Compass calibration

[Storyline]

The calibration isn't related to any particular geographic location.
It's just to let the flight controller understand which magnetic fields are part of the drone, so it can ignore these and everything else should be the earth's normal magnetic field.
My main Phantom 4 pro is nearly 2 years old and I have never calibrated anything on it.
I work it hard every week and have made >500 flights for almost 3000 km and have traveled 2200 km North/South & 4800 km East/West with it.
It never misses a beat and still performs just as it did the day I pulled it out of the box.

All interesting stuff and you are right there is a lot of misinformation regarding the compass. The thread I remember reading on the DJI forum seemed to go into quite a bit of detail but I have no idea how qualified any of the people were to comment and although I have only had my Mavic a short time I have learned not to believe in all that DJI says !
I have come across gyro compasses, mostly marine ones, a few times and all have needed a similar calibration but never in two axis before. Now you have explained it they will all be doing the same thing and that is taking their immediate environment into account. The one I use most is a steering compass on our boat and that needed 3 x very slow and large (1/2 mile) diameter circles to pass calibration but I have never had to do it again. That is accurate to within a degree or so. Of course it is surrounded by all sorts of magnetic influences, not least a half ton engine a few feet away. To be honest though I would never trust it in anger and will always use the traditional needle floating in liquid if required. The only reason we have it is that it can output data to other instruments that need it such as the autopilot.

Anyway, to get back to our Mavics, do you know how the compass is actually used by the aircraft ?
Edit: have just looked into this and as I suspected it appears to be quite complex but put simply it helps with course control. Here is good article Why would a drone need a magnetometer? Are an accelerometer and a gyroscope not sufficient?

 

[Storyline]

I am not saying that Meta4 is wrong about this calibration issue as I am just not qualified to comment but there does appear to be a lot of sources who say that calibration is needed when there is a change in magnetic variation (aka declination) which occurs with a change in location. Some of these sources are related to our size drones but others are more to do with commercial sized UAV's. The problem with the internet you just don't know the veracity of any of this and it could be that it is all based on a widely held but erroneous belief. I have read virtually nothing about magnetic anomalies that I thought caused a need but to be fair these are not that common and the only reason I know about them is that they are marked on marine charts.

The other thing I have read several times that might be relevant or not is that the compass (we have two) might be affected by close proximity to a strong magnetic force eg a loudspeaker magnet and if you suspect that, a calibration is needed. Further down the credibility stakes are bumpy landings and impacts.

So, I suppose all you can do is do what you feel comfortable with. I have read quotes from DJI who say it is necessary but any company that can allow users to download an app with an offline map function that appears to work but actually doesn't cannot be relied on

 

[Meta4]

I am not saying that Meta4 is wrong about this calibration issue as I am just not qualified to comment but there does appear to be a lot of sources who say that calibration is needed when there is a change in magnetic variation (aka declination) which occurs with a change in location. Some of these sources are related to our size drones but others are more to do with commercial sized UAV's. The problem with the internet you just don't know the veracity of any of this and it could be that it is all based on a widely held but erroneous belief. I have read virtually nothing about magnetic anomalies that I thought caused a need but to be fair these are not that common and the only reason I know about them is that they are marked on marine charts.

And all the sources that recommend frequent compass calibration don't understand what compass calibration does and why or when it might be needed.
Their beliefs about compass calibration are just superstition and not backed up by any facts.
Read my post #17 again and see if that makes sense to you.
Or check what DJi have said about calibrating the compass in the P4 pro manual:

Unfortunately DJI aren't the best at communicating clearly and they haven't updated all other manuals.
But this is the company that can't include a note in the manual to say: Never launch from steel or reinforced concrete surfaces .... even though Phantoms and Mavics are lost each week because of this.
If you doubt my qualifications to tell you this, look me up over on Phantompilots

 

[Storyline]

.....
Or check what DJi have said about calibrating the compass in the P4 pro manual:

Unfortunately DJI aren't the best at communicating clearly and they haven't updated all other manuals.
But this is the company that can't include a note in the manual to say: Never launch from steel or reinforced concrete surfaces .... even though Phantoms and Mavics are lost each week because of this.
If you doubt my qualifications to tell you this, look me up over on Phantompilots

Yes, I understand what you are saying. I find it amazing that a company that can design and more importantly reliably build such advanced machines can allow such blatant errors. Given the rapid rate of product development they can easily slip in but not to correct misinformation after they have been informed is unacceptable. .

