Sudden GPS loss, wide drift on RTH

[erkme73]

Today is my first day with the MP. I decided to take one last flight right after sunset this evening, to test out the long exposure photo mode. When I took off, I had good GPS (16), and zipped around for about 10 min. By the time I was ready to land, it was too dark to take any usable video, and LOS orientation was getting challenging - so I triggered RTH. She came in just as expected until directly above the landing point (I have the RTH altitude set to 50m). As it got to the landing point (but before descending started), I received a "Satellite positioning off. Fly with caution" error That's an understatement. It switched to ATTI mode, as I canceled the RTH, and it continued to move quite rapidly south of my position. By the time I had my bearings, it was at least 500' away from where it should have landed. I reviewed the flight on the DJI app, and it shows zero stick input from the RTH break off point until I started the manual recovery - yet it was moving away from me much faster than wind speed.

On replay, the app continues to show 16 satellites, but the bars drop from full to one little red one. That was the first time (in about 10 flights today) that I saw that happen, and of course, it happened at the worst possible moment.

So my question is, what could cause the GPS signal to go from full to zero when up at 160' on a clear sky evening?

I have several years experience with a number of Phantoms running in ATTI, so I am not unfamiliar with the drift that happens when there is no GPS hold. This was much faster - as though the MP was actually providing directional input.

I calibrated the compass after the second flight (despite receiving no warning that it needed to be performed). I noticed a large circular pattern when hovering, and figured it may have to do with the compass still being set to Shenzhen.

Ideas?

 

[msinger]

Try uploading your TXT flight log here. It might help you figure out what happened.

 

[erkme73]

Thanks @msinger. Was hoping I'd never need your help with logs - as in most cases it means a lost/destroyed device. In this case, I just need someone to pat me on the back and say, "It's ok, you just did this wrong..." Confidence badly shaken as I had a very hard time getting this thing back home... Uploading now.

Is phantomhelp.com having issues, or is it normally very slow? Maybe it's my ISP. Been that kind of a day today.

 

[erkme73]

@msinger, would mind peeking at it? Anything obvious to you? Are there more details I can pull from either the controller or the MP itself?

 

[msinger]

I'm not familiar with some of the messages in your log. The "Satellite positioning off" message appears to have caused the switch to ATTI mode. There is a lot of erratic flight behavior at times when you had your hands off the sticks (they were centered). Perhaps the "Strong Interference to Remote Controller" message had something to do with that.

 

[Stefan350]

Have you updated the firmware?


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[erkme73]

Have you updated the firmware?


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Yes, before the very first flight. It is current.

 

[MavicMinh]

Not saying it was the culprit but if you had precision landing on in low light it wont work and the drone will just hover where it should have been descending and auto land. Complete sat lost and switching to ATTI is a different story. May be a bug? Was there any breeze that drifted your Mavic once it had switched itself to ATTI mode?

 

[erkme73]

No, it was a windless night. Once it switched to ATTI, it moved at 10-12 MPH to the south without any stick movement. Just before it aborted the attempt, it created an almost perfect circle around the true point it should have landed. I don't have an issue with it being unable to land - it's the taking off on its own that bothers me. I'm at fault for flying beyond good visible light, and I'm incredibly fortunate I was able to get my bearings and bring it back. By the time I gave it stick input, it was nearly 1000' south of the auto land location.

 

[erkme73]

Well, I just received word back from DJI. If I read their reply correctly, they claim that because it lost GPS signal during the descent phase, it switched to ATTI and VPS. At that point it began drifting. When it re-acquired the GPS, it created a new homepoint, and would have tried landing there. They suggested I turn on "dynamic homepoint", thereby insuring that the MP will land where the controller is. I'm not sure that's a viable solution, as often I walk beneath trees or larger obstacles to get shade. If RTH then tries to land there, I'll have other issues. Here's DJI's response:

Thank you for contacting DJI Technical Support.

We are so sorry to hear what happened to your unit. We do understand how you feel and for the inconvenience you are experiencing right now.

Here is what happened to you, you set your home point and your GPS is working good but when you initiated the RTH and when the unit was descending you had a weak GPS signal that is why it switch to ATTI mode which detects the VPS and detected other home point, what we best recommend is before flying your unit always do the Compass calibration and to set the Dynamic home point in your App, using this dynamic homepoint the unit will detect and land where the remote controller is.

Here is an where you can set it on your App,

Hope this will help. Thank you for your time and Happy Thanksgiving.

