the imu and the drill bit is known the calculations of spatial position and velocity can be extrapolated to provide a measure of drill bit direction position and velocity the tool face deflection angle can also be calculated the imu and computer together form an inertial measurement system this system may be configured either as an attitude and heading reference system to provide directional survey data or as a full inertial navigation system ins in order to provide both directional and positional survey data the direction and or position of the drill bit are calculated with respect to a pre determined reference frame in addition the computer may be provided with depth well bore hole path length information in full inertial navigation mode depth information may be used to obtain accurate co ordinate position data by combining the inertial system data with independent depth measurements it is possible to bound the growth of inertial system error propagation fig4 shows the down hole closed loop control system 10 in the preferred embodiment of the present invention initial surface input data 11 which comprise start co ordinates and planned bore hole trajectory are input into target position means 12 together with continuous measured bore path length updates 13 surface to rotary steerable system the target position means generates target direction and or position information as.