Lost GPS mid flight, compass error also

[shadowbgd]

Ok, I had pretty scary situation few days ago.

I always power up drone, wait for GPS to get at least 12 sats, then take off. It got 16 sats, I take off, and couple of seconds later, it switched to ATTI mode, and headed toward woods at pretty decent pace. At the same time compass error also. Lucky, I was able to track it by eyesight and bring it back close to me, but it was very hard because it keep going in all directions. After agonizing couple of seconds (which seemed like eternity) it managed to get GPS signal again, and landed safely. It had clear sky, and no obstacles around.

Does anybody had similar issue? What can cause this issue?

It scared me pretty god. I have already lost my Spark because of similiar issue (lost GPS + high wind + lost track of it visually).

 

[Mossiback]

Sounds like a compass issue. It is good that you wait for 12 sats before takeoff but do you also verify that the actual heading of the MP and the map pointer agree before takeoff?

 

[shadowbgd]

Sounds like a compass issue. It is good that you wait for 12 sats before takeoff but do you also verify that the actual heading of the MP and the map pointer agree before takeoff?

No, I did not, I almost never look at the map, because I always track it by eyesight. Will do next time, good advice.

 

[Meta4]

I always power up drone, wait for GPS to get at least 12 sats, then take off. It got 16 sats, I take off, and couple of seconds later, it switched to ATTI mode, and headed toward woods at pretty decent pace. At the same time compass error also. Lucky, I was able to track it by eyesight and bring it back close to me, but it was very hard because it keep going in all directions. After agonizing couple of seconds (which seemed like eternity) it managed to get GPS signal again, and landed safely. It had clear sky, and no obstacles around.

From your description, the most likely explanation is that you launched from an area of magnetic interference that caused a yaw error which forced the drone into atti mode.
The most common cause for that is launching from a steel or reinforced concrete surface.
What can you say about your launch area?

The screenshots don't help but your recorded flight data might.
Go to DJI Flight Log Viewer | Phantom Help
Follow the instructions there to upload your flight record from your phone or tablet.
That will give you a detailed report of the flight.
Come back and post a link to the report it provides.

 

[shadowbgd]

This is link to flight record DJI Flight Log Viewer - PhantomHelp.com

 

[sar104]

This is link to flight record DJI Flight Log Viewer - PhantomHelp.com

That looks like interference at the takeoff location but, since the txt log doesn't record the magnetometer data, it's not particularly useful in these cases. The mobile device DAT file contains the definitive data.

Mobile device DAT file: How to retrieve a V3.DAT from the tablet

 

[shadowbgd]

Ok, I just read this thread: Compass error, Yaw error, IMU error and so on.

I did that, moved the drone couple of times after right after power up, to put it away from metal objects because of compass errors. Now I know to not move the aircraft after power up.\

Edit: @sar104 will try to upload DAT file

 

[shadowbgd]

This is DAT file

 

[Meta4]

This is link to flight record DJI Flight Log Viewer - PhantomHelp.com

That flight record and your later comments, fits what I suggested in post #4.
Your drone never lost GPS reception but the drone switched itself to atti mode which makes it lose GPS position holding.

Do you know what the source of magnetic interference was at the launch point?

 

[sar104]

This is DAT file

So yes - that was due to magnetic interference at the launch site. The compass (blue trace) initialized the IMU yaw (red trace) with a value of 163° (SSE). However, as soon as the aircraft took off and ascended (black trace) out of the local interference the compass value changed to around 90° (E) without any rotation of the aircraft. At that point the IMU and the compass disagreed by around 75°, hence the multiple compass errors and the switch to ATTI.

 

[shadowbgd]

@Meta4
I have attached the pictures of launch point and surroundings. I have marked the launch point with red X. There is metal all over the place, and the launch point was in middle of it.

That being said, I must say that I have fly at that same spot five times before, not a single time I got any compass warning. Not one. Also, as @sar104 has confirmed, the interference was *after* I took off, and was most distant from structures. Maybe I was lucky during previus flights, compass drift never cross the thresshold, and my moving the drone during power up was the last straw to finaly cross the thresshold of compass.

Now I know that I must check compass calibration before take off, do not move drone manualy after power up, and that compass error can turn off GPS mode. We should be able to have indication of compass interference level on the remote, as we have other information.

 

[sar104]

@Meta4
I have attached the pictures of launch point and surroundings. I have marked the launch point with red X. There is metal all over the place, and the launch point was in middle of it.

That being said, I must say that I have fly at that same spot five times before, not a single time I got any compass warning. Not one. Also, as @sar104 has confirmed, the interference was *after* I took off, and was most distant from structures. Maybe I was lucky during previus flights, compass drift never cross the thresshold, and my moving the drone during power up was the last straw to finaly cross the thresshold of compass.

Now I know that I must check compass calibration before take off, do not move drone manualy after power up, and that compass error can turn off GPS mode. We should be able to have indication of compass interference level on the remote, as we have other information.

No - you misunderstood my post. The magnetic interference was on the ground, before takeoff as you can see from the graph above. When the aircraft takes off and ascends the interference goes away and the compass changes to show the correct heading. That interference can be very local - extending as little as a few centimeters - which is why you might fly from the same general location and not always see problems.

There is no reliable way for the system to detect and warn about this kind of interference. All the compass knows about is the local magnetic field, which it measures to determine magnetic north. If that field is distorted, as it was here, then it gets north wrong, but it has no other reference to compare to. That's why it is important for the pilot to check the heading against actual north.