I do also understand the points in post 17 but all I was doing in my reply was pointing out the large number of authoritative sites who say to calibrate for deviation. Again I say it is possible they are wrong. This is quite a new industry with great pressure to get new models released so by the time the manuals are being produced for a product the design engineers are already busy on the next version. So it is possible there is some cutting and pasting going on and falsehoods being perpetuated.

I do not know how to get a positive answer. There is no point in asking DJI as I have asked a couple of technical questions recently and got pathetic replies.

 

[Meta4]

all I was doing in my reply was pointing out the large number of authoritative sites who say to calibrate for deviation. Again I say it is possible they are wrong. This is quite a new industry with great pressure to get new models released so by the time the manuals are being produced for a product the design engineers are already busy on the next version. So it is possible there is some cutting and pasting going on and falsehoods being perpetuated.

I do not know how to get a positive answer. There is no point in asking DJI as I have asked a couple of technical questions recently and got pathetic replies.

I would suggest that anyone telling you that you need to calibrate for deviation, is not an authoritative source at all.
They might project confidence but what they are doing is spreading is ignorance.
If they don't understand something as basic as the compass, what other myths are they spreading?

What calibrating does is well understood.
Over on Phantompilots, they have been discussing this in lots of detail for five years and a bunch of clever members have worked it out pretty well.
There is no doubt that magnetic deviation is irrelevant to the drone and there is no way calibrating the compass could do anything about it anyway.
Once you understand what calibrating the compass actually does, the myths just don't make any sense.

 

[Storyline]

Thanks Meta, I will head over to Phantompilots.com to have a read.
It is extraordinary though how the wrong information can be so widespread on the 'net. A quick Google search on should I calibrate my drone compass will bring up lots of sites nearly all saying that you should. As I say, some of these are big companies and respected drone sites. It just goes to show you can't always trust what you read.

I am just wondering if some of this confusion stems from the fact that I read somewhere that there are two types of electronic compass. One is called simple north seeking and the other magnetic and is designed for use in navigational systems. The same site said that a compass can use deviation data from a GPS signal. It implied that this data was not calculated on board using it's position but was actually broadcast in some way from the satellite

 

[Storyline]

I now (in layman's terms) think I understand what calibration does and that is it compensates for what are called hard and soft iron effects. When the compass is switched on it should produce a perfectly spherical 'image' that the earth's magnetic field can be measured against. At first it does not know it's north south axis so the first crude adjustment (which may be done in the factory) is to determine when the north south axis is parallel to the aircraft/sensor. This obviously only needs doing once. Then it also has to take into account these hard and soft iron effects which are caused by metals within the airframe. Hard irons do not alter the shape of the sphere but pull it one way or the other and as you calibrate it is centred. The soft iron (things like nickel and some other metals in pcb's or other components cause the sphere to deform and the two axis calibration produces a perfect sphere again. Therefore as Meta4
points out, anything you add to the airframe will mean a calibration is a good idea. In my case I added a flashing strobe so it was good to calibrate for that. I can't find anything (technical) about the need to calibrate for variation/deviation and yet so many sites say you should do it - weird !

 

[sar104]

I now (in layman's terms) think I understand what calibration does and that is it compensates for what are called hard and soft iron effects. When the compass is switched on it should produce a perfectly spherical 'image' that the earth's magnetic field can be measured against. At first it does not know it's north south axis so the first crude adjustment (which may be done in the factory) is to determine when the north south axis is parallel to the aircraft/sensor. This obviously only needs doing once. Then it also has to take into account these hard and soft iron effects which are caused by metals within the airframe. Hard irons do not alter the shape of the sphere but pull it one way or the other and as you calibrate it is centred. The soft iron (things like nickel and some other metals in pcb's or other components cause the sphere to deform and the two axis calibration produces a perfect sphere again. Therefore as Meta4
points out, anything you add to the airframe will mean a calibration is a good idea. In my case I added a flashing strobe so it was good to calibrate for that. I can't find anything (technical) about the need to calibrate for variation/deviation and yet so many sites say you should do it - weird !

