There’s no fool like an old fool...Mavic Mini lost!!

[Dan C]

Compass calibration has nothing to do with magnetic anomalies external to the drone. It just manage with internal interferences which are usually a constant till you change something in the hardware or in the payload. Imagine a jet or a high sea vessel calibrating its compasse(s) every 20 milles it travels

I just did some research, and... yes, it does have a lot to do with external factors, and that's where moving from one place to another, and having to re-calibrate, gets confusing.

It is not the geographical shift that matters; digital compasses are ratiometric and adjust automatically.

It is the change in the electromagnetic environment.

If you change something within the drone itself that alters its electromagnetic profile, you will of course have to recalibrate, but the immediate environment is indeed also a factor. Why else all this warning about trying to calibrate around rebar or steel structures? Any magnets, ferrous materials, or electrical fields can play havoc with how your drone's digital compass "sees" the environment.

I wondered why I might have to recalibrate after returning today to the same place I last flew a few days ago. I'm not an expert, but I would imagine sun activity could be a factor. My Polar Pro app warns me about the K-index, a characterization of the magnitude of geomagnetic storms. Changes in this field could trigger a recalibration.

And a very small change in launch location, even just a few meters, could make a difference. We are concerned with electromagnetic fields, and the inverse-square rule applies. A small change in distance from the source of interference can cause a very large change in the strength of the interference.

And, we must remember one more factor. DJI.

There might not be a real-world need, but if for any reason the DJI software (controller OR drone) thinks a recalibration is needed, you don't have much choice. So if the current DJI philosophy says if it registers you have changed locations by a certain amount, or are in a different country, or...or...or....

...like it or not, necessary or not, you have to recalibrate.

To summarize -- reasons you might need to recalibrate your compass:

• You change some equipment on the drone which alters its electromagnetic field; putting strobes on it, for example (batteries, and electrical currents will affect the profile).
• There is a significant difference in the electromagnetic environment where you are launching from compared to the last time/place you launched. There is a long list of causes including steel, magnetic material, electrical fields, or possibly even changes in the K-index.
• DJI software says so.

I'm quite happy to be corrected if any of this is factually incorrect, but I also hope this helps clear the air a little.

At at the end of the day, don't sweat it. If the controllers says "recalibrate", do the compass dance then get on with flying.

And the advice on doing a manual check of the compass is golden. It is always a good idea to verify instruments more than one way. Great addition to your preflight checks.

 

[sar104]

I just did some research, and... yes, it does have a lot to do with external factors, and that's where moving from one place to another, and having to re-calibrate, gets confusing.

It is not the geographical shift that matters; digital compasses are ratiometric and adjust automatically.

It is the change in the electromagnetic environment.

If you change something within the drone itself that alters its electromagnetic profile, you will of course have to recalibrate, but the immediate environment is indeed also a factor. Why else all this warning about trying to calibrate around rebar or steel structures? Any magnets, ferrous materials, or electrical fields can play havoc with how your drone's digital compass "sees" the environment.

I wondered why I might have to recalibrate after returning today to the same place I last flew a few days ago. I'm not an expert, but I would imagine sun activity could be a factor. My Polar Pro app warns me about the K-index, a characterization of the magnitude of geomagnetic storms. Changes in this field could trigger a recalibration.

And a very small change in launch location, even just a few meters, could make a difference. We are concerned with electromagnetic fields, and the inverse-square rule applies. A small change in distance from the source of interference can cause a very large change in the strength of the interference.

And, we must remember one more factor. DJI.

There might not be a real-world need, but if for any reason the DJI software (controller OR drone) thinks a recalibration is needed, you don't have much choice. So if the current DJI philosophy says if it registers you have changed locations by a certain amount, or are in a different country, or...or...or....

...like it or not, necessary or not, you have to recalibrate.

To summarize -- reasons you might need to recalibrate your compass:

• You change some equipment on the drone which alters its electromagnetic field; putting strobes on it, for example (batteries, and electrical currents will affect the profile).
• There is a significant difference in the electromagnetic environment where you are launching from compared to the last time/place you launched. There is a long list of causes including steel, magnetic material, electrical fields, or possibly even changes in the K-index.
• DJI software says so.

I'm quite happy to be corrected if any of this is factually incorrect, but I also hope this helps clear the air a little.

At at the end of the day, don't sweat it. If the controllers says "recalibrate", do the compass dance then get on with flying.

And the advice on doing a manual check of the compass is golden. It is always a good idea to verify instruments more than one way. Great addition to your preflight checks.

