Garmin GTN 650 dropped (losing) satellites

[- Poindexter -]

2014 RV7a with (2) G3X’s, GTN 650 & GMC 307.
Worked great the last (3) years I’ve owned the plane.
And, now this…
On the GTN 650 I’m getting (all) dropped satellites, which starts at about 10-15 minutes into the flight, or about the time I level off and lean out, which is typically 8K to 14‘K feet. The dropped satellite cycles back on/off for another 5-10 minutes, and then remains in LOI (dropped mode) for awhile. The satellites come all back on when I descend, and all’s back to normal. Important to know that even when all the satellites are dropped, the other NAV and autopilot functions are normal (ie - I can fly it in Heading mode and use the altitude functions). And, comm, lights have no impact.

And, yes, the antenna (I think it’s an GA 35) is installed in the FWF, on a top rack.
The wiring is not loose, nor the the unit in the rack. The coax is tight, but I have not checked the FWF connection yet.

I suspect the antenna is fault, and needs replacement. I think it’s kicking off when it gets hot on the climb and resurrects when things cool on the descent. Wanted to see if other folks have had issues with failed GA 35’s, especially in the FWF area, before I buy a replacement. And, yes, I know the antenna FWF install is not ideal.

Thanks,
Paul

NOTE: See Post #26 if you want the results/solution. If you enjoy the “classic” VAF “never-ending-popcorn-debate,” continue reading.

 

[Desert Rat]

Hey paul. If you confirm that you need a new antenna, I think I have one on the shelf I’d probably sell.

Shoot me a pm if you want to discuss at some point

 

[Avanza]

2014 RV7a with (2) G3X’s, GTN 650 & GMC 307.
Worked great the last (3) years I’ve owned the plane.
And, now this…
On the GTN 650 I’m getting (all) dropped satellites, which starts at about 10-15 minutes into the flight, or about the time I level off and lean out, which is typically 8K to 14‘K feet. The dropped satellite cycles back on/off for another 5-10 minutes, and then remains in LOI (dropped mode) for awhile. The satellites come all back on when I descend, and all’s back to normal. Important to know that even when all the satellites are dropped, the other NAV and autopilot functions are normal (ie - I can fly it in Heading mode and use the altitude functions). And, comm, lights have no impact.

And, yes, the antenna (I think it’s an GA 35) is installed in the FWF, on a top rack.
The wiring is not loose, nor the the unit in the rack. The coax is tight, but I have not checked the FWF connection yet.

I suspect the antenna is fault, and needs replacement. I think it’s kicking off when it gets hot on the climb and resurrects when things cool on the descent. Wanted to see if other folks have had issues with failed GA 35’s, especially in the FWF area, before I buy a replacement. And, yes, I know the antenna FWF install is not ideal.

Thanks,
Paul

I have a GTN650, a notch filter solved my problem. It appeard that transmitting on certain frequencys triggerd the LOI and loss of all sattelites.
It took several minutes to recover to normal mode.
Aircrft Spruce Part# 11-02204.
/The TED 4-70 notch filter is a simple low cost way to clean up the receivers frequency environment. The TED 4-70 notch filter installs quickly and easily via standard BNC bayonet couplings, in-line between radios and antenna. The 4-70 rejects in excess of 50dB of 1575.42 signal at the source. Each filter is individually tuned and checked to assure maximum performance. Of course, the 4-70 also passes strenuous environmental criteria so important in aircraft applications. Major GPS manufacturers recognize this problem and recommend using the 4-70 on new and retrofit installation./

Good luck

 

[Richard Connell]

I have a GTN650, a notch filter solved my problem. It appeard that transmitting on certain frequencys triggerd the LOI and loss of all sattelites.
It took several minutes to recover to normal mode.
Aircrft Spruce Part# 11-02204.
/The TED 4-70 notch filter is a simple low cost way to clean up the receivers frequency environment. The TED 4-70 notch filter installs quickly and easily via standard BNC bayonet couplings, in-line between radios and antenna. The 4-70 rejects in excess of 50dB of 1575.42 signal at the source. Each filter is individually tuned and checked to assure maximum performance. Of course, the 4-70 also passes strenuous environmental criteria so important in aircraft applications. Major GPS manufacturers recognize this problem and recommend using the 4-70 on new and retrofit installation./

Good luck

My understanding is that the notch filter is for other offending Coms. I don’t know about GTNs but for GTNXi Garmin specifically states its own Com does not interfere (I have a G approved combined COM/GPS antenna) and therefore does not require a NF but there may still be other offending coms or tx that do. The install manual will have details. Also it’s easy to check by following the interference test procedure in the IM. It only takes a few minutes.

