Mavic does RTH without GPS signal!!!

[Alex ROV]

Hello everybody...

I found out something very strange today... It looks like that Mavic will RTH EVEN WITHOUT GPS!!! That's a reason for fly-aways... Where (and HOW) does Mavic go without GPS???

What happened: Normally I take off and land indoors, without GPS, without any issues... I take off, go outside, wait hovering to get GPS and set my Home, then I fly further... When I come back I enter my building, loose GPS and land. Today when I lost GPS indoor, about to land, I had a warning message "RTH route will cross a NFZ..." I had no GPS signal, but my Home point was only 20m away...

Where would she go in case of RTH??? Mavicland in China??? There is no NFZ anywhere near here... And how would she go there, without GPS to guide her???

Many thanks! Have you all safe and good flights!

 

[Cyberpower678]

I've never heard of such a thing. Did it actually initiate a RTH without GPS, or is this just speculation?

 

[msinger]

If the Mavic has no GPS signal, it should auto land at its current location when RTH is initiated.

Can we check out your TXT flight log? If so, please upload it here and post a link back here.

 

[Alex ROV]

I've never heard of such a thing. Did it actually initiate a RTH without GPS, or is this just speculation?

No, she did not... It was just a quick warning on RC window... But I understand that Mavic would fly somewhere in case of RTH (my command or failsafe event)...

 

[Cyberpower678]

No, she did not... It was just a quick warning on RC window... But I understand that Mavic would fly somewhere in case of RTH (my command or failsafe event)...

Not without a GPS signal. It would land right where it is instead.

 

[Alex ROV]

If the Mavic has no GPS signal, it should auto land at its current location when RTH is initiated.

Can we check out your TXT flight log? If so, please upload it here and post a link back here.

Yes, you can, Please! I don't know how to do get the TXT log... But I will do that...
In my case it would not to good if Mavic auto land, as I can manage flight her indoors without GPS... Losing GPS is a "normal scenario" for me but now that I got this message I'm afraid that Mavic will flight somewhere without GPS trying to RTH by herself...

Many thanks!

 

[msinger]

I don't know how to do get the TXT log

See the instructions here.

 

[Cyberpower678]

Yes, you can, Please! I don't know how to do get the TXT log... But I will do that...
In my case it would not to good if Mavic auto land, as I can manage flight her indoors without GPS... Losing GPS is a "normal scenario" for me but now that I got this message I'm afraid that Mavic will flight somewhere without GPS trying to RTH by herself...

Many thanks!

It won't fly anywhere without GPS on it's own. You can rest assured. Though it is theoretically possible to implement a GPSless RTH function with reasonable accuracy. Because the drone has 2 IMUs, 2 Compasses and a FVS and VPS, and because it has detailed information on the home point in the logs when each new point is added to the log, it uses mathematical calculations to update the home point relative to it's current positions as well as accurately calculate the new compass bearing needed to get home. But alas, they didn't do that, so it's a rather big bummer as the Mavic can technically do it, but it's programmers were too lazy.

 

[Alex ROV]

See the instructions here.

Many thanks for helping...

DJI Phantom & Mavic Flight Log Viewer - PhantomHelp.com

I also upload it to Healthy Drones, as said on that tutorial vídeo. A few notes:
- My take-off and land point is 24m altitude.
- I flight indoors in OPTI Mode (a few moments ATTI) but the log says P-GPS from the take-off until landing.
- I have OA disable but it still beeps a warning (its alright).
- I have secure landing disable.
- I don't see warning messages on the log.
- I had a few warning beeps in one point when coming back, mid air... I would like to know what was that...

 

[Kilrah]

You likely were just at the borderline where the signal is considered good enough to derive a position, but that position is wrong. That would last a couple of seconds at most then it would switch to either recognising the situation and disabling GPS completely (and RTH too as such) or regain a good enough position reading.

Anyway it is pretty well documented that you should avoid any situation that you know will result in loss/regain of GPS reception with the Mavic since due to the lack of user-selectable ATTI mode it can have pretty unpredictable reactions. Countless people have crashed indoors due to this.

 

[Alex ROV]

You likely were just at the borderline where the signal is considered good enough to derive a position, but that position is wrong. That would last a couple of seconds at most then it would switch to either recognising the situation and disabling GPS completely (and RTH too as such) or regain a good enough position reading.

Anyway it is pretty well documented that you should avoid any situation that you know will result in loss/regain of GPS reception with the Mavic since due to the lack of user-selectable ATTI mode it can have pretty unpredictable reactions. Countless people have crashed indoors due to this.

We should be able to manually select GPS ON/OFF, while leaving OPTI Mode available indoors... If that also switched OFF GPS receptors we could save power in a extreme low battery situation and visually bring Mavic back...

I really was afraid of that possibility at first, Mavic repositioning when regaining GPS... But than after some experience I BELIEVE that once the GPS is lost and Mavic changes to OPTI or ATTI, she does NOT try to reposition herself if gets the GPS back... Mavic just change back to GPS mode... But I keep watching... Thanks!

 

[Kilrah]

Until GPS disables itself a bit too late, or reengages a bit too early while the position is a bit erratic and it starts drifting, at which point you absolutely can't do anything.
If you've got stable OPTI it's good, but if OPTI becomes spotty too because of light/reflections/patterns then you're in a right mess.
Since I don't trust OPTI enough I wouldn't take the risk.

 

[msinger]

@Alex ROV, your Mavic was using the GPS data up until 32:09.4 in your flight log. At that point, it was switching between P-GPS and ATTI mode for a few seconds and then remained in ATTI mode for the remainder of the flight.

