Low GPS signal / bad vision system data leading to fly-away and crash

[djules75]

This is my first post in this forum.

I initially posted my story on the DJI forum and somebody suggested I post a link here as some people might find this interesting.

The full details are in the DJI post, but long story short:

I lost my drone flying in a famous scenic spot in China (Tiger Leaping Gorge). According to the best analysis from BudWalker: following weak GPS signal, the vision system took over and provided inaccurate speed measurement to the flight controller, leading to a loss of control and my DJI Mini 3 pro crashing into the gorge's cliff wall.

Fly-away / crash - Trying to understand

 

[AeroJ]

Yes this is quite interesting and another timely reminder that flying in low canyons and gorges is rather fraught with additional risk. Sorry to hear you lost one that way. Still, at least you got a fair picture of what probably happened on the DJI forum...

 

[djules75]

Yes, I understand that the risk of low GPS is high in those conditions. However, since it was absolutely not windy, I was expecting to be able to fly it without stabilization at worse, not to completely loose control of it (as I said in the DJI post, I also fly FPV and not having stable positioning doesn't scare me).

But well... that's a costly way to learn...

 

[BudWalker]

I learned a lot with this one. With just the .txt and sketchy GPS data I assumed that it was the GPS doppler velocities that caused the problem. But, then @djules75 provided the .DAT and it was clear that it was the VIO velocities that were incorrect. We’ve seen a lot of incidents were only the .txt is available and either OSD:xSpeed or OSD:ySpeed seemed to be incorrect. It never occurred to me that the cause was incorrect VIO velocities.

Another interesting aspect is that the vision system was off when the Mini3 was flying down river and then turned on when the Mini3 turned and was facing upriver. Pure speculation on my part but I bet that the vision system found it easier to produce velocities, although incorrect, facing against the water flow. Just a theory.

 

[BudWalker]

For anybody wanting to look at this here are the log files

2024-09-17_15-27-13_FLY234.DAT

DJIFlightRecord_2024-09-17_[15-27-59].txt

 

[Zbip57]

@BudWalker Your analysis on the DJI forum was fascinating. Something sure went haywire right after the yaw to point the drone upstream, after which in sticks centred hover it started to drift downstream. It does seem at that point the VPS optical sensor started tracking the moving water.

Despite the then applied full forward right stick, it picked up speed rearward. The subsequent left stick quick input to yaw left./right worked as it should, but the full-left input on the right stick unfortunately came too late.

I made this video with my Mini-1 a year ago, demonstrating how the drone might drift away while in hands-free hover if the VPS optical sensor tracks a moving surface below it, i.e. things like flowing water, or wind ripples on the water's surface, or a floating leaf, etc. I used a patterned rug for this demonstration.

My intent in making the video was to demonstrate that it would ONLY ever behave like this in the ABSENCE of a solid GPS connection. You can see by my reaction in the video that I was STUNNED to discover that's not true at all. Even WITH a solid GPS lock, the Mini insisted on tracking and following the movement as I pulled the rug away!

That seems totally bizarre to me. Surely the IMU inertial measurement unit, plus the GPS monitoring, should sense that the drone is moving despite being commanded to hover in one spot. WHY would DJI prioritize the VPS optical sensor over what the IMU and GPS is saying?

In any case, the issue should only ever arise while in a hands-free hover. The drone should always respond properly to active stick inputs, whether in GPS mode, VPS mode, or ATTI mode. What happened in the Chinese river canyon looks like a serious flaw in the control algorithm.

To view this content we will need your consent to set third party cookies.
For more detailed information, see our cookies page.

Accept third party cookies

 

[BudWalker]

@BudWalker ........

Despite the then applied full forward right stick, it picked up speed rearward. The subsequent left stick quick input to yaw left./right worked as it should, but the full-left input on the right stick unfortunately came too late.
.......

It looks to me like the pitch forward had the proper effect. At 200 secs the Mini4 started to gain NE speed because VIO started tracking the water flow and there were no control inputs. I.e, the Mini 4 thought it was hovering but was actually moving with the water. Full pitch forward wasn't applied until about 208 secs causing the Mini 4 to decrease speed.