The Inmarsat satellites are located in geostationary orbit 22,223 miles (35,786 km) out in space.
Each satellite covers up to one third of the Earth's surface and is strategically positioned above one of the four ocean regions to form a continuous 'world-wide web in the sky'.
Every time a call is made from an Inmarsat mobile satphone it is beamed up to one of the satellites. On the ground, distributed all around the world, giant communications antennas are listening for the return signal, which they then route into the ordinary telephone network. And when someone calls an Inmarsat customer, it happens the same way - but in reverse.
The Inmarsat System consists of nine satellites in geostationary orbit.
Four of these satellites, the latest Inmarsat-3 generation, provide overlapping operational coverage of the globe (apart from the extreme polar areas). The others are used as in-orbit spares or for leased capacity.
Each Inmarsat-3 satellite also operates a number of spot beam "cells". This enables the satellites to concentrate extra power in areas of high demand and to provide services to smaller, simpler terminals.
Roaming users communicate direct via Inmarsat's satellites. Inmarsat airtime services are available world-wide through a network of about 100 leading service providers.
An extensive system of equipment manufacturers, distributors and dealer outlets ensures world-wide sales and service support.
Often service arrangements are bundled with the sale or lease of terminals. Some service providers also operate Inmarsat land earth stations (LES). There are about 40 LES in 31 countries. These stations receive and transmit communications through the Inmarsat satellites and provide the connection between the satellite system and the world's fixed communications networks