Most communication satellites can only cover a region of the earth. The spot beam technology is like a large flash light over the earth shining its light down on the earth covering a certain area. This area can be referred to as the coverage area or “foot print” of the satellite. This area could be as small as 30 square miles with a low earth orbiting satellite (LEO) or one third of the earth with a Geosynchronous (GEO) satellite which is in a much higher orbit allowing the spot beam to be much wider.
GEO satellites are located at an altitude of 35,800 kilometers above the earth and are used for high speed data, television transmission and other wideband communication.
Inmarsat uses three types of coverage related to each of their GEO satellites. Each Inmarsat satellite has a single global spot beam that covers up to one third of the earth’s surface excluding only the poles. These spot beams overlap allowing continuous service. The Inmarsat global spot beam coverage or foot print extends from latitudes of -82 to +82 degrees regardless of longitude due to the overlapping spot beams.
All the regional spot beams together offer the same coverage as the global spot beam. Regional spot beams allow satellite terminals to function with much smaller antennas. Regional spot beams were introduced with the I-3 Inmarsat satellite constellation. Each I-3 Inmarsat satellite provides four to six spot beams. The newer I-4 satellites used by the IsatPhone provides 19 regional spot beams.
Narrow spot beams are offered by the three Inmarsat I-4 satellites. Narrow beams provide coverage to several hundred kilometers of area. Narrow spot beams are much smaller than global or regional beams but are far more numerous while covering the same one third of the earth per satellite. Each I-4 satellite has around 200 narrow spot beams. Narrow spot beams allow for a smaller antenna allowing the handheld IsatPhone to be used for voice and higher data rates. The narrow spot beams provided by the I-4 satellites form the backbone of IsatPhone handheld satellite phone and the BGAN satellite terminal which is Inmarsat broadband service.
Globalstar uses a LEO constellation which unlike GEO satellites are in constant motion while orbiting the earth. They are 1,500 kilometers in altitude which is much lower to the earth’s surface compared to GEO satellites. The spot beams again are like flash lights in the sky, beam a smaller coverage area down to the earth when compared to GEO satellites. To cover large regions of the earth more LEO satellites are required over GEO satellites. There are forty satellites in the Globalstar constellation that have an overlapping coverage area while they orbit the earth. A Globalstar user can be connected from one to four satellites at one time. Globalstar satellites are constantly moving in and out of view of satellite phone users on the earth, the user is seamlessly added and removed from the calls in progress. Globalstar calls this path diversity which prevents a lower amount of dropped calls.
An advantage to Globalstar’s LEO constellation is if a number of satellites fail other satellites in the constellation can provide service. If a GEO satellite fails service could be lost in a very large region of the earth.
The Iridium LEO constellation like Globalstar’s constellation is in constant orbit. The Iridium constellation provides a much smaller spot beam than the Inmarsat I-4 satellites. The Iridium constellation consists of 66 satellites which provide total coverage to the earth with 66 overlapping spot beams in a polar orbit. The Iridium satellites are in an orbit only 485 miles above the earth requiring a large number of satellites to provide total coverage to the earth. Unlike the Globalstar satellites that can only communicate with their ground stations the Iridium satellites can communicate with each other. As an Iridium satellite orbits it can disappear over the local horizon of a user, so in this case that satellite will hand over the user to another satellite which will be transparent to the user. Satellite cross communication is used to manage the hand over.