Open Radar

 
Summary:
The device's yaw attitude changes at a constant rate even though the device is resting in a fixed position/orientation. This occurs only on some devices, both iOS 8 and iOS 9 Beta. I currently have an iPhone 5 with iOS 9 Beta installed which displays this problem every time, but it has also been seen in the field on various devices with iOS 8.

Steps to Reproduce:
1. Download sample code MotionGraphs
2. In file APLDeviceMotionGraphViewController.m: Instead of calling startDeviceMotionUpdatesToQueue:withHandler: on CMMotionManager, call startDeviceMotionUpdatesUsingReferenceFrame:toQueue:withHandler using CMAttitudeReferenceFrameXMagneticNorthZVertical as reference frame.
3. Install and start the application on a device
4. Let the device rest still with x- or z-axis vertical
5. Note the graphs presented as deviceMotion.attitude

Expected Results:
The graphs should display three curves (yaw, pitch and roll) with constant values (depending on the device's resting orientation).

Actual Results:
On some devices, the yaw curve keeps increasing/decreasing with a steady pace (and wraps at ±pi).

Notes:
With the device's x-axis vertical, yaw makes a full rotation in one minute, with the y-axis vertical it's steady as expected though. Any other angle (e.g the z-axis vertical) yields a yaw change rate somewhere between 2*pi and 0 rad/min.