|From Ocean Navigator #88 |
Explanatory notes about loran, like the one above for West Coast chains, aren?t about to disappear from charts if loran supporters succeed in keeping loran on the air.
Meanwhile, loran supporterssuch as the International Loran Association (ILA), Boat/US, the National Air Transportation Association, the Aircraft Owners and Pilots Association, the National Business Aviation Association, and others have been involved in an intense struggle to keep loran alive. “This battle has been waged behind the scenes for the past three years,” said John Beukers, a radionavigation consultant and a member of ILA.
Now, given a positive congressional mandate and some promising technical developments, loran is increasingly being seen as a sturdy, dependable pal to flashy, high-tech GPSa Robin, if you will, to GPS’s Batman.
One example of continued developments in loran technology is the work currently being done in Europe on broadcasting differential GPS corrections via loran (more on this below). The loran method of sending DGPS corrections has several interesting advantages over the present radiobeacon system used by the U.S. Coast Guard. One of the simple advantages is reliability. The loran system was designed to be robust and durable. It has an excellent on-air availability record. For example, unlike many radiobeacons that are located at exposed locations and don’t always have reliable power supplies, loran stations were set up to have reliable power. Loran stations use atomic clocks for precise frequency and timing control. And many loran stations, both in the U.S. and overseas, have been upgraded to modern, low-maintenance transmitters.
Another factor in favor of loran-based DGPS is the extensive range of loran signals compared to radiobeacons. While a 150-mile radius is considered long-range for a radiobeacon, loran signals are usable out to 700 miles or more.
And probably the best reason to use loran this way is the fact that loran can also be used as a stand-alone system for accurate lat/long position fixing (radiobeacon signals are only useful for general homing, but not lat/long fixes). In addition to being an excellent back-up to GPS, a loran fix or a single loran time difference LOP can be used as part of a combined loran/GPS mix. One of the first civil marine GPS receivers in the mid-1980s was the Trimble 10X. The 10X was able to receive both loran and GPS data and could select the best mix of these inputs. Trimble has since discontinued this product.
One of the leaders in researching this loran-based DGPS approach is the Northwest European Loran System (NELS). A combined effort of Denmark, France, Germany, Ireland, Norway, and the Netherlands, NELS is a system of four loran chains that provides coverage for the Bay of Biscay, eastern North Atlantic, Irish Sea, North Sea, Norwegian Sea, and parts of the Baltic Sea and Arctic Ocean. It is composed of four former U.S. Coast Guard stations and Jan Mayen in Norway, Ejde in the Faeroe Islands and Sylt in Germany that were stations in the Coast Guard-operated Norwegian Sea chain, two French stations at Lessay and Soustons, and three new stations, at VÃ¦rlandet and BerlevÃ¥g in Norway and at Loop Head in the Republic of Ireland. The U.S. Coast Guard turned over its four stations to the host nations on December 31, 1994.
Loran chains are attractive to many foreign governments who are leery of putting all their navigational eggs in the GPS basket. Loran is generally seen as a excellent augmentation of and alternative to GPS. It provides a regional system under European control. Loran could prove quite valuable for those crisis situations when the U.S. Defense Department might decide to further degrade or even turn off the civilian GPS signal.
The loran-based Eurofix approach makes small changes in the timing of some of the pulses as its method for sending DGPS corrections. In a loran system, each master station transmits nine pulses and each secondary station transmits eight pulses. The first two pulses from each station are transmitted normally, but the transmission time of the next six pulses is either advanced 10 microseconds, retarded 10 microseconds, or left alone. This scheme provides Eurofix with three “data states” with which to encode corrections. Thus a Eurofix-capable loran receiver will use the first two pulses for determining position and the last six pulses for picking up the 56-bit, RTCM type-9 DGPS correction. The corrections can then be sent via an NMEA 0183 interface to a DGPS receiver for calculating a DGPS position.
To take advantage of the Eurofix DGPS corrections, a mariner would need to either buy a new Eurofix-capable loran receiver or upgrade an existing receiver. This second option requires, of course, that receiver manufacturers offer a Eurofix DGPS upgrade path.
