Why do z-axis rotations reflect on y-axis in Nexus S?

I'm trying to understand the gyroscope sensor output from the Nexus S. When you rotate about the z-axis (the axis perpendicular to the screen), the z-axis correctly records the angular velocity as a result of the turn. However, the y-axis also reports spikes in angular velocity, despite no change in the orientation of the device relative to the y-axis. So if I turn around when holding the phone at a particular orientation, it appears that I have not only turned aro开发者_JS百科und, but also tilted the phone left/right (which I haven't).

Why is this? Any ideas how to compensate or correct for this?

Please note I'm talking about the raw sensor output here.

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First of all raw sensor data always contains some bias that has to be filtered out. Furtheron it is almost impossible to really suppress other rotational directions manually. If you suspect your Nexus to have a sensor bug or you want to know exactly what's going on I suggest to build your own apparatus. As inspiration a picture of mine :-)