Mavic 2 Pro out of control with incorrect gps positioning - crashed into building

[xtrapurified]

Hi guys,

new to this forum.
I just had a weird crash. I was flying slowly alongside a building in tripod mode, when suddenly drone started strafing to the right. Even though i tried to control it with sticks since it was visible to me, it still crashed to a building nearby.
I noticed when looking at the flight records that the GPS was off by at least 10-15m during the whole flight. It even shows the wrong point where it crashed on the map, since I found it on a concrete surface 15m south of the spot drawn on the map.

Luckily, other than 3 broken props, it survived the crash and everything seems to work properly.
Is there a way to check or run a test to make sure of this?

Attaching the log.

Link to the proxy file where you can see how it flew closer to the building on the left than it actually shows on the map.

 

[Meta4]

I just had a weird crash. I was flying slowly alongside a building in tripod mode, when suddenly drone started strafing to the right. Even though i tried to control it with sticks since it was visible to me, it still crashed to a building nearby.
I noticed when looking at the flight records that the GPS was off by at least 10-15m during the whole flight. It even shows the wrong point where it crashed on the map, since I found it on a concrete surface 15m south of the spot drawn on the map.
Link to the proxy file where you can see how it flew closer to the building on the left than it actually shows on the map.

There's nothing weird about your crash and the drone was not out of control.
You flew in a canyon between two buildings, close to one wall.
The wall you were close to blocked half of the sky and the satellites in that part of teh sky.
The other wall blocked most of the other satellites and you were left with a thin piece of sky above the drone and very few satellites for GPS positioning.
To have GPS positioning, you need to have a several satellites in a broad spread - and you did not.
You lost GPS positioning because you flew into a canyon and the drone lost GPS position holding.
That made it subject to drifting on any breeze and meant it had no "brakes".
Without brakes, handling of the drone is very different, but the drone is still fully controllable.
Once you lost GPS position holding, you used full joystick movements to tryu to control it, but without brakes, that caused the crash.

Here's what your flight data looks like: DJI Flight Log Viewer - PhantomHelp.com
For most of the time from 4:56 till 5:47 your GPS health dropped to 0 or 1 (you had no GPS positioning).

The positioning shown on the map is not accurate because the drone did not have enough sats to obtain good position information.

 

[Gryphon962]

Concur. I worked in the GPS Joint Program Office for 3 years: to get an accurate position you need 'good geometry' in terms of ranging signals from satellites all around and at various elevations. You flew in what is known as an urban canyon, where number of sats and the geometry of the solution would be suboptimal.

People do this all the time, usually down some river gorge or whatever.

 

[sar104]

To elaborate - the aircraft first lost its GPS position solution at 314 seconds and ran in VPS P-mode for 28 seconds. A GPS lock was reacquired at 342 seconds and the aircraft reverted to P-GPS. The final backwards run in the video was started at 377 seconds under P-GPS, but at 390 seconds the GPS lock was lost again, leading to the uncontrolled flight to the right.



The switch to ATTI mode actually didn't occur until the aircraft collided with the building at 397 seconds - it wasn't ATTI drift that caused the crash - it was active FC attempts to control the aircraft using a flawed IMU position solution.

 

[DanMan32]

That's a reason I've mentioned in the forums that there should be a way to turn off use of GPS for flight and use vision only. If GPS is unstable, then you can get yourself into lots of trouble even when vision could likely hold you steady.

 

[gnirtS]

Well you can manually enable ATTI mode with a simple assistant modification.
Vision is only going to be useful if you're also very low.

I agree it should be a standard feature though not a user modification.

 

[DanMan32]

I believe manual ATTI also disables VPS, not just GPS.

 

[TTP]

An alternative scenario could be this,
TCTW, To Close To Wall.

A phenomenon I observed while flying near a wall,.
The effect seems to work as when the wind passes an aircraft wing or a sail. You get an effect that pulls the drone into the wall, and your attempt to correct doesn't help.

Would appreciate the views on this comment.

 

[Meta4]

Would appreciate the views on this comment.

His recorded flight data shows that he had no GPS position holding.

It might be useful to look at the flight data from your incident to see if there might have been issues you weren't aware of.

 

[TTP]

TCTW, has happened to me a number of times, nowadays I stay respectfully away from walls when I fly.

Unfortunately, no flight data remains from these events.

 

[hiflyer201]

TCTW, has happened to me a number of times, nowadays I stay respectfully away from walls when I fly.

Unfortunately, no flight data remains from these events.

I think if you knew the location of the satellites you would know when you are probably going to be blocked. In an aircraft you only need three to navigate with. That's sure not the case with drones. Drifting in a confined area is no fun at all... I wish I had talked to Meta4 a couple of years ago..it would have saved me a few bucks

 

[Meta4]

I think if you knew the location of the satellites you would know when you are probably going to be blocked. In an aircraft you only need three to navigate with. That's sure not the case with drones.

You need a minimum of four satellites for a 3D GPS fix.
Using only three sats might give a 2D fix which is likely to be quite inaccurate.
DJI drones require a minimum of six sats for P-GPS flight but may need more than six to get a good spread of sats which is also necessary.

The good news is that out in the open you'll always have many more sats than the minimum and it's only in confined spots where a big chunk of the sky is not visible that you are likely to have GPS problems.

 

[hiflyer201]

Thanks Meta..Yeah..I was going to get into instrument approach requirements, WAAS, RAIM etc.....but hey..these are drones..I figured ..who cares?
It's a real wake-up call the first time you get close to a wall and the drone keeps drifting when you neutralize. I used to practice ATTI mode with my Phantom but... in time's of stress the vision narrows