Another interesting benefit of a combined GPS/loran receiver is the use of DGPS positions to “calibrate” the loran and remove some of the errors that loran signals are susceptible to. Used this way, a combined receiver could calibrate loran on the fly, bringing the accuracy of loran down to 20 or 30 meters. Once a loran receiver is calibrated it would retain its high accuracy for hours, even if GPS went off the air.
In 1997, NELS administrators tested Eurofix at the Sylt, Germany, station. According to NELS officials, Eurofix is on track to be implemented at all NELS stations in 1998.
At present, there are no Eurofix-capable receivers available. However, Locus, Inc., of Madison, Wis., a manufacturer of GPS and loran receivers for the aviation market, has announced that it will develop a combined GPS/loran receiver with the ability to use Eurofix DGPS signals.
The Coast Guard’s DGPS system uses medium-frequency (roughly 200 to 300 KHz) radiobeacons. The DGPS corrections are imposed on the radiobeacon signal using a technique called medium shift keying (MSK). These MSK corrections are then removed by a DGPS decoder box, or by internal circuitry if the receiver is DGPS-capable, and used to improve the accuracy of GPS.
Advancing technology has actually made loran more attractive as a radionavigation system. Signal processing capabilities mean that loran receivers are not limited to using a single local loran chain for navigation. The loran receiver announced by Locus, for example, can reportedly track up to 40 loran stations simultaneously. This changes the whole structure of loran coverage. “Loran stations become like satellites on the ground,” said Beukers. “Using loran this way allows you to effectively double the number of satellites available.”These loran developments come after a September 1996 Congressional directive to the Department of Transportation to find a way to keep loran on the air after the year 2000. The technical consulting firm of Booz, Allen & Hamilton was hired by the DOT to conduct a study on the merits of extending the life of loran. According to Ron Davis, task leader of the DOT loran C study, advance findings will be released in January 1998 and the loran report will be completed by March of 1998. One report that helps loran’s cause is the President’s Commission on Critical Infrastructure Protection, which was released in November of 1997. It found that sole reliance on GPS for navigation purposes was unacceptable. The report recommended a back-up system to eliminate future national vulnerability due to over-reliance on satellites. Loran supporters have been quick to point out that loran already provides such a back-up system.
With this level of study and interest, the loran system may well continue to operate after the original cut-off date of 2000. Indeed, in the last two years Congress has approved nearly $8 million in new funding for upgrades and improvement to domestic loran chains.
While the NELS researchers continue to develop their loran-based DGPS Eurofix system, the FAA has its own differential GPS system in the works. Named the Wide Area Augmentation System (WAAS), it is primarily designed to provide DGPS corrections to aircraft via geostationary satellites. Unlike the fairly low data rate of the 100 KHz loran-based Eurofix, the VHF-based WAAS signal would provide high-speed corrections. However, WAAS enhances GPS but it is not an alternative to it. If GPS signals are unavailable, aircraft won’t be able to navigate with WAAS.
“We’re looking at using loran as a back-up system to GPS,” said Bill McPartland, project engineer for loran systems at the FAA. According to McPartland, the FAA had been thinking about using a Eurofix-type loran message for sending differential loran corrections. This would be a way for increasing the accuracy of loran and making it a better back-up for GPS. However, according to McPartland, that could change when the FAA’s Joint Resource Council (JRC) meets in early January. The JRC makes decisions on where the FAA should put its resources. The council may decide to give loran a larger role in future FAA plans.
As the agency with the job of operating loran, the Coast Guard will play a big part in loran’s future. According to Cmdr. Curt Dubay, chief of the radionavigation division at Coast Guard headquarters, the 2000 cut-off date for loran hasn’t changed. “The Coast Guard position remains the same as the DOT position,” said Dubay. “Loran is currently scheduled to be shut off in 2000.” Dubay said that, if that policy is going to change, the preliminary results of the Booz, Allen & Hamilton loran study could well be the first step. “We’re all watching and waiting to see what the study says,” said Dubay. “This study could be the catalyst for changes in the radionavigation plan.”
It seems clear that not all these approaches to DGPSWAAS, marine radiobeacons, and loran-based DGPSare likely to be operated concurrently. Decisions will have to be made as to which system offers the most capability for a given price.
Since loran-based DGPS (like Eurofix) is an approach that will provide a backup navigation service should GPS signals become unavailable, it seems like the best choice for continued development and funding. Loran DGPS is a good use of resources and provides a second source of navigation data.