Compass Error - What is it about?

JB63 · Jan 12, 2017

There are many threads about this topic lately, and I thought it might be a good idea to get the community's feedback on this 'serious' issue.

I have encountered a compass error on my P3S. It happened after I upgraded a firmware. I was flying in P-GPS mode, then suddenly the AC start drifting at high speed (30 mph), switched from P-GPS to ATTI by itself, and my drone almost crashed against the neighbor's house. I also spoke to DJI regarding this and their answer was that it is normal for AC to switch from P-GPS to ATTI mode when there is a compass error (ok, what else is new?). My flight log does say that there was a compass error ... so everything checks out.

Here is what I do not understand: A compass only helps the AC with orientation relative to North (magnetic or true). Why would a compass error disable GPS? Orientation and GPS are totally NOT related. To me, if there is a compass error, the aircraft would start spinning on itself, and NOT fly-away like it did for me and many other hobbyists. Anyone understands or has an explanation for this riddle?

I also found the info below on DJI's website:

Phantom 3 Standard - Specs, FAQ, manual, video tutorials and DJI GO - DJI

22. How can I safely operate the aircraft when encountering compass error?
A compass error may occur when the aircraft is flying close to strong electric magnetic sources (e.g. power transmission lines). The Aircraft Status Indicators will blink red and yellow rapidly when a compass error occurs and the DJI GO app will display one of the following messages:
- Compass error, calibration required
This warning message indicates the aircraft is receiving abnormal compass readings. It is recommended that you power off the aircraft and re-calibrate the compass at a different location and then resume the flight.
- Compass error, exit P-GPS Mode
This warning message indicates that the aircraft is drifting severely. Bring the aircraft to a higher altitude to acquire connections with enough GPS satellites when this warning message appears. The flight controller will automatically adjust the orientation of the aircraft in the midair to mitigate the drifts. The aircraft will switch back to P-GPS mode when the automatic adjustment is completed.

beakeru2 · Jan 12, 2017

I had a very different response from DJI when I called them about my compass error...

As soon as I told them about the error.. they told me "send it back"... They (the female I spoke with) didn't even want to run me through the gamut of trial and error procedures that they normally do when I call with a firmware bug... This chick just said , "thats bad....send it back"...

....ref: mine was a p3p purchased refurbished on fleabay.. Within 15 days , had no trouble with return..

Meta4 · Jan 12, 2017

JB63 said:
Here is what I do not understand: A compass only helps the AC with orientation relative to North (magnetic or true). Why would a compass error disable GPS? Orientation and GPS are totally NOT related. To me, if there is a compass error, the aircraft would start spinning on itself, and NOT fly-away like it did for me and many other hobbyists. Anyone understands or has an explanation for this riddle?

Why would a compass error disable GPS?
Orientation (compass data) and position (GPS data) are different but related.
When the Phantom is moving and getting position information from the GPS that tells the Phantom that it is travelling in a different direction from what it's getting from compass data, The Phantom's programming can't work with the data conflict.
The solution is to drop data from one source and since the Phantom can't fly without a compass but can without GPS, it's the GPS data that gets dropped.

There have been some compass problems with Standards lately or your issue may have been related to something you did.
I'd be curious about when and where you calibrated it and where you launched from as these may be relevant factors.
You can also see the flight data to learn more and perhaps solve the mystery of your incident.
Go to https://www.phantomhelp.com/LogViewer/Upload/
Follow the instructions to upload your flight record.

JB63 · Jan 12, 2017

Meta4 said:
Why would a compass error disable GPS?
Orientation (compass data) and position (GPS data) are different but related.
When the Phantom is moving and getting position information from the GPS that tells the Phantom that it is travelling in a different direction from what it's getting from compass data, The Phantom's programming can't work with the data conflict.
The solution is to drop data from one source and since the Phantom can't fly without a compass but can without GPS, it's the GPS data that gets dropped.

