Garmin GpsMap 396 - Flight Test

Made it up in the air today. Scattered cells and rain all over the
place. Mostly, moving out and clearing. 30 seconds after take off, lost
the GPS sat lock. Took about 15 seconds to regain it. In the next 5
minutes, it lost lock several more times. I'm thinking, external antenna
is loopy. I hooked up the stubby one and it still failed on and off. I
switched to the satellite page and watched all the levels go wildly up
and down. WAAS signal was up and down, mostly down. My previous 2 GPS
units (one circa 1995) NEVER did this. On one 4 mile stretch, the signal
was down to zero and stayed there. Nothing. Zilch. Turns did not really
affect the performance. After about 15 more minutes of flying, it
settled down to a more "normal" pattern. Several sats were at the top
level and several more were 3/4 up and moving about just a little. Once
settled down, turns made a big difference. As some were lost, others
gained strength.

All during the events, the WX signal kept up fine. When it did lock
(about 50 out of 55 flight minutes), the weather display was EXACTLY
what I wanted to see. Rain intensity and direction. This was the
specific day I would use this unit for.

All looked to be in sync until landing. Again, I watched the sat signals
all march towards zero, however, they quickly recovered before losing lock.

I'm thinking this unit has an internal antenna connection defect. I'll
try it out again in the next few days and see whether it is going back
to the dealer.

Not exactly confidence inspiring. This unit may be a bad one. It
happens, I just wish it were a complete failure instead of this
intermittent crap.

I'll report on its progress.

Mike

"Mike Spera" wrote:

I'm thinking this unit has an internal antenna connection defect.

Or it could be RF interference from some source in the airplane: the
symptoms sound like it. This is exactly what one of my Cessna ARC-385
radios used to do to my portable GPS. Try tuurning off all emitting devices
one at a time the next time it happens. As a last resort, try turning off
the master. You might also try a new location for the remote antenna.

I just wish it were a complete failure instead of this intermittent crap.

Amen. I hate gremlins.

--
Dan
C172RG at BFM

I figured it was the remote or its location, even though my last 2 GPS
antennas were placed there with zero loss of signal - ever.

But, I disconnected the remote and put on the stubby (you have to remove
the power plug - another not so great design idea). It still failed in
exactly the same way. So, a different antenna and different location did
not solve the problem.

I don't have enough data yet to make the case for a DOA exchange. Since
the weather today is blotto, I will have to test tomorrow morning or
during the week.

Thanks,
Mike

I'm thinking this unit has an internal antenna connection defect.


Or it could be RF interference from some source in the airplane: the
symptoms sound like it. This is exactly what one of my Cessna ARC-385
radios used to do to my portable GPS. Try tuurning off all emitting devices
one at a time the next time it happens. As a last resort, try turning off
the master. You might also try a new location for the remote antenna.


I just wish it were a complete failure instead of this intermittent crap.


Amen. I hate gremlins.

"Mike Spera" wrote:

But, I disconnected the remote and put on the stubby (you have to remove
the power plug - another not so great design idea). It still failed in
exactly the same way. So, a different antenna and different location did
not solve the problem.

What was the location of the remote antenna? If it was on the glare shield,
the stub antenna on the yoke wasn't much farther from the radio stack.

I don't have enough data yet to make the case for a DOA exchange. Since the
weather today is blotto, I will have to test tomorrow morning or during the
week.

Yes, it could be an intermittent antenna connection or such in the unit, but
try putting the remote antenna as far back in the airplane as possible and
see what happens. Then try shutting radios, etc. down.

I''ve had no reception problems with my 396 except when using the stub
antenna in the car.

--
Dan
C-172RG at BFM

Has anyone noticed the GPS altitude and what the altimeter says become
increasingly different as altitude increases? (i.e. 10-12k+)?

This difference seems much more pronounced the higher I go. (Yes, I do
have the correct altimeter setting...) The GPS always seems to read
250-500 feet higher.

--Dan



Dan Luke wrote:
"Mike Spera" wrote:

But, I disconnected the remote and put on the stubby (you have to remove
the power plug - another not so great design idea). It still failed in
exactly the same way. So, a different antenna and different location did
not solve the problem.