That's basically correct, but a rather complicated way of looking at a very simple concept. The aircraft use 3-axis solid-state magnetometer packages, which consist of 3 orthogonal magnetic detectors aligned with what would be called the principle axes of the aircraft, forwards (x axis), right (y axis) and down (z axis). These measure the components of the local magnetic field in those directions and those data, when combined, give the direction and strength of the magnetic field.

However, the local magnetic field is, in the absence of interference from nearby ferromagnetic materials such as vehicles, rebar etc., a linear sum of the earth's magnetic field (needed by the IMU) and the aircraft's magnetic field (due to onboard ferromagnetic components and an unwanted perturbation on the earth's field). The calibration process is primarily to permit the FC to measure the aircraft's magnetic field, which stays constant relative to the magnetometers, and then use that to subtract from the total measured field in flight, leaving just the field that it needs (the earth's). It likely also serves to tune the magnetometer's scale and bias.

Once calibrated, and with no further information, the magnetometers indicate magnetic north, which differs from true north, that the IMU needs, by the local magnetic declination (a.k.a. variation), that itself varies by location. However, the calibration process does not provide any way to measure declination since true north is arbitrary. Instead, the FC computes the declination once it has a GPS position lock from a global magnetic model in the firmware, and notes that in the onboard log, e.g.

-79.686 : 2254 [L-NS][AHRS] wmm dec: 11.687672
-79.686 : 2254 [L-NS][AHRS] wmm inc: 59.062424
​

Now the FC has everything it needs to initialize the IMU yaw value and then to use the magnetometer data to correct for drift in the yaw values derived from the rate gyros during flight.

 

[Storyline]

Thanks sar104

So as the FC gets it's declination from GPS is it right to say that there is no need to calibrate when you move location ? If so, have you any idea why this is such a widespread myth ?

At the risk of repeating myself (which I will !) this has to be one big bit of misinformation. It is not just DJI that says to calibrate when you fly at at new and distant location -

Yuneec on the official site - Breeze uses a GPS and compass to keep its orientation. When you travel to a new place it is always recommended to do a compass calibration. Press “calibrate” while the drone is on the ground and follow the animations on screen.

Parrot do not specify how often to do it.

This from an agricultural survey company - Since the earth’s magnetic field changes depending on where you are, if you move your drone a large distance (several hundred miles) thiscause the drone to misread its heading and fly a few degrees off its true center, which will decrease flight efficiency and decrease the area of land that can be mapped during the flight.

 

[sar104]

Thanks sar104

So as the FC gets it's declination from GPS is it right to say that there is no need to calibrate when you move location ?

At the risk of repeating myself (which I will !) this has to be one big bit of misinformation. It is not just DJI that says to calibrate when you fly at at new and distant location -

Yuneec on the official site - Breeze uses a GPS and compass to keep its orientation. When you travel to a new place it is always recommended to do a compass calibration. Press “calibrate” while the drone is on the ground and follow the animations on screen.

Parrot do not specify how often to do it.

Correct. As @Meta4 mentioned, just changing location doesn't change anything that affects the calibration because the declination is computed, not measured. Adding, removing or changing aircraft components may necessitate calibration, as may exposing the aircraft to a strong magnetic field that changes its magnetic state or directly affects that scale/bias of the magnetometers.

And just to reinforce, if the aircraft complains of magnetic interference when powered up, then that is likely a very local (potentially just inches) effect and is a modification of the earth's magnetic field by another magnetic field. Calibration cannot help and attempting to calibrate with local interference (including magnetic jewelry or watches) will either fail or, if it takes it, potentially large compass errors in flight.

The correct recommendation (one of many that DJI has issued) is to calibrate when the aircraft requests it or when the compass status in the sensor section of the GO app shows other than green. My new M2P arrived in that state (yellow) and I calibrated it even though it was not bad enough to trigger a request.

