Controlling the trajectory of a drill string

A steering behaviour model includes build rate and turn rate equations to model bottom-hole assembly behaviour. The build and turn rate equations are calibrated by adjusting model parameters thereof to minimize any variance between actual response 118 and estimated response produced for an interval of the well. Estimated position and orientation 104 of a bottom-hole assembly along a subsequent interval are generated by inputting subsequent tool settings into the calibrated steering behaviour model. Estimated position and orientation 104 are compared to a well plan 106 with a controller 108 which determines a corrective action 110. Corrective action 110 can be converted from a build and turn rate to a set of recommended tool settings 114 by using an inverse application 112 of the steering behaviour model. As additional data 118 becomes available, steering behaviour model can be further calibrated 102 through iteration.