This illustrates much of the confusion that persists around compass calibration. Calibration has nothing to do with the earth's magnetic field - it is purely to detect and subtract the magnetic field of the aircraft itself. It does that by measuring the components of the total magnetic field (earth plus aircraft) that do not change as the aircraft is rotated (i.e. the aircraft's magnetic field). Local distortions to the earth's magnetic field don't affect calibration either, unless they are so local that they change as the aircraft is rotated.

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.

 

[Meta4]

I would imagine sun activity could be a factor. My Polar Pro app warns me about the K-index, a characterization of the magnitude of geomagnetic storms. Changes in this field could trigger a recalibration.

Forget whatever you are imagining about the Sun and the Kp Index.
None of it has any effect on your drone or its compass.

but if for any reason the DJI software (controller OR drone) thinks a recalibration is needed, you don't have much choice. So if the current DJI philosophy says if it registers you have changed locations by a certain amount, or are in a different country, or...or...or....
...like it or not, necessary or not, you have to recalibrate.

If the controllers says "recalibrate", do the compass dance then get on with flying.

Not necessarily.
If you think your controller is suggesting recalibrating the compass, check it closely.
If you put your drone down on a steel surface like the roof of a car, it flashes a warning that many users think is telling them to recalibrate the compass.
Here's part of the message:

The full message is too long for the window so it scrolls across.
It says: Magnetic Field Interference - Move aircraft or calibrate compass.
But many users just see Calibrate Compass.
In the pic above you can see the steel car roof that's just inches from the compass sensor.
That's what the compass is warning about and no amount of recalibrating anything will "fix" that.
Powering down and moving the drone is almost always the correct action.

 

[Dan C]

This illustrates much of the confusion that persists around compass calibration. Calibration has nothing to do with the earth's magnetic field - it is purely to detect and subtract the magnetic field of the aircraft itself. It does that by measuring the components of the total magnetic field (earth plus aircraft) that do not change as the aircraft is rotated (i.e. the aircraft's magnetic field). Local distortions to the earth's magnetic field don't affect calibration either, unless they are so local that they change as the aircraft is rotated.

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.

Since you are quoting my post as an example of "confusion", I must infer you disagree with my post.

With respect, it looks like you are conflating WHAT compass calibration does with WHY you might have to do it. "Calibration has nothing to do with the earth's magnetic field - it is purely to detect and subtract the magnetic field of the aircraft itself." Detect and subtract the magnetic field of the aircraft from the measured total field which includes the Earth's. It also, of course, includes other environmental influences.

The message I get from your post is that the ONLY time I should have to recalibrate is if I change something on the aircraft itself -- something that will be consistent in orientation to the aircraft.

I have never altered anything on my drone, yet have been requested to recalibrate the compass many times. I'm curious what you think the reason for that might be.

I think you are correct in what calibration does, but you have not addressed why that process might become necessary. It seems clear to me that some kind of disagreement between sensors in the drone must be the explanation. Perhaps the drone's magnetically determined north does not agree with GPS information.

I don't know for sure what is causing the problem...

But something is triggering the need for recalibration. The local environment can and does affect the measured electromagnetic field around the drone, and that to me seems a very likely suspect for needing to recalibrate.

If it is not the environment, then I am genuinely curious to hear what you think is causing the request for recalibration in the first place.

 

[Meta4]

The message I get from your post is that the ONLY time I should have to recalibrate is if I change something on the aircraft itself -- something that will be consistent in orientation to the aircraft.

That's because that's the whole message and all that matters.

I have never altered anything on my drone, yet have been requested to recalibrate the compass many times. I'm curious what you think the reason for that might be.

I think you are correct in what calibration does, but you have not addressed why that process might become necessary. It seems clear to me that some kind of disagreement between sensors in the drone must be the explanation. Perhaps the drone's magnetically determined north does not agree with GPS information.

See post #43 for one possibility.
Something steel near the compass will set off a warning.
It could be as small as a nail or screw if it's close enough to the compass.
Or it could be that you have one of the drones that DJI mysteriously programmed to demand a compass calibration at certain times or distances - for no good reason except to continue the confusion they started with their bad documentation.

 

[Dan C]

Not necessarily.
If you think your controller is suggesting recalibrating the compass, check it closely.
If you put your drone down on a steel surface like the roof of a car, it flashes a warning that many users think is telling them to recalibrate the compass.

.....

