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Instrument tips: Rude receptionInstrument tips: Rude reception

GPS interference is becoming more common—are you prepared?

When a Cessna 172 pilot flying an RNAV (GPS) approach to an Oregon airport lost GPS reception 2 miles from touchdown, a safe visual landing followed, but the scenario raised a thorny question.
Instrument Tips

Illustration by Jan Feindt

“The missed approach segment is GPS-based so I would have been unable to execute the published missed approach,” the pilot said. “The fact that a failure like this occurred, while hopefully very rare, exposes some potential problems with widely utilized and heavily relied upon GPS navigation for critical phases of flight.”

Once on the ramp, the pilot began checking avionics alerts and found multiple warning messages. Another pilot reported losing GPS while taxiing. “No notams for GPS outages were in effect at the time,” the Cessna pilot added in a filing with NASA’s Aviation Safety Reporting System.

Will such reports remain rare? The Defense Department operates the GPS system, and some GPS outages are planned events associated with security exercises. They are increasing in number and scope, and for several years, AOPA has pressed the FAA to address the potential effect on general aviation and the problem of obscure means of pilot notification.

GPS has become the heart of the NextGen modernization program, engineered into everything from RNAV instrument approaches to Automatic Dependent Surveillance-Broadcast (ADS-B) for traffic separation. With the mandated deadline less than a year away to equip aircraft with ADS-B Out to operate where a transponder is now required, GPS dependence is growing.

Pilots have documented similar experiences via the Aviation Safety Reporting System. When a Boeing flight crew lost both GPS receivers 5 minutes after hearing another aircraft report losing guidance, they sought answers in their quick reference handbook but only reached Step 2 before the procedure for transitioning to an alternate navigation source grew unclear.

That did not stop the captain—who surmised that military activity was blocking the GPS reception—from activating the alternate system.

“The crew needs more [quick reference handbook] information regarding this procedure,” the caption told the Aviation Safety Reporting System.

Back to that Cessna. A less-well-known component of the NextGen transformation is the VOR minimum operational network (MON), composed of “geographically situated VORs in the contiguous United States” needed “to provide coverage at and above 5,000 feet above ground level.”

The MON is designed to get you to an airport within 100 nautical miles to fly an instrument approach “without the necessity of GPS, DME, ADF, or surveillance,” so keep your legacy instrument flying skills sharp for the day the screens go dark.

Dan Namowitz

Dan Namowitz

Associate Editor Web
Associate Editor Web Dan Namowitz has been writing for AOPA in a variety of capacities since 1991. He has been a flight instructor since 1990 and is a 30-year AOPA member.

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