Rogue transmitter knocks out GPS signals
- By Bob Brewin
- Apr 12, 1998
An errant Air Force transmitter in upstate New York jammed Global Positioning System satellite navigation signals for two weeks earlier this year, with at least one Continental Airlines flight reporting a total loss of GPS signals and a dozen other GPS-equipped aircraft filing interference reports, according to officials from the Federal Aviation Administration and the Defense and Transportation departments.
Last year the President's Commission on Critical Infrastructure Protection (PCCIP) cited in a report concerns about the vulnerability of the GPS system to jamming by adversaries or terrorists and questioned FAA plans to rely solely on satellite-based aerial navigation and landing systems sometime after 2003. Gary Kosciusko, a staff member of the PCCIP, speaking at a DOT-sponsored GPS conference last week, said the upstate New York incident served as a real-world example "of the kinds of issues we are concerned about'' when relying on GPS as a sole source of navigation.
Developed by DOD at a cost of more than $10 billion, the GPS system provides precise navigational and positioning information from a constellation of 24 satellites, with accuracies within 100 meters for civilian users and within 25 meters or better for military users. DOT and the FAA once believed that with ground- and space-based enhancements GPS could replace all ground-based navigation aids (navaids), and both agencies already have approved GPS as an en route navaid for transoceanic aircraft flights. But the possibility of jamming the weak GPS signals forced a retrenchment from that position.
"Eroding Confidence in GPS"
Heywood Shirer, a DOT official working on development of the 1998 version of the Federal Radionavigation Plan— the navigation blueprint for the FAA, the Coast Guard and other federal agencies— told the DOT Civil GPS Service Interface Committee (CGSIC) last week that there is "eroding confidence in GPS as a sole means of navigation because of vulnerability to interference and jamming.''
GPS jammers already have appeared on the market, according to Kosciusko, who said a Russian company— which he did not identify— displayed a prototype four-watt GPS jammer at last year's Moscow Air Show. That jammer, Kosciusko added, could interfere with GPS signals "out to a range of from 150 to 200 kilometers.''
The jamming incident in New York caused the FAA to declare an interference zone of 300 kilometers, FAA officials said, although they believed the zone was actually more localized than that, centered around the Albany Very High Frequency Omni Range navigation aid.
Continental, which has equipped DC-10s in trans-Atlantic service with GPS receivers, originally believed the Dec. 30, 1997, flight, which lost all GPS signals as it descended south from Albany for a landing in Newark, N.J., had been subjected to deliberate military jamming exercises, FAA officials said.
However, according to Fran Crumm, a spokesman for the Air Force Research Laboratory Information Directorate, Rome, New York, the jamming was inadvertent, resulting from an antenna test gone awry.
In nearby Newport, N.Y., the information directorate has a research facility that tests aircraft antenna emission patterns by mounting airplanes onto pylons, running radio signals through the various antennas. These tests once were done manually but now rely on a computer-controlled radio system. On Dec. 30, the Newport facility started a test of a GPS antenna with a five-watt transmitter, stepping through frequencies with transmissions only milliseconds long adjacent to the civil GPS frequency of 1227.6 MHz.
Instead of turning itself off at the end of this cycle, the computer-controlled transmitter remained on, stuck at the 1227.6 MHz GPS frequency, Crumm said. This error was not discovered until the laboratory saw message traffic that reported a GPS outage in mid-January, he said, at which point the problem was discovered, and the transmitter was turned off.
Jerry Markey, head of the FAA spectrum office, said it was the FAA and not the Air Force that identified the Newport site by dispatching one of its flight-check aircraft from Oklahoma City to check emissions in upstate New York after receiving numerous reports of GPS interference, with the accidental jamming running from Dec. 30 through Jan. 12.
Markey said his office should have been notified of the Newport test under an agreement between DOD and the FAA, which requires such notification.
However, he added, the FAA's Boston center, responsible for air control in upstate New York, was well aware of the GPS interference early on during the two-week outage, based on pilot reports. But "they just sat on it,'' Markey said. He said that at no time did the GPS outage create safety problems, as controllers also were tracking aircraft by radar.
Markey called the incident a "real eye-opener,'' adding that his office will work to improve internal reporting procedures for GPS interference.
He also said the outage showed the importance of the directive that requires DOD to get approval from the Joint Chiefs of Staff before conducting GPS tests.
Crumm said the laboratory did not ask for such clearance because the planned tests were at "low power,'' but he added that in the future all such tests will go through the clearance procedure. The laboratory also will not rely on a computer to shut down test transmissions in the future. "We'll have more human oversight when we run a test,'' Crumm said.
One participant at the CGSIC conference, who declined to be identified, said the accidental jamming may have delivered "a real wake-up call about shutting down all other navaids and relying only on GPS. This proves jamming is easy, and even low-powered transmitters can create interference over a wide area.''