What was the location of the remote antenna? If it was on the glare shield,
the stub antenna on the yoke wasn't much farther from the radio stack.

I don't have enough data yet to make the case for a DOA exchange. Since the
weather today is blotto, I will have to test tomorrow morning or during the
week.

Yes, it could be an intermittent antenna connection or such in the unit, but
try putting the remote antenna as far back in the airplane as possible and
see what happens. Then try shutting radios, etc. down.

I''ve had no reception problems with my 396 except when using the stub
antenna in the car.

--
Dan
C-172RG at BFM

"Dan" wrote:

Has anyone noticed the GPS altitude and what the altimeter says become
increasingly different as altitude increases? (i.e. 10-12k+)?

This difference seems much more pronounced the higher I go. (Yes, I do
have the correct altimeter setting...) The GPS always seems to read
250-500 feet higher.

Your altimeter is not corrected for temperature. The GPS is not affected.

--
Dan
C172RG at BFM

This would seem to pose a safety risk in IMC, would it not? Ideally,
wouldn't altimeter settings take this into account?

I assume that since all altimeters will behave this way, ATC obviously
expects me to be at, say 10,500 as indicated by my altimeter, even
though that could be 10,900 or so MSL _actual_ altitide..

Seems a bit sub-optimal somehow... even dangerous.

I never thought the effects were so pronounced....

-Dan


Dan Luke wrote:
"Dan" wrote:

Has anyone noticed the GPS altitude and what the altimeter says become
increasingly different as altitude increases? (i.e. 10-12k+)?

This difference seems much more pronounced the higher I go. (Yes, I do
have the correct altimeter setting...) The GPS always seems to read
250-500 feet higher.

Your altimeter is not corrected for temperature. The GPS is not affected.

--
Dan
C172RG at BFM

"Dan" wrote:

This would seem to pose a safety risk in IMC, would it not?

Yes, it could. Extremely low temperatures can be dangerous to low-flying
aircraft because altimeters will read high in such conditions.

Ideally,
wouldn't altimeter settings take this into account?

Yes. Also ideally, I would be flying a new Bonanza.

I assume that since all altimeters will behave this way, ATC obviously
expects me to be at, say 10,500 as indicated by my altimeter, even
though that could be 10,900 or so MSL _actual_ altitide..

Yes.

Seems a bit sub-optimal somehow... even dangerous.

It works fine as long as everyone is using the local altimeter setting.
Watch out near the ground when it's real cold, though.

--
Dan
C172RG at BFM

Mike Spera wrote:
snip
Not exactly confidence inspiring. This unit may be a bad one. It
happens, I just wish it were a complete failure instead of this
intermittent crap.

I'll report on its progress.

Mike

Funny you should mention this. My 396 GPS reception has been flaky
lately too. Normally, the stick antenna would give me 4-5 sats indoors,
which is pretty good, while the external (powered) antenna would lock 8+
sats at higher signal indications in the same location (understandable
since it has a preamp that's powered through the coax).

Shortly after I upgraded to firmware 3.0 or 3.1 (can't remember exactly)
I took a trip to SC and had the external antenna just completely crap
out on me. No sat reception at all. I replaced it with the stick and
that got me three sats...enough for 2D positioning.

I initially attributed it to poor satellite positioning (there are times
when all the sats are behind the aircraft and the aircraft structure
shields the yoke/glareshield area), but it didn't comfort me to realize
that the Garmin 430 in the panel was locked to 10 sats with signals
pegged high.

Just today, after upgrading to 3.2, the problem isn't as bad, but the
external antenna doesn't seem to be doing the same job as it used to.

The release notes for the last several releases mention that they've
been screwing with reception issues for the XM receiver, but I wonder if
they messed something up with the GPS component firmware.

So, who wants to open a case with Garmin? If I can find two seconds to
rub together, I just might...this thing WAS running beautifully in the
2.x firmware days. I'm almost tempted to downgrade and test my theory.

-Doug

--------------------
Doug Vetter, ATP/CFI

http://www.dvatp.com
--------------------

You know, with the reception problems I am reading about here, I think I
will stick with my GPS 195 a while longer. I always get a lock with a
minimum of six to eight sats.