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Satphone roundup

Jun 20, 2013

The saga of the satellite phone is an intriguing story of ups and downs. Both Iridium and Globalstar, the two original satellite phone companies from the 1990s, went bankrupt, were sold and re-emerged in the 2000s as viable operations. In 2007, Inmarsat joined in with its own satellite phone. Now, Globalstar is selling a satphone via its Spot subsidiary called the Spot Global Phone. This new Spot-labeled phone is a good opportunity to review the satphone landscape of what you can expect from these devices.

For voyagers, the appeal of the satphone is the ability to connect when outside of cellphone range. Land-based cellphone networks use towers that are purposely limited in height so that calls can be localized and don’t interfere with calls in adjoining areas or cells. Building offshore towers to offer service to vessels outside land-based tower range is not economically viable. The expense would be too high and the number of subscribers too low. Satellite systems get around this problem by using “cell towers in the sky.” Satphones connect to satellites and the satellites retransmit the call to an earth station hundreds of miles away.

Courtesy Globalstar

Spot satphone coverage. Orange is the primary coverage area, yellow is secondary and blue is the fringe area.

Iridium and Globalstar achieved their sky cell towers using satellites arranged in low earth orbit (LEO) constellations. Each company developed their satellites and launched them into LEO — 66 satellites 483 miles above the earth for Iridium and 48 satellites 876 miles high for Globalstar. GPS satellites are at 10,000-mile-high middle earth orbit (MEO) altitude, while geosynchronous earth orbit (GEO) satellites are out there at 23,000 miles.

Getting the satellites in space is only one part of the equation, of course. Satellite phone systems are unlike high frequency SSB radios: two satphones don’t communicate directly. The signal is picked up by the satellite in orbit, but then that signal needs to get back down to the ground via an earth station where it can be patched into land-based mobile phone networks or landline networks (sometimes whimsically called “plain old telephone service” or POTS). Both the Iridium and Globalstar systems require earth stations, a site equipped with a few large dish antennas and connections to the Internet and phone networks, to pick up a user’s signal from their respective satellites. With no earth stations there is no communication. Earth station coverage, however, is not evenly distributed throughout the globe. There is the obvious problem of vast ocean areas where building a station is difficult. More than that, there simply isn’t enough telephone traffic in some of the world’s less inhabited areas, like the Southern Ocean, to make building a station economically feasible.

To get around this problem, Iridium designed its system with sophisticated satellites that not only can transmit signals down to earth stations, but can also talk to each other. The Iridium satellite knows where it is located in its orbit and also knows what earth stations are in range. If there is no station in view, the satellite transmits the user’s call to a nearby satellite in the Iridium constellation. If that satellite doesn’t have an earth station in view, it can skip the signal on to another satellite. Eventually, one of them will be in range of an earth station and can transmit the signal down to earth. This relaying ability gives Iridium truly worldwide coverage. Users at the poles, for example, can make phone calls even with no earth station in view for thousands of miles.

Globalstar took a different approach with its LEO satellites. A Globalstar satellite works as a so-called “bent pipe,” or essentially a transponder. When a Globalstar bird receives the signal from a user on the surface, it re-transmits that signal down to earth. If there is no earth station to receive the signal, the user’s call does not go through. This makes Globalstar satellites simpler and less expensive, but it also means they will not work in areas without earth stations. Luckily, much of the Northern Hemisphere is well equipped with earth stations, so Globalstar coverage is excellent in North American and European coastal waters, the Caribbean, the Mediterranean and across much of the North Atlantic. South American, Australian, Japanese and North African coastal waters are also well covered. Big swaths of the South Atlantic, the Pacific and the Indian Ocean are lacking coverage, however. Since the Spot Global Phone uses the Globalstar system, its coverage will not be truly global. It’s worth noting, however, that for many people, Globalstar’s service area will encompass most of the waters they usually sail. Also, on balance, Globalstar and Spot Global Phones tend to cost less than Iridium phones.

The other major satphone system is the IsatPhone Pro from Inmarsat. Unlike Iridium and Globalstar, Inmarsat uses geosynchronous satellites as its “cell tower in the sky.” Given their much higher altitude above the earth, Inmarsat needs only three of its latest generation Inmarsat-4 satellites to cover the earth. And because the three Inmarsat-4 spacecraft are in geostationary orbits above the equator, they “hang” over one spot on the earth. This means they are always available and in view of Inmarsat satphone users. Since the earth’s surface is curved, however, Inmarsat satellites can’t “see” users at latitudes greater than 70° north or 70° south. This is the main limitation on coverage with IsatPhone Pro.

Another geosynchronous satellite-based satphone is from Thuraya. This company provides satphone service in Europe, the Middle East and parts of Africa and Asia.

Returning to the Spot Global Phone, some observers see it not as a new satphone player per se, but as a rebranding effort by Globalstar to tie its satphone product into the company’s popular Spot line of tracking and short messaging products.
 

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