 

[Storyline]

Thanks again sar104. Without wishing to sound patronising you sound like you know what you are talking about

I, on the other hand, have no scientific background so have to rely on determining a consensus from what results Google produces and sort of sorting the wheat from the chaff ! I don't think I can think of another situation where there has been so much incorrect info about a subject.

Have you (sar104) or anyone else any idea how this myth became so widespread ?

 

[sar104]

Thanks again sar104. Without wishing to sound patronising you sound like you know what you are talking about

I, on the other hand, have no scientific background so have to rely on determining a consensus from what results Google produces and sort of sorting the wheat from the chaff ! I don't think I can think of another situation where there has been so much incorrect info about a subject.

Have you (sar104) or anyone else any idea how this myth became so widespread ?

It goes back at least to the Phantom 2 days when there were problems with the firmware and declination correction. DJI put out all kinds of strange suggestions to fix the issue, including recommendations to calibrate if changing location. The implication was that it was related to declination even though it is simply impossible to determine declination by measuring the local magnetic field. It seemed like a case of the engineers and the writers not talking to each other. Later recommendation was removed, replaced by the advice to calibrate if there were problems or the FC requested it. Then the old advice crept back in some versions of some of the manuals. It's caused endless confusion and likely a fair number of crashes.

 

[Storyline]

I suspected something like that. As I said earlier, by the time the documentation is being produced, the engineers who understand such matters have moved on to designing the next aircraft.

I still find it extraordinary that the myth has persisted and if may be a good idea if someone could write a pithy and to the point explanation of when to calibrate and make it sticky somewhere on this site. As you say, it may have caused problems as if you have a perfectly good calibration, every time you redo it needlessly it can only get worse not better !

Apols to the o/p for going o/t but I said earlier I could not think of another issue where there has been so much misinformation but actually I can.

This is to do with anchoring boats where there has always been a widespread belief that the larger the size (diameter) of your anchor chain the less chance you have of dragging your anchor. A few years ago a discussion started on a large sailing forum about this and one of the contributors was one of the leading experts in the field. He had only recently started to question this theory (which was to do with the catenary of the chain) and did some experiments. He realised that in a full gale the anchor chain would go bar taut and all the links of the chain would lift off the seabed. The weight has an effect in damping snatch loads but the only important factor was the length of the chain and hence the angle of the anchor stock to the seabed. I was involved in this discussion along with about a dozen of us. Having ridden out a few full gales at anchor I was one that was convinced that the chain diameter was vitally important. However, as soon as I read his theory with an open mind I realised this guy was right. Some people though just refused to believe it and a few got very upset and angry to have their long held beliefs questioned. There is no doubt about this anchor chain theory and yet even now several years later the vast majority of yachtsmen all design their anchoring systems using completely incorrect info ! But then 90%+ of yachtsmen think they have anchored securely when in actual fact they haven't but that is another story

I have noticed on several occasions on this and the Phantom forums that people can also get very excitable when their long held beliefs are challenged. It is almost as though they are getting personally attacked sometimes !

Edit: Sorry to waffle on (my wife says everything I write turns into War and Peace) but going back to the anchor chain thing, I have a good friend who sails with us and he was also one who believed that when the last link of chain lifted off the seabed you were in deep ****. The thing is though he has a PhD in theoretical physics and somewhere on the boat I have got two foolscap pages of unintelligible formulae where he was trying to prove he was right. He only finally believed he was wrong because another anchor guru covered his chain in load cells and underwater cameras and proved the first expert right. It is amazing to think that people have been anchoring for countless generations and it is only in the early part of the 21st century that anchoring systems have been fully understood. Now we have a cheapo gopro clone and I sometimes drop it down in its underwater housing to film how the anchor responds to wind shifts and also what wildlife is wandering around on the seabed. We call it crittercam and as there is no TV on board it is often part of the evenings entertainment. Believe me, you don't want to live on the seabed - it's a vicious world down there ! Actually it's not too bad, crabs are amazingly stupid and seem to spend more time trying to eat the camera housing and the light than the bait.

