A satellite telephone, satellite phone, or satphone is a type of mobile phone that connects to orbiting satellites instead of terrestrial cell sites. Depending on the architecture of a particular system, coverage may include the entire Earth, or only specific regions.
The mobile equipment, also known as a terminal, varies widely. Early satellite phone handsets had a size and weight comparable to that of a late 1980s or early 1990s mobile phone, but usually with a large retractable antenna. More recent satellite phones are similar in size to a regular mobile phone while some prototype satellite phones have no distinguishable difference from an ordinary smartphone. Satphones are popular on expeditions into remote areas where terrestrial cellular service is unavailable.
A fixed installation, such as used shipboard, may include large, rugged, rack-mounted electronics, and a steerable microwave antenna on the mast that automatically tracks the overhead satellites. Smaller installations using VoIP over a two-way satellite broadband service such as ASTRA2Connect Maritime Broadband bring the costs within the reach of leisure vessel owners. Internet service Satellite phones have notoriously poor reception indoors, though it may be possible to get a consistent signal near a window or in the top floor of a building if the roof is sufficiently thin. The phones have connectors for external antennas that are often installed in vehicles and buildings. Some systems also allow for the use of repeaters, much like terrestrial mobile phone systems.
In some countries ruled by oppressive regimes such as Burma, possession of a satellite phone is illegal. Their signals will usually bypass local telecoms systems, hindering censorship and wiretapping attempts. In Australia, residents of remote areas may apply for a government subsidy for a satellite phone.
Satellite Phone Networks
Geosynchronous services
Some satellite phones use satellites in geosynchronous orbit, which are meant to remain in a fixed position in the sky at all times. These systems can maintain near-continuous global coverage with only three or four satellites, reducing the launch costs. However the satellites used for these systems are very heavy (approx. 5000 kg) and therefore very expensive to build and launch. The satellites sit at an altitude of about 22,000 miles (35,000 km); a noticeable delay is present while making a phone call or using data services due to the large distances from their users. The amount of bandwidth available on these systems is substantially higher than that of the Low Earth Orbit (LEO) systems; all three active systems provide portable satellite Internet using laptop-sized terminals with speeds ranging from 60 kbits to 512 kbits per second (Kbps).
Another disadvantage of geostationary satellite systems is that in many areas—even where a large amount of open sky is present—the line-of-sight between the phone and the satellite is broken by obstacles such as steep hills and forest. The user will need to find an area with line-of-sight before being able to use the phone. This is not the case with LEO services: even if the signal is blocked by an obstacle, one can wait a few minutes until another satellite passes overhead.
* ACeS: This small regional operator provides voice and data services in East Asia using a single satellite.
* Inmarsat: The oldest satellite phone operator, founded in 1979. It originally provided large fixed installations intended for use on ships, but has only recently started to enter the market of hand-held phones in a joint venture with ACeS. The company operates eleven satellites with another planned for launch in 2010. Coverage is available on most of the earth's surface, notably excepting polar regions.
* Thuraya: A system based in the UAE. Three satellites are currently in active service that provide coverage to the most of Eurasia, Africa and Australia. There is some degree of coverage overlap between adjacent satellites within the network.
* MSAT / SkyTerra: An American satellite phone company which uses equipment similar to Inmarsat, but plans to launch a service using hand-held devices in the Americas similar to Thuraya's.
* Terrestar: Satellite phone system for North America
* ICO Global Communications: A satellite phone company which has launched a single geosynchronous satellite which is not yet in active service.
Low Earth orbit
LEO telephones utilizes LEO (low Earth orbit) satellite technology. The advantages include providing worldwide wireless coverage with no gaps. LEO satellites orbit the earth in high speed, low altitude orbits with an orbital time of 70–100 minutes, an altitude of 640 to 1120 kilometers (400 to 700 miles), and provide coverage cells of about (at a 100-minute orbital period) 2800km in radius (about 1740mi). Since the satellites are not geosynchronous, they must fly complete orbits. At least one satellite must have line-of-sight to every coverage area at all times to guarantee complete coverage. Depending on the positions of both the satellite and terrestrial user, a usable pass of an individual LEO satellite will typically last 4–15 minutes on average; thus, a constellation of satellites is required to maintain coverage (as is done with Iridium, Global Star, GPS, and others).
Two such systems, both based in the United States started up in the late 1990s but soon went into bankruptcy after they failed to gain the number of subscribers required to fund the large satellite launch costs. They are now operated by new owners who bought the assets for a fraction of their original cost and are now both planning to launch replacement constellations supporting higher bandwidth. Data speeds for current networks are between 2200 bit/s and 9600 bit/s using a satellite handset.
* Globalstar: A network covering most of the world's landmass using 44 active satellites; however many areas are left without coverage since a satellite must be in range of an earth station. Satellites fly in an inclined orbit of 52 degrees; as such, polar regions cannot be covered. The network went into limited commercial service at the end of 1999 .
* Iridium: A network operating 66 satellites in a polar orbit that claims coverage everywhere on the earth's surface. Commercial service started in November 1998 and fell into bankruptcy soon after. Notably radio cross-links are used between satellites in order to relay data to the nearest satellite with a connection to an earth station.
Tracking
LEO systems have the ability to track a mobile unit's location using doppler shift calculations from the satellite. However, this method can be inaccurate by tens of kilometers. On some Iridium hardware the coordinates can be extracted using AT commands, while recent Globalstar handsets will display them on the screen.[8]
[edit] Proposed systems
* Teledesic: An ill-fated company backed by Bill Gates which planned to provide broadband internet using a network of 840 LEO satellites, it ended up launching only one test satellite.
* Ellipso: Start up that entered a partnership with ICO.
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