Differential GPS corrections widely available

From Ocean Navigator #92
September/October 1998
Technological solutions to real-world problems often aren’t based on the most technically elegant approach, but rather on whatever works right now for the least amount of money. Differential GPS (DGPS) correction broadcasts via marine radiobeacons are a good example of this. Indeed, from a cost/benefit point of view, the radiobeacon solution works well enough that marine radiobeacons are being used worldwide as the preferred method for disseminating DGPS corrections. This emerging standard for DGPS has important ramifications for world-girdling ocean voyagers.

Fully automatic differential beacon receivers, like the Trimble NavBeaconXL above, will lock on to the strongest source of RTCM SC-104 differential corrections and output them to a DGPS receiver.
   Image Credit: Courtesy Trimble Navigation

In the U.S., DGPS corrections are broadcast by the Coast Guard’s network of marine radiobeacons. The corrections allow mariners with DGPS receivers and radiobeacon receivers (DGPS-capable beacon receivers can be bought separately or are often built into high-end DGPS units) to get 10-meter GPS accuracy in many areas.

For the U.S. Coast Guard, the decision to base its differential GPS broadcasts on marine radiobeacons made sense. A national system of radiobeacons already existed, giving the Coast Guard a distribution network without having to build any new broadcast stations. And the Coast Guard already had authorization from the FCC to use those frequencies, so the process of getting clearance for a new set of frequencies was unnecessary. Finally, the system was designed so that the corrections could be broadcast without affecting the function of the radiobeacons as navigational aids (although one might be hard pressed to find a mariner who actually still uses radiobeacons for navigation).

The ready availability of the radiobeacon network, along with prompt work ironing out the technical details by the Coast Guard’s Research and Development Center in Groton, Conn., allowed the Coast Guard to get its DGPS system up and running fairly quickly. The operational requirements for the system were defined in early 1992. Initial operational capability was declared on January 30, 1996. Full operational capability of the system should be set by late 1998. The Radio Technical Commission for Maritime (RTCM), a non-profit organization in Alexandria, Va., that sets standards in marine radio, published a standard format for DGPS messages called RTCM SC-104. Given the RTCM SC-104 standard, marine electronics manufacturers can build beacon receivers that will work throughout the system. A DGPS beacon receiver captures the DGPS message from the radiobeacon (the DGPS messages are modulated onto the radiobeacons using a technique called medium shift keying or MSK), then routes it, via the NMEA 0183 interface standard, into a GPS receiver. Then, inside the GPS receiver, the corrections are applied to raw GPS position calculations, taking out the inaccuracy imposed on the system by the Department of Defense. Other nations, wanting to set up their own DGPS systems, looked at their own radiobeacons and decided to follow suit. “There are up to 30 countries using this approach,” said Bill Adams, president of RTCM. “It [the radiobeacon/RTCM SC-104 combo] happened to be the first thing on the scene, and it worked.” Some of the nations using RTCM SC-104 include Sweden, Finland, Canada, Australia, India, Denmark, Iceland, Belgium, Poland, Singapore, the U.K., and Ireland. Leica Geosystems, a manufacturer of GPS receivers and DGPS reference stations, has built five DGPS stations in China and recently was awarded a contract for four additional DGPS beacon stations.

What all this means for the ocean voyager can be summed up in one word: standardization. To put it another way, you can take your DGPS receiver to many places in the world outside the U.S., and it will provide you with differential corrections. “Any user who has an automatic differential receiver can just sail into that area and it will automatically pick up that differential beacon,” said Stuart Tolman, Leica’s sales manager for the Americas. “If he has a manual beacon receiver, he’ll have to tune it to the nearest DGPS beacon.” Either way, the key is that just about every nation that has installed DGPS equipment is offering free corrections in a standard format (the British initially opted for a subscription-based DGPS system, but they have since backtracked and are now installing a freely available RTCM SC-104 system).

Thus, the international radionavigation community has embraced the radiobeacon approach to DGPS. It undoubtedly works, and it is available now. But is sending DGPS corrections via low-frequency/medium-frequency radiobeacons the best way to go? In those places where a radiobeacon network already exists, a cost/benefit analysis would probably come out in favor of radiobeacons. But what about in those areas where there is no established net of beacons? In China, for example, the new DGPS stations will use marine radiobeacon frequencies, but won’t function as navigational radiobeacons in the classic sense.

Just as there are advantages to LF/MF beacons, there are disadvantages, too. Their signals are susceptible to interference from thunderstorms. And their range is limited (the higher a radio frequency, the more easily it is absorbed by the atmosphere). Typically, radiobeacons will have a range of 50 to 200 miles. This means that, to provide coverage for thousands of miles of coast, quite a few stations are needed. Service to the U.S. East Coast (Maine to Virginia Key, Fla.), for example, requires 11 stations. Compare this to the coverage capabilities of lower-frequency loran signals. Three loran stations (at Caribou, Maine; Nantucket, Mass.; and Carolina Beach, N.C.) provide substantially the same coverage when loran is used for sending DGPS corrections (see Eurofix, below).

Tied to the range issue is the question of redundant coverage. Due to the short range of radiobeacons, in the U.S. the Coast Guard has set up a system that has little overlapping coverage. When a beacon has an outage, there is no second station to provide DGPS corrections.

Thus, while radiobeacons have certain advantages as a method of DGPS-correction delivery, they also have their drawbacks. What about other methods for getting differential corrections to users? In Europe, researchers have been developing a loran-based system for sending DGPS messages (see “Loran expected to stay alive,” Issue No. 88). This method shifts the transmission times of six pulses in each pulse group. By advancing or retarding each pulse by 10 microseconds, DGPS correction data can be encoded on a loran signal. This DGPS broadcast technique (which also uses the RTCM SC-104 message structure) is scheduled to be incorporated into the Northwest European Loran System this year. To take advantage of this loran-based DGPS service, users will need to purchase a new Eurofix-capable loran receiver. Right now, however, no such receivers are commercially available.

In the U.S., the Coast Guard had planned to shut off its loran network on December 31, 2000. However, complaints from the marine and aviation user communities and loran equipment manufacturers have been heard on Capitol Hill. Congress requested that the Coast Guard find a way to keep loran going. So, on June 29, 1998, the FAA, the Coast Guard, and the Dept. of Transportation agreed to extend loran system operation to at least 2008, and possibly beyond should funding be available. The Coast Guard reportedly has provisional plans to spend $109 million in fiscal year 2002 for system improvements with an estimated $40 million more required in 2006-2007.

Another method for sending DGPS corrections involves satellites. The FAA is still working on its Wide Area Augmentation System (WAAS) DGPS service. One part of WAAS involves geosynchronous satellites broadcasting DGPS corrections throughout North America and adjacent coastal waters. Obviously, a radiobeacon DGPS receiver wouldn’t be capable of receiving these UHF or SHF frequencies. WAAS is still very much a work in progress, though. Changes in prime contractors and other problems have pushed back WAAS deployment.

There are plenty of other methods of DGPS correction deliverymany FM music stations are engaged in the side business of broadcasting DGPS corrections using an arcane technique called FM subcarrier.

For marine users, corrections broadcast via radiobeacons, or at radiobeacon frequencies, have become the standard approach worldwide. And more locations are being added to the DGPS coverage list all the time. This is great news for those mariners who have invested in a DGPS receiver.

By Ocean Navigator