Precise position tracking

In the following link a quick 3d model of my prototype for a mouse.

http://home.wanadoo.nl/bogers22/bol.jpg

The difference with a normal mouseball is the axis through it. I want a precise position calculation of the axis, which is free to move in any direction.

I have tried an optical mouse sensor, placed on the bottom. The problem here is that it sometimes drifts so the whole system is ruined since it is a relative system.

The since of the ball in the image is approximately 5 cm. It should be as cheap as possible. Any suggestions??
Thanks