I had exactly the same error this week on my brand new Phantom 3 4K, driven from Android DJI GO app version 3.0.0. Everything connected fine (camera pictures show on app, I can move gimbal with RC sticks), but I can't fly because the Sensors tab shows no IMU data (blank, or all zeros), and "Calibrate IMU" just hangs forever at 0%.
The IMU bug seems to be a
known problem with factory firmware on the Phantom 3 4K, and the fix is just to upgrade the firmware. But the current app won't upgrade the bird's firmware--status shows "Latest version!", but the About panel shows Firmware: n/a, and the secret Downgrade Firmware button in the front page shows Equipment:Loading Firmware Version:Loading.
Last night I fixed this, by manually downloading a Phantom 3 4K binary firmware file from this blind-guessed URL:
http://dl.djicdn.com/downloads/phantom_3_4k/P3XW_FW_V01.05.0040.bin
When I copied this to the camera's microSD card, and started the bird, it went into firmware update mode (d-d-d-d four beeps and green-red camera LED flashing) for 20 minutes, and ended in successful update mode (camera LED solid green and D---- D-D beep).
Afterwards, my Phantom 3 4K was able to calibrate the IMU successfully, and I was able to fly and take video!
The update log file P3XW_FW_RESULT_AB.txt shows it succesfully updated the firmware:
========== 2014.01.01 00:03:47 remo-con disconnect======
Packet: P3XW_FW_V01.05.0040.bin
Upgrading ...
Result: Success.
Strangely, I still see Firmware: n/a in the app, but I can confirm from /etc/firmware_version on the bird that it updated from "software version:01.00.09.38" (factory firmware?) to "software version:01.00.09.56", which might not be fully up to date, but does work!
It's frustrating that (1) the bird shipped with unusable firmware, (2) the firmware can 'only be updated from the app', but (3) the app does not seem to actually update firmware, only the manual microSD process from an undocumented source. Does anybody know where I can download other Phantom 3 4K firmware images, such as the latest supported version?