Compass Calibration

[Meta4]

The only time you should clabrate is when you travel to new locations or if you receive a warning.

That's a popular myth.
Compass calibration has nothing to do with distance from anywhere.
Read the first post in this thread to find out what compass calibration actually does and why travelling to new sites makes no difference to compass calibration:

A short explanation of compass function, calibration and errors.

BACKGROUND The aircraft compass is often misunderstood, both in terms of function and purpose, but it is important to understand what the compass measures, why, and what happens to that information. At the most basic level the purpose of the compass is to measure the earth’s magnetic field in...

mavicpilots.com

 

[Tofino]

I used to get a warning to calibrate the compass every time I took off from the driveway. But now that I fly out of the backyard no warnings or compass calibrations are needed. Must have been the rebar in the cement.

 

[Ol' Sparky]

Ok, so the consensus is that it is not necessary to calibrate when asked by the app? Can we agree on that?

I'm not sure of the consensus...but if the app asks for a calibration, I'd do it. It asks for a reason. I would not think of lifting off after ignoring the warning. If the app asks every time you fly, something is not right.

 

[Thomas B]

I'm not sure of the consensus...but if the app asks for a calibration, I'd do it. It asks for a reason. I would not think of lifting off after ignoring the warning. If the app asks every time you fly, something is not right.

My first move is to power down and reposition the drone a few feet from the original spot. Almost always works.

 

[Meta4]

I used to get a warning to calibrate the compass every time I took off from the driveway. But now that I fly out of the backyard no warnings or compass calibrations are needed. Must have been the rebar in the cement.

The compass was warning of the steel in the cement.
But there was nothing wrong with the compass and recalibrating wouldn't have done anything to "fix" the problem it was warning about.

if the app asks for a calibration, I'd do it. It asks for a reason. I would not think of lifting off after ignoring the warning. If the app asks every time you fly, something is not right.

You'd think so, but DJI's message has very poor wording.
Almost every time you see the Calibrate Compass warning, there's nothing wrong with the compass.
It's either warning you about the magnetic environment of the launch location or you have one of the models they have programmed to ask for a completely unnecessary recalibration after 30 days or some distance from the last flight.

 

[Endeavor]

It depends, I only calibrate if the drone demands it. My Mavic2z never but the MA almost always needs calibration. So far the Mini 2 is 50% of the time. I see no reason to calibrate unless the drone shows the need.

 

[Richie K]

Seems I am forced to calibrate my Mavic Air more than 50% of the time or it just won't take off. Only flying in the wide-open spaces. I am also sure to be about 20 yards away from my car but does not seem to help?

Cannot remember the last time I calibrate my Phantom 3 or my Mavic Pro. I go from Ca. to Az. In USA over 400 hundred miles . I look at the map on the screen and it shows my current location every time.

 

[LA 2020]

I read a few compass calibration threads which were helpful. Most were about being required to calibrate every single flight. There was no clear consensus though as to whether or not it’s actually really necessary. So, since I’m asked to calibrate every single flight, is it safe not to do so?

Usually you only need to calibrate if you have moved quite a distance from your last calibration or if you've been around a magnetic disturbance. When your drone is powered up don't take it around large metal things like cars. Shut off the drone before being in those areas.

 

[sar104]

Usually you only need to calibrate if you have moved quite a distance from your last calibration or if you've been around a magnetic disturbance. When your drone is powered up don't take it around large metal things like cars. Shut off the drone before being in those areas.

Simply exposing the aircraft to a magnetic field of the kind associated with a vehicle, for example, doesn't cause any persistent problem whether it is powered up or not, unless the field is sufficiently strong to permanently change the magnetic state of the aircraft. If that happens then recalibration, or sometimes even degaussing, is required.

If the aircraft is powered up and exposed to a magnetic field then it may or may not give a magnetic interference/calibration request, depending on the exact situation. As mentioned in link above, there are several triggers:

1. The measured magnetic field is outside the expected range of field strength;
2. The aircraft location is more than 50 km from its last flight (model and firmware dependent criterion);
3. The date is more than 30 days from the last flight (model and firmware dependent criterion).

