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Someone at the marina told me that there's a warning out about TV antennas messing up GPS. Please explain. B.T., via e-mail
That someone is correct. Late last fall the U.S. Coast Guard and Federal Communications Commission issued a joint warning concerning certain models of active TV antennas, the kind that look like big Frisbees and are designed to pull in regular TV signals as opposed to satellite ones. The problem is a bad transistor that got into some of the amplifier sub assemblies used by Tandy, Radio Shack, and Shakespeare. In certain conditions, when actually amplifying a TV signal, this part goes a little wacky and becomes a miniature transmitter on the GPS frequency.

Fairly few antenna units were manufactured with the faulty piece (Shakespeare estimates that only 400 left its factory), but if you have an active TV antenna, you should definitely check it out. For starters, turn on your GPS at the dock with the TV antenna turned off, then turn on the antenna, tune to a TV station, and see if your position changes or gets lost. You might also go to the U. S. Coast Guard safety site (www.uscgboating.org) to get a list of affected models. If you have one, you'll find that the manufacturer has a program to further identify and replace defective units.

Overall, this is not a huge problem, but there is an aspect to it that all navigators should consider. The authorities have documented cases when the faulty antenna didn't just jam the GPS signal but actually caused false positions and even affected GPSs on other boats within 2,000 feet of the bad transistor. This confirms the fears of many, including some observations I made in my April 2002 column called "The Dark Side of GPS." As marvelous and 99.9 percent reliable as GPS is, it is always a good idea to use independent means--radar, Loran, soundings, visual bearings, etc.--to double-check your position.

Got a marine electronics question? Write to Electronics Q&A, Power & Motoryacht, 260 Madison Ave., 8th Fl., New York, NY 10016. Fax: (917) 256-2282. e-mail: PMYElectronics@primediamags.com. No phone calls please.

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