The GPS Steering function standard in the System 55X, and optional for all other S-TEC autopilots, dramatically improves enroute and approach GPS navigation tracking. Normal NAV tracking uses heading data and course deviation shown as CDI or HSI needle deflection. GPSS by S-TEC flies the roll steering commands output by many of the newer GPS Navigators. Theoretically, the GPS computer always knows where it is located and, based on the flight plan programmed by the pilot, where it is going. The GPS computer processes this information into right and left steering commands. These are then sent to the autopilot that flies the airplane in response to these commands navigating the GPS course very accurately. Currently, some GPS navigators have roll steering capability for enroute flight and limited approach transition procedures.