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Compass errors. Why it happens
1031 29 2018-9-28
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Hellsgate
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Many threads have been made in this and many other forums that have been the direct result of compass errors.
These include flyaways uncontrollable flight lost drones and many more but many have gone unexplained and may or may not relate to this topic.
So this thread relates only to how a magnetic field can affect your drones compass.
In this discussion ill talk about the phantom 4 pro compass but many drones if not all will be affected in the same way as the phantom does.
In the phantom your compass is located in the landing legs of your drone in other models it may be located in different positions but in nearlly all cases the compass will be located as far away from the electronics and motors as possible.
The reason manufacturers do this is so that the compass built into the drone is as far away from any magnetic fields as possible.
The down side of this positioning  is that the compass sensor is located at the closest proximity to the ground.
This close proximity to the ground and the very sensitive nature of the compass makes it very susceptible to magnetic fields.
Magnetic fields can be caused by many objects that can include but not limited too rebar reinforced concrete, wire fences, glass top outdoor tables and of course cars.
How does a magnetic field affect your drone?
Well the answer is very simple many objects made from steel such as fences, cars, concrete rebar or many buried objects made from similar materials can create an influence or distort the earths natural magnetic field some objects such as some steels and  magnets similar that seen in the video below create or have there own magnetic field *note not all metals do this
This sometimes very weak disturbance is enough to cause the compass in your drone to give an incorrect reading.
Watch this short video i made to show what effect a very small magnet has on a regular hand held compass.

You can see from the video that even a small magnet in this phone case can cause a significant error in the compass reading even at quite a distance.
Now large metalic objects like cars and fences can cause this effect at even larger distances.
So inside your drone we have many different circuits that do many different things the two circuits that get affected are the compass circuit and the imu circuit.
When you place your drone on or near a metal object and power it on lets say the bonnet of your car the compass will assume that the magnetic field it is detecting is the earths natural magnetic field.
The drone will automatically check its orientation against its gps position if it finds a discrepency it will give a compass error msg.
The dji app will ask you to move the drone or to recalibrate the compass.
Ok so you see this error pop up on your screen as part of your pre flight check list what should you do?
Well there are a few things you should do.
1. First and foremost DONT recallibrate the compass.
2. Power down the drone and controller.
3. Move your drone to a new location.
4. Power everything back up and try again.
5. If you get another compass error go back to step 1.
6. if after 3 - 4 tries you still recieve a compass error you should abandon any attempts to fly in that area, take the drone to a known good area to fly without errors and recheck your drone by doing a test flight.
7. If you still recieve a compass error in a known good area you should contact dji support.
How does the imu and compass interact?
The compass is a simple  yet sensitive electronic compass.
The imu can best be described as the brains of your drone, it detects the motion of the drone in 3 dimensional space wether it be forward backwards sideways etc etc the imu detects every motion the drone makes. The imu gathers information from sensors on the drone in this case the compasses.
Here is a very common scenario that happens far to often you see a compass error come up so you recalibrate the compass the procede to take off.
Your imu and compass are working perfectly untill your drone clears the influence of the magnetic disturbance, once this happens the compass will detect a shift in the magnetic field and send this information to the imu, the imu then compares this information from the compass to that from the gyroscopes the gyro info given to the imu is telling it that the drone has not made any moves in relation to the info given by the compass.
To put that in plain language the compass thinks the drone has turned around but other info given to the imu tells it the the drone has not turned around now we end up with a conflict within the imu. The imu will automatically favor the compass reading over the gps readings as the drone can still fly without gps but cannot fly without a compass.
So now the imu will disable your gps and put the drone into atti mode, this mode for those that aren't aware is a manual flight mode with no positioning stabilisation from the gps.
Your drone now has an incorrect compass so it no longer knows which direction it is facing and no gps so its now susceptible to wind gusts and will not break when you take your fingers of the controlls.
When you try to fly forward you may end up flying sideways or in some totally random direction. You may try pressing RTH and hope that the drone will return home but instead it flys of in a completely new random direction this is commonly referred to as a flyaway there are many threads on here refering to flyaways and one that msinger has recently posted about how to avoid a fly away i strongly recommend every pilot read his recommendations.
There is a misconception around the drone community that you should do a compass calibration prior to every flight or that the compass should be calibrated whenever you have any form of error even gimbal errors i have seen on here that people recalibrate there compass when there gimbal is not facing straight forward.
Let me put these misconceptions to rest here and now your compass has absolutely nothing to do with the gimbal or the gps system.
Once you have a good accurate compass calibration you should never need to do another one unless you modify your drone by adding accessories.
I have personally witnessed my boss whom religiously does the drone dance (Compass calibration) prior to every flight after warning him not to he lost control of his drone and flew his 3 month old p4 straight into a brick wall at 60 kph. Obviously he no longer has a drone...... if he only had listened to good advice.