Thanks for that, though I've never seen that message. I have an Air, and the message looks different to that.

Also, what I'm generally seeing is in the setup screen. When I power up, the DJI Go opens that screen, and high on the list is the compass calibration. There is where I see the red indicator that I need to calibrate. There is no text here -- just the red tag.

Of course, if I close that window, I'll something on the main screen to, but it looks a bit different to yours.

-----

And in any case, you have made my point for me. As I said to @sar104 I think you guys are confusing the WHAT with the WHY. sar104 gave an excellent explanation as to what calibration does, and how it does it.

And while your point is that merely moving the drone before launch might be all that is NEEDED, you have still handily reinforced my argument about WHY the message to recalibrate comes up at all.

And clearly, the message is that THE ENVIRONMENT can readily trigger this message. In your case, the expanse of ferromagnetic material we call steel which makes up the roof of your car.

 

[Dan C]

That's because that's the whole message and all that matters.

See post #43 for one possibility.
Something steel near the compass will set off a warning.
It could be as small as a nail or screw if it's close enough to the compass.
Or it could be that you have one of the drones that DJI mysteriously programmed to demand a compass calibration at certain times or distances - for no good reason except to continue the confusion they started with their bad documentation.

Great. Thank you for confirming that my post about WHY you might be asked to recalibrate is, in fact, quite accurate after all.

Ergo, environmental factors.

Cheers.

 

[Meta4]

Great. Thank you for confirming that my post about WHY you might be asked to recalibrate is, in fact, quite accurate after all.

Ergo, environmental factors.

Try reading what I wrote again.
The compass doesn't need calibration at all because of environmental factors.
No amount of recalibrating will "fix" those environmental factors.

Go back and read what sar104 wrote.
He really knows the subject, unlike most opinions you'll read on the forum.

 

[Dan C]

Try reading what I wrote again.
The compass doesn't need calibration at all because of environmental factors.
No amount of recalibrating will "fix" those environmental factors.

Go back and read what sar104 wrote.
He really knows the subject, unlike most opinions you'll read on the forum.

I have nothing but respect for sar104. In fact, I hold him in some awe.

But you both seem to have this idea that I'm arguing something I am not. I'm coming at this from the human factor, or human interface, if you will.

Ever since I've owned my drone, I've seen this issue of recalibration. Here today I've addressed why this keeps coming up.

I appreciate your input, and sar104's, and what I think I should then add is this...

Ladies and gentlemen, just because because your DJI Go asks you to recalibrate, doesn't mean you have to. Moving your drone somewhere else may be all that is required.

However, the pilot might be atop a geographical feature rich with ferromagnetic content, and there is not anywhere practical to "move to".

So my message is simple. If the software asks you to calibrate, do the compass dance (only takes a few seconds) then get on with flying.

You and sar104 are no doubt correct about the (lack of) necessity of recalibrating, but so what?

If the app refuse to launch the drone until you clear the error, then you gotta do it. Or try to find somewhere (if you can) where it doesn't complain, but if that's not practical, well...

Finally, at least for me with my MA, I'll move away from obvious problems, like the cell tower I hadn't noticed before. But usually, it is clear that no suitable launch position is going to be any different from where I am now -- like it's all concrete and all going to have the same material in it.

So if it asks me to recalibrate, I do it.

And I've experienced no fly away events, no location or orientation problems, no RTH issues.

Even if unnecessary, it doesn't seem to have caused any harm calibrating when the software asks for it.

That's real, gentlemen.

And. one. last. time. I've only been talking about why the message might appear. That's it. Don't read any more into it.

 

[sar104]

Since you are quoting my post as an example of "confusion", I must infer you disagree with my post.

With respect, it looks like you are conflating WHAT compass calibration does with WHY you might have to do it. "Calibration has nothing to do with the earth's magnetic field - it is purely to detect and subtract the magnetic field of the aircraft itself." Detect and subtract the magnetic field of the aircraft from the measured total field which includes the Earth's. It also, of course, includes other environmental influences.

The message I get from your post is that the ONLY time I should have to recalibrate is if I change something on the aircraft itself -- something that will be consistent in orientation to the aircraft.

I have never altered anything on my drone, yet have been requested to recalibrate the compass many times. I'm curious what you think the reason for that might be.

I think you are correct in what calibration does, but you have not addressed why that process might become necessary. It seems clear to me that some kind of disagreement between sensors in the drone must be the explanation. Perhaps the drone's magnetically determined north does not agree with GPS information.

