Weird behaviour in DeadReckoning mode while GPS outage is not constant

[tipoman9]

INAV/SPEEDYBEEF405WING 8.0.0 Jun 28 2024 / 00:18:11 (60a4b71)
Got GPS lock on the ground, though not as many satellites as usual.
Took off and immediately lost GPS - there were two symbols "ES" instead of satellites count on the lower right.
Didn't want to risk in these circumstances, so circled the plane overhead in LoS control.
Decided to test RTH without GPS, the plane has a magnetometer, calibrated carefully.
Both times the plane started porpoising with audible motor bursts, so I had to disengage for fear of not getting it stalled (it's an ArPro on the heavy side with 10x21700 cells).
During the flight, the GPS coordinates were constantly changing. There were moments when the GPS lock was recovered for several seconds, then lost again for minutes. The speed changed from 70km/h to 10km/h in a second.
Can these sparse GPS coordinates at irregular intervals cause some problem with inertial navigation?

LOG00064_ArPro_GPS_Outage_DeadReckoning.zip

https://youtu.be/dDb1sNGmgcI

BlackBox log provided.

[Jetrell]

The logs a bit messed up.. Seeing random jumping from 5 to 35 Sats with HDOP being all over the place. Is indicating a lot of GNSS band interference, which would be causing the speed related issue you're seeing.
Looking at the 5 second interval EPH spikes.. Would you happen to have your Avatar VTX mounted near the GNSS module ?
If its a V1 VTX, you want at least 20cm separation between them. If its a V2 VTX, you want at least 5 cm.

Both times the plane started porpoising with audible motor bursts, so I had to disengage

That part is related to this known issue.

[stronnag]

Pretty much "game over" for the log ....

Given the location, there is also the possibility of external GNSS interference.

The jumps in location during the replay are quite disconcerting.

[tipoman9]

I mapped the recorded coordinates and the results are ... weird. If I read them carefully, it seems I landed 1.5km away from the take-off location, while in reality I landed on the same spot where I'd taken-off.
During last summer I've experienced spoofing here, but the last few months have been relatively free of GPS problems.
In fact I thought the cause of the GPS troubles I had came from the VTx PowerAmplifer being set to very high power levels (I did increase it before the flight) that make it work in saturated mode.
After I landed, I turned it off and got 19 satellites in less than a minute.
So I won't attribute this to intentional GPS spoofing, it was rather my plane jamming itself.
But in this case I think the GPS should stop provide data, rather than provide wrong coordinates
This seems to impact INAV stabilization somehow - look at the AHI drift at the landing

BTW.
If you are interested, here is how intentional gps spoofing looks like - Seems I live in an interesting location for testing GPS
https://youtu.be/2yU2Ft2V39c
Log files for the video above - just look where my humble plane was suddenly teleported...
Crazy_GPS_in_flight.zip