On Sept. 10, 2001, the DOT’s Volpe Transportation Center in Cambridge, Mass., warned that the very low-powered signals of the global positioning system (GPS) were vulnerable to both unintentional and deliberate jamming. Tests showed that powerful jammers could overcome GPS signals at ranges of 300 to 400 miles from the jamming source. High-powered jammers are not the only threat: several years ago a small half-mile-range $50 “truckers’ special” jammer blocked FAA GPS landing guidance tests for more than a year at Newark Airport before being traced and confiscated. Those jammers are now illegal.
To counter the GPS jamming threat, the FAA, DOD and several other government and industry specialists established a Position, Navigation and Timing (PNT) committee to assess countermeasures against GPS jamming and “spoofing,” where more powerful, but false, signals are transmitted. The agency released its finding on the subject at its PNT Symposium last October.
Various PNT solutions to the loss of GPS have been studied over the past several years, involving combinations of VOR, DME, radar and certain ADS-B features (while recognizing that ADS-B positions are themselves GPS-derived) but none has offered sufficient cost/benefits to justify national adoption. A total upgrade to the earlier Loran-C, named eLoran, was also proposed but not seriously considered. In fact, while eLoran worked well, many felt that continuing to describe a new system as a Loran (which simply stands for long-range navigation) inevitably suggested a cleaned-up version of its aviation Loran-C predecessor, an occasionally quirky performer.
The British have since given eLoran the respectability it needed. The English Channel between England and France has been described as the world’s busiest seaspace. Just 22 miles at its narrowest, and handling 24-hour two-way traffic flows from supertankers and giant container ships to private yachts and power boats, the channel is rigorously monitored and controlled, and GPS is mandatory. But as large vessel collisions would be disastrous, and GPS interference or jamming would be intolerable, the Brits have installed eLoran stations around their coast.
At press time AIN learned that the Army Contracting Command has issued a Request for Information (RFI) covering eLoran receivers. RFIs are common contracting precursors, and are neither contracts nor formal requests for price and delivery quotations. Their purpose is to determine what industry can provide, either currently or potentially, along with their individual products’ characteristics versus those specified by the RFI.
The RFI “provides an outline for the potential use of eLoran stand-alone and integrated (with GPS) receivers in Army and other Department of Defense (DOD) maritime, aviation and/or vehicular platforms and for position and timing purposes…The Army is interested in leveraging recent technology developments in the industry mainly for adding eLoran capabilities into Army Assured Positioning, Navigation and Timing (APNT) solutions.”
Besides technical details, the RFI requests rough order of magnitude per unit receiver costs, assuming a 50,000-unit order.
Accurate Enough?
Differential eLoran tests by Dutch hydrographers at Rotterdam produced +/-5 meter lateral accuracies, versus +/-2 meters from GPS/Waas/SBAS in LPV applications. GPS receivers look for the four best positioned satellites of all those visible to derive their location. eLoran receivers look for the best combination of signals from the eLoran ground transmitter network. Positives: eLoran is essentially unjammable and offers a range of as much as 1,000 miles. Negatives: there are no airborne eLoran receivers or operational NAS ground stations–yet.
Will that change? At the October PNT Symposium, the Space-Based PNT National Executive Steering Group initiated a Complementary PNT “Tiger Team,” its prime purpose being to “Re-explore eLoran as a back-up GPS technology.” The Tiger Team was to brief the PNT Executive Committee on December, but its report had not been released at the time this was written. o
FAA Findings on GPS Jamming
• The FAA is dependent on GPS for navigation, surveillance and network/infrastructure timing.
• The majority of NextGen benefits currently rely on GPS and this reliance will increase in the future.
• GPS relies on a puny signal and is therefore vulnerable to unintentional or intentional interference. The GPS signal is almost a billion times weaker than other navigation signals (DME, VOR, ILS and so on).
• Inexpensive jamming and spoofing equipment and user tactics are widely available on the Internet.