I don't know for sure what is causing the problem...

But something is triggering the need for recalibration. The local environment can and does affect the measured electromagnetic field around the drone, and that to me seems a very likely suspect for needing to recalibrate.

If it is not the environment, then I am genuinely curious to hear what you think is causing the request for recalibration in the first place.

Yes - your post was incorrect. No - I'm not conflating anything. You are conflating the need for a compass calibration with an app request for a compass calibration. And yes - the only reason that a compass recalibration is every necessary is if the magnetic state of the aircraft itself changes. Various DJI firmware versions, however, have included requests to calibrate if you are over 30 days from the last calibration, have moved more that 50 km from the previous flight, or if the FC detects magnetic field strengths outside the normal range.

Abnormal field strengths could indicate a change in the magnetic state of the aircraft, or could indicate that there is a significant local distortion of the earth's magnetic field. Calibration can fix the first problem, provided the aircraft is not too highly magnetized (in which case it will need demagnetizing), but cannot fix the second problem - only changing location can fix that.

 

[sar104]

Great. Thank you for confirming that my post about WHY you might be asked to recalibrate is, in fact, quite accurate after all.

Ergo, environmental factors.

Cheers.

If, by environmental factors, you mean factors external to the aircraft, then your assertion is incorrect. Calibration cannot fix those.

 

[sar104]

I have nothing but respect for sar104. In fact, I hold him in some awe.

But you both seem to have this idea that I'm arguing something I am not. I'm coming at this from the human factor, or human interface, if you will.

Ever since I've owned my drone, I've seen this issue of recalibration. Here today I've addressed why this keeps coming up.

I appreciate your input, and sar104's, and what I think I should then add is this...

Ladies and gentlemen, just because because your DJI Go asks you to recalibrate, doesn't mean you have to. Moving your drone somewhere else may be all that is required.

However, the pilot might be atop a geographical feature rich with ferromagnetic content, and there is not anywhere practical to "move to".

So my message is simple. If the software asks you to calibrate, do the compass dance (only takes a few seconds) then get on with flying.

You and sar104 are no doubt correct about the (lack of) necessity of recalibrating, but so what?

If the app refuse to launch the drone until you clear the error, then you gotta do it. Or try to find somewhere (if you can) where it doesn't complain, but if that's not practical, well...

Finally, at least for me with my MA, I'll move away from obvious problems, like the cell tower I hadn't noticed before. But usually, it is clear that no suitable launch position is going to be any different from where I am now -- like it's all concrete and all going to have the same material in it.

So if it asks me to recalibrate, I do it.

And I've experienced no fly away events, no location or orientation problems, no RTH issues.

Even if unnecessary, it doesn't seem to have caused any harm calibrating when the software asks for it.

That's real, gentlemen.

And. one. last. time. I've only been talking about why the message might appear. That's it. Don't read any more into it.

To address your question about a distorted magnetic field, but no way to move out of the distortion, only the Mavic 2 is capable of dealing with that (not sure about the Mini). If the magnetic field changes on takeoff without any rotation detected by the rate gyros then the FC will reset the IMU yaw to the new magnetic yaw.

Compass error demonstration

 

[Meta4]

You and sar104 are no doubt correct about the (lack of) necessity of recalibrating, but so what?
If the app refuse to launch the drone until you clear the error, then you gotta do it. Or try to find somewhere (if you can) where it doesn't complain, but if that's not practical, well...

Reaclibrating your compass doesn't fix anything.
As has been explained several times in this thread, calibrating the compass only identifies and measures the magnetic fields that are part of your drone.
That's all it does.
Think about that some more
Identifying and measuring the mag fields that have already been identified and measured before isn't going to change anything.
It isn't going to solve an issue caused by trying to launch from on top of a steel object.
How could it?

Finally, at least for me with my MA, I'll move away from obvious problems, like the cell tower I hadn't noticed before. But usually, it is clear that no suitable launch position is going to be any different from where I am now -- like it's all concrete and all going to have the same material in it.

Your cell tower isn't likely to be a problem unless you are close enough for it to affect the compass - probably just a few metres.
And if you are launching from concrete, don't fool yourself that it's all teh same.
Here's what it looks like inside:

Moving the drone just an inch this way or that can make a big difference to how much steel is within range of your drone's compass.

