A system for monitoring the position and orientation of a downhole tool assembly having multiple beacons. In a preferred embodiment the first and second beacons are supported by the downhole tool assembly. Both beacons are adapted to transmit signals that are indicative of the orientation and position...http://www.google.ca/patents/US7373993?utm_source=gb-gplus-sharePatent US7373993 - System and method for locating and tracking a boring tool
