Apparatus for determining the path of a well bore during drilling, comprises an inertial measurement unit (12) for providing data from which position, velocity and attitude can be derived, the measurement unit comprising a plurality of inertial sensors mounted on a platform assembly which is, in use,...http://www.google.ca/patents/US6714870?utm_source=gb-gplus-sharePatent US6714870 - Method of and apparatus for determining the path of a well bore under drilling conditions
