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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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