(2) and (3) generally don't mean that there is anything wrong with the calibration, and can be ignored provided that the aircraft indicated heading is correct - i.e. agrees with the direction that the aircraft is actually pointing. All the Mavics that I've tested will still arm the motors, which dismisses the warning.

If it enters a magnetic field during flight then it will simply throw a compass error and possibly switch to ATTI mode, depending on the duration and strength of the interference.

But note that it is also perfectly possible for significant, problematic magnetic interference to go unnoticed at power up if the total field strength is not out of range. This situation is the main cause of compass problems and unstable flight, where the FC cannot tell that there is interference and the IMU yaw value then gets incorrectly initialized. Once the aircraft takes off and leaves the region of magnetic interference, the compass heading changes to the correct value but the IMU which, after initialization, is using the rate gyros to detect heading changes, stays incorrect.

Some of the later models/firmware now have algorithms to detect this sudden change in magnetic yaw on takeoff, and reset the IMU yaw accordingly. It works, but I would not rely on it, and it also requires that the aircraft is not moved after power up and prior to takeoff.

 

[Starz]

Ok, so the consensus is that it is not necessary to calibrate when asked by the app? Can we agree on that?

I’m still not convinced that what you are seeing is asking you to calibrate the compass. Post number 7 ?
I‘ve been wrong on several occasions, but I’ve also seen some people that were reading some wording in the app incorrectly.

 

[In The Zone]

I think I've only calibrated my compass 3 times in the last year with 650 flights so far and not a single issue.

 

[Me262]

I always get the calibration warning if I'm flying my drone next to my car. I generally ignore it and the warning dissapears after takeoff

 

[In The Zone]

I always get the calibration warning if I'm flying my drone next to my car. I generally ignore it and the warning dissapears after takeoff

That's very odd because, I have a very large carport and it's metal, I take off under it and even fly from under it to keep me out of the Sun and keep the glare off the screen I have never had any issues with compass calibrations. I have also landed on the roof of my car and I have taken off from the roof of my car never had any problems. Could I just be lucky, I don't know?

 

[PimV]

Some of the later models/firmware now have algorithms to detect this sudden change in magnetic yaw on takeoff, and reset the IMU yaw accordingly. It works, but I would not rely on it, and it also requires that the aircraft is not moved after power up and prior to takeoff.

@sar104:
- The link attached to " It works," is dead. Do you have another URL to this webpage?
- I want to compliment you for your clear and accurate article "A short explanation of compass function, calibration and errors". And also for your experiment with a disturbing magnetic field: "Compass error demonstration". Both very illuminating!

For me two lessons stand out:
- Check the heading of the direction indicator as soon as the aircraft reaches an altitude of ca. 10 metres.
- Never calibrate the compass if you are not absolutely sure there are no magnetic disturbances in the vicinity of the aircraft.

I find it strange that DJI orders to calibrate the compass as soon as its measurements are 'out of bounds', because there is no way the system can know whether this is caused by environmental causes or by causes connected to the aircraft. In the former case it is even dangerous to recalibrate the compass because compensations will be made to counteract magnetic deviations which will not be present anymore after takeoff.

Electric currents cause magnetic fields. Once I got a compass calibration message in a meadow about 2 metres away from an electric fence (for cattle, only a few wires). On YouTube I have seen video's where drones are used to inspect high tension electrical transport cables, and the masts carrying them, in close proximity. I know these lines cause deviations to hand-held compasses on the ground, so I wonder how those inspections can be done.

 

[slup]

...For me two lessons stand out:
- Check the heading of the direction indicator as soon as the aircraft reaches an altitude of ca. 10 metres.

Electric currents cause magnetic fields...drones are used to inspect high tension electrical transport cables...in close proximity. I know these lines cause deviations to hand-held compasses on the ground, so I wonder how those inspections can be done.

