GPS drops out on Garmin GNC355

jd21476

jd21476

Line Up and Wait
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jd21476
Ive been chasing a problem with my GNC355 for quite some time. I had it installed last year but unfortunately the installer passed away shortly after the install so I cant really go back to him for a fix.

Anyway, when I key the mic to talk for any longer than 2 seconds, the GPS signal drops out and I get a GPS signal lost. It will then reacquire the signal and come back online after a few more seconds. Obviously this is not going to work for any type of IFR flying and it has delayed me from working on any IFR approaches.

I thought that maybe it was the fact that I had an old Narco CP136 audio panel but since I had plans ot replace it in the near future, I didnt put too much thought in to it. I recently had the Narco swapped for a PS Engineeering 450b but the issue has now followed to the new audio panel.

Any ideas?
 
jd21476 said:
Any ideas?
Click to expand...
Do you have the installation manual? I seem to remember there being a statement on the minimum distance you can have the GPS antenna from the COM antenna. You might want to break out your tape measure. Beyond that, perhaps the COM antenna wiring is old and is leaking signal out to the GPS. Or the GPS antenna wiring is bad and is admitting the COM signal.
 
I just read an article about a guy with a similar issue but it only happened for him anytime he was near the 121.5 freq. It turns out it was his ELT interfering and he swapped it for a new 406ELT and it fixed it. Our local tower is 120.7 so Im going to try a freq far out on the spectrum and see if it still does it. If not then a new ELT may be in order.
 
Do you have just the single COM? Does the drop out happen if you use the 2nd COM? If it was an older 2nd radio that was causing this dropout, I'd say you probably need a harmonic filter on the other radio, but it sounds like its the GNC's COM that is causing the dropout? Was the COM antenna wiring installed/replaced with RG-142/RG-400 (i.e. not still using the old RG-58?) Was the GPS antenna installed at least 2 feet from any COM antenna (note, this can be difficult on smaller aircraft, but should have a best effort for maximum separation distance). Is there a decent ground plane around the GPS antenna (Garmin wants at least a 7.5" distance all the way around the antenna). And like already mentioned, harmonic re-radiation from older ELTs is a definite possibility. Can temporarily remove the ELT from the aircraft and see if it goes away. FYI, there is a VHF COM interference check in the install manual that should have been done that would have found this at install time.
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Yeah, my avionics shop that installed the new audio panel mentioned the COM interference check but I dont think the original installer ever did that.
 
Distance between antennas is important. I would give the coax cabling for all the antennas a good inspection before anything else happens however.
 
I had that happen on my 182. They put filters on the GPS antenna. They said writhing a certain range of frequencies it blocks out the GPS reception. Especially on 121 range. They put the filters on in under an hour, haven’t had the problem since.
 
Someone I know had to replace their old radio because of this. They put in GTR 225 and the problem went away.
 
I had this issue with my GNX375 specifically at lower frequencies around 121.xx. It was interference from my old ELT. Filter on the Com eliminated my problem.
 
So I ran a test and found that my GPS will drop out if I key the mic on and channel from 120.6 thru 121.8 with COM2 on. If I turn COM2 off then it does not drop out on any channel.
 
jd21476 said:
So I ran a test and found that my GPS will drop out if I key the mic on and channel from 120.6 thru 121.8 with COM2 on. If I turn COM2 off then it does not drop out on any channel.
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What do you have as COM2?
 
So I had the GPS issue fixed about a year ago and my GPS has not dropped out since. Today I took a long flight from Northern NV to Socal and it started to happen again. It was a got day. Any ideas?
 
GPS outage NOTAMS?

Remove the ELT for 90 days, keep flying, and see if it still does it. If so reinstall the ELT, then remove the Narco for a while.
 
Do I have to remove the ELT completely or can I just unplug the antenna?
 
So I turned off my ELT and unplugged the antenna but that did not fix the issue. Should I try removing it completely?
 
Just had the same issue with a new garmin 175 install. I have Narco radios, com1 and com2. Each time I would let up, I get the gsrmiN LOI screen. My installer found a filter and place them on each radio, resolving the problem. At some point though, it appears the narco radios are getting old and need repair or replacement.
 
jd21476 said:
So I turned off my ELT and unplugged the antenna but that did not fix the issue. Should I try removing it completely?
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Is all this testing being performed on the ground AND away from buildings or in the air? I'm almost positive its the old Narco stuff...
 
jmarine225 said:
Just had the same issue with a new garmin 175 install. I have Narco radios, com1 and com2. Each time I would let up, I get the gsrmiN LOI screen. My installer found a filter and place them on each radio, resolving the problem. At some point though, it appears the narco radios are getting old and need repair or replacement.
Click to expand...
Do you know what type of filter they needed?
 
From the install manual

2.4.4.5 Interference of GPSOn some installations, VHF COM transceivers, Emergency Locator Transmitter (ELT) antennas, and Direction Finder (DF) receiver antennas can re-radiate harmonics that can potentially interfere with the GPS antenna. The GTN unit 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 635/650/750COM antenna), ELT antenna, and DF receiver antenna is important.Use the following guidelines, in addition to others in this document, when locating the GTN unit and its antennas.• GPS/WAAS antenna—Locate as far as possible from all COM antennas and all COM transceivers (including the GTN unit COM), ELT antennas, and DF receiver antennas. The GPS/WAAS 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 unit 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 635/650/750 transmitter.If a COM is found to be radiating, the following can be done:1. Replace or clean VHF COM rack connector to ensure good coaxial ground.2. Place a grounding brace between the GTN unit, VHF COM and ground.3. Shield the VHF COM wiring harness.2.4.4.6 COM and NAV Antenna Installation ConsiderationsInstall the COM and NAV antennas according to the manufacturer’s recommendations, and obtain FAA approval. (Other FAA approval for installation of the COM and NAV antennas are required, and is not a part of this STC.) Ensure that the COM and NAV antennas are electrically bonded to the aircraft. Avoid running other wires and coaxial cables near the NAV antenna cable. If there are two separate COM antennas, attempt to run the COM coaxial cables independently of each other and provide zonal separation as much as possible in the aircraft to avoid loss of both COMs in a single event
 
Can I install the notch filter? I hate taking my plane into an avionics shop because everytime they want to rrouble shoot and spend hours trying to diagnose a known issue.
 
Ok, so I fixed the problem and Im posting this in case others have a similar issue.

I ordered the inline notch antenna filter (thanks Floyd) as pointed out above. Prior to installing it I disconnected and removed my ELT. No change. Then I pulled my Comm2 out of the panel and set it on the rear seat. The issue went away which confirmed it was an interference from Comm 2. I then installed the inline notch filter on the antenna of comm2 at the radio and it was all fixed.
 
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