To counter the possibility of jammed and  “spoofed” signals, the authors of a U.S. Department of Transportation report on GPS vulnerability released on September 10 recommend that GPS not be relied upon as the sole source in critical applications, including precision approaches. As a backup, the authors said that an upgraded loran-C could play a significant part in the nation’s federal radionavigation system, but stressed that a prompt commitment to loran-C is required.

The report, produced by the DOT’s Volpe Transportation System Center in Cambridge, Mass., was the main topic of discussion at a public “outreach” meeting held at the FAA’s Washington headquarters on October 5. The meeting’s objective was to offer a public review of the document, to discuss its implications for future GPS use and to solicit industry comments, which would be assessed alongside studies requested from several government agencies by Transportation Secretary Norman Mineta. The results will be analyzed and presented at a follow-up public outreach meeting, planned for early next year.

GPS Jammers Abound

The review was presented by Volpe’s Dr. Jim Carroll, one of the report’s authors. While Carroll discussed the nondeliberate GPS interference sources, such as ionospheric activity and unintentional transmissions in the GPS frequency band, his main thrust concerned deliberate, hostile disruption of the GPS signals. He noted that the probability of signal loss has heretofore been regarded as low, but stressed that the consequences of such losses are high, both in aviation and in many key terrestrial government and industrial applications that rely on the system’s very accurate timing. Aviation applications include navigation and precise timing for the future VHF Nexcom and various ATC datalinks (such as ADS-B), and to synchronize ground radar networks.

But Carroll stated that since GPS is primarily a military system, there is “a fairly large GPS disruption industry out there.” Jamming techniques are well known and jammers are “fairly easy” to build, from large powerful units to small devices concealed in Coke cans. (Three years ago, AIN attended a DOD briefing showing an airport fence, with the usual windblown garbage in the long grass beside it, including a Coke can. “There,” said the speaker, pointing to the soda can, “is a battery-powered GPS jammer.”)

Carroll also stressed the danger of “spoofing,” where false GPS signals could slowly divert an aircraft off track, undetected by the pilot. This could be hazardous during an approach. Current civil receiver designs cannot counter spoofing and few provide immediate failure warnings, Carroll noted, so he proposed that new certification standards be developed.

Assessing Risk

But since the system’s overall vulnerability “cannot be diminished,” he said an assessment of the risk involved in its use in various applications was necessary. For example, unlike a precision approach, GPS loss is not a safety-of-life issue for hikers or in automobile navigation. For each application it will be necessary to determine the level of risk and assess additional costs required to reach acceptable levels. “The risks are not going to go away,” said Carroll, who proposed that additional pages be added to aircraft operations manuals to ensure that pilots understand and know what to do if signal-loss problems occur.

But in civil aviation, Carroll said, “Safety is number one. Risk can be reduced, as long as we don’t rely on GPS as a sole source in critical applications.” He reviewed candidate backup systems and stated that Volpe researchers believe that the upgraded and “revitalized” loran-C, with its new “all in view” receivers and its potential to broadcast GPS WAAS integrity signals, could play “a useful role in the future systems mix.” But a prompt commitment to continue loran-C was required, since that system could easily be shut down in the relatively near future.