What's giving your AC the heading direction during flight isn't mainly data from the compass ... it's from the IMU.

At power on of the AC the IMUYaw direction needs to be initiated to a start value that corresponds to the actual direction the AC is pointing ... if the AC is pointing to east the IMUYaw needs to have a value of 90 degrees for example. Without the assistance of the compass the IMU will not know that.

So if the compass says 90 at power on ... the IMUYaw will be initiated to that also. And if the compass is correct in that power on moment (i.e. the compass is properly calibrated & not magnetic disturbed) the pointing direction of the IMU will say 90 degrees also & correlate to a AC that is pointing to east in reality.

So after that power on moment with it's initialization of the IMU the compass have served it's main purpose ... from here on it's the IMU that shows the heading direction ... so if the AC flies into magnetic disturbance later in the flight the compass will be deflected but that doesn't matter as it's the IMU that tells the direction & the IMU will not be deflected by magnetic disturbance flying close to those high tension electrical transport cables in your example.

I started up by writing "... isn't mainly data from the compass". The reason is that a lot of other sensor data slowly feeds in corrections to the IMUYaw direction during flight ... such as the compass, but also the gyro. These corrections will take time & the correction values normally need to agree from the other sensors if corrections shall be made.

So ... it's when you power on the AC you need to have a undisturbed compass ... once the AC is fully on not much can happen if the compass gets deflected later on.

The check you mention ... doesn't need to be performed on 10m height. Power on the AC in your hand well away from hidden magnetic sources under ground ... once the live view are up in your mobile device you know that the IMU have been initialized & you can place it on ground. THEN ... check that the drone icon on the map in your app is pointing equal in relation to other objects in the map as the drone does in reality ... if not, abort launch attempt, POWER DOWN & move away, power up again and repeat.

 

[sar104]

@sar104:
- The link attached to " It works," is dead. Do you have another URL to this webpage?
- I want to compliment you for your clear and accurate article "A short explanation of compass function, calibration and errors". And also for your experiment with a disturbing magnetic field: "Compass error demonstration". Both very illuminating!

For me two lessons stand out:
- Check the heading of the direction indicator as soon as the aircraft reaches an altitude of ca. 10 metres.
- Never calibrate the compass if you are not absolutely sure there are no magnetic disturbances in the vicinity of the aircraft.

I find it strange that DJI orders to calibrate the compass as soon as its measurements are 'out of bounds', because there is no way the system can know whether this is caused by environmental causes or by causes connected to the aircraft. In the former case it is even dangerous to recalibrate the compass because compensations will be made to counteract magnetic deviations which will not be present anymore after takeoff.

Electric currents cause magnetic fields. Once I got a compass calibration message in a meadow about 2 metres away from an electric fence (for cattle, only a few wires). On YouTube I have seen video's where drones are used to inspect high tension electrical transport cables, and the masts carrying them, in close proximity. I know these lines cause deviations to hand-held compasses on the ground, so I wonder how those inspections can be done.

Apologies for the broken link - now fixed. It points to this page:

Compass error demonstration

Ok my apologies, I don’t own a Mavic but I know how my phantom standard and i1 reacted in these exact same locations, pix below. With a compass error. I was totally wrong for flying in these areas but I clearly remember how my drones reacted after take off. I had to walk out of the block to keep...

mavicpilots.com

I agree that the error message is less helpful than it could be, since "move" and "calibrate" conflates completely different problems and functions.

While it is correct to say that electric currents produce magnetic fields, that statement needs some additional clarification. The magnetic field, ?, around a conductor is proportional to the current, ?, and inversely proportional to the distance from the conductor, ?:

? = µ₀?/2π?​

Power lines operate at high voltage and relatively low current, and don't produce a significant magnetic field unless the you get very close to the conductors. Additionally, many are AC rather than DC, which leads to a time-averaged zero field. Any fields around pylons and lines are likely due to the ferromagnetic steel pylons rather than the lines themselves.

Electric fences also operate at high voltage and very low current, and will not cause a noticeable magnetic field.