I'll say one thing about this thread, confusing as all get out and left me scratching my head. I thought I would apply a bit of common sense by first defining what the IMU is;

Inertial Measurement Unit (IMU) and Attitude Heading Reference System (AHRS). Incorporating the latest MEMS sensor technology, the VN-100 combines 3-axis accelerometers, 3-axis gyros, 3-axis magnetometers, a barometric pressure sensor and a 32-bit processor. Along with providing calibrated sensor measurements, the VN-100 also computes and outputs a real-time 3D orientation solution that is continuous over the complete 360 degrees of motion.
Source; http://www.vectornav.com/products/vn-100

This may not be the part that is in the Mavic but we get the idea of what the IUM does.

DJI tells us the IMU was calibrated at the factory and unless notified by the app, re-calibration is not required.

As long as the position of the IMU chip remains the same with respect to the airframe, I can see absolutely no reason to recalibrate the IMU.

Regarding the advice from DJI's support to recalibrate before each flight or any of the ambiguously worded advice about re-calibrating the IMU, now that we have a better understanding what the IMU contributes to the aircraft, we can make a logical decision. Simply leave it alone unless the apps tell us otherwise. First line support people do not have the exposure and likely do not have the knowledge base to form a conclusion about the IMU's aircraft integration. If I spoke with a DJI engineer that actually had involvement in the software and hardware design of the IMU circuit, that would be different.

The IMU is barely mentioned in the Mavic  User Manual, so apparently DJI doesn't expect you to calibrate the IMU unless the app prompts you.  However, with a new Mavic right out of the box, I found it desirable.  The new Mavic was drifting, even in no wind. IMU showed yellow, but the app did not prompt for calibration. Strangely, when I initiated the calibration the app told me not to touch the aircraft during the process.  Got to 95%, then it just sat there and never completed. I restarted app & Mavic - now I have an IMU error  & it prompts me to calibrate the IMU. This time I got the correct Mavic IMU five step calibration prompts. Process completed successfully.  Calibrated the compass, test flew - and now no drift - aircraft is rock solid, can now land on a small surface with no trouble.
    One other thing: when the IMU was messed up, both IMU and compass were demanding calibration.  Do the IMU first - it is impossible to calibrate the compass when the IMU is messed up.

How long does it take to calibrate IMU?

Kashish Chawla Posted at 2018-2-16 16:08
How long does it take to calibrate IMU?

A couple of minutes or so. Done on a level surface. That is what makes it difficult to calibrate in the field. I usually recalibrate after firmware update.

Aardvark Posted at 2018-2-16 16:45
A couple of minutes or so. Done on a level surface. That is what makes it difficult to calibrate in the field. I usually recalibrate after firmware update.

what to do if it is stuck on step 2 for a long time

Kashish Chawla Posted at 2018-2-21 20:56
what to do if it is stuck on step 2 for a long time

Have a close look at the Mavic position shown on the screen and try again, make sure the aircraft is lying exactly the same way.

A true IMU (Inertial Measurement Unit) calculates it's position in free space by using precision accelerometers to calculate a position that is independent of the location of the earth.
That's why it drifts as the Earth rotates when your 747 flies from Europe to USA but as it's less than 50 km it's acceptable as radar and ILS kicks in.

Unfortunately dji can not afford this technology so all co-ords are computed from GPS co-ords so not really an IMU because all readings are calculated from take off point and that depends on GPS satellites.

* hex *