One embodiment the invention includes a method of determining an orientation of a sensor. The method includes measuring a local magnetic field vector and a local gravity vector and using those measurements to determine the orientation of the sensor. Embodiments can include measuring the magnetic field vector and the local gravity vector using quaternion coordinates. Another embodiment includes measuring a local magnetic field vector, a local gravity vector, and the angular velocity of the sensor. These three vectors are processed to determine the orientation of the sensor. In one embodiment the three vectors can all be measured in quaternion coordinates. Another method embodiment includes determining a local gravity vector by providing a acceleration detector, moving the detector from a start point to an end point over a time period, and summing acceleration measurements over the time period. The local gravity vector is calculated using the summed acceleration measurements.