The most universal and available communication tool on aircraft is satellite communications equipment.
Principle of satellite communication organization is shown on Fig.121. Satellite communication system consist from great number of subscriber terminals, communications satellites placed in geostationary or polar orbits and ground control stations and communication, providing switching of channels with ground system.
Satellite services are provided by three operators : Inmarsat, Iridium and Globalstar.
The most popular software for proving the needs of aviation communications satellite system is Inmarsat.
Space segment of Inmarsat consists from geostationary satellites that located at altitude is equal to 35.600 km. Geostationary satellites rotates around the Earth with the same speed as the Earth, that’s why it seems stationary from ground.
Communication channel with geostationary satellites is much more stable than with satellites at other orbits, because:
-during all communication session with geostationary satellites( session can be as long as wanted) for terminal there is no need to switch from one satellite to another;
- geostationary satellites don’t fly beyond the mountain or the horizon and a data link channel is not interrupted.
Fig. 121. Satellite communication system links
Space segment of Inmarsat consist from four main and one stand-by satellites of the third generation. Besides, four satellites of the second generation is already working. Main satellites got names according to the areas over which they are located:
- Indian ocean satellite(IOR) 64.5° East longitude
- Pacific ocean satellite(POR) 178° East longitude
- East-Atlantic satellite(AORE) 15.5° West longitude
- West Atlantic satellite(AORW) 54° West longitude
Three geostationary satellites is located uniformly along all Equator longitude, it is enough to cover 98 per cent of the Earth surface with the help of his global rays. Out of service are only nearest Polar Regions. Since in system engaged four satellites, their areas of service is overlap, and in many countries we can see just two or three Inmarsat satellites.
Inmarsat satellite communications includes several subsystems
- Inmarsat-A - the ancestor of the system;
- Inmarsat-B - digital standard Inmarsat A;
- Inmarsat-C – duplex low-speed data transmission by using simple portable terminals;
- Inmarsat-M – first in the world portable satellite phone;
- Inmarsat Mini-M - most widespread standard of Inmarsat;
- Inmarsat Fleet - economic global communication and rapid access to actual business information;
- Inmarsat-M4 (GAN) - work in a global area network (GAN);
- Inmarsat R-BGAN - high-speed access in Internet and corporate networks;
- Inmarsat BGAN - broadband communication in any, even the most remote place in the world.