Odd radio - GPS incompatibility

[prathman]

In one of our '06 Vibes we have a Garmin GPS-V on top of the dash and have sometimes experienced poor reception. Today I finally got around to checking what causes the problem. Turns out that GPS reception rapidly deteriorates whenever the stock radio (single CD, no M&T) is tuned to AM stations from 540 to 560 kHz. Surprisingly the reception remains poor even if the radio is turned off as long as it was last tuned to that frequency range. I wonder if the tuner portion of the radio is still powered even when the radio is switched off. When the ignition is turned off the GPS reception remains poor for about 20 seconds before returning to normal - presumably the radio circuits still retain some power for that amount of time. Reception is fine when the radio is set to any frequency in the FM band.

[shemp]

"All portable electronics must be turned off for the duration of the flight as they may interfere with our navigation system"

[ClunkClunk]

As far as I know, GPS uses ~1.1-1.5GHz frequencies, so I wonder what's causing the interference.

[prathman]

Yes, the GPS signals are a little above 1.5 GHz so it's certainly not a direct interference with them. I suspect the problem is somewhere in the intermediate amplifier stages where you get much lower 'beat' frequencies by combining the original signal with an internally generated one. What made the problem hard to pin down was that turning the radio off didn't eliminate it. Apparently the radio tuner circuits remain powered and create the interference.

[northvibe]

yes the vibe radio is always on standby power....so that theory is it could be