Compass calibration

[hph]

Calibration is an easier concept for the masses.
Compensation is what is actually occurring.
The process of rotating the magnetometer in two axis allows for the characterization of the hard and soft iron distortions created by the components in and on the aircraft. Then an algorithm creates a polar compensation matrix correcting them.

I guess it’s easier to accuse folks of propagating BS than actually looking into it.

If you wish here’s just one article to read...

Hard and soft iron magnetic compensation explained

I hope you read this paragraph within your reference: "Earth ambient magnetic field as a function of zip code can be obtained from the online National Geophysical Data Center magnetic field calculator. But for some applications, you may not even care what the local value is. (That's why i really don't care when I recal the drone's compass 'fore flight.) The important point to take from the discussion above is that we are leveraging the symmetry of the data set to drive a solution. Compass orientation is determined by ratios of the 3 dimensions calculated. If those values are off from the expected values by a multiplicative constant, the ratios still hold."

Please note the noun, "constant!" Where these "multiplicative constants" occur gives rise to the maps showing magnetic declination with the familar isogonic lines.

OK, I don't know the answer to this question, but since you appear to be learned, I'll ask you, "Just how much is the drone's "compass" affected if the operator calibrates it where the mag dec is +6.0º and then flies it without recal where the mag dec is +7.0º? The difference in one degree of longitude at Greenwich is roughly 53 statute miles.

That of course, changes with a change in latitude. Greenwich is at 51.5ºN. That's 6 deg. in latitude further N than the latitude of Montreal, Canada. So, as you come S from 51.5º, the mileage for 0.5º in latitude increases until the Equator is reached. All of the continental US is at a latitude less than 49º30'N.

Please pardon the length of my discourse, but what I'm trying to show is just how different from true N a magnetic compass will indicate with position.

 

[sar104]

Then why does my P3A require me to recalibrate the compass before flight when I am in NC altho it was previously calibrated in SC? And vice-versa?

Are you saying that the aircraft is telling you that you need to recalibrate each time that you move between those two locations? If so, then I have no idea why it is doing that unless during travel you are exposing it to a significant magnetic field that is causing a change in the magnetization of components on the aircraft.

In addition to the fact that there is no theoretical physical basis for that to be necessary, I can say that I've travelled thousands of miles and never needed, or been required, to recalibrate.

 

[sar104]

I hope you read this paragraph within your reference: "Earth ambient magnetic field as a function of zip code can be obtained from the online National Geophysical Data Center magnetic field calculator. But for some applications, you may not even care what the local value is. (That's why i really don't care when I recal the drone's compass 'fore flight.) The important point to take from the discussion above is that we are leveraging the symmetry of the data set to drive a solution. Compass orientation is determined by ratios of the 3 dimensions calculated. If those values are off from the expected values by a multiplicative constant, the ratios still hold."

Please note the noun, "constant!" Where these "multiplicative constants" occur gives rise to the maps showing magnetic declination with the familar isogonic lines.

OK, I don't know the answer to this question, but since you appear to be learned, I'll ask you, "Just how much is the drone's "compass" affected if the operator calibrates it where the mag dec is +6.0º and then flies it without recal where the mag dec is +7.0º? The difference in one degree of longitude at Greenwich is roughly 53 statute miles.

That of course, changes with a change in latitude. Greenwich is at 51.5ºN. That's 6 deg. in latitude further N than the latitude of Montreal, Canada. So, as you come S from 51.5º, the mileage for 0.5º in latitude increases until the Equator is reached. All of the continental US is at a latitude less than 49º30'N.

Please pardon the length of my discourse, but what I'm trying to show is just how different from true N a magnetic compass will indicate with position.

Calibration has nothing to do with declination and, in fact, has no way to detect it. That's trivially obvious if you consider the calibration process. Declination, as you are aware, is the angular difference between the horizontal component of the earth's magnetic field and the direction of true north. Although true north points to the axis of rotation, on the ground true north is essentially an arbitrary direction. More importantly, the aircraft has no way to know where true north is - all it can detect is magnetic north - and so no amount of rotation of the aircraft will allow it to determine declination. Instead, declination is stored in the firmware as a global model.

The purpose of the calibration process is quite different - it is primarily to detect the components of magnetic field that don't change as it is rotated, which are its own magnetic field. It needs to know that in order to subtract them from the magnetic field that it measures in flight, leaving just the components of the earth's magnetic field that it needs to determine its orientation with respect to magnetic north and, by subtracting declination, from true north.

