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Special Report: GPS: Simulating GPS Outages

By Charlotte Adams | December 1, 2001
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The Federal Aviation Administration (FAA) plans to begin simulating the impacts of Global Positioning System (GPS) outages on the U.S. air traffic control (ATC) system next year. FAA intends to simulate jamming events ranging from 30 to 50 minutes in length, but not a protracted, days-long problem or a spoofing (false signal) threat. This is a first step toward determining how to handle and recover from a GPS signal outage in an en-route area feeding into a busy terminal area.

The U.S. Transportation Department’s (DoT’s) recently published assessment of GPS vulnerability highlighted the importance of the test process. According to the report’s authors, after the results of the simulation of a GPS outage over a major piece of airspace–like an entire en-route center–are made available, "a redetermination of the vulnerabilities and risks of the NAS [National Airspace System] can be made."

Evolutionary Plan

Initiation of these human factors studies has been delayed by major changes in FAA thinking on ground-based navaids, funding issues, and uncertainties about future aircraft equipment mix and GPS augmentation systems. There was a simulation plan in 1999, based on rapid decommissioning of ground-based navaids, but this had to be shelved when policy changed.

FAA plans two separate studies over the next two years, assessing the impacts of a GPS outage in sections of a representative en-route area and in a terminal area, respectively. A study on the interplay between the two areas may be necessary but has not been scheduled. If, in the first two simulations, current ATC procedures are found to be inadequate or the controller workload is found to be overburdening, FAA will look at other simulation scenarios to develop mitigations, says an agency program official.

In the post-Sept. 11 context, however, FAA is reassessing its assumptions and may modify the approach. "The plan will be evolutionary," he says, allowing for changing conditions and assumptions.


The agency plans not to run en-route and terminal area simulations back-to-back. "We want to learn from [the first] simulation what the good, the bad and the ugly is to make corrections and apply lessons learned to the terminal [simulation]," the official says. The GPS Outage En-Route Simulation (GOERS) is scheduled for August/September of 2002. Why not sooner? FAA has to reassess its thinking, select the ATC site whose recorded traffic will be used, and obtain simulation lab time. The GPS Outage Terminal Simulation (GOTS) is planned for the same time period in 2003.

GOERS will assess the impacts of a signal loss affecting three area sectors within an Air Route Traffic Control Center (ARTCC) over an area 100 nautical miles in radius. Real, recorded traffic data will be used in the offline exercise. The GOERS project will use a mix of air transport, regional and general aviation (GA) aircraft.

Air traffic control today is not dependent on GPS. "In today’s environment, if you had GPS interference, you’d probably have a minimal impact," the FAA official says. "Except for GA, it would be a no-brainer."

In a GPS-dependent scenario, however, a loss of navigation over 100 nautical miles would be considered severe, the official says. Such an event could require controllers "to provide either radar vectoring or some other type of rerouting capability with conventional navaids." A controller, for example, could divert an aircraft to an airport with a conventional navaid system, i.e., ILS, VOR or NDB.

The GOERS simulation will start with baseline scenarios reflecting current, normal conditions. Later scenarios will introduce various levels of outages and aircraft equipment configurations. Controller workload in the baseline scenarios will be compared with the workload in the subsequent scenarios. The agency has identified four candidate ARTCCs, from which one will be selected. The FAA Technical Center in Atlantic City, N.J., will host the exercise.

As the en-route simulations progress, they will incorporate projected decreases in the numbers of ground-based navaids and increases in aircraft GPS equippage. The exercise will not build in estimates of future traffic volume, although this is subject to change. The effort will try to determine whether controllers, using existing tools and procedures, can handle serious outages.

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