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GPS
deaf jam


When the Global Positioning System was being developed in the 1970s and early 1980s, one of the system’s advantages, as professed by the U.S. Department of Defense, was its superior resistance to enemy jamming. This was during that forgotten struggle, the Cold War, when the enemy in question, the Soviet Union, was regularly jamming U.S. Voice of America broadcasts to Eastern Europe. In fact, GPS isn’t so resistant to jamming as was originally stated.
If your GPS positions don’t seem to make sense, check to see if your TV set is on. According to the Coast Guard, certain brands of marine TV antennas can interfere with GPS signals. This only occurs with some land-based TV antennas, not satellite TV units.
Tests have shown that GPS can be jammed with relatively simple transmitters. (A functional GPS jamming transmitter was reportedly offered for sale by a Russian company at the 1997 Moscow Air Show.)


by Tim Queeney


Now it turns out that TV-antenna-equipped GPS users might be more concerned about TV broadcasts than jamming from the international communist conspiracy (the big-name players like the Soviet Union and East Germany have become free agents, but China, North Korea, Cuba and Vietnam are still officially on the ICC roster). On Dec. 16, 2002, the U.S. Coast Guard issued a safety notice that marine television antennas may interfere with GPS and cause inaccurate fixes or cause a receiver to lose satellite signals entirely. Maybe your mother was right when she told you too much TV was bad for you. At home, overindulging in TV might turn you into an overweight couch potato, Receive NOW, pay LATERbut on your boat, it seems a TV antenna deafening your GPS could adversely affect your vessel’s health.

The problem arises from certain TV antennas with built-in preamplifiers that were manufactured with a subpar component. Recently, the Federal Communications Commission and the Coast Guard received reports of TV antennas interfering with GPS operation. When the FCC investigated the situation in the lab, it found that certain models of terrestrial broadcast TV antennas had preamps built with electronic elements that squirt out radio-frequency interference under certain conditions. “The problem is with one of the transistors in the preamp,” said Bill Cairns, an electrical engineer at the Office of Spectrum Management at Coast Guard headquarters. According to Cairns, when this preamp element receives TV signals, it goes unstable and resonates, becoming like a miniature transmitter. Like any transmitter, it has a resonant frequency, but it also transmits less-powerful secondary waves, called harmonics, on nearby frequencies. It is one of these harmonics that interferes with or sometimes even blankets the L1 GPS frequency at 1575.42 MHz.

According to Cairns, the problem occurs when the preamp in the TV antenna is powered and receiving TV signals. Should the power to the preamp be disconnected or if you are offshore and out of range of land-based broadcast stations, there should be no problem with your GPS performance. And not all TV antennas with preamps cause the problem.

“Only certain preamps with a particular chip in them,” Cairns said. In its safety notice, the Coast Guard cited models by Tandy, Shakespeare and Radio Shack. For a detailed list of models affected (spelling is correct). The Coast Guard stated that not every model in the list is defective, and the problem may not be limited to the models listed. “We’re not sure how big a problem it is,” Cairns said.

According to Shakespeare, only about 400 units of its SeaWatch model 2040 and 2050 units are suspect. “We’ve determined that there are 150 of the 2040 models that potentially have the problem,” said Don Henry, director of marine products group, “and potentially 250 of the 2050 models.” Readers with these models can check the date codes on their units: The 2040 models with a manufacturing date code of 02A00 stamped on the power supply and 2050 models with 03A00 stamped on the gray control box should be checked. Call Shakespeare at 800-800-9008 for instructions. Shakespeare has offered to repair or replace all the affected units. “If it has that code, we will take care of it,” Henry said. “What we will do is replace the preamp in the antenna or replace the entire unit if we think that is the better solution.”

It should be stated that this is only a problem with a small number of TV antennas that contain the defective chip. Most marine TV antennas do not emit this spurious radiation. And it is also only an issue with land-based broadcast TV — satellite TV antennas don’t suffer from this problem, since they operate on different frequencies and use different electronics packages than land-based broadcast TV antennas.

Another issue raised by this problem is the danger of letting GPS do your thinking for you. Relying uncritically on GPS for navigation could put you, your crew and your boat in possible danger from a wide variety of malfunctions. Certainly, no one who put one of the suspect TV antennas on their boat had any idea of possible interference with their GPS receiver. Yet such an electronic interaction could have been taking place, unknown to the user. If the boat was on autopilot without an alert watchstander, the result could have been serious. Who knows what other types of errors might creep into a GPS position due to the many electronic devices now common on voyaging sailboats?

The safer way to navigate is to use GPS but also do a “truth check” on the position data you are getting by looking at other nav inputs like radar, loran (if you are lucky enough to have one), visual bearings, soundings, a dead-reckoning plot, and simply by looking around once in a while and correlating your position with the chart.




Ocean Navigator Feature Story
Ocean Navigator Feature Story

article (May/June 2003)


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