Do not rely exclusively on GPS navigation

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I have to take a vigorous exception to Mr. Barry Syrett’s letter, “Sailor says GPS is all he needs” (Issue #114, May/June 2001), in which he argues that one can trust GPS for uninterrupted service, so it can be used for “autopilot” control. I strongly caution new sailors that the GPS system will indeed fail, and fail often enough, if only briefly, to preclude its use guiding an automatic piloting system.

Both of his two fundamental tenets are without merit. Mr. Syrett clearly does not have a thorough-enough knowledge of aviation navigation systems, relying instead on what might be called "popular myth"; and I have to also suspect that Mr. Syrett really has not accumulated many actual hours using GPS. I not only have a 12-channel GPS receiver on my boat, but also another automobile 12-channel GPS that I constantly use in my car (and assiduously transfer to whichever of my cars that I use at any time); so I have as many hours of year-round GPS experience as anyone might have.

First, Mr. Syrett bases much of his arguments on the fact that commercial and general aviation (and, I would add, military application in cruise missiles, etc.) has had such success with GPS; he is obviously not aware that these aviation systems do not solely depend on GPS, but rather interface a number of components, including inertial guidance systems that continue to predict the plane's position in the event of interruptions in continuous GPS signal reception (acting essentially as an electronic dead-reckoning plot). SA-off, DGPS, and WAAS, mentioned by Mr. Syrett, all improve the precision of the GPS positions, but do nothing to improve the reliability of receiving the satellite signals.

Second, my personal experience observing GPS reception a few hours each day of the work week throughout the year, longer periods on vacation trips in a car, and many hours at a time on my boat, assures me that GPS reception is not trustworthy. At least a couple of times, sometimes more often, each week I find that I lose my position along with a warning from my GPS unit - and this occurs in both of my GPS units (different highly regarded brands). These interruptions may last from less than a minute to several minutes in duration, and certainly contraindicates adequate reliability to turn over control of a boat's autopilot to it. Indeed, many of these interruptions can be attributed to disturbances of the atmosphere through which the signals must propagate - and the ionosphere layer is very sensitive to solar disturbances (e.g., flares), which are currently at the "solar maximum" of an 11-year cycle. Thus, frequent GPS interruption can be expected for several years to come before a relative lull for this reason alone, and not including others.

Indeed, if the aforementioned airplanes depended solely on GPS (as Mr. Syrett would have us believe), planes would be raining down on us daily. Sailors should never rely solely on electronic navigation, as they would by turning control of an autopilot to a GPS. Electronic navigation is a wonderful adjunct, but a compass and a chart and good charting practices (including but not limited to electronic information) should be the mainstay of navigation.

If the editors will indulge me just a little more space, I would like to add that I found this May/June issue one of the best ever - full, as always, of invaluable information and advice. I particularly want to praise Capt. Prentice "Skip" Strong, his crew, and his article, "Unlikely salvage vessel recovers NASA barge." After all the articles in this and earlier issues criticizing the "unprofessional" practices of freighters - viz., keeping a proper lookout, etc. - and my opinion of them declining, it was heartening to meet such a responsible and admirable ship's captain. I certainly hope that he is typical, and not the exception.

By Ocean Navigator