Flew my 172M today and experienced some truly bizarre GPS behavior.

On preflight my portable GPS (Garmin 296), with external antenna and
aircraft power securely attached, initialized just fine, acquired
several satellites and went right into NAV mode. After runup and right
before takeoff, it showed total loss of satellite signals and even
after cycling power several times, would not pick them up again.

With the utmost confidence in my pilotage skills (sarcasm intended) I
took off and hoped to see valid GPS signals in flight. No dice. we
fired up the copilot's Garmin 195 portable, and with a fresh load of
batteries and the external antenna connected tried to pick up a signal.
Once again, nothing. This persisted on both portable units for a good
10 minutes, including a climb up to 4500' MSL.

I tried individually cycling power on the vast avionics array (dual
ancient NARCOs, King DME, pre-Cambrian ADF and King XPNDR) and lo and
behold, when the transponder was turned off and then back on, both
portable GPS units came to life, acquired multiple satellites and
worked perfectly. Looking back, the 296 probably pooped out when the
transponder went on in the runup area.

On the trip back, there was no problem on either portable unit when the
transponder was turned on.

Has anybody ever seen the 296 (or their own portable) act like this??

> wrote:
> Looking back, the 296 probably pooped out when the
> transponder went on in the runup area.
>
> On the trip back, there was no problem on either portable unit when the
> transponder was turned on.
>
> Has anybody ever seen the 296 (or their own portable) act like this??

On a couple of occasions I've had a Garmin portable (a GPS III Pilot first,
later a 296) "flatline" due to interference from a Cessna NAV-Com, but only
during transmission. Both times I swapped out the radio and that fixed the
problem.
--
Dan
C-172RG at BFM

> > wrote:
>>Has anybody ever seen the 296 (or their own portable) act like this??

My Garmin 196, and before it, my Garmin 90 would both crap out when the NAV
radio was tuned to a certain VOR frequency.

My 296 did that the other day also. It was only finding 2 sats. It did this
of and on the whole trip. I have never even and a blip with this unit
before.
> wrote in message ...
> Flew my 172M today and experienced some truly bizarre GPS behavior.
>
> On preflight my portable GPS (Garmin 296), with external antenna and
> aircraft power securely attached, initialized just fine, acquired
> several satellites and went right into NAV mode. After runup and right
> before takeoff, it showed total loss of satellite signals and even
> after cycling power several times, would not pick them up again.
>
> With the utmost confidence in my pilotage skills (sarcasm intended) I
> took off and hoped to see valid GPS signals in flight. No dice. we
> fired up the copilot's Garmin 195 portable, and with a fresh load of
> batteries and the external antenna connected tried to pick up a signal.
> Once again, nothing. This persisted on both portable units for a good
> 10 minutes, including a climb up to 4500' MSL.
>
> I tried individually cycling power on the vast avionics array (dual
> ancient NARCOs, King DME, pre-Cambrian ADF and King XPNDR) and lo and
> behold, when the transponder was turned off and then back on, both
> portable GPS units came to life, acquired multiple satellites and
> worked perfectly. Looking back, the 296 probably pooped out when the
> transponder went on in the runup area.
>
> On the trip back, there was no problem on either portable unit when the
> transponder was turned on.
>
> Has anybody ever seen the 296 (or their own portable) act like this??

I had a Garmin 95 XL that lost signal every time the VOR was tuned to
108.8. Finally figured out that it was the associated DME channel. ie,
when my DME transmitted, a harmonic wiped out GPS reception.

Another wierd one: Apollo (ii Morrow) UPSAT GarminAT (they keep changing
names) GX-60, IFR installation. Whenever I started up on the north ramp at
SFB (Orlando-Sanford), the GPS would not initialize. I had to wait until
airborn, turn it off and back on again, and no problem. Starting up
anywhere else on that airport, or at any other airport, no problem. The
dealer at the north ramp of SFB could never figure the problem. I finally
cured the problem when I stopped using that FBO (for other reasons).

"" > wrote in :

> Flew my 172M today and experienced some truly bizarre GPS behavior.
>
> On preflight my portable GPS (Garmin 296), with external antenna and
> aircraft power securely attached, initialized just fine, acquired
> several satellites and went right into NAV mode. After runup and right
> before takeoff, it showed total loss of satellite signals and even
> after cycling power several times, would not pick them up again.

L. R. Du Broff wrote:
> I had a Garmin 95 XL that lost signal every time the VOR was tuned to
> 108.8. Finally figured out that it was the associated DME channel. ie,
> when my DME transmitted, a harmonic wiped out GPS reception.

108.8 is/was also one of the freqs that would cause my old Garmin 90 and current
Garmin 196 to lose reception. DGB