A satellite sitting in a geosynchronous orbit will appear to hang over a specific point on the Earth’s surface. This fortunate aspect of orbital physics has spawned a huge satellite communications industry. Geosynchronous satellites are used as relay stations for sending communications traffic, television, and data from point to point on the Earth. Advances in satellite and antenna technology mean that voyaging sailors can have satellite voice, data and even TV signals on the average sailboat. One of the key advances has been the use of satellite spot beams.
The early adopters of satcom were large commercial ships. This made sense since the first generation of satellite antennas were heavy enough to sink a voyaging sailboat. The main reason the antennas were large and heavy rests with the with the way satcom signals were transmitted down to Earth.
The satellites of the marine communications consortium Inmarsat provide a good example. The first two generations of satellites transmitted their signals to the ground using a “global beam.” This means that the energy of the satellite transmission was spread out over the entire area of the Earth’s surface in view (from about 70° N to about 70° S, with a 150° spread of longitude). This approach maximizes the coverage area, but it also weakens the received signal because all the available signal strength has to be spread out over this area. Thus, with a global beam, you needed to have a large “dish” antenna to scoop up enough signal out of the noise. In addition, the wide coverage area of the beam means that those users near the edge of the coverage area have antennas that are looking at the satellite with a low elevation angle and thus are trying to “listen” to the satellite through a thicker slice of electrically noisy atmosphere. To top it off, the second-generation Inmarsat 2 satellites, with an initial power rating of 1,200 watts, were less powerful than the current third-generation Inmarsat 3 satellites. The first Inmarsat 3 spacecraft was launched in April 1996, and the last spacecraft was rocketed into position in February of last year.
With a power rating of 2,800 watts, the newer, more powerful Inmarsat 3 satellites actually have up to eight times more power available compared with the Inmarsat 2s. This power gain comes from more efficient power amplifiers on the satellites and more efficient antennas. While the added power of the Inmarsat 3s makes them more effective, there is another technical development that allows smaller, lighter dish antennas to be used with the system. The Inmarsat 3s use a technique called spot beams to direct signal energy into tight beams that can be aimed at areas of high usage.
“Spot beams give you more energy per square foot,” said Vasilis Riginos, former head of the transponder department at Comsat Labs and now a consultant. “On a satellite, energy is at a premium; there is no outlet to plug into. So you want to squeeze as much power out of the satellite as you can.” So, not only is more power available, but the satellite’s five spot beams use that energy more effectively (the Inmarsat 3 satellites also have a single global beam to support users of Inmarsat’s older A, B, and M services). In addition to these advantages, the Inmarsat 3s have power-management capabilities and can dynamically reallocate both RF power and bandwidth among spot beams.
Another advantage to spot beams is something called frequency re-use. Since the coverage area for each spot beam is much smaller than the coverage for a satellite’s global beam, users in adjacent beams can use the same frequency without interfering with each other. This same technique is also used in cell phone systems in which cell phone users re-use frequencies in each cell without causing interference.
For the mariner, the result of more power and spot beams is that the antennas used for the Mini-M voice communications service are considerably smaller than those required for Inmarsat’s other voice services: A, B, and even M. A number of marine electronics companies offer Mini-M satcom products: KVH Industries offers the TracPhone 25 for $6,995; O’Gara Satellite Systems has the Boatfone for $7,995.
For true globe-girdling voyagers who want a small antenna with voice satcom capability there is a drawback to the Mini-M system: it does not provide global coverage. This isn’t as bad as it sounds because the spot beams are directed to areas most often visited by voyagers. But significant sections of the South Atlantic, South Indian, and South Pacific oceans are not covered.
For voyagers who want to watch TV on their boats, some of the technical advances used on board Inmarsat communications satellites have also been employed in building direct broadcast satellite (DBS) TV. The DBS satellites, like the Inmarsat 3s, are more powerful than previous generations of communications spacecraft. The combination of higher power, spot beams, and a main “shaped” beam means that the signal-to-noise ratio is improved. The result is that the receiving antenna can be smaller, and the added power means that there is more signal to power through the rare interference caused by heavy rain. The DBS companies studied weather patterns and rainfall amounts and determined that certain areas of the country receive more rain than othersfor example, the Pacific Northwest and Florida. So, part of the DBS satellite development involved fitting the spacecraft with phased-array antennas that can send a shaped beam down to Earth. This shaping puts more energy into those areas that experience more rain. According the DBS satellite people, very heavy rain should only cause eight hours of rain problems in a year. Like Inmarsat Mini-M, DBS TV satellite coverage doesn’t extend over all ocean areas. However, some DBS TV-equipped boats have been able to pick up signals up to 200 miles offshore.
The ability to use a smaller antenna is good news for mariners, too. Of course, land-based users have the advantage of a stable platform for their antennas. But improved stabilization equipment and techniques makes it possible to receive DBS TV on board a yacht, even in substantial sea states and in all weather conditions short of heavy downpours.
KVH Industries (401-847-3327, www.kvh.com), for example, has a new system for receiving DBS TV broadcasts from DirecTV and the DISH network called the TracVision 3. The 30-pound antenna package has a 19-inch-diameter dome containing a stabilized dish antenna. This antenna actively moves to counteract the effects of boat motion and to maintain lock with the satellite. KVH uses its own method for tracking the satellites. “We don’t track the satellites just based on what the satellite receiver is telling us,” said Jeff Hawes, lead design engineer for satellite systems. “We use a broadband RF energy detector that will remain locked on the satellite 1 1/2 to 2 db below the point where the satellite receiver loses lock.” This means that when the signal gets strong again the KVH unit can begin supplying picture immediately without having to reacquire the signal.
The biggest difference between the previous TracVision II and the new TracVision 3 is the addition of KVH’s GyroTrac digital gyro sensor. This unit makes the TracVision antenna that much more capable in tracking the DBS satellite signal. The GyroTrac unit handles most of the boat motion, and then a smaller gyro controller built into the antenna assembly can easily counteract the rest of the motion. The TracVision 3 has a suggested retail price of $5,995.seatel.com). Its model 1898 has a 20-inch radome enclosing an 18-inch antenna; the unit weighs 33 pounds. According to Sea-Tel, the 1898 unit uses solid-state inertial sensors for pointing the antenna and keeping it stable in a seaway. The manufacturer’s suggested price is $4,995.
The advent of powerful satellites using spot beams and shaped beams has meant that voyagers on medium-sized boats can make phone calls and watch TV using antennas that are smaller and lighter than ever.
