Mobile satellite services (MSS) refers to networks of communications satellites intended for use with mobile and portable wireless telephones. There are three major types: AMSS (aeronautical MSS), LMSS (land MSS), and MMSS (maritime MSS).
A telephone connection using MSS is similar to a cellular telephone link, except the repeaters are in orbit around the earth, rather than on the surface. MSS repeaters can be placed on geostationary, medium earth orbit (MEO), or low earth orbit (LEO) satellites. Provided there are enough satellites in the system, and provided they are properly spaced around the globe, an MSS can link any two wireless telephone sets at any time, no matter where in the world they are located. MSS systems are interconnected with land-based cellular networks.
As an example of how MSS can work, consider telephones in commercial airliners. These sets usually link into the standard cellular system. This allows communication as long as the aircraft is on a line of sight with at least one land-based cellular repeater. Coverage is essentially continuous over most developed countries. But coverage is spotty over less well-developed regions, and is nonexistent at most points over the oceans. Using an MSS network, the aircraft can establish a connection from any location, no matter how remote.