In Phantom 3 Release note:
https://dl.djicdn.com/downloads/phantom_3/en/Phantom_3_Professional_Release_Note_en_20160613.pdf
Firmware update that touches the compass is this one:
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2016.3.15 Overview: 1. All-in-One firmware version updated to:v1.7.0060 2. DJI GO app iOS version updated to: v2.7.0 3. DJI GO app Android version updated to: v2.7.1
What’s New: 1. Improved compass accuracy.
2. Fixed issue of image being corrupted in rare occasion.
Notes: The firmware can be downgraded to v1.6.0040.
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Can the latest firmware be downgraded, does anyone know?