Regarding Direction calculation using Gyroscope,Magnetometer and Accelerometer
Hi: If the probe is not moving you don't need the gyro, the 3-axis magnetometer and the 3-axis accelerometer are all you need to determine the orientation of the probe.

It turns out that the Earth's magnetic field in Northern California is stronger in the vertical direction than in the horizontal direction so the magnetometer by itself can not determine direction.  The companies that make integrated sensor ICs have app notes that go into detail on how to use the accelerometer to correct the magnetic readings and may even have the firmware for that.
http://www.prc68.com/I/Sensors.shtml#EMF