Test flights of a GPS receiver to replace TACAN began in 1996. Each orbiter has one GPS receiver. After RTF, ramp-up to single string GPS use will commence. Endeavour is undergoing TACAN removal and upgrade to three string GPS. GPS is not used for ascent and is powered on prelaunch to support an emergency landing. GPS has limited use for orbital operations and is not used for Rendezvous and docking. GPS will supplement or replace TACAN, but cannot be used as an MLS replacement. GPS enhances Shuttle navigation and improves safety. State-of-the-art GPS receivers have capabilities that would be useful in supporting automated/autonomous missions
