En route and terminal GPS failures simulated

 - April 17, 2008, 11:22 AM

Goers and GOTS describe two critical FAA programs planned for later this year and early next year, respectively. Goers stands for GPS outage en route simulation, while GOTS is similar, but with terminal replacing en route.

Each program will assess how well air traffic controllers can cope with GPS outages in IMC, handling a mix of aircraft with different levels of avionics equipage and with current and future reduced levels of conventional navaids. Goers and GOTS were first conceived in 1999, but received major impetus from the September 10 GPS interference report produced by the Volpe National Transportation Systems Center and from September 11.

Each program will simulate three separate scenarios covering today’s conditions and two future situations. The current scenario posits that all conventional navaids are operating, but with 10 percent of aircraft depending on supplementary GPS navigation. The second scenario, based on the 2013 to 2015 period, assumes that all conventional navaids are still operating but that 80 percent of aircraft will be GPS equipped. The third scenario, which looks beyond 2015, assumes that the 80-percent GPS equipage continues, but that conventional navaids have been reduced by 50 percent.

The results will be used by the FAA to develop procedures to maintain safe separations during such outages. Additionally, the data produced will provide insights toward the planning of GPS backup facilities as conventional ground-based aids are progressively reduced in the future.

The simulations will be performed at the FAA’s Technical Center in Atlantic City, N.J., and will be based on actual flight plans of the current traffic mix in three sectors of the Jacksonville (Fla.) ARTCC. Jacksonville Center personnel will staff the controller positions, while Technical Center pilots will act as “pseudo pilots” responding to controller instructions.

The interference source will jam GPS signals over a radius of 130 nm and will be assumed to be airborne, meaning there will be no low-level “escape” routes, such as would exist below the line-of-sight signals emanating from a ground-based interference source. All aircraft will lose GPS navigation, and IMC will be assumed to exist throughout the test airspace.

Resolution of the GPS outage scenarios is expected to be accomplished by inertial/FMS aircraft reverting to Rnav, VOR-equipped aircraft flying point-to-point between existing VORs and other aircraft depending on radar vectors and ATC instructions to follow dead-reckoning tracks.

To handle the outages effectively, controllers must first determine the approximate boundary of the area affected by interference, which currently can practically be obtained only from pilot reports. Simulation officials already recognize this will be unsatisfactory in a real situation–a large number of “we have just lost GPS” voice reports could clog the channels at the very time they are most urgently needed by controllers for vectoring and other instructions. A dedicated “GPS lost” transponder code would likely soon be created.

One FAA official suggested to AIN that the interference radius of 130 nm chosen for the simulations was too conservative and should be made larger, and that eventually it should include intermittent transmissions, some emanating from a moving source, to simulate more realistic threats. A GPS outage in the Phoenix area last December, caused by an emitter’s being accidentally left on over a weekend at a defense contractor’s facility, created continuous interference out to 180 nm; more recent DOD jamming tests in Alaska extended over a 320-nm radius. And even when an unauthorized emitter is located, the FAA cannot immediately shut it down, since it  turns out that interfering with navaids is a federal offense requiring the coordination of several agencies, including the FBI. As a result, while the culprit at Phoenix was quickly pinpointed, the investigative paperwork took five days to complete. Ninety minutes later, the emitter was off the air. Perhaps predictably, the FAA official was apprehensive about the future length of the bureaucratic process under the new Department of Homeland Security.

In the Atlantic City Goers and GOTS simulations, normal communications and radar systems will remain fully operational during the GPS outage. However, loss of navigation due to GPS interference is not the FAA’s only concern. Agency officials have also conducted internal reviews of the potential effect of hostile acts against ATC’s communications and surveillance capabilities, which, with navigation, form the essential triad of services required for safe traffic control and separation under IMC.

It was concluded that loss of communications would be the least disruptive, since NORDO procedures are already in place and well understood by pilots. On the other hand, surveillance was believed less likely to be compromised, due to the extensive duplication of civil and military radars over much of the continental U.S., coupled with the increasing use of TCAS. (But ADS-B would be adversely affected by the loss of GPS.)

But examination of scenarios involving simultaneous attacks on combinations of any two “legs” of the triad drew one chilling conclusion. Should there be a loss of both communications and navigation facilities under IMC, leaving just the radar operating, then according to one recent FAA document, “controllers would become spectators” of the ensuing events.