The microwave scanning beam landing system (MSBLS) was a Ku band approach and landing navigation aid used by NASA's Space Shuttle.[1][2][3][4][5] It provided precise elevation, directional and distance data which was used to guide the orbiter for the last two minutes of flight until touchdown. The signal was typically usable from a horizontal distance of approximately 28 km and from an altitude of approximately 5 km (18,000 feet).