Microwave Scanning Beam Landing System Wikipedia
The Microwave Scanning Beam Landing System (MSBLS) is a Ku band approach and landing navigation aid formerly used by NASA's space shuttle. It provides precise elevation, directional and distance data which was used to guide the orbiter for the last two minutes of flight until touchdown. The signal is typically usable from a horizontal distance of approximately 28 km and from an altitude of approximately 5 km (18,000 feet).
MSBLS installations used by NASA have to be certified every two years for accuracy. Since 2004, the Federal Aviation Administration works with NASA to execute this verification; previously, only NASA aircraft and equipment were used. Testing of the Kennedy Space Center's MSBLS in 2004 revealed an accuracy of 5 centimeters.
The shuttle landing approach started with a glide slope of 19 degrees, which is over six times steeper than the typical 3-degree slope of commercial jet airliners.