I replied asking if it is to be expected to suddenly in inexplicably lose GPS signal (despite showing 16 satellites) when flying in clear, cloudless skies at 200+ feet.

 

[phaeton]

I had a similar incident occur last week, when I was out flying. I was flying and had the Mavic up about 30m and hovering when I got the message that GPS signal was lost. The Mavic then veered South rapidly and slow yaw to the right. The Mavic went about 50m south, while I was trying to work the controls to keep height and get control back. Within about 30 secs, GPS signal was reinstated and I was able to bring the Mavic back.

For cause, I was attributing it to a bad compass calibration. When I first got the Mavic, I got a prompt to calibrate and I calibrated in my office (re-bar in the pad below and metal desk right beside) so a big no-no. I did have a 40 min (2 battery) flight the next day without incident, but I had GPS signal the whole time. Anyhow, I still need to calibrate the compass, but I'd be interested to find out if there's another cause.

 

[Kilrah]

Invalid compass data does cause what you experienced, not a very clear/helpful way to notify it but technically since compass is invalid it can't use GPS data anymore, and thus signals no GPS and signal bars down even if the root cause isn't actually GPS-related.

 

[The Editor]

Yes, as Kilrah mentions above, just because the aircraft switches out of GPS doesn't mean it has lost GPS.
The algorithms are written in such a way that if you have a bad calibration then DJI have decided that switching to Atti mode temporarily is preferable to the aircraft going into TBE. This would be catastrophic for new inexperienced flyers and so it is thought Atti the lesser of two evils.

 

[erkme73]

Yes, as Kilrah mentions above, just because the aircraft switches out of GPS doesn't mean it has lost GPS.
The algorithms are written in such a way that if you have a bad calibration then DJI have decided that switching to Atti mode temporarily is preferable to the aircraft going into TBE. This would be catastrophic for new inexperienced flyers and so it is thought Atti the lesser of two evils.

But does that mean when GPS comes back it automatically creates a new homepoint wherever it happens to be?

 

[Mako79]

But does that mean when GPS comes back it automatically creates a new homepoint wherever it happens to be?

It shouldn't create a new home point unless you instruct it to or turn it off and on (which is impossible in midair).
GPS>ATTI is compass skitzing out and the Mavic defaulting to ATTI to prevent flyaway. Symptoms is the massive arc you see. That's a giant size toilet bowl effect its trying create.

Go to an open grassy field and take your pants off and redo a compass calibration. AND REMEMBER, there is no need to do compass calibrations at every flight-sight. Only do it if stated or if you have travelled 50km+ where the magnetic declination may occur.

 

[niclariv]

How is it that some people can get away with never calibrating it? Even the manual and quick start guide don't say to calibrate it before the first flight, yet obviously it's travelled more than 50km from where it was made


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[erkme73]

How is it that some people can get away with never calibrating it? Even the manual and quick start guide don't say to calibrate it before the first flight, yet obviously it's travelled more than 50km from where it was made


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And, I actually did calibrate in an open grassy area on the second day that I had the MP. I received no errors, but did notice some TBE. Despite this, a couple days later, I get this flight (that started this thread). I just recorded my phone's screen to show the stick inputs in the last few minutes of the flight.

Initially, before I initiated the RTH, it was making an ever-increasing circular flight (TBE)... It then came home, but just before starting the descent above the homepoint, it bugged out:

 

[Kamilo]

Hi guys. Had same problem with controlling the Mavic as well. Looks that it is only happening while battery is around 25% or less and very close to landing area + ~ 50'. Kind of scary experience.

 

[erkme73]

Well this is the latest response from DJI support. I can't tell if I'm just not understanding their answer, or if they're simply not addressing my concerns.

Thank you for providing these information's. With regards to your concern, one factor that caused the GPS signal dropped is the interference within the area. The unit shifted to Atti mode when GPS is low or lost and use VPS Barometer to stabilize but it is still GPS assisted and will continue to Return to your set home point, however, you have mentioned on the initial email that it was dark and did not notice how far the unit drifted away. While the unit is in Atti Mode it uses VPS barometer and Vision sensor on the bottom of the Aircraft to locate the homepoint, since it is dark the VPS Vision Sensor cannot locate the recorded or exact Home point Location.

We recommend you to fly in a different location without any interference and check the flight mode every flight.

I mean, I understand that ATTI mode will drift, but at 12mph? And the seemingly random movements w/o stick input? Confidence shaken.

 

[bluesgeek]

Similar here too. RTH and directly overhead at about 40m. Wide open field on a hilltop. GPS lost then came back.