RTH Obstacle Check - defeated by the sun (potential bad situation)

[davmcw]

There are a thousand ways to screw up and I stumbled upon one yesterday. Fortunately I avoided any kind of a problem but it could have been bad. The RTH feature has worked flawlessly for me many times but this time I discovered a way it could trap my MP in limbo indefinitely (I think). Here is what happened:

1) I flew a few hundred feet away in an open field and decided to RTH (no emergency, I just wanted to return to home quickly and with little effort on my part). I initiated the RTH.

2) My MP spun around and ascended to the 80M (260 feet or so) my RTH height was set to. It was now facing west into the sun at 5pm.

3) It moved toward home for 5-10 ft and then stopped as if it had hit a wall. It then backed up 15 feet and ascended 20 ft.

4) It then repeated # 3) above until it was 20 feet or so further away and at a height of 380 ft.

5) I hit the pause button and flew it home.

6) I immediately relaunched the MP and tried it again and the same exact thing happened upon initiating a RTH...lurch forward...step back...ascend...repeat...repeat...repeat.

7) I hit pause, flew it home, turned it off, restarted, and tried it again...this time the RTH worked flawlessly.

At this point I had no idea what the issue was and was uneasily writing it off as a glitch. I slept on it and this morning it came to me after reviewing the flight log...my final and successful RTH was with the MP facing south while the first two problematic RTH's were facing west into the setting sun. It was no glitch but rather the MP reacting to a perceived obstacle as the "RTH Obstacle Check" mode is supposed to. How long it would have carried on like this, I don't know but I can imagine a scenario where the MP really needs to come home (lost signal plus low battery) and it can't because it is trying to return into the sun and the RTH Obstacle Check algorithm won't let it.

The RTH Obstacle Check can be turned off in the advanced settings section of the Visual Navigation Settings menu. Mine will be off from now on and assuming I have my RTH height set appropriately I should have no problem.

If I'm wrong on this one someone let me know. I tried uploading a phone video of the flight path log but it was too big but it shows just what I described above.

 

[dsd317]

Interesting find. I'll definitely have to test this out when I get a chance. Thanks for sharing your experience.

 

[Strafe1]

I think you will also have to turn off the main OA in addition to the RTH OA. Not 100% sure on this. I'm pretty sure the reverse is true though, that if the main OA is off and RTH OA is on, OA will be activated regardless.

 

[davmcw]

I think you will also have to turn off the main OA in addition to the RTH OA. Not 100% sure on this. I'm pretty sure the reverse is true though, that if the main OA is off and RTH OA is on, OA will be activated regardless.

I typically fly with OA off specifically because of the sun induced false alarms and the speed penalty of operating with active OA. But, as you point out, if you don't also turn off the RTH OA you in effect have OA on during a RTH event.


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[sonof40]

Informative and well written.

 

[SkySailorMan]

A very similar thing happened to me on one of my first flights - on manual activation of RTH, the Mavic turned into the sun. I watched the obstacle registration on the screen, but mine didn't pause and reverse like yours did. No, mine started going straight up like a rocket, and it went way, WAY past the max altitude I had entered. Watching THAT on screen had my adrenaline pumping let me tell you. I learned a good hard lesson about orientation during RTH.

 

[Daniel Marlin]

Wondering if having a polarizer/ nd filter would help with this since it protects from the glare...

 

[AlanTheBeast]

Wondering if having a polarizer/ nd filter would help with this since it protects from the glare...

Interesting thought. I was thinking of extending sun shades out a few mm.

 

[sonof40]

A very similar thing happened to me on one of my first flights - on manual activation of RTH, the Mavic turned into the sun. I watched the obstacle registration on the screen, but mine didn't pause and reverse like yours did. No, mine started going straight up like a rocket, and it went way, WAY past the max altitude I had entered. Watching THAT on screen had my adrenaline pumping let me tell you. I learned a good hard lesson about orientation during RTH.

You should send that flight info to DJI.

 

[davmcw]

A very similar thing happened to me on one of my first flights - on manual activation of RTH, the Mavic turned into the sun. I watched the obstacle registration on the screen, but mine didn't pause and reverse like yours did. No, mine started going straight up like a rocket, and it went way, WAY past the max altitude I had entered. Watching THAT on screen had my adrenaline pumping let me tell you. I learned a good hard lesson about orientation during RTH.

