Some folks have experienced calibration issues due to magnetic interference.  I will offer a scenario for how it happened to me, how it worked again, which may help others.  And I hope to hear from you for potential solutions because I don't think it is completely solved yet.

My case:

1. I'm in the UNited States and I have a Mavic Pro, but purchased the Mavic Air for yet more portability, considering that I travel frequently. The Mavic Air packs smaller than Mavic Pro and is lighter.  I have about 70-80 flights of experience and never had a problem flying the Mavic Pro.  I've flown the Mavic Air three times.

2. When I got the Mavic Air I flew it in my back yeard a couple of times and packed it for a trip to Brazil.  When I arrived in Brazil, I upfdated all firm ware, everything was up to specs, and on my first attempt it had the compass calibration problem due to magnetic interference.  I was at a beach, in the middle of nowhere.  Therefore, there was no magnetic interference.  I attempted to fly it wearing the same accessories I always have with me (phone, watch, etc).

3. After dozens of calibration attempts and two different phones it was a no go.  It never calibrated.  I signed up to this forum to get ideas from you: calibrate it higher from the ground, remove watches, go away from remote control when calibrating. You name an idea, I tried it and it never worked.  I did not attempt the idea of using magnets to demagnetize the compass - that would be beyond the manufacturers instructions, therefore I did not consider it appropriate.

4. After all attempts failed, I packed it and that was that.  Today, back in the United States, I took it out for a try, before I would return it to DJI and requested my money back. I started the Mavic Air first, then the remote, and then connected the phone and app.  The Mavic Air was blinking its rear green light normally.  But a message came up on the app indicating: your wi-fi is from a foregin country, reset and try again later (or something to that order).  Without me doing anything, the lights on the Mavic Air turned yellow and than green again and the message disappeared from the phone.  The Mavic Air was ready to fly with no codes, no calibration problems whatsoever. It flew and was very stable.

CONCLUSION:  
In my case (not sure if it applies to others here) the compass calibration problem was not magnetic interference.  It was a wi-fi problem.  Who knows what the specific problem is, is it a programming and firmware issue? Is it that it does not operate on Brazilian bandwidth for some specific issue?

QUESTION TO DJI AND OTHER INHOUSE EXPERTS:
What is really the problem here?
Why was it indicating a compass problem when the issue was wi-fi?
Why isn't it providing the appropriate indication of what the problem is, so that we can try to solve it with solutions that apply to the problem?
How can I prevent this from happening again? Are there solutions to this problem?
Why it doesn't work in Brazil?  Are there other similar problems in other countries? I will need to retun it because the main reason for purchasing it was to use outside of the US.
Why did it automatically re-set for US wi-fi and was able to operate wthout problems? Why didn;t it re-set automatically to Brazil's wi-fi?
Is this an Apple vs Android issue?  I was using an Apple phone (tried both a 5S and a 6S with the most recent update for the app at that time - May 5th an 6th).
One of the phones, the 6S had the US chip and was on "airplane mode"
The other phone, the 5S had a brazilian chip and was not on airplane mode.

Thanks.

You can install magnetic field meter app to verify is any magnetic interference around.

i'm looking forward to hearing the answers to those questions...

I think it's best you power on the Remote Controller syncing and binding it to your smart device BEFORE you power on and connect the drone.

GDL Posted at 2018-5-18 07:24
You can install magnetic field meter app to verify is any magnetic interference around.

If you read my post you would have learned magnetic interference was likely not the issue. Thank you for your response.

davidmartingraf Posted at 2018-5-18 09:25
I think it's best you power on the Remote Controller syncing and binding it to your smart device BEFORE you power on and connect the drone.

In model scale race drones the proper binding process is to always have the Transmitter powered on first, then turning on the drone to bind it.

davidmartingraf Posted at 2018-5-18 09:27
In model scale race drones the proper binding process is to always have the Transmitter powered on first, then turning on the drone to bind it.

As I explained, I tried all options of what was turned on first, nothing worked.  Upon returning to the US, just today, I turned the drone first, then the remote and then the phone. And it worked.  

Anyway, because I TRIED ALL DIFFERENT OPTIONS WITH TWO DIFFERENT PHONES OF WHAT I TURNED ON FIRST, I know the order by which I turned it on was not the issue.  Thank you!

We're sorry for the troubles. Could you connect the drone with DJI Assistant 2 and export the flight logs when the compass cannot be calibrated? I'd like to forward to our engineers for analysis.

I have problem yesterday to calibrate compass with Android. Once change to iPhone the problem gone. Looks like MA don't like Android.

MAIRDA Posted at 2018-5-18 09:27
If you read my post you would have learned magnetic interference was likely not the issue. Thank you for your response.

