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RE: GPS INTERFERENCE FROM RADIOS

On a recent flight to Brantford, Ont. We lost satellite connections for both GPSs from just south of Shelbourne to almost Brantford.We have experienced this in the same general area (northwest and west of Toronto) on two previous occasions. It really is the only area where we have had a problem.Why? Has anyone else found the same experience? I was using two GPSs; Garmin 196 and 296.

GORDON RIDDLE

ed: Portable GPS systems can be subject to interference from certain aircraft communication radios on some frequencies in the VHF spectrum.

Pat Field from Brant Aero www.brantaero.com advises that simply selecting a frequency on the nav/com can swamp the GPS signal on many portable GPSs. He also says that older nav/coms generate frequencies and harmonics of those internally that are strong enough to emit a signal externally that can interfere with the GPS and that when certifying GPS for IFR use, there are at least a dozen frequencies that have to be tested - lower and upper end of the VHF band, in order to make sure the GPS does not lose too much signal.Gordon was communicating with Toronto Terminal on 119.3 at the time of his GPS coverage loss, so it could be that his Narco Mark 12D radio was the culprit. Several GPS user’s manuals contain cautions about interference but they are nonspecific, except to say that if you experience difficulties, try moving the GPS further away from the radio.

I experienced the same loss of GPS coverage in an aircraft with a Narco 12D and Garmin 295 when the suction cup antenna was used and I was broadcasting on a lowend VHF frequency. The antenna was located in the centre of the windscreen, just above the Narco radio. Moving the antenna to the right side of the cockpit solved the problem.

Gordon’s interference experience points to the need to use portable GPS receivers as a supplemental device only. Keeping a map handy and/or following VOR airways can minimize issues when sudden GPS outages occur. If you experience this with your GPS, try moving the GPS antenna or installing an external antenna.

— KEVIN PSUTKA

 

RE: NORDO and RONLY

I have to disagree with Mario Gasparovic’s letter published in the October 2010 COPA Flight on his views on inclusion of regulations for NORDO and RONLY. I agree with him that all pilots should carry a transceiver on board and use it. In my experience most pilots do have built-in transceivers and those that don’t carry portable transceivers.However, just because you have a transceiver on board does not preclude the fact that power can fail leaving your transceiver useless. I have been in a situation on a long cross-country returning from the U. S. where the transmitter failed even though I could still receive.

Regulations specifying the procedures for flying without a means of radio communication or in a situation where you can only receive are absolutely necessary. Stuff happens.

HUBERT WREN