 

[bud01234]

There's no need to calibrate the compass before flying.
Do you have a good reason to want to calibrate the compass?

How's this:
Last week I calibrated and flew from a spot and RTH worked flawlessly. (I enjoy watching the precision landing... amazing!) I Then I moved to a spot about 5 miles away and didn't bother to calibrate. Flew my mission and hit RTH. The mavic pro immediately took off in a bee line for the previous home spot 5 miles away! I was able to cancel RTH and fly manually back to the correct location.
On the way home it suddenly occurred to me...If I'd lost connection and a RTH was automatically initialized, I might not have had the opportunity to cancel the RTH.

 

[Meta4]

How's this:
Last week I calibrated and flew from a spot and RTH worked flawlessly. (I enjoy watching the precision landing... amazing!) I Then I moved to a spot about 5 miles away and didn't bother to calibrate. Flew my mission and hit RTH. The mavic pro immediately took off in a bee line for the previous home spot 5 miles away! I was able to cancel RTH and fly manually back to the correct location.
On the way home it suddenly occurred to me...If I'd lost connection and a RTH was automatically initialized, I might not have had the opportunity to cancel the RTH.

How's that?
It's confused and impossible - that's how it is (unless you left the Mavic powered on while you drove to the second launch point).
You've confused calibrating the compass with recording a homepoint but there's no link at all between them.

Calibrating the compass enables the flight controller to measure magnetic fields and work out which ones are part of the drone and which is the earth's normal magnetic field.
Whenever your Mavic powers up and acquires enough satellites to get a good GPS location fix, it records that as it's homepoint.
The homepoint is lost as soon as you power the drone down and next time you power up, it has no homepoint until it gets a GPS fix again.

Whatever your Mavic was doing, it was not going back to a previous homepoint because that's impossible.
If you want to find out what your Mavic was doing, the recorded flight data would probably show it.
To look at that go to DJI Flight Log Viewer - Phantom Help
Follow the instructions there to upload your flight record from your phone or tablet.
Come back and post a link to the report it gives you.

 

[DanMan32]

Even flight playback in the Go app will show where the H point was set.
It is possible to update the H point during flight (I think you can program one of the buttons for it) or dynamic update for self tracking situations, perhaps one of these occurred?

 

[jeplane]

Yes, it will. I have not calibrated my MP compass in months. I understand it is recommended if you are changing location by several hundred miles but there are some that scoff at that too.

For the record, I never calibrate the compass, and I fly the drone all over the US as an airline pilot. So I could be flying it in California, and some hours later in Florida as an example.

Never has been an issue.

What I do as part of my preflight is to verify the sensors thru the go4 app, and confirm that the numbers are low/ in the green for interference, and making sure the drone orientation matches the map.

Only twice did I calibrate: once when I noticed the drone drfiting with no winds and no inputs from me, and another time when I took off from a metal structure near by, which apparently really confused the drone...

Calibrating before every flight?
Don’t mess up something not broken.

Calibrating when starting hundreds of miles away?
I have proof that is isn’t nessecary either...

 

[BossBob]

How's this:
Last week I calibrated and flew from a spot and RTH worked flawlessly. (I enjoy watching the precision landing... amazing!) I Then I moved to a spot about 5 miles away and didn't bother to calibrate. Flew my mission and hit RTH. The mavic pro immediately took off in a bee line for the previous home spot 5 miles away! I was able to cancel RTH and fly manually back to the correct location.
On the way home it suddenly occurred to me...If I'd lost connection and a RTH was automatically initialized, I might not have had the opportunity to cancel the RTH.

Fairly simple. You’d neglected to check that a new home point had been recorded. There is no need to constantly calibrate the compass.

 

[Meta4]

Fairly simple. You’d neglected to check that a new home point had been recorded. There is no need to constantly calibrate the compass.

No .. whether he checks or not, his drone is going to record a new home point as soon as it gets a GPS location fix.
It just happens automatically.
If you are impatient and launch before the home point is recorded. you are going to get GPS before you fly a short distance and the drone would record it's home point there.
Old home points are not stored and until the drone gets GPS, it has no home point.