Patent ID: 7251567

Claim:
A horizontal directional drilling system comprising: a horizontal directional drilling machine; a drill string connectable to the horizontal directional drilling machine; a downhole tool assembly supported on the drill string; a sensor assembly supported by the downhole tool assembly for movement therewith; wherein the sensor assembly transmits at least one sensor assembly signal containing sensor assembly orientation information; and a monitoring system adapted to monitor the position and orientation of the downhole tool assembly, the monitoring system comprising: an antenna assembly adapted to detect the sensor assembly signal; and a processor assembly adapted to determine an orientation of the sensor assembly using the sensor assembly signal detected by the antenna assembly, to predetermine a calibration factor indicative of the orientation of the sensor assembly relative to a known downhole tool assembly orientation, to later determine an orientation of the downhole tool assembly using the sensor assembly signal and the predetermined calibration factor while the downhole tool assembly is in use.