 

[Dan C]

Reaclibrating your compass doesn't fix anything.
As has been explained several times in this thread, calibrating the compass only identifies and measures the magnetic fields that are part of your drone.
That's all it does.
Think about that some more
Identifying and measuring the mag fields that have already been identified and measured before isn't going to change anything.
It isn't going to solve an issue caused by trying to launch from on top of a steel object.
How could it?

Ah, I see your mistake now.

You are forgetting that the purpose of determining the drone's magnetic field is to adjust what the sensors see in total, and correctly determine where magnetic north is.

N = F - D
where N = magnetic north, F equals total field sensed, D equals the field of the drone. Okay, not mathematically that simple perhaps, but conceptually representative.

If you start your drone up, and it detects that north isn't where it should be (by comparing to GPS information?), then it knows something has changed.

Either F has changed, or D has changed. Or both.

YOU know D hasn't changed, but the drone does not until it calibrates again.

And that is why I believe the environment can trigger a request to recalibrate.

It "fixes" the problem of the drone not knowing what has changed.

 

[Meta4]

I thought I saw you say ....

I'm quite happy to be corrected if any of this is factually incorrect

You are particularly resistant to correction.

Ah, I see your mistake now.

The only mistake is yours.

You are forgetting that the purpose of determining the drone's magnetic field is to adjust what the sensors see in total, and correctly determine where magnetic north is.

I never forgot because that's not what calibration is for.
Calibration doesn't determine where magnetic north is
Compass calibration just (for the last time .. please take this in) identifies and measures the magnetic fields which are part of the drone.
It doesn't identify or measure mag fields outside of the drone, whether they be the earth's regular mag field or local anomalies.
Any mag field that isn't identified as part of the drone is just the rest.
Whether is a magnetically contaminated area or not, the calibration does not determine or correct or fix anything.

 

[Bigbird48]

I guess were never going to find out what really happened as I don't see his logs posted yet ?

 

[Meta4]

Ah, I see your mistake now.

Since you are finding the concept difficult to grasp, I'll try a different tack.

In a past life, before the days of GPS, I lived aboard and cruised on a yacht like this one:

Having an accurate compass is an extremely important thing when cruising between small islands that could be hundreds of miles apart.
But the yacht has all kinds of magnetic influences that can affect the compass.
Things like the engine, electric motors, pumps, electric cables etc.
To ensure that my compass was correctly giving the directional information I needed, I engaged the services of a licensed compass adjuster.
He carefully positioned some small magnets around the ships compass to correct and compensate for the magnetic fields which were part of the yacht.
Then the compass would properly show correct bearings.

But if I was to tie up to a steel wharf, the magnetic field of the wharf would have caused a false compass reading.
The compass correction could not account for external magnetic influences.

DJI compass calibration is the digital equivalent of what the compass adjuster did for my boat.
If you put your drone down close to an extraneous magnetic field, the compass might warn you that it's detected a field that doesn't fit the normal range but you can't correct for that.
All you can do is to get your compass further away from the magnetic contamination.

 

[Dan C]

I thought I saw you say ....

You are particularly resistant to correction.

The only mistake is yours.

I never forgot because that's not what calibration is for.
Calibration doesn't determine where magnetic north is
Compass calibration just (for the last time .. please take this in) identifies and measures the magnetic fields which are part of the drone.
It doesn't identify or measure mag fields outside of the drone, whether they be the earth's regular mag field or local anomalies.
Any mag field that isn't identified as part of the drone is just the rest.
Whether is a magnetically contaminated area or not, the calibration does not determine or correct or fix anything.

From post #42. sar104's words...

"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."

Please note the section I bolded. If that process produces anomalous data, the drone cannot know why.

IT IS POSSIBLE for the drone to be modified, and its field changed. IT IS POSSIBLE that the stored data describing the drone's field is corrupted. So the software requests a calibration.

You do the process, and it determines the drone's field as described. At this point it is irrelevant whether the drone's field changed or not. The drone knows what its field is right now.

All that matters is that when the drone detects an anomaly regarding what it thinks is magnetic north, it COULD be necessary to recalibrate.

Maybe DJI should give you an option to say, "I confirm the drone's field data is correct". Take it up with them, I didn't write the software.

For the last time, the reason WHY a recalibration might be requested could be external or internal. Performing the calibration is the only way for the software to verify it knows what the drone's field characeritcs are right now.

Period.

 

[Meta4]

From post #42. sar104's words...

"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."

Please note the section I bolded. If that process produces anomalous data, the drone cannot know why.

IT IS POSSIBLE for the drone to be modified, and its field changed. IT IS POSSIBLE that the stored data describing the drone's field is corrupted. So the software requests a calibration.

