Handheld GPS Works Fine, Except in One Particular Aircraft

When I attach it to the yoke and put the antenna on the dash over the panel,
it refuses to get any satellites--- none. Yet it works fine in another 172
and in a Taylorcraft -- and in all the other airplanes I've used it in.
Now why is that? It will work as long as the master and radios are not on
and the engine not running. It's a Trimble Flightmate Pro, ancient but
adequate for me. TIA for satisfaction of my burning curiosity.

" jls" wrote in message
...
When I attach it to the yoke and put the antenna on the dash over the
panel,
it refuses to get any satellites--- none. Yet it works fine in another
172
and in a Taylorcraft -- and in all the other airplanes I've used it in.
Now why is that? It will work as long as the master and radios are not on
and the engine not running. It's a Trimble Flightmate Pro, ancient but
adequate for me. TIA for satisfaction of my burning curiosity.


Something in that 172 is generating a signal strong enough to "jam" the GPS.
I suspect a harmonic of the LO (local oscillator) in one of the receivers in
the aircraft. Did you try tuning the Navs, the Coms and any other receiver
(DME) to a different frequency. That will change the LO frequency and
possibly move the harmonic outside the GPS receiver's input bandwidth. Also
turn the transponder off to see if the LO in the transponder is the culprit.
If that does not work, then turn off one electronic device at a time.....and
narrow it down to the offending device. If you have a split master switch,
you can run for a short time on the aircraft battery to rule out some sort
of high frequency oscillation in the voltage regulator. Good luck hunting.

J. Severyn

"J. Severyn" wrote in message
...

" jls" wrote in message
...
When I attach it to the yoke and put the antenna on the dash over the
panel,
it refuses to get any satellites--- none. Yet it works fine in another
172
and in a Taylorcraft -- and in all the other airplanes I've used it in.
Now why is that? It will work as long as the master and radios are not
on
and the engine not running. It's a Trimble Flightmate Pro, ancient but
adequate for me. TIA for satisfaction of my burning curiosity.


Something in that 172 is generating a signal strong enough to "jam" the
GPS.
I suspect a harmonic of the LO (local oscillator) in one of the receivers
in
the aircraft. Did you try tuning the Navs, the Coms and any other
receiver
(DME) to a different frequency. That will change the LO frequency and
possibly move the harmonic outside the GPS receiver's input bandwidth.
Also
turn the transponder off to see if the LO in the transponder is the
culprit.
If that does not work, then turn off one electronic device at a
time.....and
narrow it down to the offending device. If you have a split master
switch,
you can run for a short time on the aircraft battery to rule out some sort
of high frequency oscillation in the voltage regulator. Good luck
hunting.

J. Severyn


Thanks for the helpful comment. I'll check it out.

There have been cases of an intermediate frequency of transponders causing
this type of interference.

Jerry in NC

" jls" wrote in message
...
When I attach it to the yoke and put the antenna on the dash over the
panel,
it refuses to get any satellites--- none. Yet it works fine in another
172
and in a Taylorcraft -- and in all the other airplanes I've used it in.
Now why is that? It will work as long as the master and radios are not on
and the engine not running. It's a Trimble Flightmate Pro, ancient but
adequate for me. TIA for satisfaction of my burning curiosity.