Could someone explain why it's been advised to to calibrate the IMU??
I've had the p3 and p4 and only did the IMU after a firmware update .

Calibrate the IMU is to guarantee the performance of your drone.
It's doesn't hurt to do the IMU calibration.

IMU "Inertial measurement unit"
This was not written by me , but was an answer to a question I posed.
As far as I see it, IMU calibration on a level surface updates a table of values the flight controller software uses as a reference for a level stationary hover. From there the craft responds predictably to flight commands.  It also likely measures any sensor noise and thermal drift so that these technical imperfections are accounted for when using IMU sensor data in flight. - this is likely why there must be no vibrations during the calibration process.


Bad IMU calibration could cause drift and attitude issues as the flight controller fights to hold the craft in what it thinks is the correct attitude as opposed to the correct physical attitude.


The IMU usually has 2 types of sensors – angle and acceleration and in turn 3 sensors of each type measuring in the X,Y and Z axis. These sensors can, through vibration, aging, impact etc, drift in their response over time and thus an IMU calibration will establish a new reference for the IMU’s level/stationary state that the flight controller can work with to restore stable flight.


Think of calibration as the bringing back into line the measured craft attitude with the true physical craft attitude.


As to how often an IMU calibration is needed, I wouldn’t like to say; certainly after any impact or if there are suspicions the craft can’t hover in a level attitude in a windless environment. Also after a firmware update is a good opportunity just in case the stored values get corrupted or are incompatible with the later firmware.

I did put this piece together because at least 1 person seemed to have a problem, I think Bad IMU.

Its also good to mention that the IMU is the reason why you are able to shut off the motors using just the throttle down command when on the ground. The IMU reads that the aircraft is unable to move any further, and after three seconds it determines that is has officially landed and can shut off the motors safely.