I have read your post, but I do not see how Magnetic Intereference warning was false due to bad WiFi for following reason.
The data being transmitted between Drone and Smartdevice is sent via WiFi (802.11_) networking standards.  7-Layer OSI model (see picture below)
  
WiFi is radio carrier version of electrical signals sent over Ethernet Wire or light signals sent over Fiber cable, at Layer #1 (Physical).  WiFi does extend into Layer #2 (Data Link)
There is error checking and error handling done at various layers.  Layer #2 (Data Link) handles errors for Layer #1 (Physical)
There is also error recovery and error handling done at Layers #3 (Network) and #4 (Transport).  Layer #6 (Presentation) transforms and encrypts data as to type.

If there were problems with WiFi transmissions, the Layers below #7 (Application -- aka GO-4) app should have:
Alerted Application layer that data is corrput
Caused errors at Presentation layer (decrypting / change format)
Requested packet be resent
Attempted to correct packet
Detected packet errors

For WiFi transmission problems to cause only a false Magnetic Intereference warning would be extremely unlikely.
It would really have to be a specific Bug in GO-4 app, where status of read request involving only compass data was not checked, and data from previous read of buffer was assumed to be compass data.

DJI Susan Posted at 2018-5-20 20:05
We're sorry for the troubles. Could you connect the drone with DJI Assistant 2 and export the flight logs when the compass cannot be calibrated? I'd like to forward to our engineers for analysis.

I will be glad to.  Is there a quick tutorial for how to connect to DJI Assistance 2? Thanks!

HedgeTrimmer Posted at 2018-5-20 21:32
I have read your post, but I do not see how Magnetic Intereference warning was false due to bad WiFi for following reason.
The data being transmitted between Drone and Smartdevice is sent via WiFi (802.11_) networking standards.  7-Layer OSI model (see picture below)
  

What I know:

I've flown my Mavic Pro in Brazil before. Twice.  Never had a problem.

I took the the Mavic Air to Brazil and I was never able to perform the request compass calibration.  I was in the middle of nowhere... on a beach.  Then I tried at different places in Brazil, never was able to perform the compass calibration.  

Then I bring it back to the United States, took it out of the case, same battery as last trial, same charge, and on first attempt it recognized an "wi-fi signal from another country" and it needs to re-connect (or something to that order).  It fixes itself (I did not do anything to it except look at it :-)) and then in less than 5 seconds it is ready to fly without needing compass calibration. And I flew it briefly just to test and it was very stable.

New data:  

I took it to San Diego a couple of days ago, so it was on another plane ride.  When I set it up to fly it in San Diego it requested a compass calibration.  I did it once and it did not work, then I changed my phone setting to "airplane mode" and the Mavic Air calibrated on first try on this mode.  I flew it without problems.

That's what I know... Therefore, without knowing all these layers you describe, my guess is that it is a communication issue.  I say it based on the experience i have and the wi-fi from foreign country message I got.  That's all.

I think MavAir compass calibration sensors are set very sensitive, many been asked to calibrate almost every time it starts, I’ve had calibration warnings numerous times and on checking compass values in the app everything looked fine, once I had trouble calibrating, and just from switching on and off aircraft all was good.
So maybe something needs fixing with firmware, but phones also seem to be a problem, it does seem to be a problem for some, I have also traveled long distance with no problems, so just a bug somewhere.

MAIRDA Posted at 2018-5-21 08:57
What I know:

I've flown my Mavic Pro in Brazil before. Twice.  Never had a problem.

I changed my phone setting to "airplane mode" and the Mavic Air calibrated on first try on this mode.  

Need to clarify something.  
Are you flying Drone via WiFi from your smartdevice or flying Drone via RC that is cable to your smartdevice via USB cable?

Not sure about different (LG, Samsung, ...) Android based phones.
On iPhones, switching on / off ''Airplane Mode'' can disable / re-enable Cellular Communications (GSM, CDMA), Bluetooth, and WiFi.
Additionally, ''Airplane Mode'' will carry-over (remember) what was disabled and enabled last time ''Airplane Mode'' was changed.

With an iPhone it is possible that switching on ''Airplane Mode'' disable Cellular Communications and Bluetooth; while leaving WiFi still enabled.  Or some other combination.

Being DJI has never divulged even rough process of what goes on during a drone's Calibration, all we can do is guess as to whether drone communicates with smartdevice to get information such as magnetic field deviation for current location and current time.

HedgeTrimmer Posted at 2018-5-21 10:18
I changed my phone setting to "airplane mode" and the Mavic Air calibrated on first try on this mode.  

Need to clarify something.  

Via wi-fi.
(Phone is connect via cable to RC).

Thanks!

MAIRDA Posted at 2018-5-21 08:46
I will be glad to.  Is there a quick tutorial for how to connect to DJI Assistance 2? Thanks!

Please check the first part the tutorial video of P4, you will see how to connect the drone with DJI Assistant 2 in the beginning.