Conventional “old-fashioned” high-frequency radios are quite capable of communicating worldwide, just as they did throughout most of the 20th century. But they suffer from two main problems.
The first is that radio communication over distances of more than a few hundred miles can be achieved only by reflecting radio waves off electrically charged layers in the Earth’s atmosphere. This tends to weaken and distort the signal, sometimes making voices incomprehensible and faxes illegible. The second is that frequencies that are good at bouncing are very bad at handling data at the kind of speeds we have come to expect.
In satellite communication, atmospheric reflection is replaced by artificial satellites that receive and retransmit radio signals. This allows the use of much higher frequencies—which is particularly useful for high-speed data—and produces a much cleaner and stronger signal.
Satcom systems can be divided into two main groups: those using satellites in low earth orbits (LEOs) and those using much higher “geostationary” (GEO) satellites.
It’s relatively cheap and easy to launch a satellite into a low orbit a few hundred miles above the earth’s surface. But a satellite in low orbit has to “fly” quite fast to avoid crashing back to earth, and the area over which it can transmit and receive radio signals, known as its footprint, is limited. Iridium satellites, for example, orbit at an altitude of 484 miles and have a footprint 2,920 miles in diameter.
Geostationary satellites, by contrast, are in orbits 22,236 miles above the equator. At this altitude, it takes each satellite exactly one day to go round the earth, so it appears to hover in place. The theoretical footprint of a GEO satellite is huge—10,000 miles across—and can potentially cover well over 40 percent of the earth’s surface. However, getting a satellite up into such a high orbit is expensive, and sending radio signals to and from it requires either a lot of power or a highly directional (and accurately targeted) antenna. Another drawback is that a quarter-second delay is created as the radio signals travel all the way up to the satellite and back.
[ GLOBALSTAR ]
Globalstar uses a constellation of 40 satellites in low earth orbit. These do no more than receive calls and retransmit them to one of Globalstar’s 24 ground stations (called “gateways”) from which they are connected to the land telephone system, and vice versa.
Globalstar telephones are not much bigger than ordinary telephones, do not require sophisticated stabilized dish antennas and have negligible transmission delay. But calls can only be made or received when a satellite can see a ground station and the user terminal at the same time. This limits Globalstar’s geographical coverage: coastal waters of the United States, Europe and Australia are well covered, but there is a gap in the middle of the north Atlantic and almost no coverage in the Pacific and Indian Oceans. At the moment, technical problems are also limiting the times during which the system is operable. Details vary, but service is typically unavailable for about 30 minutes in each hour.
Globalstar is good for voice calls, short text messages and low-speed data such as emails. For many customers its overwhelming advantage is its price: the latest handsets cost less than $1,000, and older units are retailing for as little as $300. There are a variety of airtime packages available, typically working out at less than $1.00 per minute.
[ IRIDIUM ]
Iridium has a constellation of 66 LEO satellites that fly lower and faster than those of Globalstar. Its satellites, however, are far more sophisticated and can pass calls from one to another—even re-routing a call while it is in progress—so coverage is worldwide.
Like their Globalstar counterparts, Iridium phones are scarcely bigger than ordinary domestic telephones, will work without stabilized antennas and have negligible transmission delay. Last year Iridium also launched a new “broadband” service called OpenPort. This provides three independent phone lines and data connections at anything up to 128 kilobytes per second through an antenna that is just 22 inches in diameter and 9 inches high.
As with ordinary cell phones, the price of an Iridium phone or an OpenPort terminal varies depending on the length of the contract and how much you expect to use the service. Hardware and airtime generally cost more compared to Globalstar: expect to pay between $1,000 and $1,500 for a phone, or about $4,000-$7,000 for an OpenPort terminal. Airtime generally costs $1-2 per minute and data costs average $10-15 per MB.
[ INMARSAT ]
Inmarsat is the oldest of the satcom services and is the only one introduced specifically for marine use. Set up over 30 years ago as an intergovernmental organization to provide telephone service to ships at sea for safety purposes, it is now an independent company operating 11 satellites, all of them in geostationary orbits that provide a range of different services.
Each of Inmarsat’s geostationary satellites can “see” a vast swath of the earth’s surface. However, both the satellites and the on-board terminals must be able to transmit and receive over vast distances. That’s why early Inmarsat terminals needed such big antenna domes.
A couple of years ago, Inmarsat launched the third, and final, satellite in its I4 constellation. Compared with the I2 and I3 satellites still in service, the I4 satellites are more powerful and sophisticated, with a 30-foot reflecting dish on each satellite directing the transmissions from 120 separate transmitting aerials into hundreds of high-power “spot beams.”
Thanks to these focused beams, Inmarsat has been able to introduce a new range of services called Fleet Broadband. These include voice, text messaging and data service, plus an emergency number that connects directly to the nearest search and rescue center.
Fleet Broadband 150 is probably most suitable for the majority of sailboats, because the dome is about a foot in diameter and weighs between 5 to 10 pounds, depending on the particular model you choose. The total hardware price is about $5,000 to $7,500. At 150kbps, its data connection is slow compared to domestic broadband, but it’s fine for email and occasional web surfing.
A Fleet Broadband 250 dome is two or three inches bigger than an FB150 dome and costs about twice as much. But FB250 almost doubles data connection speed to a pretty healthy 284kbps, and includes fax service and—if you really want it—streaming video.
Fleet Broadband 500 is intended primarily for commercial vessels. Its data speed is almost 432kbps, and the service offers voice, fax and text messaging through a two-foot dome. But the dome weighs about 50 pounds and costs about three times as much as the FB150.
As with all satellite services, airtime and hardware deals vary widely, but expect to pay about $1.00 per minute for voice service and $10-12 per MB for data service.
[ VSAT ]
The term VSAT doesn’t refer to an operating company or satellite constellation. The initials stand for Very Small Aperture Terminal—a rather unhelpful, and some might say misleading, description of any satellite receiving dish less than 10 feet across. VSAT terminals are mainly used ashore by companies such as Walgreens, Walmart and CVS to transmit sales and ordering information, such as your credit card payments, from an individual store to their head office.
What sets VSAT apart from other satellite systems is that you don’t buy airtime by the minute or megabyte, you buy a share in the capacity of a particular satellite. It’s like renting a private parking space, rather than buying a ticket to park in a public lot.
VSAT terminals are technically more sophisticated than those used for Inmarsat or satellite TV, so prices are high, with even the smallest terminals costing about $30,000. VSAT airtime isn’t cheap, either—contracts start at several hundred dollars per month—but you have unlimited use of the capacity you purchase. KVH, for instance, supplies hardware and airtime for both Inmarsat and VSAT packages, and reckons that VSAT is more economical for anyone likely to use more than about 300MB per month during a three-year contract.