Gradual decreasing of satellites number with gaining altitude.

[Maxim8609]

Hello everyone. Recently I was flying in forest park with lake. Decided to climb higher and suddenly my drone switched to ATTI mode. I performed descending and landing swiftly.
Then I found out with my flight log that satellites number was decreasing gradually with altitude gaining - from 16 to 5 and that's why the aircraft switched to ATTI mode. And there weren't errors like "Compass error" or "Magnetic field interference. Exit p-gps mode."
During descending the satellites number increased and GPS signal was regained.
The location is near the city but I flew over the lake in forest park. The space above and around drone and sky was clear. I also flew there a few times earlier and hadn't experienced troubles like this. I always had 18-20 satellites on altitudes about 200-400 feet.

Here's the flight log. DJI Flight Log Viewer - PhantomHelp.com
Satellite number decreasing starts on 0m 54.1s

Why this "connected satellites number" decreased? Could GPS signal be muffled at some altitude? Any thoughts.

Thank you.

 

[sar104]

Ground-based GPS jamming and spoofing has been reported before in the Moscow area - that would typically get worse with altitude in the affected area.

 

[Maxim8609]

Ground-based GPS jamming and spoofing has been reported before in the Moscow area - that would typically get worse with altitude in the affected area.

Yeah, I think so too. Didn't assume though that this jamming can take place even slightly outside Moscow.

 

[N017RW]

Long shot----
There is a parametric setting in the GPS engine for 'elevation mask'. It's not user accessible. SVs at low angles are subject to signal fade and atmospheric noise.

Possibly at the time and place of your flight many of the visible SVs were low (enough) on the horizon and were thus ignored.

Try another flight there if possible to see if the problem repeats.

 

[sar104]

Long shot----
There is a parametric setting in the GPS engine for 'elevation mask'. It's not user accessible. SVs at low angles are subject to signal fade and atmospheric noise.

Possibly at the time and place of your flight many of the visible SVs were low (enough) on the horizon and were thus ignored.

Try another flight there if possible to see if the problem repeats.

That would not be altitude dependent, so the decrease with altitude would have to have been pure coincidence. Possible though.

 

[N017RW]

I believe he said sats decreased with altitude increase...

"Then I found out with my flight log that satellites number was decreasing gradually with altitude gaining..."

 

[Maxim8609]

I believe he said sats decreased with altitude increase...

"Then I found out with my flight log that satellites number was decreasing gradually with altitude gaining..."

Yes, exactly.

 

[sar104]

I believe he said sats decreased with altitude increase...

"Then I found out with my flight log that satellites number was decreasing gradually with altitude gaining..."

Yes - that was my point. Fewer satellites with increasing altitude would not be explained by elevation masking - that would have to have been coincidental.

 

[N017RW]

How so, on a theoretical level?

At say a 7-degree mask, increasing altitude would further mask more and more SVs as the horizon (artificially) rises. Thus fewer SVs 'available'.

Where's my misunderstanding?

 

[sar104]

How so, on a theoretical level?

At say a 7-degree mask, increasing altitude would further mask more and more SVs as the horizon (artificially) rises. Thus fewer SVs 'available'.

Where's my misunderstanding?

Ah - I see what you are getting at. But for an increase in altitude of just a few hundred feet, the elevation angle to distant satellites is not going to change significantly at all. Parallax is very small if the observer moves a few hundred feet when the object is hundreds of kilometers away.

 

[embayweather]

Is it possible that with the lake and the trees that there may have been some signal interference with the satellite signal being bounced off, say, the lake, and interfering with the main signal. A sort of channel interference