Is Apple's iPhone magnetometer calibration working properly?

Question

I'm currently developing an iPhone App (on iPhone 5, iOS 7, Xcode 5) which requires a very accurate determination of the current attitude. The "attitude" of CMDeviceMotion does not fulfil these requirements because Apple's sensor fusion algorithm seems to rely too much on the gyroscope which drifts away rather fast (in my experience). That's why I decided to read out the bare sensor data and later I want to combine it within a sensor fusion algorithm by myself.

When asking for magnetometer data one has two possibilities:

1. via CMMagnetometerData in CMMotionManager

2. via CMCalibratedMagneticField in CMDeviceMotion about which Apple says

The CMCalibratedMagneticField returned by this property gives you the total magnetic field in the device’s vicinity without device bias. Unlike the magneticField property of the CMMagnetometer class, these values reflect the earth’s magnetic field plus surrounding fields, minus device bias.

In principle (2.) is exactly what I want.

There is a very simple test if magnetometer data is calibrated properly. For simplicity one can restrict oneself to two dimensions. When the device lies on it's back, the combination B_x^2 + B_y^2 must be constant, independent of the direction the device is pointing to. It must just equal the horizontal component of the Earth's magnetic field (assuming no other fields in the vicinity of the device). Thus, when performing a 360 degrees turn of the device which lies on it's back, the measured data B_y over B_x should display a circle. See here for details.

Now the point: the data of CMCalibratedMagneticField does NOT result in a circle!

Does anyone have an explanation for that? Or does anyone know, how the CMCalibratedMagneticField comes about? Is the magnetometer calibrated in the sense of the link from above when performing the "eight-shaped" movement of the device or what is the movement good for?

Btw. why the "eight-shaped" movement and not flipping the device around it's three axis, which would allow a calibration as described in the link from above?

I would be very glad for any clarification with this issue... Thanks!

Answer 1

There is a problem with the magnetometer in iOS 7, it has an error of +-7º. Try using the 7.1 beta version.

EDIT

The magnetometer has zero-drift over time, but is pretty inaccurate for sudden changes in position. The accelerometer and gyroscope on the other hand adjust quickly for sudden changes but, being inertial sensors, they lose accuracy over a period of time.

So when CMCalibratedMagneticField tries compensate for your rotational motion it uses data from the gyroscope and accelerometer. This is when the accelerometer and gyroscope's +-7º error creeps in and throws your circle off track. Check this answer and this wikipedia article for more info.

As regards to the figure of eight: Both do the same thing, they orient the "North" of your device in each direction in hope of cancelling out magnetic interference. Flipping your device along all three axes will work better but it is harder to perform and not as easily understood by the user.

Hope this helps.