The guidance system on any spacecraft is vital.  Without it, you cannot rendezvous with another object in space or reenter the earth's atmosphere safely and in the desired location. In fact, it is highly unlikely that one could attain orbit without a guidance system.

    X-33 and X-38 were both designed to be autonomous, making the guidance system even more important. Both incorporate Global Positing System in addition to their more traditional inertial navigation systems. Active radar, much like the ones onboard fighter aircraft, is also tied to the guidance system. Active radar is used primarily during docking, as its application is limited elsewhere. One would assume that active radar could alert the crew to micrometeorites, but 95% of micrometeorites that are lethal to a spacecraft do not pick up on active radar.

    The implementation of GPS allows for the spacecraft to determine its exact position in space without relying on data from ground tracking stations. The use of onboard tracking data allows the onboard computers to calculate abort paths in real time, anticipating failures in advance and automatically taking the safest course of action. The abort modes will be available for all modes of operation, including launch, orbit, and reentry. The Guidance computers can also calculate landing locations in real time so that in the event of an emergency while docked to the station, it is ready to reenter as soon as the hatch is closed.

    Overall, this next generation guidance system will provide an extra level of safety for the crew and be more cost effective since ground tracking stations are no longer necessary. All guidance is 'internal,' not requiring any proprietary hardware or support.