satellite-based mobile communications
In addition to terrestrial mobile communications, there is also satellite-based mobile communications with Iridium and the Inmarsat system. Satellite mobile communications enable worldwide portability of the terminal equipment and the telephone number. This means that the subscriber can use a terminal anywhere in the world and can be reached under one and the same telephone number.
The prerequisite for satellite mobile communications is direct communication between the mobile station and the satellite, which requires a line-of-sight link for the duration of the connection. Only LEO satellites are suitable for this service. Only memory-oriented systems do not require line-of-sight.
Since the communications costs and also the costs of the terminal equipment for mobile satellite communications are relatively high, its use is recommended only where terrestrial networks are not available or where the user operates worldwide and does not want to be confronted with different international standards and different terminal equipment. However, as with Iridium, regional areas may be blanked out if the countries concerned have not joined agreements with satellite operators.
Depending on the satellite location, the link between the satellite and mobile stations is characterized by a shorter or longer transmission distance, which has a direct impact on signal propagation time. For GEO satellites such as the Inmarsat satellites, which are positioned at an altitude of 36,000 km, propagation times of 320 ms to 350 ms occur, but with the delay times of the electronics. This means a propagation time of about 700 ms for the entire communication path from the transmitting mobile station via the satellite(s) to the receiving satellite telephone, resulting in a degradation of voice communication. For LEO satellites, such as those of the Iridium system with an altitude of about 800 km, the delay times correspond to those of terrestrial networks.
The mobile satellite systems are connected to the terrestrial mobile networks and the wireline fixed networks via gateway stations, so that direct communication from fixed networks or mobile networks takes place directly to the mobile satellite networks.