FAA report outlines GPS transition strategies

 - May 7, 2008, 5:17 AM

After extensive industry consultation, the FAA has recently completed a document outlining its proposed strategy for transition from today’s terrestrial navaids to GPS, including proposed procedures to minimize the effect of GPS jamming. The document, called the “Navigation and Landing Transition Strategy” and recently obtained by AIN, was prepared in response to Transportation Secretary Norman Mineta’s directive earlier this year to various government operating components to submit their individual GPS transition strategies, including their proposed methods to mitigate GPS interference, for inclusion in an overall federal action plan (AIN, April, page 88).

In turn, the action plan was instigated after publication by the DOT’s Volpe Transportation Center on September 10 last year of its report on GPS interference vulnerability, and the events of the following day.

The new strategy contains no real surprises in terms of equipage: it had always been the FAA’s plan eventually to transition from ground- based navaids to satellite navigation and landing guidance, on the grounds of satnav’s better performance and its cost benefits to users and, with it, a gradual reduction in ground-based navaids. And the document confirms the expected FAA position that “sole means” GPS in IMC is ruled out for the foreseeable future, and that backup systems of one type or another will be mandatory for IFR operations.

However, the new document’s language and tone reflect the changed realities of the post-September 11 world. Where occasional GPS interference had previously been regarded by the FAA as a somewhat improbable event, the strategy’s specific focus is now on required goals in the event of GPS disruption. The three main goals cited are safely recovering aircraft; sustaining capacity and efficiency of commercial flight operations; and retaining navigation equipment used by the
DOD for homeland defense. (Tacan, therefore, and its integral DME component, will not be included in any reduction in ground-based navaids.)

The strategy document states that “intentional interference is expected to be targeted at air commerce,” but adds that “if commercial aviation has sufficient backup capability to continue to operate in the presence of interference, the threat is significantly diluted.” That is, if terrorist attempts to jam GPS were not having any obvious effect on aircraft movements, then the incentive to continue jamming the signals could diminish. But it also cautions that “interference to GPS may not have aviation as its only target,” pointing out that while interference attacks may be directed elsewhere, aviation could still be affected.

Reduced Ground-based Navaids

As in previous FAA forecasts, the satnav transition plan is coupled with a reduction in the number of ground-based navaids, since the FAA cannot afford to operate their previous numbers and the substantial cost of supporting civil use of GPS. The main reductions are now seen in VORs, where the current 1,008 stations in the continental U.S. would gradually fall to 471, strategically located to provide uninterrupted coverage at 5,000 ft and above, and thereby allowing flight to continue should GPS be lost. However, VORs in Alaska would all remain in service, as would long-range NDBs in Alaska and the Gulf of Mexico. These navaid reduction strategies are to be evaluated at the FAA Technical Center in Atlantic City, N.J., during simulations of GPS interference scenarios in the terminal and en route areas (AIN, July, page 89).

All DMEs would remain in service to support commercial and corporate aircraft. But the 1,275 ILS installations in the NAS would be gradually cut back as GPS approaches become more commonly used, with the proviso that one system would always remain as a basic backup at each airport currently equipped with ILS. And the nation’s 117 Cat II or Cat III ILS installations would remain operational. (Some FAA officials note that there are now questions about the actual cost benefits of Cat III GPS systems vs retaining Cat III ILS.)

Interestingly, the strategy document devotes an extensive section to loran-C, describing it as “the best theoretical backup to GPS.” Loran is seen potentially as an integrated part of a GPS or GPS/WAAS package, but for it to meet backup requirements it must meet FAA nonprecision approach requirements of RNP-0.3, meaning ±0.3 nm on final, throughout the NAS. Also, the several improvements proposed by the industry that will lead to “enhanced” loran performance must be confirmed.

In March the FAA and Coast Guard established a Loran Integrity Performance Panel, similar to one set up in 2000 for WAAS, and including some of the same experts. The strategy document states that the panel “is 90-percent confident that enhanced loran will be able to provide RNP-0.3 integrity over virtually the entire continental United States and much of Alaska.” (FAA flight tests of enhanced-technology loran units, covered in a separate report, coupled with independent industry tests, indicate that the equipment appears to meet the FAA’s requirements.)

Finally, inadvertent GPS interference remains a serious problem for the FAA, and the strategy report described a large number of incidents where jamming signals were accidentally transmitted over wide areas. Primarily, these originated
at military establishments or contractors’ facilities where equipment was left on–over a weekend, in one instance–after specific tests were completed.

While not discussed in the FAA document, this inattention would be of concern to aviation, even if intentional jamming by U.S. adversaries were not a problem. But one system expert has suggested that it could be simply prevented by the use of low-cost general aviation GPS receivers, modified to sound a simple audio alert to warn test technicians that their interfering source equipment was still radiating signals after their tests were finished.

The DOT GPS action plan is due to be completed later this year.