It sounds to me more likely that it’s a faulty antenna for some reason or another otherwise other com interference would have shown up in preceding years. It could also be a connector issue too. 10y sitting in that environment could easily have compromised the connector. Poor wire support etc.

I know this is contentious…. But…It gets **** hot up at the top of a cowl and I’m sure there’s good reasons G says not to put them there. I know lots of people swear it’s fine and I get that. But these are the sort of problems that may well not show up for many many years or hours in service as the fleet ages.
Guys who say yeah I’m out of phase 1 (kidding but you get the idea) and it’s fine may not have all the data. There’s also plenty of people that will attest to sub par performance as a minimum.

Maybe it’s a good time to start a new poll or ask the question like Paul says.
Even external antennas aren’t immune to environmental degradation.
FWIW 1xGA35 external here with 16y ~1700h on it and one with 2y on it. No issues.

 

[- Poindexter -]

First, thank you for the replies. I’ll run the interference test.
I flew XC this morning for 1-1/2 hours at 3,500’ and there were no dropped satellite. And, the weather was crappy (overcast, MVFR).
interesting that it only drops the satellites when I’m at higher altitudes (above 6K’).

 

[Walt]

Electronics don't like heat; I think everyone knows that.
So, if you mount them FWF you're just asking for trouble, (not to mention the increased attenuation and reduced view of the sky).
Mounting something so important in such a harsh environment is foolish IMO.
'Pretty' vs 'functional'.

 

[9GT]

Electronics don't like heat; I think everyone knows that.
So, if you mount them FWF you're just asking for trouble, (not to mention the increased attenuation and reduced view of the sky).
Mounting something so important in such a harsh environment is foolish IMO.
'Pretty' vs 'functional'.

I originally made a nice shelf under my -9A cowling and had my 650 antenna mounted there along with 3 of the tiny Garmin puck antennas. I read so many posts and PIREP's here on the forum of builders that had great success doing it this way. After it was done, every time I looked at it common sense kept nagging me that this is a bad idea. After a couple months of this, I tore it out. The 650 antenna is now mounted on the fuselage aft of the slider and the puck antennas industrial velcro mounted on the glare shield.

 

[PilotjohnS]

My 9a’s GNX375 antenna (same as the 650 antenna) is mounted just aft of the rear canopy. It turns out the antenna is thin enough that the slider slides over it. This location provides easy access from the baggage area

 

[Carl Froehlich]

My understanding is that the notch filter is for other offending Coms. I don’t know about GTNs but for GTNXi Garmin specifically states its own Com does not interfere (I have a G approved combined COM/GPS antenna) and therefore does not require a NF but there may still be other offending coms or tx that do. The install manual will have details. Also it’s easy to check by following the interference test procedure in the IM. It only takes a few minutes.

But - this was a “known” problem for the GTN-650 and I experienced this on my first RV-10 build. How the 650 interact with other stuff when transmitting on the 650 is the culprit. Glad to hear they actually addressed the problem with the GTNXi series - I have one coming for the new RV-10 build.

I suspect however that since the OP did not have the problem for the first three years that his other assumption about cooking the active GPS antenna is his lead suspect.

Carl

 

[bjdecker]

But - this was a “known” problem for the GTN-650 and I experienced this on my first RV-10 build. How the 650 interact with other stuff when transmitting on the 650 is the culprit. Glad to hear they actually addressed the problem with the GTNXi series - I have one coming for the new RV-10 build.

I suspect however that since the OP did not have the problem for the first three years that his other assumption about cooking the active GPS antenna is his lead suspect.

Carl

Verbiage from the 650 TSO manual, dated 2-2019 (revision S) "The GTN COM does not interfere with its own GPS section."

 

[Carl Froehlich]

Verbiage from the 650 TSO manual, dated 2-2019 (revision S) "The GTN COM does not interfere with its own GPS section."

No - not true. I even have confirmation from Garmin about this problem. But I was an early adopter (2012) so perhaps by 2019 they fixed it.

The problem was the 650, during transmit, produced harmonics that were re-radiated by other stuff (Garmin first pointed me to the ELT antenna). For me it was the Comm #2 radio and associated wiring (even though it was just in receive mode). So if taken literally, the GTN-650 itself may not have interfered with its GPS section. Unfortunately few if any install the 650 as the only avionics in a plane.