As for your comments and questions about latitude and longitude, you seem to be confusing heading (direction) with lat/long (position). The aircraft does not use the magnetic data in any way to attempt to determine position - it is only used to determine the orientation of the aircraft.

 

[Meta4]

You definitely need to define "normal" here. FX, if you change flying locations, you should always recalibrate compass. If you've been around electrical equipment, like ground transformers, you should recalibrate.

Normal ? I haven't ever calibrated the compass on my P4 pro which I've had for 15 months and made hundreds of flights with over a wide area.
That's normal circumstances.
If you add or remove equipment from the Phantom, that's not normal and a recalibration is advised.

Read the manual!!!

OK ... here's what a recent manual says about when the compass needs calibrating:

Previous manuals had very poor wording that created a lot of misunderstanding and superstitious behaviour.
There is no need to ever calibrate the compass in normal circumstances.
Not for a new Phantom out of the box, not for moving 10 miles, 100 miles or 1000 miles.

Then why does my P3A require me to recalibrate the compass before flight when I am in NC altho it was previously calibrated in SC? And vice-versa?

Your Phantom doesn't require any recalibration - you just think it does.

An understanding of what compass calibration does is helpful.
Sar explained it pretty well in post #23 above.

 

[RunningWithScissors]

There is no need to ever calibrate the compass in normal circumstances.
Not for a new Phantom out of the box, not for moving 10 miles, 100 miles or 1000 miles.

After reading all of the closing arguments above, I believe the take-away is, your Phantom knows when it needs to be calibrated and will alert you to that. Is that a fair statement?

 

[sar104]

After reading all of the closing arguments above, I believe the take-away is, your Phantom knows when it needs to be calibrated and will alert you to that. Is that a fair statement?

In my opinion that is correct. However, it may also be worth considering recalibration if you add or change any components on the aircraft, or if you see unusual compass behavior not attributable to external interference.

 

[N017RW]

Agree with s-104 above....

My contention and advice has always been to re-compensate whenever you add, relocate, or remove metallic items or accessories on-board the aircraft.

 

[hph]

Are you saying that the aircraft is telling you that you need to recalibrate each time that you move between those two locations? If so, then I have no idea why it is doing that unless during travel you are exposing it to a significant magnetic field that is causing a change in the magnetization of components on the aircraft.

In addition to the fact that there is no theoretical physical basis for that to be necessary, I can say that I've travelled thousands of miles and never needed, or been required, to recalibrate.

It does travel within the confines of an SUV. How does yours travel over the thousands of miles w/o having to recalibrate? I find that to be miraculous if you go to areas of different magnetic declination (I don't think that's theoretical physics!) differing by several degrees. You're not going across the Equator, are you? I'm unsure as to what you're flying. Mine's a P3A, which has compass sensors in all four legs.

 

[sar104]

It does travel within the confines of an SUV. How does yours travel over the thousands of miles w/o having to recalibrate? I find that to be miraculous if you go to areas of different magnetic declination (I don't think that's theoretical physics!) differing by several degrees. You're not going across the Equator, are you? I'm unsure as to what you're flying. Mine's a P3A, which has compass sensors in all four legs.

As I said in post #23 - this has nothing to do with declination. It simply cannot determine declination by itself - it's not physically possible. It gets declination from a digital model of the earth's declination that is in the firmware. If you look at the DAT logs you can see the record of declination being applied.

I have several P2s that suffered from the original j-hook problem, and were how I first became aware of the subtleties of compass problems while working with DJI trying to fix the issue. Currently I fly a P4 and a Mavic. Both have travelled by road and air and I've never recalibrated either of them. But that's anecdotal and is not what should be convincing you - it really is a simple matter of physics as I explained above. DJI dropped the advice to recalibrate if you change location. It seems to have crept back sporadically via published advice and even a couple of manual versions, but it's simply wrong - whoever is randomly writing that stuff is not talking to the technical staff.

 

[N017RW]

Remember these craft are not flying cross country nor does any one flight cover so much distance as to have a navigational issue with varying declination.
Since these flights are, for the most part, relative to the take-off location and the system looks-up the local dec. all the system needs is a good polar response from the magnetometer to maintain the heading(ings) required for a few mile flight from take-off location (VLOS notwithstanding).

Since non-polar magnetometer responses are the result of ON-BOARD distortions, under normal circumstances, once they are characterized and compensated for there's no need to do so again until changes are made on-board the aircraft.