The more I think about it the less use I have for obstacle avoidance ("OA") for the type of flying I do. Waypoint missions are subject to OA false alarms when facing the sun putting the whole carefully arrived at mission timetable in jeopardy, RTH can be turned into a death trap for the MP (in this bizarre example at least) and speed is sacrificed. Unless I'm flying in, around and amongst obstacles the OA has more ways to harm me than it does to help.


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[markagarcia]

Wondering if having a polarizer/ nd filter would help with this since it protects from the glare...

Unlikely, the front Obstacle Avoidance system utilizes data from the 2 sensors to the left and right of the camera(the two things that look like eyes on the front of the Mavic)- NOT data from the actual camera so putting polarizing lens on the camera would not help.

 

[sonof40]

Yes, I turn mine OFF and still wish for an ATTI mode switch.

 

[Xcel]

I typically fly with OA off specifically because of the sun induced false alarms and the speed penalty of operating with active OA.

OA speed penalty? I haven't heard about this one, are we talking top speed or just faster acceleration? How much faster is it in normal mode without OA?

 

[davmcw]

The more I think about it the less use I have for obstacle avoidance ("OA") for the type of flying I do. Waypoint missions are subject to OA false alarms when facing the sun putting the whole carefully arrived at mission timetable in jeopardy, RTH can be turned into a death trap for the MP (in this bizarre example at least) and speed is sacrificed. Unless I'm flying in, around and amongst obstacles the OA has more ways to harm me than it does to help.


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Plus, flying with OA on makes it difficult to overcome headwinds because of the speed and angle of attack limitations associated with OA...a real killer in waypoint missions and just generally a buzz kill. Funny how the feature I thought I HAD to have only seems useful in limited situations and harmful in ways I couldn't have anticipated and now is something I could definitely live without.


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[AlanTheBeast]

The RTH Obstacle Check can be turned off in the advanced settings section of the Visual Navigation Settings menu. Mine will be off from now on and assuming I have my RTH height set appropriately I should have no problem.

If I'm wrong on this one someone let me know. I tried uploading a phone video of the flight path log but it was too big but it shows just what I described above.

I've come to the same conclusion.

 

[rydfree]

This is a well documented characteristic of the Mavic on all forums . Disable OA in the RTH menu if you feel that you will be returning to home facing the sun . It uses the two sensors facing forward that look like eyes as someone stated , not the camera . I disable OA in the general menu as well as in the RTH menu as I have no use for it . Just select a RTH altitude that will be above all objects in your RTH path .

 

[davmcw]

OA speed penalty? I haven't heard about this one, are we talking top speed or just faster acceleration? How much faster is it in normal mode without OA?

It depends but top speed with OA on is maybe 20mph. With OA off maybe 30mph. In sport mode 40+mph. Try it. The governed speed with OA is presumably to give the MP time to react to obstacles detected. Really it is an angle of attack limitation meaning that a 15mph wind is difficult to make forward progress against with OA on.


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[Xcel]

Thanks, I normally just flick it into sport mode if I notice the speed dropping in a headwind. Going to see how it feels to fly it in normal mode without OA tomorrow.

 

[davmcw]

This is a well documented characteristic of the Mavic on all forums . Disable OA in the RTH menu if you feel that you will be returning to home facing the sun . It uses the two sensors facing forward that look like eyes as someone stated , not the camera . I disable OA in the general menu as well as in the RTH menu as I have no use for it . Just select a RTH altitude that will be above all objects in your RTH path .

Yes, the OA's sensitivity to light is well documented. That's one reason why I fly with OA off. However, what was less easily anticipated was the little land mine I laid for myself by not ALSO turning off the RTH Obstacle Check. Seems like a "do no harm" kind of a setting. Who doesn't want an obstacle check during the most vulnerable of maneuvers...an emergency RTH? However, it is much more likely that I'll get stuck in this do loop when someday I'm inevitably facing the sun during an unplanned RTH than that it will actually save me from an obstacle if, as you pointed out, my RTH height is set appropriately.

 

[davmcw]

Thanks, I normally just flick it into sport mode if I notice the speed dropping in a headwind. Going to see how it feels to fly it in normal mode without OA tomorrow.

That works. There is a Youtube video somewhere of a guy doing a speed test with OA on vs OA off (non sport mode) and he got approximately the numbers I quoted. I did a similar test and got similar results.