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.
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.
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).
GPS system cannot serve as a sole source for position location or precision
timing for certain critical applications,” it states.
for positioning and precision timing are necessary for all GPS applications
involving the potential for life-threatening situations or major economic or
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.”
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.
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
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.
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”.
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.
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”.
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”.
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
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”.
“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
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”.