I don’t know for a fact but I suspect this issue is what drove Garmin to specify a minimum coax length to the GPS antenna. The extra length providing some attenuation of these harmonics.

Carl

 

[bjdecker]

Just quoting chapter and verse from the book - if Garmin’s tech pubs are wrong, so be it.

And the "minimum" RG-142/RG-400 cable length requirement was introduced with the WAAS receivers (400W, 420W, 430W, et al) in 2005/6. I remember a bunch of heartburn when my initial install with the GNS430 only had a couple of feet of coax, then went through the upgrade program and then was "required" to add 6' of wire.

They really wanted to get to 1.5dB loss (minimum).

 

[dmattmul]

I prefer to let Gamins operating documents speak for themselves. The vast majority of their electronics (GSU 73, GSU 25, GMU 11, GMC 507, GAD 29 etc.,etc. ) use 158 F as the upper limit. The GA 35 has a maximum of 185 F but not sure if that's operating or storage. Even the GDU 460 has an upper operating limit of 140 F. I have 2 ring type thermocouples FWF that I can place where I want and have never seen temps above 125 F on my antenna shelf and yes, I've monitored after shutdown, (In Florida summers) It's easy to check signal strengths by reviewing Garmin metrics cowling on vs off. Any reduction of view can be tested by rotating the aircraft 90, 180 and 270 degrees and reviewing performance metrics. Lastly even Vans uses FWF in their plans. (See pic) I'm very comfortable with mine, FWF and accept the risk.

 

[Walt]

I prefer to let Gamins operating documents speak for themselves. The vast majority of their electronics (GSU 73, GSU 25, GMU 11, GMC 507, GAD 29 etc.,etc. ) use 158 F as the upper limit. The GA 35 has a maximum of 185 F but not sure if that's operating or storage. Even the GDU 460 has an upper operating limit of 140 F. I have 2 ring type thermocouples FWF that I can place where I want and have never seen temps above 125 F on my antenna shelf and yes, I've monitored after shutdown, (In Florida summers) It's easy to check signal strengths by reviewing Garmin metrics cowling on vs off. Any reduction of view can be tested by rotating the aircraft 90, 180 and 270 degrees and reviewing performance metrics. Lastly even Vans uses FWF in their plans. (See pic) I'm very comfortable with mine, FWF and accept the risk.

Following the Gamin Install manual is what you should be doing, not trying to second guess them with their operating limitations.
Personally, who cares if you lose ADS-B position on an RV12, you might however have an exciting time if you lose WAAS on a real approach.
And I'm pretty sure you haven't done testing of signal strength/integrity in flight, in all parts of the country/world.
Comparing your ground tests at your airport against the Garmin certification tests, well that is just silly.
I accept risk each time I fly, but I always prefer to reduce risk whenever possible, especially when there is no cost to doing so.

 

[Jonnyb]

First, thank you for the replies. I’ll run the interference test.
I flew XC this morning for 1-1/2 hours at 3,500’ and there were no dropped satellite. And, the weather was crappy (overcast, MVFR).
interesting that it only drops the satellites when I’m at higher altitudes (above 6K’).

I don’t want to sound stupid, but could it be possible that you have water in your coaxial cable that’s freezing as you go higher? I don’t know where your freezing level is, or the routing of your cable.

 

[- Poindexter -]

could it be possible that you have water in your coaxial cable that’s freezing as you go higher?

Thanks Jonnyb, and good observation. It is a very short coax run from the unit to the FWF (fire wall forward), so even though I may be in freezing conditions (OAT), it should not affect that short of a run since it is cabin and FWF temps.

My original question to the group was if folks have seen failures with these antennas. I know, and have read, that FWF rack mount should be avoided and I’ll have to scrub airplane bellies for the rest of my life. But appreciate some advice and solution. I’m on an extended North America tour, but parked for 10 days, so I’ll let you know what I find.

Thanks,
Paul

 

[Walt]

Thanks Jonnyb, and good observation. It is a very short coax run from the unit to the FWF (fire wall forward), so even though I may be in freezing conditions (OAT), it should not affect that short of a run since it is cabin and FWF temps.

You do know that you are required to have a minimum of 6' of RG400 between antenna and GTN?

 

[KatieB]

You likely cooked the antenna. At top of climb it is hot and that’s probably what’s happening. From talking with various people with similar complaints, I’d say, 3-4 years is all you’ll get out of a GA 35 under an RV cowl. I get why people put them there, but as Walt said, it’s a bad idea.

