, we find satellites the orbit of Middle-earth (MEO). Seen from Earth, they move slowly, taking about 6 hours to circle the Earth. Therefore, they must be followed when moving in the sky.
Since they are lower than GEO, they occupy less space on the ground and require less powerful transmitters. Currently, they are used for navigation systems, but not for telecommunications. A constellation of about 30 GPS satellites (Global Positioning System) orbiting about 20,200 km is an example of an MEO satellite.
Low-Earth Orbit Satellites
Decreasing the altitude called the LEO satellites (low Earth orbit). Because of its rapid movement, many of them are necessary for a complete system. On the other hand, satellites being very close to the ground, the ground stations do not need a lot of energy and the delay of signal transmission is limited to a few milliseconds. The startup cost is also significantly cheaper. There are two examples of satellite constellations for the telephone service: Iridium and Globalstar.
The Iridium satellites are located at an altitude of 750 km in circular polar orbits. They are located at the gate from north to south with a satellite every 32 degrees of latitude. Each satellite has a maximum of 48 cells (spot beams) and a capacity of 3,840 channels, some of which are used for positioning and navigation and others for data and voice transmission.
The Globalstar is another Iridium design. It is based on 48 LEO satellites but uses a switching scheme different from that of Iridium. While Iridium relays satellite-to-satellite calls, requiring sophisticated satellite switching equipment, Globalstar uses the traditional curved tube design.