The services of mobile satellite communications have grown markedly in recent years, so it is estimated that nearly a thousand of orbiting satellites covering the globe in 2004.
Establish a system for global coverage is not yet easy. The hefty investment, the order of billions of dollars, are only possible through the formation of large international conglomerates. Experience shows that the viability of projects is not easy to achieve. Examples of Iridium and Globalstar, the two main operators, both having faced flaw tortuous processes and records having been saved by last minute the show.
With variable-sized portable terminals but, in essence, with an average weight around 200 grams, aesthetic and functional approach of GSM mobile phones combine satellite usually the orbital network connection with the possibility of roaming with GSM networks.
Thus, depending on the operating mode is chosen, the user can make calls by satellite, GSM (when available) or let the device choose the best solution. When you make a call based on the satellite network, the mobile contact the nearest spacecraft orients the call, depending on the case, directly or indirectly through one or more satellites in the same constellation, for a gateway ( tracking station) in the soil. The gateway takes care of inserting in the conventional cable network.
The networks that offer mobile services satellite work, depending on the type of satellite orbit, in two ways: using geostationary constellations and using non-geostationary satellites.
Non-geostationary systems
Non-geostationary systems, like those used by Iridium and Globalstar, using satellites in low orbits (700 to 1500 Km above the surface) or medium (10000 km, as the ICO). On the move permanent, these satellites may have orbital periods - rotation around the Earth - as small as 100 minutes. Thanks to its proximity, the advantage of not need very powerful emitter, and both may offer little more mobile than conventional GSM, only with larger antennas.
Since they are in motion at every instant zone of the crust must be covered by at least one, usually more (average 2). The user establishes the call with a satellite and that, when it disappears over the horizon, it transfers the call to another. This technique makes it possible to avoid masking by buildings and trees, or due to the morphology of the terrain or movement of the user, so that there is always coverage.
Geostationary Systems
Another view of the satellite coverage is used by the geostationary systems. What is a geostationary satellite? It is basically a device placed in such space in the space ponto acquires sync with the earth movement itself, permanently covering an area of the globe. For a user on the ground, a geostationary satellite will always have the same relative position in the sky. It is, for example the case of satellite television channels issuers.
But, as with TV receivers, systems based on a constellation geostationary case of Inmarsat and Thuraya, forcing the user to use mobile units bulky.
This is due to de facto geostationary orbit, usually on the ground Ecuador, only be possible to approximate distances to the Earth of 36,000 km
In this problem the added problem that due to the breaking storm that takes the signal between your phone, satellite and ground station that receives and relays it to the recipient, and vice versa, this system tends to place a small effect delay in communication.