 

[dmattmul]

You likely cooked the antenna. At top of climb it is hot and that’s probably what’s happening. From talking with various people with similar complaints, I’d say, 3-4 years is all you’ll get out of a GA 35 under an RV cowl. I get why people put them there, but as Walt said, it’s a bad idea.

What is the temperature rating from Garmin for a GA 35?? (And they are conservative and that's ok) Vans engineers seem to think it is ok under the cowling for a RV-12, why? (They are ok replacing the antenna every 3-4 years?)

Data is compelling not speculation.......(Likely, I'd say ??) E-mags, P-mags are in an environment that has a much hotter conditions (50 F vs top of cowling), I've measured.

 

[KatieB]

What is the temperature rating from Garmin for a GA 35?? (And they are conservative and that's ok) Vans engineers seem to think it is ok under the cowling for a RV-12, why?

Data is compelling not speculation.......(Likely, I'd say ??) E-mags, P-mags are in an environment that has a much hotter environment (50 F), I've measured.

RV-12 has liquid cooled heads. It’s probably cooler under that cowl than your RV-7. I don’t work for Garmin anymore so I don’t have access to any specs for TSO products, but Garmin has always advised against that location for the reasons Walt mentioned earlier.

 

[AZSteve053]

Sent you a message (conversation)

 

[g3xpert]

Just to ensure one datapoint is accurate, the GA 35 is tested to 85C, per DO160 requirements.

We of course stand by our guidance to avoid mounting the antenna for your IFR navigator underneath the cowling. This ensures the best possible installation for what is a very important piece of equipment in your aircraft.

Thanks,

Justin

 

[lr172]

Just to ensure one datapoint is accurate, the GA 35 is tested to 85C, per DO160 requirements.

We of course stand by our guidance to avoid mounting the antenna for your IFR navigator underneath the cowling. This ensures the best possible installation for what is a very important piece of equipment in your aircraft.

Thanks,

Justin

Not to put words in Garmins mouth, but seems more likely it is tested for several hours at 85* and not several years. The implication Katie made was that heat over the long haul (3-4 years of use) kills the antenna. This is generally consistent for electronics in general. Unless they are specially built to withstand heat, heat tends to reduce lifespan.

 

[Brantel]

Don't assume it is the antenna. I destroyed a perfectly good one when I did that.

Check the cable and the connections first. I had a tiny little wire strand whisker cause all kinds of intermittent issues once. Get the ohm meter out and check for shorts and opens. Wiggle the connectors around on the cable while monitoring. Check for 5V at the antenna end of the cable while wiggling things around.

I also had two GPS board failures. One was on a 430W and the other was on a GTN 650. Once the units heated up, they would lose satellites. There are ways to troubleshoot this using onboard diagnostics within the box.

Finally if you can't find anything wrong with the cable, connections, GPS board etc. Swap the antenna out with a known good one and give it a try.

 

[mburch]

seems more likely it is tested for several hours at 85* and not several years.

To expand on that, if I recall correctly, the DO-160D test requirement is 3 hours duration.

 

[- Poindexter -]

Problem solved.
Replaced the GA35 with a new one and all is back to working well.
As a reference:
740 hrs TT.
Plane is (10) years old, but the last (3) years (since ownership), I’ve put on (600+) hrs.

I’m on an extended trip around US/Canada, so don’t have time with an alternate mount (currently on a rack in the FWF).
I’ll look at alternative mounting when I get back. But, just wanted to reference for others.

 

[dmattmul]

Problem solved.
Replaced the GA35 with a new one and all is back to working well.
As a reference:
740 hrs TT.
Plane is (10) years old, but the last (3) years (since ownership), I’ve put on (600+) hrs.

I’m on an extended trip around US/Canada, so don’t have time with an alternate mount (currently on a rack in the FWF).
I’ll look at alternative mounting when I get back. But, just wanted to reference for others.

Would be interested in actual temperatures where your GA35 is mounted. As reported my temps upper FWF (Behind baffles) are maximum 125 F. (Yes, after shutdown) Have measured temps glareshield in excess of 125 F. Upper surface temps in Florida regularly are higher than 125 F. I still contend FWF could be an environmentally safer place for an antenna but would love to see your data. Garmin at least tests there GA-35 to 185 F (Limited time) others (RAMI) state their operating temps to 208 F. I'm at almost 700 trouble free hours (Florida-Ohio), last 400 hrs. Florida. I get it Garmin can't guess at temperature environments to interpret long-term operating levels.

Poindexter, can I purchase your GA3? PM sent.

(And do appreciate the follow-up)