There have been some compass problems with Standards lately or your issue may have been related to something you did.
I'd be curious about when and where you calibrated it and where you launched from as these may be relevant factors.
You can also see the flight data to learn more and perhaps solve the mystery of your incident.
Go to https://www.phantomhelp.com/LogViewer/Upload/
Follow the instructions to upload your flight record.

Thank You. While I agree that the GPS and Compass data 'might' be related, this is true only during motion (because of kinematics). In my specific case, the AC was only hovering and was not undergoing any motion. I've been using professional grade GPS (e.g. RaceLogic VBox and SoMat D-200) and other fancy sensors in my line of work for many years and all of this is rather perplexing. In my specific situation, the series of events is like this:
1. I flew the AC many times before, never an issue
2. I updated the firmware
3. I go out, launch the app. Somehow this time the app DOES NOT ask for compass calibration, so I did not do it. (In the past, I calibrated the compass before every flight)
4. I started in P-GPS mode, got the AC to about 3-4 feet height and hovered for a short while.
5. I pushed the AC up (climbed) to about 20, hovered again (there was a slight breeze, nothing serious)
6. Then suddenly, the AC takes off sideways and almost crashed into the neighbor's house.
7. I somehow managed to regain control, brought it down, and that was the last time I flew my drone.

I then downloaded the log file, and it shows clearly that there was a compass error, followed by P-GPS being disabled, and replaced by ATTI mode. This lasted few but very long seconds. The log says that the AC reached a horizontal speed of about 20 mph (rather scary).

No, I did not do any crazy maneuvers to cause any of this, the AC was just hovering at some height when it took off.

Back to GPS vs. Compass. I agree that the GPS trajectory can be used to compare how the AC is moving vs Compass direction and the rest, to give feedback the the AC, but GPS is NOT very precise so I am not sure how the interconnection plays out.

There are few flavors of high precision GPS that I use at work, these include RTK and D-GPS (Real Time Kinematics and Differential-GPS) but I do not think the DJI has such fancy sensors.

I chatted with DJI, and she lady mentioned the switch from P-GPS to ATTI mode is normal AC behavior when there is a compass error. Still, to me, why would there be a sudden compass error in the first place? The lady also mentioned possibly the firmware update did not execute properly and that is something I have yet to try.

Meta4 · Jan 13, 2017

JB63 said:
I chatted with DJI, and she lady mentioned the switch from P-GPS to ATTI mode is normal AC behavior when there is a compass error. Still, to me, why would there be a sudden compass error in the first place? The lady also mentioned possibly the firmware update did not execute properly and that is something I have yet to try.

Unfortunately DJI "support" people are not a good source for actual information.
Your firmware update had zero effect on anything compass related.
I'd still like to know about your compass calibration and launch site if only to eliminate that from the usual suspects.
Can you post a link to the Phantomhelp analysis?

Phantom-Paul · Jan 13, 2017

The bottom line is that with the Phantom series you get enough compass errors it will automatically switch to attitude mode only. It doesn't matter what is technically possible that's just the way the Phantom works. What we really need to find out is what caused the compass errors. Bad calibration , electrical interference, taking off from metal impregnated surface, intermittent fault on phantom compass etc.
It is a sad fact that it could happen any time your flying so it pays to be alert and also practice flying in atti mode just to give you a fighting chance.

After you did the FW update did you do all the callibrations again before flying?
Sorry got to ask the obvious because it wasnt mentioned in your post just to eliminate that.

Sent from my SM-G900I using PhantomPilots mobile app

Meta4 · Jan 13, 2017

Phantom-Paul said:
The bottom line is that with the Phantom series you get enough compass errors it will automatically switch to attitude mode only. It doesn't matter what is technically possible that's just the way the Phantom works. What we really need to find out is what caused the compass errors. Bad calibration , electrical interference, taking off from metal impregnated surface, intermittent fault on phantom compass etc.
It is a sad fact that it could happen any time your flying so it pays to be alert and also practice flying in atti mode just to give you a fighting chance.

