Open Radar

 
Summary:
When using Core Motion with CMAttitudeReferenceFrameXTrueNorthZVertical some devices consistently return a CMErrorTrueNorthNotAvailable (error code 102) in the in the CMDeviceMotionHandler block. It can be reproduced in 100% of the cases on affected devices. A reset of the iOS settings solves the problem.

Steps to Reproduce:
Run the pARk sample app on an affected device.

Expected Results:
The labels should move on the screen depending on the orientation of the device.

Actual Results:
The labels stay in the center of the screen and never move. (Screenshot: http://d.pr/i/dyjI)

Regression:
This happens at least since iOS 6.0. It happens only on a tiny fraction of devices but on those devices it is 100% reproducible. We've reports from only a handful of devices out of a user base of many thousands (the Augmented Reality mode in Where To? http://itunes.apple.com/app/id314785156?mt=8).

Notes:
To actually obtain the NSError object the pARk app has to be modified to use 

-startDeviceMotionUpdatesUsingReferenceFrame:toQueue:withHandler: instead of
-startDeviceMotionUpdatesUsingReferenceFrame:

This is the NSError received:
	Error Domain=CMErrorDomain Code=102 "The operation couldn’t be completed. (CMErrorDomain error 102.)"

After receiving the error callback, no further callbacks are received. cmMotionManager.deviceMotion is always nil.

The error (CMErrorTrueNorthNotAvailable) hints at a Core Location problem. Therefore we had the customer test Core Location on the affected device and verified it's working correctly: CMLocationManager updates are received and MKMapView displays the blue userLocation dot.

We tried a little modification of the pARk app: We replaced CMAttitudeReferenceFrameXTrueNorthZVertical by CMAttitudeReferenceFrameXMagneticNorthZVertical and it worked fine. So only the CMAttitudeReferenceFrameXTrueNorthZVertical mode is affected.

Then we had the customer reset "All Settings" using General => Reset => All Settings and the problem disappeared. Apparently there is some corrupt system setting that triggers this bug.