In 1997 the President’s Commission on Critical Infrastructure Protection, which was charged with examining threats to our national security, recommended an assessment be made of the vulnerability of the U.S. transportation infrastructure if it had to rely on GPS. Subsequently, the DOT was directed to assess the “risks to civilian users of GPS-based systems, with a view to basing decisions on the ultimate architecture of the modernized National Airspace System.” In turn, the DOT assigned this task to its highly respected Volpe Transportation Systems Center in Cambridge, Mass.
The Volpe study, released last month after lengthy internal delays, suggests that sole-means GPS may be much further away than previously believed. “There is a growing awareness within the transportation community that the safety and economic risks associated with loss or degradation of the GPS signal have been underestimated,” the report states, and to ensure optimum safety it advocates the “use of backup systems and procedures to GPS in applications where the consequences of losing GPS are unacceptable.”
The first part of the Volpe Center report, describing specific threats to GPS, was submitted to the DOT in mid-1999, and the complete 113- page report, including techniques to combat these threats, was submitted in the summer last year. However, although the report was unclassified, the DOT held up its release until last month, despite outside requests, including several from former FAA Administrator Langhorne Bond.
DOT and FAA insiders have informed AIN that the delay was caused by the need to edit Volpe’s original text to meet objections filed by the DOD and FAA, the latter on the grounds that its conclusions would undermine aviation industry confidence in the agency’s plan to use GPS as the nation’s sole-means navigation system. But one FAA official went further, telling AIN that “if it weren’t for the likes of Langhorne Bond, you’d never have seen it.”
The DOT’s announcement covering the report’s release stated: “The study notes that GPS is susceptible to unintentional disruption from such causes as atmospheric effects, signal blockage from buildings and interference from communications equipment, as well as to potential deliberate disruption. It contains a number of recommendations to address the possibility of disruption and ensure the safety of the national transportation infrastructure.” The statement appeared to suggest that while GPS had some vulnerabilities, those could be fixed.
Ironically, the report’s release and the DOT’s covering statement were made on September 10. On that day, undoubtedly, the report’s assertion that GPS would become “a tempting target that could be exploited by individuals, groups or countries hostile to the U.S.” would have been regarded as a remote possibility, as would the report’s claim that “the GPS signal is subject to degradation and loss through attacks by hostile interests.” The report also referred to “long-term GPS outages that could be caused by hostile actions far less overt than full-scale war.”
But 24 hours later, on September 11, the possibility of hostile attacks on GPS no longer seemed remote.
GPS Weaknesses
Essentially, the Volpe report examined the full range of GPS vulnerabilities, from natural disturbances to intentional signal disruptions, all of which could potentially cause temporary or longer-term loss of signals. It stated that the most critical GPS deficiencies were its extremely low-powered signals and its single civil frequency
These deficiencies would be corrected in DOD’s GPS III, which will transmit at higher power over two dedicated civil frequencies, which will virtually eliminate natural interference and overcome cheap, pocket-sized jammers. However, it noted that GPS III was not expected to become operational until between 2012 and 2015. Until then, GPS would remain extremely vulnerable to interference, although the report also reviewed various mitigating techniques, primarily antenna concepts, being developed.
It therefore recommended “a comprehensive analysis of GPS backup navigation and precise timing options, including VOR/DME, ILS, loran-C, inertial navigation systems and operating procedures.” In this connection, the report also recommended that the FAA and the U.S. Coast Guard continue their loran-C modernization program, stating, “If it is determined that loran-C has a role in the future navigation mix, the DOT should promptly announce this to encourage the electronics manufacturing community to develop new loran-C technologies.” Presumably, this referred to development work with all-in-view receivers and combined GPS/loran units.
But the report’s concerns were not limited to navigation, and it equally underlined the need for back ups for critical communication systems such as the FAA’s Nexcom digital radios and various aviation datalinks, which rely on GPS for timing synchronization. “This is recognized within the FAA, which is planning a backup system and/or operational procedures to mitigate the consequences of loss of the GPS signal,” the report stated. It also noted that the nation’s power distribution grids, telecommunications and other critical systems currently depend on GPS timing, but pointed out that loran-C previously performed this function, and could therefore also be used as a backup in these applications.
The report also dealt with human factors in the design of GPS avionics, including the lack of standardization of control functions compared with other cockpit equipment, and the lack of indicators warning of possible signal interference. On the first, it noted that “this factor becomes more important during stressful situations such as troubleshooting failing GPS performance during a critical flight phase.” On the second, it cautioned that natural disturbances could be used by saboteurs as a disguise, where users warned in advance of natural disturbances “would be more likely to dismiss observed anomalies as harmless when they may not be harmless.” It mentioned that during government tests, deliberate GPS interference “caused a convoy of helicopters to ignore obvious visual cues and fly off in the direction indicated by an inaccurate GPS receiver.”
Much has been written about the dangers of cheap, easily concealed GPS jammers falling into the wrong hands. With considerable prescience, the report states, “It seems inevitable that in the future some of these disruption systems will be available from international arms merchants. A single device that could disrupt military and civil operations worldwide would be attractive to malicious governments and groups.”
The Volpe report is written almost entirely in nontechnical language, and is available on the Internet at www.navcen.uscg.gov/news/FinalReport-v4.6.pdf.