Anyway im going to sum this up now and simply say this if you choose not to take the advice of many many good experienced pilots that have advised not to recalibrate your compass then you do so at your own risk i can only hope that you dont hurt any innocent people or property when you end up in a flyaway situation.

2018-9-28
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RedHotPoker
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I would be very careful, with powerful magnets anywhere near my drones compass.

Good post though. Thanks for sharing this.

RedHotPoker
2018-9-28
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Hellsgate
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RedHotPoker Posted at 2018-9-28 00:05
I would be very careful, with powerful magnets anywhere near my drones compass.

Good post though. Thanks for sharing this.

I guess im just tired of hearing about people recalibrating their compass everytime they fly and or have some unrelated random error
2018-9-28
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RedHotPoker
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Hellsgate Posted at 2018-9-28 00:08
I guess im just tired of hearing about people recalibrating their compass everytime they fly and or have some unrelated random error

Yes, it is one of those regular posts that continue to show up here.


Too bad, it’s such a mysterious component of our drones makeup.


RedHotPoker
2018-9-28
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Hellsgate
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I dont and have never claimed to be a highly experienced pilot but i have taken on board advice from pilots like msinger,  geebax, yourself, cetacean just to name a few and have never ever had an issue with my drones appart from unrelated gimbal problems.
I find it is a problem that so many pilots choose to ignore this advice then come on here and complain about their drone flying into a tree or disapearing of into the distance without a trace.
2018-9-28
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RedHotPoker
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Hellsgate Posted at 2018-9-28 00:19
I dont and have never claimed to be a highly experienced pilot but i have taken on board advice from pilots like msinger,  geebax, yourself, cetacean just to name a few and have never ever had an issue with my drones appart from unrelated gimbal problems.
I find it is a problem that so many pilots choose to ignore this advice then come on here and complain about their drone flying into a tree or disapearing of into the distance without a trace.

Yes, Geoff, Ken and Mike are all very well versed in this stuff. ;-)

I do feel fortunate, to have never had an issue, that wasn’t pilot error related,... haha
Very lucky, to have been granted a completely trouble free model.


RedHotPoker
2018-9-28
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Labroides
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Well the answer is very simple nearlly every metalic object has or creates a disturbance in the earths natural magnetic field.
You've repeated one of DJI's mistakes from their manuals.
DJI use the word metal and it's quite incorrect.
You can launch from copper, aluminium, bronze, zinc, lead, silver, gold etc, etc without any effect on the compass.
Metals are not the problem, ferromagnetic metals (iron and steel) are.
2018-9-28
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A CW
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Very detailed and thorough post - thanks for sharing   
2018-9-28
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ALABAMA
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Good read.  Thanks.
2018-9-28
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Hellsgate
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The term was a generic one meaning the metal of a car or wire fence for example not from an exotic metal, that for the purposes of this discussion will have as you have stated no effect upon the compass whatsoever therefore they need not be mentioned.
But i will reword that section.
You should also have noticed that i put a note at the end of the sentence stating that not all metals do this. But i have reworded it
2018-9-28
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solentlife
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Good post but I must make a comment about 'direction'.

A Compass gives the direction a unit is facing. It does not give the direction it actually moves.

With most moving objects - this creates a situation where leeway / drift gives different direction actually moved than the compass says. Its possible to calculate the 'vector' as I had to do for many years as a professional navigator.
But a drone has a unique ability to alter its thrust of various motors to create movement in a direction ... such that it does not normally point in a different direction to movement.

IMU is a unit that uses info from many different sources : Accelerometers, Gyros, GPS, Compass to pass solutions to Flight Controller. Flight Controller basically is told by these which way the model is moving / attitude and it then gives commands to comply with your commands.

GPS - this is one of the most misunderstood items of all ... it does not detect direction of movement. GPS is purely a unit that calculates successive positions and then passes to a computation algorithm that compares them to arrive at direction and speed.

OK - we move on and we can now see where this acts to decide if acceptable or not.

If it was a normal airplane / boat  - any wind / current etc. would create that heading difference and you would suffer many IMU / Compass warnings as they cannot agree. GPS positional data would be saying you are moving xxx degrees direction, but Compass would say the unit is facing yyy degrees.

Because the Drone can maintain pointing in the direction of movement by varying the motors, we can now see that the IMU receiving Compass and all other sensor data including GPS can have a good relationship of generally agreeing.

It is now relatively easy to understand why IN FLIGHT you can have errors display if any get out of acceptable range.

But that's in flight ... how does it know there is error when its not even taken off ... it hasn't moved ?  The above GPS and IMU cannot tell you anything because no movement has occurred to create data. The only unit that is actually saying anything is the Compass. But again what the Compass says in terms of direction is not enough to create an error. So what does ?

