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GTN 650 Loss of Satellite Coverage with Com Frequency

Chris Engler

Active Member
Scratching my head over this one :confused:....I have a GTN 650 in an RV-8 and for the past 20 hours or so I've noticed a loss of GPS satellite coverage at certain times....most often on the ground an KFRG where I fly into weekly. The nav/com has been rock solid for 380 hours TT. At first I thought it was intermittent and would occasionally loss of coverage while enroute. Bought a new antenna and it made no difference.

After doing some research, I noted that some folks were having similar trouble while transmitting on certain frequencies (most notably around 121.5).

The other day I had the satellite tracking screen cued up while my active frequency was 122.8 (lots of satellites with a strong signal) and had 121.5 loaded in "standby" and as soon as I switched 121.5 to active it completed killed all satellite tracking and I lost navigation. I repeated the test several times with the same results. I then tried a few frequencies close to 121.5 (including KFRG ground at 121.6) and sure enough, all sat coverage was lost. once I got above 122.0, I could maintain sat coverage but the signals were clearly diminished the closer I got to 121.5

The REALLY strange part is I don't need to transmit on any of the trouble frequencies....simply load them into the "active" window.

Any similar experiences out there and/or suggestions for a fix?

Thanks,

Chris
 
From the GTN IM:
3a.
The GPS antenna should be mounted no closer than two feet from any VHF COM antenna or any other antenna which may emit harmonic interference at the L1 frequency of 1575.42 MHz. An aircraft EMC check (reference VHF COM interference check in Post Installation Checkout procedures) can verify the degradation of GPS in the presence of interference signals. If an EMC check reveals unacceptable interference, insert a GPS notch filter in line with the offending VHF COM or the (re-radiating) ELT transmitter.

3.5.6 Interference of GPS
On some installations, VHF COM transceivers, Emergency Locator Transmitter (ELT) antennas, and Direction Finder (DF)
receiver antennas can re-radiate through the GPS antenna. The GTN COM does not interfere with its own GPS section.
However, placement of the GPS antenna relative to a COM transceiver and COM antenna (including the GTN COM
antenna), ELT antenna, and DF receiver antenna is critical.
Use the following guidelines, in addition to others in this document, when locating the GTN and its antennas.
• Locate GPS antenna as far as possible from all COM antennas and all COM transceivers (including the GTN COM),
ELT antennas, and DF receiver antennas
• The GPS antenna is less susceptible to harmonic interference if a 1.57542 GHz notch filter is installed on the COM
transceiver antenna output
• Locate the GTN as far as possible from all COM antennas
If a COM antenna is found to be the problem, a 1.57542 GHz notch filter, Garmin P/N 330-00067-00, may be installed
in the VHF COM coax, as close to the COM as possible. This filter is not required for the GTN transmitter.
If a COM antenna is found to be radiating, consider the following:
• Replace or clean VHF COM rack connector to ensure good coax ground
• Place a grounding brace between the GTN, VHF COM and ground
• Shield the VHF COM wiring harness
 
Yep - “known” GTN-650 problem. I got bit on the GTN loosing GPS signals when transmitting on the GTN radio. I was eventually able to fix this during a panel mod to replace my COMM #2 with the then newly release Dynon radio.

Carl
 
Chris,

Is the GTN puck sitting close to say a Dynon puck?

I suffered for a year with exactly the issue you describe. The problem frequency was 121.2. Notch filters did not work in my case.

The solution that worked for me was to move the GTN puck from forward of the windshield to behind the cabin. The Dynon puck remained forward of the windshield. Before cutting metal you will be able to temporarily move the puck and ground test it on your problem frequencies.

Moving the GTN puck also allowed the prescribed length of coax to run in a straight line, rather than sit coiled beside the GTN unit.

Since the reconfiguration the GTN operation has been flawless.

Hope this helps.
 
Thanks for the thoughts and suggestions. I'm running GRT EIFS systems which interface to the GTN 650 for Navigation. Other antennas (COM, ELT, and ADSB) are all 10+feet away from the 650 and associated GPS antenna. The GPS antenna is however located just a few inches away from the 650 itself.

Today I removed the GPS antenna and moved it around while watching the 650 satellite screen and with the COM set to 121.5. As I moved the GPS antenna away from the 650, the satellite tracking improved greatly....even when just moved a few inches further away from the 650 than it's prior position. It seems the problem may be solved but anxious to fly in to KFRG to confirm.

It's interesting that the problem presented after 380 hours of no issues and without transmitting on a problem frequency. While antenna positioning was the same before and after the problem, I did have the transponder software updated and reconfigured a few settings in the 650 and EIFS to get a compliant ADSB report about the time the satellite tracking trouble started....hard to say what the root cause was but antenna repositioning seems to be the fix!
 
I had the same issue the my 355. Turned out to be my Nav lights. Found it by turning everything off, then turning one item on at a time and testing the Comm. I wound up running shielded wire to all my Nav lights. Never had to move my antennas.
 
Monitor the GPS satellite screen on the GTN. Turn off other components one at a time, and watch your satellite reception. You can also key your mic on the suspected frequencies. If it is interference with other equipment, you will instantly see a reduction in satellite reception. I've seen interference from other GPS antennas, especially the small antennas you might use with a portable or G5, that was 4 feet away.
 
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