GPS jamming still a major concern

Aviation International News » March 2002
July 14, 2008, 7:47 AM

The DOT Volpe Center’s September 10 report on the vulnerability of GPS to jamming and other interference, in addition to the events of the following day, have greatly heightened national concerns about the security of the satellite system and the degree of dependence that should be placed on it as the backbone of our future ATC system. These concerns were reflected in several of the presentations and much of the discussion at the annual technical meeting of the U.S. Institute of Navigation (ION) in San Diego in January.

Since September 11, military and civil government officials, including the FAA, have been conducting internal, but classified, assessments of the extent to which short- or long- term non-availability of GPS would affect their operations. ION attendees had hoped that a DOT presentation scheduled for the meeting would shed some light on these assessments, but Heywood Shirer, DOT radionavigation program manager, was only able to tell them that while an action plan for civil GPS users had been prepared in December, it had still not been signed off in late January by Transportation Secretary Norman Mineta.

This was disturbing to those who had attended an October DOT Outreach meeting on the subject, at which a public review meeting was promised for early this year, only to have it canceled without explanation. While Shirer’s discretion was understandable, one senior FAA official told AIN that “sole-means GPS is no longer in our vocabulary.”

Some civil organizations have also been vocal in demanding a review of the government’s GPS policy. One of these, the newly formed Heritage Foundation, whose members include a cross section of top-level industrialists, and former high- ranking politicians and military officers, issued in December a detailed manifesto–from its own homeland security task force–that called upon President Bush to enact seven specific priorities to ensure protection of the nation’s infrastructure.

While the group’s top priority was to require the reorganization of affected federal agencies, its second priority was to “designate the GPS frequencies and network as a critical national infrastructure.” In reviewing the system’s vulnerability and the threats it faces, the foundation called upon the President to assign lead agency status to the Department of Defense, and require the agency then to “accelerate modification of current-production satellites to include more robust signals,” and to launch satellites at an increased rate “to augment the fragile constellation currently in operation.”

The DOD may have been listening. At the ION meeting DOD spokesmen described the new GPS modernization program, which next year will begin to launch satellites with greater signal power, making them harder to jam. But the modernized satellites will have significantly reduced orbital lives, according to John Clark of The Aerospace Corp.

As for the “fragile constellation,” DOD Lt. Col. John Anton noted that of the 28 GPS satellites currently in orbit, 18 are beyond their expected design life, with 12 being “one component away from nav mission failure,” and 10 being “one component away from bus failure.” However, while allowing that GPS is “an aging constellation,” Anton assured attendees that such “single thread” operation from satellites that were originally launched with triple-redundant electronics is not really as dire as it seems. The component failure-mode process is well understood, he said, and with four spare satellites in orbit (24 are optimum for civil use, while 21 is the military’s minimum), close monitoring at the USAF’s GPS Master Control Center forestalls surprise failures.

More civil-user-community involvement was the thrust of another DOD presentation at ION, but while civil participation could be detected here and there within the acronym-rich military organization charts presented, attendees could not avoid reading a key statement in bold type that “the DOD’s processes can accommodate civil involvement.” After the presentation, several commented that they would have been more comforted to have seen “will” rather than “can” in the statement.

GPS III is slowly making its way toward the heavens, with its first launch now forecast for 2009–a four-year slippage from earlier estimates. With higher power, superior anti-jamming capabilities, more satellites in different (but still undefined) orbits, and the ability to direct even higher-power beams at enemy targets, GPS III promises to be the military’s 800-lb gorilla in future conflicts.

While the higher power will help civil users, its more important feature will be its transmission of a second civil frequency, called L5, that will virtually eliminate ionospheric interference and produce consistently high accuracy. More precisely, L5 is actually the third civil GPS frequency, since a second frequency, L2, will be initially available from earlier satellites by 2005. But L2 will be outside the band “protected” for civil aviation, and its use will therefore not be approved by the FAA. It is also important to note that the replacement of earlier satellites by L5 satellites, to provide advanced worldwide capabilities throughout the complete constellation, will take many years. As a result, avionics manufacturers are far from announcing civil unit production plans.

Finally, the future of the FAA’s wide-area augmentation system was often debated during the meeting’s coffee breaks. Since its original raison d’etre of providing nationwide Cat I approach guidance appears to be drifting farther away in time, mainly due to delays in providing a satisfactorily placed GEO satellite to cover the central U.S., many observers believe its days may be numbered as potentially less costly alternatives, such as improved satellites and even loran, are starting to attract attention.

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