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GPS vulnerability and the future of loran

Jan 1, 2003

Many mariners undoubtedly consider loran (a contraction for long-range navigation) an obsolete system ready for shutdown. However, loran's future seemed a little brighter on Sept. 10, 2001, when the Volpe National Transportation Systems Center in Cambridge, Mass., released a report that detailed the vulnerabilities of relying solely on GPS as a positioning source. This vulnerability seemed to be savagely underscored the next day with the atrocities in New York and Washington, D.C.

The study detailed the various disruptions to GPS that can make it unreliable or unusable. These range from unintentional disruption, like atmospheric effects and interference from communications devices, to intentional disruption by terrorists, via jamming, spoofing or attacking GPS system monitors or the control center itself. The concept of terrorists disrupting GPS might have seemed excessively paranoid before the attacks of Sept. 11. After those events, however, protecting GPS, primarily the network of ground monitors and the GPS control center in Colorado, has to be an important priority for the federal government.

To be fair, short of outright attack, GPS outages are a rare event, and boats are far less vulnerable to GPS loss than aircraft. However, relying wholly on GPS for your navigation needs might not be wise. Should the GPS signal be lost, you might find yourself in a tight situation. For example, a report published in the Aug. 29, 2001, edition of Aviation Daily told of several jetliners crossing the Atlantic July 28, 2000, which, according to their GPS receivers, were roughly 108 nm off course. One of the GPS satellites, PRN22, suffered an onboard clock failure and was providing erroneous nav data. According to the Aviation Daily report, PRN22 was broadcasting bad data for upwards of an hour before the GPS control center uplinked a message to the satellite and set its status to non-useable.

The thrust of the report is that putting all your navigational eggs in one basket isn't the best approach, especially since a secondary radio-navigation system already exists in North American waters: loran. The national loran system recently went through an expansion and modernization, making it an up to date and very effective system for both marine and aircraft navigation. The national loran system is in place, it's operating and it's accurate. For mariners, GPS and loran make an excellent combination, with one system complementing the other. GPS, for example, has high absolute accuracy with worldwide availability, while loran has excellent repeatable accuracy and full coverage of U.S. waters.

Loran, as it now stands, is a highly capable radio-navigation system, but it can be made even more effective using current technology. Loran receivers were originally designed to use only the master and secondary stations from a single loran chain. The next line of development was the idea of allowing receivers to use sets of stations from different chains at the same time. The latest thinking in loran is to synchronize all stations, master and secondary, to a single, absolute time reference and allow receivers to dispense with chains altogether and use signals from all the stations it can receive at any given time to calculate an even more accurate position. This improvement should make loran even more valuable to mariners and aviators.

Even though loran is an operational system that has recently been expanded and modernized, interested mariners may have a hard time purchasing a loran receiver. And there are currently no combined GPS/loran receivers on the market. Loran supporters, like the International Loran Association, have stated that without a firm commitment by the Department of Transportation (DOT) for the continued operation of loran, marine electronics manufacturers aren't going to make the capital investment to produce combination GPS/loran receivers.

The Volpe Center study seemed to agree with this assessment when it recommended continuing "the Loran-C modernization program of the FAA and the Coast Guard until it is determined whether Loran-C has a role as a GPS backup system. If it is determined that Loran-C has a role in the future navigation mix, DOT should promptly announce this to encourage the electronics manufacturing community to develop new Loran-C technologies."


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