After you did the FW update did you do all the callibrations again before flying?
Sorry got to ask the obvious because it wasnt mentioned in your post just to eliminate that.

Sent from my SM-G900I using PhantomPilots mobile app

Despite popular opinion to the contrary, no recalibrations are required after a firmware update.

Sent from my SM-G900I using PhantomPilots mobile app

Phantom-Paul · Jan 13, 2017

Are you willing to risk your bird on that ? (Or worse)
A thorough going over after major changes doesn't take long and how many times have the settings in the app changed either back to default or worse to complete garbage after a major update.
Common sense says as a pilot you should do a thorough pre take off check including all instrument readings and I will agree with you that if the compass readings are in the norm then I definitely would not do unnecessary calibrations.
However we have no idea what the new firmware does to the hardware during install. Does it / will it reset the hardware , will it clear out previous settings ? Its something you cannot see or check so best bet is start as if its a virgin aircraft and follow the process from beginning to end for a known base setting.

Sent from my SM-G900I using PhantomPilots mobile app

Mako79 · Jan 14, 2017

I think the OP has taken a bad compass snapshot (some pilots compass calibrate at every location). This can be a bad thing if surrounded by metal structures. Think of this, true north at ground level with metal structures is different 400ft up. This is why its important to get a clean primed compass in an open grassy field free any influences.

If your craft is still showing GPS NUMBERS (more than 7) but switching to ATTI then there is a high chance that the Phantom is getting erroneous compass data and attempting to do a flyaway. The safe guard is to switch back to primitive ATTI mode to prevent the flyaway.

JB63 · Jan 14, 2017

Meta4 said:
... since the Phantom can't fly without a compass but can without GPS ...

Not sure I agree with the logic behind this decision from DJI (assuming they setup such priority on purpose) ... What is so important from compass readings, that the drone can't do without? ... If I have to choose, I'd drop the compass and keep the GPS ... Anyone knows?

Meta4 · Jan 14, 2017

JB63 said:
Not sure I agree with the logic behind this decision from DJI (assuming they setup such priority on purpose) ... What is so important from compass readings, that the drone can't do without? ... If I have to choose, I'd drop the compass and keep the GPS ... Anyone knows?

A Phantom can't fly straight without compass so it's not so easy - that's the way it's programmed..
But unless you have a weird hardware error, there's no excuse for getting compass errors.
The only times I see compass errors are when I put the Phantom down on a car roof or reinforced concrete or descend to within a metre of a steel roof to demonstrate a compass error.
I've never seen one in normal flight.
But I see accounts of compass errors every week from people that calibrate-every-time and screw up a perfectly good compass calibration.

JB63 · Jan 15, 2017

I flew my bird few times from my front yard, and everytime I did a compass calibration BEFORE the fligt. This specific day (with the compass error) the dji-go app did not ask for calibration. Also, I did update the firmware the night before. The update itself did not go right the first time. It somehow aborted then asked me to repeat, which I did, and then it quickly said it was ok, and that the firmware was successful. After I spoke with DJI, they suggested I check files on the SD, and when I did, it said in there that the firmware update failed. I've been reading a lot about the firmware update being very flaky and even DJI acknowledges that it could abort few times. What bothers me here is, I followed the upgrade to the letter, yet I managed to get an error, and the app said the upgrade was successful, whereas the file on the SD card said it failed ... This is rather upsetting that DJI has a very flaky/sensitive firmware update procedure, and that a diagnostic is NOT documented anywhere. I now need to redo the firmware, and check before trying to fly.

Tim Dawes · Jan 15, 2017

For about two months DJI Go app has indicated the compass on my P3s needs calibration and when I do always indicates failure. I returned the bird twice for repair afterwhich I received the same reading on the app but the lights indicated it had done so. When I called DJI again they told me they had, in fact, swapped out my P3 for brand new ones the two times I returned it and the must be something with my Galaxy S4 (hadn't been in the past). I've cleared the cache and uninstalled/reinstalled and updated firmware but still get compass calibration failure.
Litche app says the P3 calibrates ok.
I've since flown with no problem (Go app still reads same).....weird!