Magnetic Flux. In simple terms - this is the 'lines' of magnetic energy that influences the 'needle' or electronic detection in your compass. It usually lies within a reasonable range of values but if you have for example a concrete / rebar structure / iron ore underground / steel ship etc. that has magnetic properties, then that Flux is now significantly altered at that position.

The calibration of compass on the Phantom is not to align the direction of the compass ... its actually what we in the trade call 'swinging the compass' to see the effect through 360 degrees ... The difference being that with the Phantom we are looking at the calibration routine 'recording' the horizontal and vertical components through complete revolutions.

This means that when you start up in a 'bad area' - the recorded data is different to the flux detected and bingo - Error is displayed. It has nothing to do with compass direction or the GPS position as nothing has moved.

I have tried to translate into easily understood terms to give the idea. The subject of Compasses and Magnetics is actually a highly complex subject.

Nigel
2018-9-28
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Hellsgate
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solentlife Posted at 2018-9-28 05:16
Good post but I must make a comment about 'direction'.

A Compass gives the direction a unit is facing. It does not give the direction it actually moves.

Thanks for clarifying in further detail.
2018-9-28
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Hello. Thank you for sharing this informative information with us. This will really help our co-pilots in regards to your topic. If there's any other concern or inquiries, feel free to contact us. Thanks for choosing DJI and have a safe flight.
2018-9-28
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Mark The Droner
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Here's the best compass post I've read - everything's in the OP

https://phantompilots.com/thread ... plete-primer.32829/
2018-9-28
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Geebax
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Mark The Droner Posted at 2018-9-28 11:45
Here's the best compass post I've read - everything's in the OP

https://phantompilots.com/threads/compass-calibration-a-complete-primer.32829/

A very detailed post, but it does not agree with what I believe is the reason for compass calibration. I am convinced that the idea of compass calibration was created back in the days of the earliest Phantoms in order to compensate for items being carried by the aircraft. Originally you were required to fit your own camera, so the aircraft assembly was not a fixed thIng, therefore you did a compass calibration to allow the aircraft to identify what was the earth's magnetic field as against the magnetic field of the aircraft assembly itself. It is the only explanation that makes sense.
2018-9-28
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Mark The Droner
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Geebax Posted at 2018-9-28 16:01
A very detailed post, but it does not agree with what I believe is the reason for compass calibration. I am convinced that the idea of compass calibration was created back in the days of the earliest Phantoms in order to compensate for items being carried by the aircraft. Originally you were required to fit your own camera, so the aircraft assembly was not a fixed thIng, therefore you did a compass calibration to allow the aircraft to identify what was the earth's magnetic field as against the magnetic field of the aircraft assembly itself. It is the only explanation that makes sense.

Yes, I agree the post is at least a little dated.  Thanks for your comment.  
2018-9-28
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solentlife
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What tends to be overlooked in such as the highlighted OP post .... by Mark (I am not pointing at Mark - but the post he highlights) ....

An object that has any magnetic state will be affected by other magnetic objects / influences around it. What happens are the 'lines' of Flux around the object get bent / altered such that the combined value changes and the object then experiences a change in effect on it.

Just because a DJi P3 / P4 is a designed complete unit with dedicated camera etc. - does not necessarily mean that its magnetic state is completely constant. What it means is that in normal use - its magnetic state is subject to a reasonable acceptable range as it flies. watch the IMU Compass numbers ... they are not constant are they. They flicker around an average figure.

There are 3 factors that primarily affect any magnetic based compass :

Variation - this is the offset of degrees caused by geographical location of Magnetic North not being at the North Pole. This location is slowly moving in fact and Variation has an annual calculated rate of change noted for your location along with the nominal value of Variation stated for a specific date.

Deviation - The effect of the unit itself and its surrounding structure and the offset it causes to the compass reading. Ships, vehicles etc. should have a Deviation table as the degrees alters with the angle across the earths magnetic flux lines. (refer to opening lines in this post).

Anomalies - These are factors such as iron ore underground, buried pipeline / cables, any external influence in fact that is outside of the norm. Even you as the person standing next to compass with keys or other items in pocket !

(I have purposely left out the term Declination - as this in my world of Navigation immediately makes me look to the sky !! Declination in astronomical terms is the celestial equivalent of Latitude).

Its actually a very involved subject and still today is not fully understood by scientists.  We still today scratch the surface of the subject and have found ways to compensate.

Nigel
2018-9-29
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msinger
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Here's a more up to date compass calibration guide:
DJI Phantom Compass Calibration Guide
2018-9-29
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RedHotPoker
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That’s good stuff.
Thanks for sharing it...