 

[RunningWithScissors]

Is it just me, or is every thread an educational opportunity to learn the wonders of these self-propelled lighter-than-air craft that we boldly launch skyward towards the heavens in the earnest hope that they return to us safe and unbroken?

 

[hph]

As I said in post #23 - this has nothing to do with declination. It simply cannot determine declination by itself - it's not physically possible. It gets declination from a digital model of the earth's declination that is in the firmware. If you look at the DAT logs you can see the record of declination being applied.

I have several P2s that suffered from the original j-hook problem, and were how I first became aware of the subtleties of compass problems while working with DJI trying to fix the issue. Currently I fly a P4 and a Mavic. Both have travelled by road and air and I've never recalibrated either of them. But that's anecdotal and is not what should be convincing you - it really is a simple matter of physics as I explained above. DJI dropped the advice to recalibrate if you change location. It seems to have crept back sporadically via published advice and even a couple of manual versions, but it's simply wrong - whoever is randomly writing that stuff is not talking to the technical staff.

I refer you to the latest version of the DJI Mavic Pro manual, v. 2.0, of last December:

https://dl.djicdn.com/downloads/mavic/Mavic Pro User Manual V2.0-.pdf

 

[dirkclod]

Ya could have at least copy and pasted what you wanted all to see instead of the whole manual .
All I know is that since we got the Go App unless it tells me to do a compass cal I don't and have had 4 phantoms and on my second Mavic now that use the Go App .I haven't even had to calibrate em when I got em .
Now do occasionally have a compass error and it says to recal but I just move it over abit and that goes away but now I do get those when in the back of my truck bed as thats what I use to take off most of the times .
Guess it's just to much steel in those 90 GMC 4x4's

 

[N017RW]

DJI’s Owners Manual content has already been acknowledged along with the caveat that the message has changed over the years.

No one is arguing what has been published just its validity.

What then is the point of citing such?

 

[sar104]

I refer you to the latest version of the DJI Mavic Pro manual, v. 2.0, of last December:

https://dl.djicdn.com/downloads/mavic/Mavic Pro User Manual V2.0-.pdf

This discussion has become circular and you don't seem to be at all interested in discussing the underlying issues. If you want to calibrate every time you move location then do so. I have nothing more to add.

 

[dirkclod]

This discussion has become circular

Nothing new to ya is it .

 

[hph]

Ya could have at least copy and pasted what you wanted all to see instead of the whole manual .
All I know is that since we got the Go App unless it tells me to do a compass cal I don't and have had 4 phantoms and on my second Mavic now that use the Go App .I haven't even had to calibrate em when I got em .
Now do occasionally have a compass error and it says to recal but I just move it over abit and that goes away but now I do get those when in the back of my truck bed as thats what I use to take off most of the times .
Guess it's just to much steel in those 90 GMC 4x4's

I apologize to those affected by the whole manual being in the link. I was on the page for "compass calibration" when I copied and pasted the URL. I assumed too much, literally! OK, you need a Ford pickemup with the Al alloy bed! Wonder if the side rails are that, too? I wouldn't have one, myself, but that has been the #1 vehicle sold in the US for some time. Those big GMCs are monsters in comparison to the F-150. We are off the subject. Watch out for "bad words" from the mod!!!

 

[Paul Sw]

This discussion has become circular and you don't seem to be at all interested in discussing the underlying issues. If you want to calibrate every time you move location then do so. I have nothing more to add.

I watch this thread often and contribute rarely. I find the technical level fairly low with lots of misconceptions and technical errors. However the posts in this thread by sar104 and N107RW are the exception. They show an exceptional understanding of both the physics involved and the detailed operation of the Phantom system. A ray of sunlight!

Paul

 

[hph]

I watch this thread often and contribute rarely. I find the technical level fairly low with lots of misconceptions and technical errors. However the posts in this thread by sar104 and N107RW are the exception. They show an exceptional understanding of both the physics involved and the detailed operation of the Phantom system. A ray of sunlight!

Paul

 

[WV. Rootman]

Ya could have at least copy and pasted what you wanted all to see instead of the whole manual .
All I know is that since we got the Go App unless it tells me to do a compass cal I don't and have had 4 phantoms and on my second Mavic now that use the Go App .I haven't even had to calibrate em when I got em .
Now do occasionally have a compass error and it says to recal but I just move it over abit and that goes away but now I do get those when in the back of my truck bed as thats what I use to take off most of the times .
Guess it's just to much steel in those 90 GMC 4x4's

You would do fine in West Virginia