This study involved refining a vehicle for computer guidance and GPS position control. The vehicle was powered by a small diesel engine coupled to a hydrostatic drive transmission adaptable to computer control. By interfacing GPS and a computer with the control portions of the vehicle, it was controlled automatically with no driver interference. The computer interface for the vehicle was built to use TTL level inputs and outputs on a standard parallel port configuration. All control aspects of the vehicle were dictated by two parallel ports with vehicle position and heading read simultaneously through two nine pin-serial connectors. This interface configuration was flexible so the vehicle’s software control could be executed by a variety of computers, not just one built specifically for the application.A hydraulic power steering valve driven by a single bidirectional DC motor controlled the electro-hydrostatic steering system. An optical encoder and counter interface chip monitored the angle of the steering wheels and fed that information to the computer. The vehicle’s heading was read from an electronic compass while the vehicle’s position was read from a differentially corrected GPS data acquisition system. The vehicle control program used the angle of the front wheels, the vehicle’s heading, and the vehicle’s position to calculate a turning solution to match the vehicle’s heading with the correct bearing for the next point. This control system guided the vehicle to within 3.58 m (11.74 ft) using a +/- 5 m (16.4 ft) accuracy differential GPS configuration and to within 6.42 m (21.05 ft) using a differential GPS setup with a +/- 10 m accuracy.