 

[shadowbgd]

Ah, I see, so the 163° was incorrect heading, and the 90° was the correct heading.

Well, I was thinking about how system currently detect that something is wrong, by how much IMU and Compass disagree. Having simple scale, or preemtive warning that something is odd, and that aircraft is about to switch to ATTI, will be helpful. Because on the display, when it displayed magnetic interference error, it was already in the ATTI mode, and I was already in panic. In the logs there are compass errors after takeoff, but were non on display. If I knew that there is compass error, and that will result in ATTI mode very soon, I would land immediately.

 

[sar104]

Ah, I see, so the 163° was incorrect heading, and the 90° was the correct heading.

Yes - that's what the data indicate. Do you recall if the aircraft was pointing east at takeoff?

Well, I was thinking about how system currently detect that something is wrong, by how much IMU and Compass disagree. Having simple scale, or preemtive warning that something is odd, and that aircraft is about to switch to ATTI, will be helpful. Because on the display, when it displayed magnetic interference error, it was already in the ATTI mode, and I was already in panic. In the logs there are compass errors after takeoff, but were non on display. If I knew that there is compass error, and that will result in ATTI mode very soon, I would land immediately.

The app warned you of the compass errors immediately that the compass and IMU began to disagree - the log entries are copies of what is displayed on the screen. There really is no more that it can do.

 

[Mossiback]

...Now I know that I must check compass calibration before take off, do not move drone manualy after power up, and that compass error can turn off GPS mode. We should be able to have indication of compass interference level on the remote, as we have other information.

You can move the drone manually after power up without causing problems. I hand launch and part of my takeoff routine is to point my MP in different directions to confirm that the pointers on the map and "radar" display match my actual heading. If they do not, that is a confirmation of magnetic interference (or a screwed up compass on my MP).

If launching from the ground, at least confirm that the direction your Mavic is pointing is accurately shown on the map before taking off.

 

[shadowbgd]

Yes - that's what the data indicate. Do you recall if the aircraft was pointing east at takeoff?

Honestly, I don't recall I only recall that I moved it and rotated it couple of times after power up, and before flight, but I don't recall orientation But anyway, I will pay more attention to compass before flight. I learned a lot in this thread (and others too). It scared me that much, that after that "incident" I didn't fly after anymore. I'm really bad pilot in ATTI mode. I already lost Spark in january because of ATTI.

The app warned you of the compass errors immediately that the compass and IMU began to disagree - the log entries are copies of what is displayed on the screen. There really is no more that it can do.

I don't recall seeing messages about compass error before ATTI, but then again, I was keeping my eyes mostly on the drone, not the screen. My friend told me that his expirence is that Android app does not always show all messages. And I know, as matter of fact, that android APP, has issues (C1/C2 buttons, Manual Focus...) and maybe this is also issue, but I cannot confirm, I was primarly looking at the drone, not screen that flight.

@Mossiback thank you, will keep your advice in mind in future. This is my third DJI drone, compass was always non issue, always spot on, so I grew pretty confident in it. But as my local DJI dealer told me today: "you fly them, they listen very good, you get confidence over time, and then one day they don't listen, and it gets scary. People who don't drive drones, don't know how stressfull it is. Every. Single. Flight."

 

[jeplane]

Prevent a Crash or Flyaway with Your DJI Drone (The Complete Guide) | Phantom Help

I found some good advice in this link. Pls go thru it. Taking off with magnetic interference is listed, and sadly, it is quite common...

 

[sar104]

Honestly, I don't recall I only recall that I moved it and rotated it couple of times after power up, and before flight, but I don't recall orientation But anyway, I will pay more attention to compass before flight. I learned a lot in this thread (and others too). It scared me that much, that after that "incident" I didn't fly after anymore. I'm really bad pilot in ATTI mode. I already lost Spark in january because of ATTI.



I don't recall seeing messages about compass error before ATTI, but then again, I was keeping my eyes mostly on the drone, not the screen. My friend told me that his expirence is that Android app does not always show all messages. And I know, as matter of fact, that android APP, has issues (C1/C2 buttons, Manual Focus...) and maybe this is also issue, but I cannot confirm, I was primarly looking at the drone, not screen that flight.

@Mossiback thank you, will keep your advice in mind in future. This is my third DJI drone, compass was always non issue, always spot on, so I grew pretty confident in it. But as my local DJI dealer told me today: "you fly them, they listen very good, you get confidence over time, and then one day they don't listen, and it gets scary. People who don't drive drones, don't know how stressfull it is. Every. Single. Flight."

The first compass error message was at 3.3 seconds, and it switched to ATTI after 9 seconds, so you would not have had much time to react anyway.

 

[Meta4]

I have attached the pictures of launch point and surroundings. I have marked the launch point with red X. There is metal all over the place, and the launch point was in middle of it.

The steel that caused the problem was invisible and much closer than you realise.
It was in the reinforced concrete surface you launched from.

That being said, I must say that I have fly at that same spot five times before, not a single time I got any compass warning. Not one.

Launching from reinforced concrete surfaces is a lucky dip.
Here's what's inside that concrete:

You can't see where the mesh is, how deep it is and where the crossing points or overlaps are or where they might have used thick steel rod.
It's possible to position the Mavic's compass so that it's not significantly affected but if you keep coming back and buying tickets in the steel mesh (un)lucky dip, one day, you'll get a losing ticket.

It's a good idea to avoid launching from reinforced concrete surfaces altogether.