The three onboard microwave scan beam landing systems are airborne Ku-band receiver/transmitter navigation and landing aids with decoding and computational capabilities. The MSBLS units determine slant range, azimuth and elevation to the ground stations alongside the landing runway. MSBLS is used during terminal area energy management, the approach and landing flight phases and return-to-launch-site aborts. When the channel (specific frequency) associated with the target runway approach is selected, the orbiter's MSBLS units receive elevation from the glide slope ground portion and azimuth and slant range from the azimuth/distance-measuring equipment ground station.

The orbiter is equipped with three independent MSBLS sets, each consisting of a Ku-band receiver/transmitter and decoder. Data computation capabilities determine elevation angle, azimuth angle and orbiter range with respect to the MSBLS ground station. The MSBLS provides highly accurate three-dimensional navigation position information to the orbiter to compute state vector components for steering commands that maintain the orbiter on its proper flight trajectory. The three orbiter Ku-band antennas are located on the upper forward fuselage nose. The three MSBLS and decoder assemblies are located in the crew compartment middeck avionics bays and are convection cooled.

The ground portion of the MSBLS consists of two shelters: an elevation shelter and an azimuth/distance-measuring equipment shelter. The elevation shelter is located near the projected touchdown point, with the azimuth/DME shelter located near the far end of the runway. Both ends of the runway are instrumented to enable landing in either direction.

The MSBLS ground station signals are acquired when the orbiter is close to the landing site and has turned on its final leg. This usually occurs on or near the heading alignment cylinder, about 8 to 12 nautical miles (9 to 13 statute miles) from touchdown at an altitude of approximately 18,000 feet.

Final tracking occurs at the terminal area energy management ''autoland'' interface at approximately 10,000 feet altitude and 8 nautical miles (9 statute miles) from the azimuth/DME station.

The MSBLS angle and range data are used to compute steering commands until the orbiter is over the runway approach threshold, at an altitude of approximately 100 feet. If the autoland system is used, it may be overridden by the commander or pilot at any time using the control stick steering mode.

The commander's and pilot's horizontal situation indicators display the orbiter's position with respect to the runway. Elevation and azimuth are shown relative to a GPC-derived glide slope on a glide slope indicator; course deviation needles and range are displayed on a mileage indicator. When the orbiter is over the runway threshold, the radar altimeter is used to provide elevation (pitch) guidance. Azimuth/DME data are used during the landing rollout.

The three orbiter MSBLS sets operate on a common channel during the landing phase. The MSBLS ground station transmits a DME solicit pulse. The onboard MSBLS receiver responds with a DME interrogation pulse. The ground equipment responds by transmitting a return pulse. A decoder in the onboard MSBLS decodes the pulses to determine range, azimuth and elevation. Range is a function of the elapsed time between interrogation pulse transmission and signal return. Azimuth pulses are returned in pairs. The spacing between the two pulses in a pair identifies the pair as azimuth and indicates which side of the runway the orbiter is on; spacing between pulse pairs defines the angular position from runway centerline. The spacing between the two pulses in a pair identifies the pair as elevation, and the spacing between pulse pairs defines the angular position of the orbiter above the runway.

The elevation beam is 1.3 to 29 degrees high and 25 degrees to the left and right of the runway. The azimuth/DME beam is zero to 23 degrees high and 13.5 degrees to the left and right of the runway.

Each RF assembly routes range, azimuth and elevation information in RF form to its decoder assembly, which processes the information and converts it to digital words for transmission to the onboard GN&C; via the multiplexers/demultiplexers for the GPCs.

Elevation, azimuth and range data from the MSBLS are used by the GN&C; system from the time of acquisition until the runway approach threshold is reached. After that point, the azimuth and range data are used to control rollout. Altitude data are provided separately by the orbiter's radar altimeter.

Since the azimuth/DME shelters are at the far ends of the runway, the MSBLS can provide useful data until the orbiter is stopped. Azimuth data give position in relation to the runway centerline, while the DME gives the distance from the orbiter to the end of the runway.

Each MSBLS has an on/off power switch on panel O8 and on the channel (frequency) selection thumbwheel on panel O8. Positioning the MLS 1, 2 and 3 switch provides power to the corresponding microwave scan beam landing system. MSBLS 1 receives power from main bus A, MSBLS 2 from main bus B and MSBLS 3 from main bus C. Positioning the channel 1 , 2 and 3 thumbwheels selects the frequency (channel) for the ground station at the selected runway for the corresponding MSBLS.

Redundancy management mid-value-selects azimuth and elevation angles for processing navigation data. The three MSBLS sets are compared to identify any significant differences among them.

When data from all three MSBLS sets are valid, redundancy management selects middle values of three ranges, azimuths and elevations. In the event that only two MSBLS sets are valid, the two ranges, azimuths and elevations are averaged. If only one MSBLS set is valid, its range, azimuth and elevation are passed for display. When a fault is detected, the SM alert light is illuminated, and a CRT fault message is shown.

Each MSBLS decoder assembly is 8.25 inches high, 5 inches wide and 16.16 inches long and weighs 17.5 pounds. The RF assembly is 7 inches high, 3.5 inches wide and 10.25 inches long and weighs 6 pounds.

The MSBLS contractor is Eaton Corp., AIL Division, Farmingdale, N.Y.