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Loss of GPS signal on descent

Over my last 10 flights, I’ve lost GPS signal on both GTN navigators when descending through FL140. All satellites are lost, and it takes a couple of minutes to get them back. It took me by surprise the first time, and especially the fact that the autopilot (STEC3100) has to reinit its ADHARS after the GPS comes back on line; now I’m expecting it it’s not a problem – but I still need to understand why it does that and get it fixed. Has anyone had anything like that before? No work has been done to those units or anywhere near the antennas in a couple of years.

Last Edited by denopa at 13 Aug 19:53
EGTF, LFTF

Was there any fancy terrain around? did you also lost the signal on your tablet EFB?

Then it could be something about the airframe & antennas…

Paris/Essex, United Kingdom

Might you might have been transmitting on a frequency in the range of 120 to 122 MHz or 131.2 MHz +/- 1 MHz?

Edit, the reason I ask is that what you describe sounds like Com interference where transmitting on one of these frequencies kills GPS signal for short period of several minutes, but when no longer transmitting on the frequency, GPS is restored. One common source is an ELT system re-broadcasting harmonics. Even though most ELT have been updated to 406 MHz systems, most still retain 121.5 MHz for close in location.

Last Edited by NCYankee at 13 Aug 21:20
KUZA, United States

Sorry should have specified – the iPad didn’t miss a bit and started using the internal GPS.

Yes there was terrain (I’ve been crossing the Alps, and the Abrugges, quite a lot recently !) but that’s never been a factor for GPS for me in the past.

Good lead on the frequencies, I’ll monitor that on my next higher level flight.

EGTF, LFTF

I’ve read just today about something maybe comparable in the Comanche Society Journal, which is, it happened in a Piper Comanche.

He noticed in any descent that his GNS530 shut down. Issue was, that during normal operation electric power is delivered from the alternator, over the alternator bus. But during descent he reduces power to so low a setting (and additionally uses, flaps, gear, lights, and so on, all drawing current), that current flows from the battery over the battery bus instead. These are two separate installations, and only connect somewhere behind the panel or even at the device. But here came a faulty ground (corroded or so) and the voltage dropped too low, leading to a restart of the GNS530.

Germany

denopa wrote:

Good lead on the frequencies, I’ll monitor that on my next higher level flight.

You don’t have to wait until your next flight to see if this is an issue. On the ground, get a good GPS fix outside the hangar. Watch the GPS page and in particular the one showing the satellite bars. Set your com to the following frequencies and press-hold the transmit for 35 seconds. The bars may temporarily collapse, but you should not experience an integrity check on the satellite view and one should not lose GPS position. Test each of these frequencies:

Try every frequency between 121.150 MHz to 121.25, and 131.200 MHz 121.175 MHz 131.350 MHz. The lower frequencies are most likely to cause interference as they are close to 121.5 which the ELT is tuned to,

Last Edited by NCYankee at 16 Aug 14:41
KUZA, United States

After ADS-B and associated GPS installation on my plane, with an older com radio, running that test on the ground killed the GPS signal completely and instantly the moment the PTT switch was pressed, with recovery taking a long time. A notch filter in the com antenna cable fixed the issue equally quickly.

Last Edited by Silvaire at 16 Aug 15:57

Silvaire wrote:

After ADS-B and associated GPS installation on my plane, with an older com radio, running that test on the ground killed the GPS signal completely and instantly the moment the PTT switch was pressed, with recovery taking a long time. A notch filter in the com antenna cable fixed the issue equally quickly.

It’s no coincidence that the installation manual for panel-mount GPSs include such tests… E.g. from the Garmin GTN 650Xi IM:

1. Tap GPS Status.
2. Verify at least seven satellites are acquired by the GTN Xi.
3. Verify the GPS “LOI” flag is out of view.
4. Select 121.150 MHz on the COM transceiver to be tested.
5. Transmit for a period of 35 seconds.
6. Verify the GPS “LOI” flag does not come into view.
7. Repeat steps 5 and 6 for the following frequencies:
121.15 MHz 121.17 MHz 121.20 MHz 121.22 MHz 121.25 MHz 131.20 MHz
131.22 MHz 131.25 MHz 131.27 MHz 131.30 MHz 131.32 MHz 131.35 MHz
For VHF radios with 8.33 kHz channel spacing, in addition to the frequencies listed for 25 kHz COM channel spacing, include the following:
121.185 MHz 130.285 MHz 121.190 MHz 131.290 MHz
8. Repeat steps 4 through 6 for all remaining COM transceivers installed in the aircraft.

ESKC (Uppsala/Sundbro), Sweden

It happens all the time with me, flying in Ukraine, west of UKHH. People say there is a military base and they jam all signal in the area.
It usually lasts for 5-10 min and then I’m starting to get satellites back. It could be something similar in your case.

Ukraine
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