satellite communications (Satcom)
In satellite communications (satcom), messages are transmitted via satellites as part of both individual and mass communications(telecommunications). In the case of mobile satellite services, a distinction is made between systems with geostationary satellites and those with satellites that change position.
GEO satellites, i.e. those with geostationary orbits (GEO), are positioned at a fixed position above the equator line at a distance of about 36,000 km from the Earth. The fixed position of GEO satellites allows the satellite radio to be permanently aligned with the satellite between the transmitter and receiver of an earth station with parabolic antennas. GEO satellites are mainly used for broadcasting, but also for satellite Internet.
Different satellite orbits for different services
In addition to geostationary satellites, there are also Non Geostationary Satellite Orbit( NGSO). All other satellite systems belong to this group. For example, the MediumEarth Orbiter ( MEO) satellites. They orbit the Earth and have a flight altitude of between 10,000 km and 15,000 km. The Low Earth Orbiters( LEO), which orbit the Earth at relatively low altitudes of 700 km to 1,500 km, and the HEO satellites with their elliptical orbits.
For satellite transmission, the International Telecommunication Union( ITU) has defined special satellite bands that have large transmission capacities. A satellite network can be a point-to-point link between satellite and earth station, between satellite and mobile station, or between two satellites. Thus, the transmission from the satellites to the terminal can refer to stationary, fixed, but also mobile terminals. In addition, these systems can be used for broadcast communications. A disadvantage in data communication is the long delay times between the ground station and the satellite, which are caused by the long transmission paths. For geostationary satellites, the signal propagation time is approximately 320 ms to 350 ms per link, including latency in the satellites. This delay impairs voice communication. Due to this delay, the ITU recommends limiting the satellite sections in a telephone connection to one satellite section and only allows two satellite sections in exceptional cases.
Satellite orbits and delay times
There are several systems in earth-orbiting satellite systems for mobile satellite communications. The best known is Iridium, which began operations in 1998 with 66 earth-orbiting satellites. However, it was shut down again in 2000 due to insufficient demand for mobile communications. There is also Globalstar for satellite-based mobile communications, which began service in 1999 and is based on 48 satellites. The Globalstar satellites orbit the earth at an altitude of 1,400 kilometers
In addition, there is the ICO system for mobile voice communications. This system uses 10 MEO satell ites, but they orbit the earth at an altitude of 10,400 kilometers. Later, the Ellipso system will be launched, with six satellites above the equator and another ten in two tilted orbits.
LEO satellites for broadcast services
LEO satellites are also ideal for high-speed, short-latency data communications. For example, as communications satell ites for satellite mobile communications and for satellite Internet. There are some interesting projectsin this area, such as SpaceX's Starlink, which is expected to consist of over 40,000 Earth-orbiting LEO satellites when completed, OneWeb with over 6,000, and Amazon 's Kuiper project with over 3,000 LEO satellites, to name a few.
Earlier projects such as Skybridge and Teledesic have already been cancelled.