You do the process, and it determines the drone's field as described. At this point it is irrelevant whether the drone's field changed or not. The drone knows what its field is right now.

All that matters is that when the drone detects an anomaly regarding what it thinks is magnetic north, it COULD be necessary to recalibrate.

Maybe DJI should give you an option to say, "I confirm the drone's field data is correct". Take it up with them, I didn't write the software.

For the last time, the reason WHY a recalibration might be requested could be external or internal. Performing the calibration is the only way for the software to verify it knows what the drone's field characeritcs are right now.

Period.

No ... you still aren't getting it.
You don't want to get it because you have your own ideas that can't be supported by the facts
I've tried (and tried) but you have your own ideas about what you want it to do rather than what it actually does.
There's no hint of any willingness to be corrected.
I give up.
Perhaps @sar104 has some more to add (but I guess it would be a waste of effort if he tried).
Good luck with your ideas

 

[Dan C]

Since you are finding the concept difficult to grasp, I'll try a different tack.

In a past life, before the days of GPS, I lived aboard and cruised on a yacht like this one:

Having an accurate compass is an extremely important thing when cruising between small islands that could be hundreds of miles apart.
But the yacht has all kinds of magnetic influences that can affect the compass.
Things like the engine, electric motors, pumps, electric cables etc.
To ensure that my compass was correctly giving the directional information I needed, I engaged the services of a licensed compass adjuster.
He carefully positioned some small magnets around the ships compass to correct and compensate for the magnetic fields which were part of the yacht.
Then the compass would properly show correct bearings.

But if I was to tie up to a steel wharf, the magnetic field of the wharf would have caused a false compass reading.
The compass correction could not account for external magnetic influences.

DJI compass calibration is the digital equivalent of what the compass adjuster did for my boat.
If you put your drone down close to an extraneous magnetic field, the compass might warn you that it's detected a field that doesn't fit the normal range but you can't correct for that.
All you can do is to get your compass further away from the magnetic contamination.

Why is it okay for you to be condescending?

Never mind...

*sigh* Okay, to take up your analogy...

The compass adjuster = the calibration routine.
The little magnets he added = the profile electronically stored in the drone.

The actual electromagnetic profile is the same; no analogy required.

After a long period away from your boat, you return with plans to go sailing, but you note that your compass is off. You are familiar with this area, and don't need verification -- you know where magnetic north is in this part of the world, and you know your compass is not pointing to it. Or maybe you simply see it doesn't agree with your GPS. Doesn't matter. You know it's wrong.

By your logic, you will assume it's the metal (or something) of the dock, and assume it will be fine once you move away.

But there are two other possible explanations. One of those magnets has moved/fallen off (who knows why -- some kids snuck on board and messed with it -- doesn't matter). Or something changed about your boat.

Now, being an intelligent person, you know nothing has been changed about your boat. You didn't replace your diesel engine with a different sized one, or bring anything significant onboard, or take it away. You are confident that everything is the same.

And maybe you can determine your compass has not been tampered with. Perhaps there is some security seal. Again, use your imagination. Point is, you are confident that neither your boat's field has changed, nor the calibration of your compass. It is perfectly reasonable for you to conclude that the dock, or something else close by, is responsible. And off you go.

DJI Go is not, cannot be, so intuitive. It cannot know if you added strobes last night. It cannot know if something damaged the data stored in it's memory.

It only knows that when it uses the stored electromagnetic profile to adjust what is being currently read by its sensors, that the answer cannot be correct. By comparing to GPS? I don't know. I simply assume it knows somehow.

So to eliminate those internal possibilities, it requests a calibration.

I'm NOT saying the calibration is needed.
I'm NOT saying that moving the drone a little won't clear the problem.

I am only saying that, logically, so long as the anomaly exists, the only way for a piece of software to eliminate those possibilities is for the pilot to perform a calibration.

And that it does no harm to recalibrate.

Despite your low opinion of my intelligence, I quite understand your analogy, and I can only say that, if I had a totally automated catamaran (which would be my preference over a yacht), and I was trusting it to get me safely to a distant location, and it detected an anomaly and asked my to go pull a 360 out in the bay before heading out of sight of land...

...I'd be okay with the small inconvenience just for peace of mind.

Even though I was 99.99% sure it wasn't necessary.

There's always that little chance the problem is internal, and I wouldn't want to end up lost at sea.

Or lose my drone, for that matter.