The GlobalStar satellite network delivers service utilizing the following technology elements:
LEO Satellites
Unlike conventional satellites Globalstar uses Low Earth Orbit satellites, positioned 1500km above the earth, to power to its network. In contrast a typical communications satellite would be placed in an (GEO) orbit at 35000km. The LEO satellites of Globalstar (and Iridium) are designed to deliver mobile satellite phone services while their higher orbit counterparts are best suited for high-speed data like video transmission (ex. televising the Olympics).
LEO satellites are uniquely qualified to provide reliable hand-held and vehicle-mounted satellite phone service providing cell phone quality globally no cell phone towers required.
Voice propagation delays inherent to higher-orbit GEO systems (like Thuraya) produce echo effects and for broadband require bulky, stationary, directional antennas which must remain fixed and pointed at a specific satellite. Because LEO satellites don’t require this type of equipment, users of a Globalstar satellite phone won’t encounter these issues—they will simply feel like they are using a cell phone with superior connection even in remote locations.
Path Diversity
Path Diversity is a signaling method that combines multiple signals of varying power strengths into a single, coherent signal. Subscriber terminals are consequently able to operate with a connection to a single satellite though typically two to four satellites will be overhead.
In the Globalstar network, subscriber satellite phones will often communicate with potentially 3 satellites simultaneously and will combine these signals into a single static-free channel. Satellite phones may alter their power levels to compensate for shadowing and interference, averaging a power range of between 50 and 300 mw.
Satellites are in continuous motion passing in and out of view, however they will be seamlessly added and dropped during calls in progress, reducing the risk of call failure. This feature enables Globalstar to provide service with less potential for signal latency due to blockage from buildings, terrain or other natural features.
Satellite CDMA - just like a World Cell Phone
Globalstar utilizes a version of Code Division Multiple Access (CDMA) technology based upon the IS-95 CDMA standard to provide high quality, voice, data, and fax services. CDMA utilizes digital transmission methods whereby subscribers share time and frequency allocations, and are assigned by unique assigned codes. Signals are separated by the receiving handset that accepts only signal energy from the desired circuit. Undesired signals are ignored as merely being noise. This technology allows a large number of subscribers to simultaneously access a single frequency channel with reduced interference resulting in an exponential increase in capacity when compared to analog systems such as Frequency Division Multiple Access (FDMA).
Using CDMA is like being at a party where all the ‘guests’ are talking in the same room, at the same time and every conversation in the room is being carried out in a different language that you do not understand. They would all sound like noise from your perspective, however, If you knew an appropriate language, you could imagine filter out all of the other unwanted conversations, listening only to the conversation of interest to you. CDMA filters traffic in a similar way. CDMA also maintains a ‘control’ process where the listener at the same party can request higher volume or even signal other people/conversations to speak more softly.
Globalstar has actually combined the best of FDMA and CDMA with spectrum modulation which supports multiple subscribers simultaneously, so they can share frequencies with other CDMA systems. The result of this satellite phone service is superior call quality with fewer dropped calls.
 Back to top
|