I'm not sure why your flight log shows your Mavic was flying in P-GPS the entire time. I've seen the same thing occur in a few other flight logs.

Here are the times where the flight modes were switching in the flight log:

...
32:09.3 GPS:YES
32:09.4 GPS:NO
...
32:09.8 GPS:YES
...
32:10.0 GPS:NO
32:10.1 GPS:YES
...
32:12.0 GPS:NO
...
32:13.0 GPS:YES
...
32:13.8 GPS:NO

 

[Alex ROV]

Until GPS disables itself a bit too late, or reengages a bit too early while the position is a bit erratic and it starts drifting, at which point you absolutely can't do anything.
If you've got stable OPTI it's good, but if OPTI becomes spotty too because of light/reflections/patterns then you're in a right mess.
Since I don't trust OPTI enough I wouldn't take the risk.

I never let go on sticks when indoor, I have my experience flying submarine robots over 12 years and I know we shouldn't rely on auto positioning systems... I only flight indoor moving on my way to open area or coming back to landing point, always under strong direct winds and around obstacles. It keeps switching between OPTI and ATTI but it's AMAZING how much Mavic is stable, anyway!

 

[Alex ROV]

@Alex ROV, your Mavic was using the GPS data up until 32:09.4 in your flight log. At that point, it was switching between P-GPS and ATTI mode for a few seconds and then remained in ATTI mode for the remainder of the flight.

I'm not sure why your flight log shows your Mavic was flying in P-GPS the entire time. I've seen the same thing occur in a few other flight logs.

Here are the times where the flight modes were switching in the flight log:

...
32:09.3 GPS:YES
32:09.4 GPS:NO
...
32:09.8 GPS:YES
...
32:10.0 GPS:NO
32:10.1 GPS:YES
...
32:12.0 GPS:NO
...
32:13.0 GPS:YES
...
32:13.8 GPS:NO

This flight I had "ATTI" on RC screen only for 2 secs when I was taking off... Than I flew up 40cm and it changed to OPTI... I flew out doors and got "No GPS" with green background on top status bar until it changed to GPS at same time I the home was set. Mavic (and RC screen remained GPS) all the way until I got back inside and OPTI was shown...

I noted that when outside, hovering 10m away from 12 floors building, at 6th floor altitude (24m) she goes up and down like 2 m range, fast... At this area Healthy Drones website tools shows red "weak GPS" signal... I checked IMU and compass, they were green. KP index was 3.

 

[JLane]

Just to be safe, when you're flying into or out of the indoor space, you should probably set your Mavic to Land or Hover in case of Signal Loss, instead of RTH.

 

[Kilrah]

I never let go on sticks when indoor

The problem is that your sticks will do nothing.
If poor GPS or opti signal tells the Mavic it's moving right fast, it will move left fast to correct more than you can compensate with the sticks.

Really, DO read the indoor crash threads. There are at least 2 per week.

 

[Alex ROV]

The problem is that your sticks will do nothing.
If poor GPS or opti signal tells the Mavic it's moving right fast, it will move left fast to correct more than you can compensate with the sticks.

Really, DO read the indoor crash threads. There are at least 2 per week.

That is a very good info! All others auto positioning systems that I know go disable when there's any input on sticks... Pilot always has priority over these systems. Thanks for clarify!

 

[JLane]

That is a very good info! All others auto positioning systems that I know go disable when there's any input on sticks... Pilot always has priority over these systems. Thanks for clarify!

Unfortunately, the Mavic seems to sometimes trust its systems more than it should. eg. When GPS signal is marginal, or there's compass interference, sometimes it would really be best to be in ATTI Mode, but the Mavic will hold onto GPS mode for a while. During times like this, the Mavic's flight controller can "fight" with your stick inputs, making them feel almost unresponsive (based on user reports).

To minimize the risk of this happening when indoors, you want to make sure the conditions are ideal for VPS to work in OPTI mode. That means making sure lighting is really good, and making sure the floor surface is not uniform and not reflective. As long as the Mavic can use the VPS system, that should avoid ATTI Mode problems.

 

[Robbyg]

@Alex ROV, your Mavic was using the GPS data up until 32:09.4 in your flight log. At that point, it was switching between P-GPS and ATTI mode for a few seconds and then remained in ATTI mode for the remainder of the flight.

I'm not sure why your flight log shows your Mavic was flying in P-GPS the entire time. I've seen the same thing occur in a few other flight logs.

Here are the times where the flight modes were switching in the flight log:

...
32:09.3 GPS:YES
32:09.4 GPS:NO
...
32:09.8 GPS:YES
...
32:10.0 GPS:NO
32:10.1 GPS:YES
...
32:12.0 GPS:NO
...
32:13.0 GPS:YES
...
32:13.8 GPS:NO

I am a bit confused by the way you indicate time. The flight lasted 14Min 32.8Sec. For reference are you using Ticks or something else?

I looked at the Graphs and I have seen this same pattern on my own Mavic when it is indoors. From the information I have looked at I came to the conclusion that if the IMU detects almost no movement it seems to keep the flight state in GPS mode even if the GPS signal has dropped out.

So far I have only seen this happen when the GPS drops out for very short periods and the craft has minimal movement. It might be DJI has programmed in a hysteresis effect in case the GPS lock is going in and out very fast. I think this flight demonstrates that happening.

I sampled the last bit of the Flight.

GPS Data


Movement during that period


Rob