The Globalstar system [GLO96, GAF94, MAZ93, ROU93] has 48 low earth orbit satellites in eight planes. The constellation is designed for 100% single satellite coverage between ±70° latitude, and 100% dual or higher satellite coverage between 25° to 50° latitudes. Globalstar will employ path diversity combining to mitigate blocking and shadowing; up to three satellites may at any one time be used to complete the call.
Globalstar chose Qualcomm’s terrestrial CDMA technology for the mobile link, and for the feeder link FDM uplink and FDMA downlink. As for any satellite CDMA system, the chosen feeder link approach is bandwidth-consuming; each beam will require a full 16.5 MHz portion of the feeder link for full re-use between beams, due to the CDMA technique. CDMA was chosen to increase capacity on the mobile link through frequency re-use and voice activity detection, for the ability for spectrum sharing and for improved multipath performance. Globalstar offers data rates at 1,200, 2,400, 4,800 and 9,600 bps, and the vocoder rate is allowed to drop down to 1,200 bps when no voice activity is detected. This reduces interference and increases capacity, while maintaining synchronisation and conveying background comfort noise. Globalstar’s antennas are shaped for elliptical beams aligned with the satellites velocity vector to increase the time a user stays within each beam.
The Globalstar system provides interconnection to the PSTN/PLMN (Public Switched Telephone Network/Public Land Telephone Network) through 100 to 210 ESs which will each interface an MSC for extension of terrestrial cellular call processing. Globalstar will sell access to the Globalstar system to local service providers, which will have an exclusive regional right to offer the Globalstar service, as well as an obligation to obtain necessary regulatory approval. Calls will only be established through satellite(s) when connections cannot be made over the terrestrial network. All calls that are connected through the Globalstar system will be connected through the regional ESs, giving the local service provider additional revenue and enabling local regulatory authorities to maintain control. Two satellite operations control centres (SOCCs) will track and control the satellites through TT&C units located in various ESs. Additionally, two ground operations control centres (GOCCs) are designed for dynamically allocating capacity among nearby regions, coordinating information received from the SOCCs, and collecting information for billings to service providers.
For further information on Globalstar, see the Globalstar entry in Lloyd’s satellite constellations.
http://personal.ee.surrey.ac.uk/Personal/L.Wood/constellations/overview.html