JB63 · Jan 18, 2017

It appears to me this compass error issue is here to stay, and we all need to learn how to fly in ATTI mode, to try and save the bird should an issue happen in the middle of a flight.

Sent from my iPad using PhantomPilots

sinxa · Jun 29, 2017

JB63 said:
Here is what I do not understand: A compass only helps the AC with orientation relative to North (magnetic or true). Why would a compass error disable GPS? Orientation and GPS are totally NOT related. To me, if there is a compass error, the aircraft would start spinning on itself, and NOT fly-away like it did for me and many other hobbyists. Anyone understands or has an explanation for this riddle?

I see you have missunderstanding on positioning.
To be clear, positioning take 2 sensors into account which are GPS and COMPASS.
If one or both of them fail, then there will be no positioning available.
You have miss understood as GPS=POSITIONING. No, its not like that.

You can imagine your self, GPS is like you got a map, and compass is like you know where is you're heading at the moment.

If you know your position on the map, and you need to go some where else on the map, how can you decide which way to go if you don't know which dirrection are north or south or west or east?
It applied vice versa. If you know your current heading, but you dont have any idea where are you on the map right now, how would you do to get into your desired destination?

Mix_ · Jun 30, 2017

sinxa said:
I see you have missunderstanding on positioning.
To be clear, positioning take 2 sensors into account which are GPS and COMPASS.
If one or both of them fail, then there will be no positioning available.
You have miss understood as GPS=POSITIONING. No, its not like that.

You can imagine your self, GPS is like you got a map, and compass is like you know where is you're heading at the moment.

If you know your position on the map, and you need to go some where else on the map, how can you decide which way to go if you don't know which dirrection are north or south or west or east?
It applied vice versa. If you know your current heading, but you dont have any idea where are you on the map right now, how would you do to get into your desired destination?

I think, no - GPS gives just XYZ position (ok, Lat, Lon and Elev above geoid).
As far I remember from using old-good consumer gps'es just months after cancellation of SA on may of 2000, in case the unit doesn't has built-in electronic compass, the North indication works plain based of position change. So, no movement - no real North direction/orientation.

With The Birds · Jun 30, 2017

JB63 said:
Not sure I agree with the logic behind this decision from DJI (assuming they setup such priority on purpose) ... What is so important from compass readings, that the drone can't do without? ... If I have to choose, I'd drop the compass and keep the GPS ... Anyone knows?

The compass is required not purely to determine heading, it is also used to determine yaw rate. I'd rather be comfortable flying in atti mode without trying to compensate for random spinning of the AC.

lb650 · Jul 4, 2017

Meta4 said:
A Phantom can't fly straight without compass so it's not so easy - that's the way it's programmed..
But unless you have a weird hardware error, there's no excuse for getting compass errors.
The only times I see compass errors are when I put the Phantom down on a car roof or reinforced concrete or descend to within a metre of a steel roof to demonstrate a compass error.
I've never seen one in normal flight.
But I see accounts of compass errors every week from people that calibrate-every-time and screw up a perfectly good compass calibration.

Well I'm thinking there must be different variations of compass error, because probly 50% of the time I get compass errors both in pre flight & during With no affect on flight or auto switching to atti mode, I have tried all solutions to correct the error from making sure you have no metal on your person to turning my cell phone off,(I use an iPad Air 2 3G ) to calibrating in an open field miles from nowhere. I don't calibrate unless it asks. It will ask for calibration in a field totally open,no power lines, no nothing, so I do it & all is good then 5-10 min in to flight the compass error comes up, but has no affect on control of the AC. Even when I'm out 2 km AC still functions perfect