Unexplainable Altitude/Velocity Variations During RTH

[MathAddict]

32 degrees, 10 mph winds. full battery. I have 52 flights on my MA2 in the past two months.

I went for a flight at lunchtime from the same location I do 96% of my flights. The GPS signal was weak, but after about one minute it showed the correct location on the map so I ascended to 394 feet and left it there and flew around a bit testing an ND filter.

At no time was the wind an issue, not in the slightest.

After the lowering battery level initiated an auto RTH I sat back and let the little drone come back as it always has. My fingers were off the joysticks. The drone made a bee-line back to me. Then...

The DJI Fly app indicated that it was climbing to 430 feet. WTF? It then went down to 387 feet, then up to 440 feet. It’s velocity also dropped to just a few mph, then 18, then zero, then ... I left it like that for a few seconds hoping it would come to its senses, then cancelled the RTH and took manual control, after which it responded as expected.

After uploading the log to AirData I can see that 16 seconds before it started its first climb that I had the following status:

No GPS.ly with caution. (sic)
GPS signal weak. Hovering unstable.

(0) I recorded the screen on my phone during the flight and after playing it back I can see that it had either 18 or 19 GPS satellites locked during the entire flight.

(1) During the RTH the obstacle avoidance icon kept bouncing between mostly green then red, then repeat. How can there be obstacles at that altitude?

(2) Could no/weak GPS cause the drone to not maintain RTH altitude?

(3) Could it also explain the velocity variations during RTH of bouncing between 22 mph and zero until I took manual control?

(4) Why the weak GPS messages in the log when the Fly app said 18/19 satellites?

Guesses are welcome, but please label them as such.

Best,

Jim

 

[DanMan32]

The sun or sun reflection often tricks obstacle avoidance to think there's an obstacle in front. One of the features of OA with RTH is that if MA encounters an obstacle, it will try and fly over it unless it reaches set max altitude. It will also stop but when it levels or tilts back to brake, the sun is no longer seen as an obstacle, so you get stop and starts.

 

[MathAddict]

The sun or sun reflection often tricks obstacle avoidance to think there's an obstacle in front. One of the features of OA with RTH is that if MA encounters an obstacle, it will try and fly over it unless it reaches set max altitude. It will also stop but when it levels or tilts back to brake, the sun is no longer seen as an obstacle, so you get stop and starts.

Hi DanMan. It was partly sunny and the ground is snow covered, so that could fit. I do have avoidance set to go around though.

 

[Meta4]

(2) Could no/weak GPS cause the drone to not maintain RTH altitude?

Your drone does not use GPS for altitude

(3) Could it also explain the velocity variations during RTH of bouncing between 22 mph and zero until I took manual control?

Probably not

(4) Why the weak GPS messages in the log when the Fly app said 18/19 satellites?

This is a false alarm that's showing up repeatedly in RTH since a recent update.

 

[Gbower]

I just finished a flight and on RTH my Air 2 started back fine and then started to climb to the max altitude with me doing nothing. The Air 2 then slowed and stopped and then started returning again and then stopped. I then went manual and brought it home safely. All updates have been done. I have noticed this a couple of times before on old versions but today was a bit concerning. I had lots of satellites and good signal. Wind wasn't a big issue. It was fairly cool out but I've flown in colder. I looked at the data and can't see anything out of the ordinary for the full flight. Going to be too cold to fly until next week so will have to wait to try it out again.

 

[airbuspilot]

Your drone does not use GPS for altitude

Probably not

This is a false alarm that's showing up repeatedly in RTH since a recent update.

How does the Mavic Air 2 determine its altitude, if not through GPS?

 

[Meta4]

How does the Mavic Air 2 determine its altitude, if not through GPS?

Just like most real planes .. with a barometric sensor.

 

[Meta4]

I just finished a flight and on RTH my Air 2 started back fine and then started to climb to the max altitude with me doing nothing.

The usual cause of this is the obstacle avoidance identifying a low sun as an obstacle to avoid.
If you could fly manually in a different direction, that suggests it's OA that's caused the issue.

I had lots of satellites and good signal. Wind wasn't a big issue. It was fairly cool out but I've flown in colder.

None of those would have any effect on altitude issues.

 

[airbuspilot]

Just like most real planes .. with a barometric sensor.

Can you show a reference in a DJI manual? Just curious.

 

[Meta4]

Can you show a reference in a DJI manual? Just curious.

No .. the manual leaves out many interesting information.
But the detailed flight data confirms that it's barometric data that's providing altitude information.
And you still have altitude data when flying in GPS-free environments.

 

[Prismatic]

How does the Mavic Air 2 determine its altitude, if not through GPS?

GPS at the consumer level does not provide very accurate altitude data. Military GPS is another matter entirely.

 

[Thomas B]

GPS at the consumer level does not provide very accurate altitude data. Military GPS is another matter entirely.

DJI drones use barometer altitude above 10-20’ or so.

 

[airbuspilot]

DJI drones use barometer altitude above 10-20’ or so.

Source?

 

[Thomas B]

Source?

DJI

 

[airbuspilot]

DJI

A specific source? Like a manual or written document?

 

[Thomas B]

A specific source?

Manual and DJI website.

 

[Meta4]

A specific source? Like a manual or written document?

All recent DJI drones have the barometric sensor which is used for all altitude data.
They also have short range downward facing sensors that measure the distance to the ground or whatever is below the drone when there is something within their limited range.
That height is a separate measurement.
It's displayed in the app (for some models at least) and recorded in detailed data, but not used for general flight functions.

 

[Meta4]

A specific source? Like a manual or written document?

DJI don't provide comprehensive technical information but over time analysis of recorded flight data gives a good understanding of (some of) what's going on under the hood.

 

[airbuspilot]

Manual and DJI website.

What page of the Mavic Air 2 user manual? I couldn't find the barometric sensor reference.

 

[Thomas B]

What page of the Mavic Air 2 user manual? I couldn't find the barometric sensor reference.

Sorry.... but answer is above.