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I've had many occurrences when one faulty GPS antenna can appear to work correctly but can cause other nearby (within 20m) GPS antennas to fail. This is why modern installation standards call for resilient GPS antennas to be sited as far apart as possible - I usually aim for 60+m, but I have the luxury of working on large buildings! On an aircraft, you'll be lucky to get 5m between the aerials, hence I'm wondering if you have a faulty GPS antenna. I believe you've already had one replaced - but maybe the other one needs to be replaced too.
Murgatroyd liked this
I thought I'd asked about chargers...though that may have been on a similar thread for someone else's similar problem.

Yes, check chargers or anything which may cause interference. There was an issue some years back whereby tuning in certain frequencies on a certain radio would cause the GPS to go out.
Murgatroyd liked this
Like many, lockdown has prevented much flying this year, I've taken on board all the comments and tonight I had a total GPS failure once more: which is good as it wasn't intermittent.

So, I had another bash at checking device operation to monitor the effect...and:-

(I placed another thread regarding my Skyecho not picking up a GPS signal. I now know how, but the 'why' still evades).

Todays flight:
-Both built in GPS have no satellite reception.
-Skyecho after extended warm up, taxi and ten minutes of flight: still no GPS.
- Eventually I switch off the in-board devices one by one and found:- GNS430W off=secondary GPS unit working fine. Skyecho working fine.
-Switch back on GNS and within five seconds, all other GPS signals lost. I cycled this three times and it certainly seems like when the GNS doesn't receive a signal: none of the GPS receivers get a signal.

That's how the 'other' system fails. I would be keen to learn why this fault is occurring, the powering of the GNS clearly effects the signal for all GPS receivers. My question is, is it likely that the fault is within the GNS and I can get this bench tested, or should the focus still be on the GNS to cage connection, cage to harness wiring connections, antenna to GNS cage wiring, Antenna?

I do want this fault rectified, but just like my toothache at the dentist, every time the aircraft has been in for service, the GPS works fine. But now I have something to go on. But that's it.
(No chargers, 4G kit switched off: this is the fault-generating unit)

:? :? :? :? :? :? :? :? :? :? :?

Suggestions welcomed, TIA.
There was at least one batch of defective Garmin antennas, and I would have thought that issue would be sorted by now, but my understanding is that although Garmin acknowledged the problem, they failed to issue any sort of SB, so the defective units still exist in various suppliers and distributors without any indication that they shouldn't be used.
I have heard of people who got a replacement from Garmin, but the removal and replacement costs will still be yours - just be thankful you didn't have to pay for an avionics tech to track down the intermittent problem!
Murgatroyd liked this
I had a problem with my GPS occationally dropping out. Most of the time, the GPS was rock solid. After trying a number of things it turned out to be the VOR receiver.
When I selected the WAL VOR frequency, the GPS signal drops away. All other selected VOR frequencies had no effect on the GPS.
It was completely repeatable both in the air and on the ground.
I am not an expert on RF and interference but it was suggested that my NAV unit with the WAL frequency selected could be emitting RF interference which is a harmonic of the GPS signal. The GPS aerial and VOR aerial are about 1m apart so fairly close.
My GPS is a Garmin 795 and the Nav/com is an MX170c (approx 15 years old) . I use a Trigg TY96 (833) for comms now but the MX170 is still usefull for NAV (except WAL).
Thanks, the antennae is the place where my focus will be shifting. GNS unit has been bench tested over the past few days and behaves perfectly. It’ll get stuck back into the panel and over to the avionics chap as soon as I confirm which antenna was changed last time.
Once the unit is confirmed reliable, I’ll report back on any fault found.
VOR was working fine when GPS was getting blocked. As was dme and adf. I know they all use separate antennae but I also know they are all closer together than they ought to be.

New antenna purchased. Will be fitted next week. GNS been bench tested and software updated. All working fine.
No ELT on this aircraft. PLB only. After speaking again, with the avionics chap i think we could be onto something with the new antenna. Its purchased now and once fitted I'll be making a point of trying it out: now that I've switched to SkyEcho for my location in Skydemon, I'll get to know right away if its still misbehaving, very quickly. I will report back and hope it does indeed boil down to the second GPS antenna replacement.