|From Ocean Navigator #84 |
Four-satellite coverage of Inmarsat 3 global beams (large ellipses) and the combined coverage of the Mini-M spot beams (shaded areas). Note lack of Mini-M coverage in Southern Hemisphere.
The full four-satellite constellation of the new generation 3 units is now in place: Atlantic East, Pacific Ocean, Indian Ocean, and the newest satellite, Atlantic West.
Previous generations of Inmarsat satellites used a single large beam that covered a wide area from high northern latitudes to high southern latitudes. The generation 3 satellites also have a single large beam, but they can focus up to five smaller beams, or spot beams, to cover specific areas. This capability means that,rather than trying to cover a huge area with a single beamand therefore spreading out the available transmission energy over that large area, a spot beam allows you to concentrate power into a smaller area. This fact, alongwith the overall higher power capability of the generation3 transmitters (up to an eightfold increase on L-band frequencies compared to the Inmarsat 2 satellites), means that you don’t need to use such a large “bucket” to gather enough satellite signal for communication. This translates into a smaller, lighter antenna and lower cost. And those lowered physical requirements mean that these types of antennas can be placed on smaller boats.
Inmarsat has always offered a mix of services to appeal to different users. Until now, the only service option that fell between the big-ship Inmarsat A and B services, on the one hand, and text-only Inmarsat C service, on the other, was Inmarsat M. Like A and B, M uses a stabilized dish antenna, but a much smaller one than its big-ship cousins. However, while an M antenna unit is smaller than the large A and B units, it still is large and heavy for any boat smaller than 50 feet or so. The new Mini-M service, which is primarily a voice system but can also send 2.4 kbps fax and data, made possible by the new satellites, fills a slot between the M and C services. The difference in size and cost for Mini-M equipment compared to M requirements is substantial.
A typical M antenna assembly, for example, is 22 inches in diameter, weighs 44 pounds and typically sells for about $18,000. By contrast, the smallest Mini-M antenna from KVH Industries in Middletown, R.I., is called the Inmarsat Tracphone 25. This antenna unit is 10 inches in diameter, weighs about 10 pounds, and costs roughly $7,000. “The unit consumes 30 watts in listen mode, 40 watts in talk mode, and 50 watts in fax or data mode,” said Jim Dodez, vice president of marketing for KVH. Mackay Communications in Edison, N.J., the U.S. distributor of Nera products, offers a laptop computer-sized mini-receiver called World-phone that sells for $3,995. Because this unit doesn’t have a stabilized antenna, it can only be used on land. However, for an additional $3,500 you can purchase a 10-pound stabilized antenna unit called the Worldphone Marine antenna, which is 10 inches in diameter.
Clearly, the size and cost of these units makes an Inmarsat Mini-M satcom unit more attractive to the smaller-boat owner. The price for connect time is about $2.80 a minute, depending on whom you use as a service provider.
There is a downside to Mini-M, however. Unlike the wide beams that are used for the other Inmarsat services, the Mini-M spot beams are focused on global land masses. The beams do overlap onto ocean areas, but there are large ocean areas that are not covered by Mini-M spot beams. “There’s a big hole in the Indian Ocean, a hole in the Pacific, and a hole in the South Atlantic,” said Carl Sederquist of Quest Telecom in Ellsworth, Maine. Also, there is little to no coverage in the Southern Ocean. Because of these coverage gaps, you can’t rely on Mini-M as a truly worldwide system. Still, there is coverage for just about all the areas in which most voyagers operate.