Long-awaited DOT report blasts sole-source GPS navigation

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US civil aviation should not rely on the Global Positioning System (GPS) as a sole-source means of navigation, a long-awaited report from the Department of Transportation’s Volpe research center concludes.

The Volpe National Transportation Systems Center in Cambridge, Massachusetts was asked by the President’s Commission on Critical Infrastructure Protection in October 1997 to assess the implications of GPS signal interference and disruption for civilian users.

The commission also asked Volpe to assess how GPS signal vulnerability should affect federal decisions on the mix of radionavigation systems and the final architecture for the modernization of the US National Airspace System (NAS).

“The GPS system cannot serve as a sole source for position location or precision timing for certain critical applications,” it states.

“Backups for positioning and precision timing are necessary for all GPS applications involving the potential for life-threatening situations or major economic or environmental impacts.

“The backup options involve some combination of: 1) terrestrial or space-based navigation and precision timing systems; 2) onboard vehicle/vessel systems; and 3) operating procedures. Precision timing backups including cesium clocks or Loran-C for long-term equivalent performance, or rubidium or quartz clocks.”

The Volpe Center presented its original draft to the DOT back in the summer of 2000, but the DOT has kept a tight lid on its contents until now. In recent months accusations have flown from high-profile critics such as former FAA Administrator Langhorne Bond – a skeptic of GPS sole-source navigation – that the DOT and FAA were covering up the report.

In a recent speech to an Air Traffic Control Association conference in Dublin, Bond publicly repeated rumors the two agencies were trying to pressure the Volpe Center to redraft portions that would question the FAA’s safety oversight performance.

Much of the Volpe report was said to call into question the FAA’s entire research and development and capital spending effort on new radionavigation systems, aimed as they are at relying exclusively on GPS for their positional signals.

But in the report’s executive summary the only possible indication that the Volpe Center might have come under pressure from the DOT and FAA to change its draft comes in the recommendation that maintenance of safety when GPS is lost “may not necessarily require a backup system for every application”.

“Requiring a GPS backup will involve considerable government and user expense”, it notes, adding: “The transportation community [should] determine the level of risk of each critical application is exposed to [and] what level of risk each application can accept”, as well as the associated costs.

Volpe’s report finds that even when the GPS signal is augmented by systems such as the local area augmentation system (LAAS), “use of GPS can still be disrupted and transportation services thus impaired”.

Disruption can be avoided “by awareness, planning and supplementing GPS with a backup system or operational procedures when it is used in critical applications”, which the report describes as those “in which the consequences of GPS loss could be catastrophic without ensuring that mitigating options are available”.

In addition to being “susceptible to unintentional disruptions”, the report warns: “The GPS signal is subject to degradation and loss through attacks by hostile interests.” Potential attacks “cover the range from jamming and spoofing (falsifying) of GPS signals to disruption of GPS ground stations and satellites”.

As a result, the Volpe Center recommends the DOT “encourage the development of affordable vehicle-based backups such as GPS/inertial receivers, and, in the event Loran-C becomes a viable backup to GPS, aviation-certifiable Loran-C receivers and GPS/Loran-C receivers”.

Furthermore, “all GPS receivers in critical applications must provide a timely warning when GPS positioning and timing signals are degraded or lost”. The FAA should conduct “a comprehensive analysis of GPS backup navigation and precise timing options including VOR/DME, ILS, Loran-C, inertial navigation systems and operating procedures”.

Volpe recommends the FAA and US Coast Guard continue their modernization program for Loran-C – a system the FAA had hoped to abandon – until it is determined whether Loran-C can have a role as a GPS backup system. If the FAA finds that it does, the agency should promptly say so publicly “to encourage the electronics manufacturing community to develop new Loran-C technologies”.