GPS woes

[RO13FLY]

I have a GTN 650 and GNS430W both installed by the same major avionics outfit. Although I have never had any previous issues with the 430 in the three years prior to joining it with the 650, both units have failed navigating at exactly the same time. It has happened about three times and in different locations. The error message I get on the 650 refers to ariel grounding if I remember correctly. I don't think it can be loss of signal as neither box went into dead reckoning mode and my handheld Garmin worked fine.

Has anyone had similar issues with their panel mounted GPS?

[A and C]

I had the same issues with a King KLN89B, the GPS signal worked most of the time and occasionally failed.

It turned out the problem was in the antenna line, I can't remember if it was at the antenna or box end but the line had gone intermittent.

The fix was cutting the line a few mm shorter and remaking the connector.

[Fly4Business]

Do you use the pilotaware DIY ADS-B solution? That caused similar occasional outages on my panel mount GPS after WAAS update, but none on the 430 before. It appears the circuits of the RaspPi and the TV stick interfere with WAAS coverage and sad to say, they are not really meant for cockpit use.

[Pilot DAR]

You might have a wiring location problem. If a wire, even not at all related to the avionics, is run close to comm and GPS antennas, there can be interference of the GPS caused by the comm. During flight tests for EMI, I test for this, and have found it. The following is an extract from my test plan. You can perform this test yourself on the ground (considering frequency allocation of local ATC)

5.2 Test Frequencies
When testing VHF Comms as a Source, the test frequencies shown below will be selected when assessing the GPSs as the Victim.

121.150, 121.175, 121.185, 121.190, 121.200, 131.250, 130.285, 131.275, 131.290 131.300 MHz.

The lack of interference from VHF radios will be demonstrated on the completed GNSS installation by tuning each VHF transmitter to the frequencies listed below and transmitting for a period of 35 seconds, or the longest uninterrupted transmission possible for that transmitter, while observing the signal status of each satellite being received. Degradation of individually received satellite signals below a point where navigation is no longer possible is not acceptable


Test each of the frequencies. I have seem all the satellite signal strength bars drop right off when the comm was transmitted on just one of those frequencies. They are specifically chosen, as they are a harmonic of the GPS frequencies.

The organization who did the avionics installation, or any subsequent wiring changes to the aircraft, should be doing this test post installation. If they have a surprised look on their faces when you describe your doing this test to them, you should have a heart to heart chat with them about the completeness of their work.

[RO13FLY]

That's very helpful, thanks.

I had the same issues with a King KLN89B, the GPS signal worked most of the time and occasionally failed.

It turned out the problem was in the antenna line, I can't remember if it was at the antenna or box end but the line had gone intermittent.

The fix was cutting the line a few mm shorter and remaking the connector.


Thanks for this as well. I spoke to Garmin today as well and they suspect an antennae issue either with grounding or with a faulty antennae which could be radiating and causing both to lose signal. They have also suggested the installers give them a call tomorrow to discuss so let's see what happens...

[Curlytips]

As title :)

[RO13FLY]

As title :-)