8D3B5A9C-A539-44CB-894F-D8B193B79F22.jpeg

Better than getting the tattoo... haha


RedHotPoker

2018-9-29
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Geebax
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msinger Posted at 2018-9-29 19:12
Here's a more up to date compass calibration guide:
DJI Phantom Compass Calibration Guide

That is a very good guide. The only thing I would add to it is the advice to never degauss the compass in your aircraft. There is nothing in a solid state magnetometer or compass chip that is magnetic or is even influenced by the degaussing process.
2018-9-29
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solentlife
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Geebax Posted at 2018-9-29 20:27
That is a very good guide. The only thing I would add to it is the advice to never degauss the compass in your aircraft. There is nothing in a solid state magnetometer or compass chip that is magnetic or is even influenced by the degaussing process.

I have to agree with G ... apart from one point ... people get confused about what a Magnetometer is ... it is NOT a directional instrument - but a meter to show flux strength. Archaeologists and others use magnetometers to survey with .. as example.

But I think there is another risk with a degaussing instrument : You have no idea what effect it can have on a Chip or other sensitive electronics .... a degausser usually has strong magnetic field and can lead to strange results ... possibly.

Nigel
2018-9-30
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Mark The Droner
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I've always thought degaussing was something that could be done when nothing else worked.  I've seen posts from people who say degaussing solved their problem.  Personally, I've never done it - I've never had to do it.  But I always thought it was sound knowledge to use some day when nothing else worked.  So how is it possible that it does nothing at all helpful when people have been discussing its benefits for years?

Here's just one thread, a more recent one, of many.

https://phantompilots.com/thread ... ved-i-think.133607/
2018-9-30
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Geebax
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solentlife Posted at 2018-9-30 03:41
I have to agree with G ... apart from one point ... people get confused about what a Magnetometer is ... it is NOT a directional instrument - but a meter to show flux strength. Archaeologists and others use magnetometers to survey with .. as example.

But I think there is another risk with a degaussing instrument : You have no idea what effect it can have on a Chip or other sensitive electronics .... a degausser usually has strong magnetic field and can lead to strange results ... possibly.

Yes, degaussing could induce electric current in an area where it is not wanted. Good point.
2018-9-30
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Geebax
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Mark The Droner Posted at 2018-9-30 04:00
I've always thought degaussing was something that could be done when nothing else worked.  I've seen posts from people who say degaussing solved their problem.  Personally, I've never done it - I've never had to do it.  But I always thought it was sound knowledge to use some day when nothing else worked.  So how is it possible that it does nothing at all helpful when people have been discussing its benefits for years?

Here's just one thread, a more recent one, of many.

My only explanation is probably one I would not want to put up on this forum. But people swear that all sots of things have benefits they dont.

It used to be a relatively common practice to degauss ships, but it was not done for the benefit of the compass, it was to stop tiggering of magnetic fuses in torpedoes or mines.

And it used to be done for aircraft too, although not commonly. But you took the compass out of the aircraft first.
2018-9-30
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Hellsgate
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Well instead of all these scientific explanations how about we keep it simple for the purposes of thus discussion.
The fact stands that the compass can be adversely affected by outside influences by placing the drone on or near some metal objects like cars, fences and reinforced concrete. This can and has on many occasions caused a dangerous situation called a flyaway. I created this post to try and educate people to the dangers of such a situation in the hopes that even if it saves one drone or person from being hurt then i will be extremely happy.
2018-9-30
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Geebax
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Hellsgate Posted at 2018-9-30 16:53
Well instead of all these scientific explanations how about we keep it simple for the purposes of thus discussion.
The fact stands that the compass can be adversely affected by outside influences by placing the drone on or near some metal objects like cars, fences and reinforced concrete. This can and has on many occasions caused a dangerous situation called a flyaway. I created this post to try and educate people to the dangers of such a situation in the hopes that even if it saves one drone or person from being hurt then i will be extremely happy.

True. And thank you for posting this.
2018-9-30
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Hellsgate
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Geebax Posted at 2018-9-30 17:09
True. And thank you for posting this.

Im just trying to help. I may not be the most knowledgeable person and my explanations may lack perfect details but my intentions are in the right place.
2018-9-30
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Geebax
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Hellsgate Posted at 2018-9-30 17:34
Im just trying to help. I may not be the most knowledgeable person and my explanations may lack perfect details but my intentions are in the right place.

Absolutely. Thank you for doing this.
2018-9-30
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Siambuddhas Group
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glad to read it...well written
2018-9-30
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solentlife
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C'mon ... You laid a very good foundation for a good thread ...

I know for myself - I appreciated that and wanted to add a few blocks onto your foundation.

As with near all threads ... they all generate more info - some good .. some bad ... its what they are there for.

Cheers
Nigel
2018-9-30
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