A system for controlling from a path of a first vehicle a second vehicle to a complementary path, the system comprising a first vehicle sub-system having a memory for time and (X, Y) position co-ordinates of the first vehicle's path; a GPS receiver for determining time and actual (X, Y) position coordinates of the first vehicle, means for comparing the first vehicle's actual position with path position at determined times as test path errors and means for transmitting the test path errors and a second vehicle sub-system having a memory for time and (X, Y) position coordinates of the second vehicle's path; a GPS receiver for determining time and actual (X, Y) position coordinates of the second vehicle, means for receiving the test path errors, means for computing a modified path of the second vehicle taking account of the test path errors and means for controlling the second vehicle.