The major telecommunication satellite constellations are: Globalstar, Inmarsat, and Iridium. These are also the three major satellite phone services.

Globalstar launched commercial services in 1999 and today assert that their handsets are the most widely used in the world. Customers can use their phones in more than 100 countries on six continents, as well as from most territorial waters and several mid-ocean regions. Globalstar signals are received by the company's constellation of 48 Low Earth Orbiting (LEO) satellites and relayed to ground-based gateways, which then pass the call on to the terrestrial telephone network.

(Terrestrial-of or relating to the earth and is a term used in satellite technologies to distinguish between what is out in space and what is located on the earth's surface.)

Globalstar satellite phone service is delivered through special multi-mode phones, which work just like traditional cellular phones when in an area with cellular coverage. When the user needs to communicate from outside the area covered by ground-based systems, the phones easily switch to Globalstar satellite mode. Satellite and cellular communications are both available in one phone.

Inmarsat was the world's first global mobile satellite communications operator and states it is still the only one to offer a mature range of modern communications services to maritime, land-mobile, aeronautical and other users. Formed 20 years ago as an international maritime organization, Inmarsat was reorganized as a limited company in 1999 and has broadened its customer base.

Inmarsat's primary satellite constellation consists of four Inmarsat I-3 satellites in geostationary orbit. A fifth spacecraft that can be brought in to provide additional capacity currently backs these up. Between them, the main "global" beams of the satellites provide overlapping coverage of the whole surface of the Earth apart from the poles. So, with Inmarsat coverage, it has become possible to extend the reach of terrestrial wired and cellular networks to almost anywhere on Earth.

Iridium claims to be the "only provider of truly global, truly mobile satellite voice and data solutions with complete coverage of the Earth (including oceans, airways and Polar regions). Eighty-six percent of the world's landmass and all of its oceans are in areas with inadequate landline service. Iridium addresses these situations by providing coverage in all ocean areas, air routes and all landmasses-even the Poles.

Sixty-six low-earth-orbiting (LEO) satellites provide global coverage, which allows users to make and receive calls virtually anywhere in the world. The only requirement is a clear line of sight to the sky. Their services are aimed most heavily towards business users in heavy construction, defense/military, emergency services, maritime, mining, forestry, oil and gas and aviation.