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Should you trust your GPS?

Nov 9, 2007

To the editor: I will never again trust my GPS chartplotter completely. On a recent voyage, it would have run me aground if I had believed it.

I was taking my sailboat from her Chesapeake Bay homeport to a yacht yard in the West River.  The GPS chartplotter was running and recording the current track. Everything was going well for about an hour as I motored north. After rounding the buoy marking the entrance to the West River, I noticed that the chartplotter display did not seem quite right. Over the next hour, the discrepancy got progressively greater. Just before arriving at my destination and shutting down the instruments, the GPS chartplotter showed me sailing over land about a quarter mile south of my actual position. After an hour at the yard, I turned the instruments on and sailed away, retracing my track down the West River, with the GPS chartplotter working correctly. If I had been depending on GPS on the inbound course, I would have been in serious trouble. Fortunately, I was in familiar waters in clear weather and navigating visually.

Was I in a “GPS dead spot?” GPS does not work in a dead spot due to unintentional interference from a land-based transmitter. Usually, the GPS receiver simply loses its fix and issues an alert that is hard to ignore. Most boaters are aware of these dead spots and they are small enough that they are more of a curiosity than a danger. In any event, the West River is not a dead spot.

Was interference the cause? Interference tests may be conducted at locations and times listed on the U.S. Coast Guard Navigation Center website (www.navcen.uscg.gov). There was a window of possible interference tests at Patuxent River, Md., on the day I observed the problem, with the comment that “GPS navigation signal will be unreliable within an 83-nautical mile radius of Patuxent River at 40,000 feet altitude.”  I had been within 35 miles of Patuxent River and the area of unreliable signal might have extended to sea level.  On the one hand, the Coast Guard told me that there had been no interference testing at Patuxent on the day in question, so that did not explain the problem. On the other hand, it was disquieting to learn that system testing can produce an “unreliable navigation signal.”

Had the GPS receiver and display on my vessel failed temporarily in some way? Possibly, but I simply do not know. I was too busy at the helm to troubleshoot my system or turn on my backup GPS. However, if there was a temporary equipment failure, my system did not give any indication of a problem with the navigation data, so what caused the problem is a moot question.

Had there been a failure in the GPS satellite system? This is a distinct, but not definite, possibility. According to the U.S.C.G. Navigation Center website, one of the satellites was taken out of service a couple hours after the events I observed. It is possible that the anomaly I observed was related to the initial failure of the satellite before the problem was detected and the satellite taken out of service. On the one hand, the Coast Guard told me that nobody else had reported a problem, they monitor satellite health closely, and it is unlikely that a failure would have gone unrecognized for two hours. On the other hand, failure of a satellite at about the time of the anomaly is quite a coincidence.

I do not know what happened. Clearly, something failed in the overall system, my receiver equipment, or the satellites or health monitoring. I would have been at least a quarter mile off course if I had blindly followed the GPS. Worse, the system failed to alert me to the fact that something was wrong. Identifying the cause of the error would be interesting, but irrelevant. If I had not observed the environment visually or by radar, I would not have known there was a problem and I would have been in serious trouble.

The conclusion is that you should not depend entirely on any single piece of equipment, especially a complex system like a GPS chartplotter. When navigation is critical, verify GPS with simpler, more dependable, systems like your eyeballs or radar.

– Philip G. Gallman lives in northern Virginia with his photographer wife, Minnie. They sail on Chesapeake Bay on their sloop Dolly G. Gallman has a Ph.D. in engineering and applied science, both from Yale University, and spent 27 years in military radar and communications systems.

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