From Boat U.S. Magazine
November 2002

 

GPS Interference Warning

Most GPS receivers are normally so reliable and accurate that skippers take for granted their signals are dead on. However, the U. S. Coast Guard has recently issued A Local Notice to Mariners about potential disruption of GPS signals from some portable televisions. This occurswhen a GPS receiver inadvertently becomes a transmitter and radiates a signal that interferes with reception of the GPS signal.

The interference involves a number of television antennas often used on boats. (See list at bottom.) The offending receiving antennas contain signal amplifiers used to strengthen weak TV signals. Due to an undetected design flaw in the amplifier, a strong TV signal may make the amplifier oscillate, and, acting as a transmitter, radiate an unwanted signal into the surrounding area. The inadvertent transmission can be very weak but still strong enough to interfere with GPS operation.

The problem may originate right on board or on a boat some distance away. If anything out of the ordinary appears on the GPS display, treat the position data with suspician, advises electronics expert Chuck Husick. If you have a TV antenna with an internal amplifier on your boat, the first troubleshooting step is to remove all power from the antenna by disconnecting the coax cable.

However, even in close proximity to a powerful TV transmitter, enough energy may be present at the antenna to cause interference even with the power removed. One way to check is to move the boat a mile or two while observing the GPS for a return to normal operation. Another option is to remove the offending antenna from your boat.

Reports of electronic interference show how unwise it can be to rely upon a single source of navigation information. A properly corrected magnetic compass, an up-to-date chart and remaining aware of your navigation situation can save the day when electrons start lying to you.

According to the Coast Guard, the television antennas suspected of causing interference to GPS receivers are:

• TDP (Tandy) Electronics MINI STATE Electronic Amplified UHF/VHF TV antenna Model No. 5MS740

• Radio Shack Corp. Long Range Amplified Omni Directional TV Antenna, Model No. 1501624

•TDP Electronics Model No. 5MS921 (looks like Radio Shack 15-1624)

• Shakespeare Corporation Sea Watch Model No. 2050

• TDP Electronics Model No. 5MS 750

• Thompson RCA (not confirmed, but TDP sells to them)

Note: Questions or concerns may be directed to:
Antennacraft Customer Care
PO Box 1005, Burlington, IA 52601
Phone 800-553-2377
(Updated 19 July 2006)

Read More:
USCG Public Affairs Press Release - "MARINE TELEVISION ANTENNAS MAY INTERFERE WITH NAVIGATION SYSTEMS" ( 02 Jan 03)
USCG SAFETY ALERT - "SAFETY ALERT - TELEVISION ANTENNAE INTERFERENCE WITH GPS" ( 16 Dec 02)