The two radar altimeters on board the orbiter measure absolute altitude from the orbiter to the nearest terrain within the beamwidth of the orbiter's antennas.

The RAs constitute a low-altitude terrain-tracking and altitude-sensing system based on the precise time it takes an electromagnetic energy pulse to travel from the orbiter to the nearest object on the ground below and return during altitude rate changes of as much as 2,000 feet per second. This enables tracking of mountain or cliff sides ahead or alongside the orbiter if these obstacles are nearer than the ground below and warns of rapid changes in absolute altitude.

The two independent RAs consist of a transmitter and receiver antenna. The systems can operate simultaneously without affecting each other. The four C-band antennas are located on the lower forward fuselage. The two receiver/transmitters are located in the middeck forward avionics bays and are convection cooled.

Each RA transmits a C-band (4,300 MHz modulated at 8.5 kHz) pulse through its transmitting antenna. The signal is reflected by the nearest terrain, and the leading edge of the return radar echo is locked on by the RA through its receiving antenna. The altitude outputs by the RA are analog voltages that are proportional to the elapsed time required for the ground pulse to return, which is a function of height or distance to the nearest terrain. The range output of the RA is from zero to 5,000 feet. The RA will not lock on if the orbiter has large pitch or roll angles.

The onboard GPCs process the data for the autoland mode and touchdown guidance after the orbiter has crossed the runway threshold from an altitude of 100 feet down to touchdown. If the autoland mode is not used, the GPCs process the data for display on the commander's and pilot's altitude/vertical velocity meters from 5,000 feet.

The commander and pilot can select RA 1 or 2 for display on their respective AVVI. The commander's radar al tm 1 and 2 switch is located on panel F7, and the pilot's switch is located on panel F8. The radar altimeter on/off 1 and 2 power switches are on panel O8. Positioning radar altimeter 1 to on provides electrical power to RA 1 from main bus A; positioning radar altimeter 2 to on provides electrical power to RA 2 from main bus B.

The display scale on the commander's and pilot's AVVI raw data recorder indicators ranges from 5,000 to zero feet. Altitude is displayed on a moving tape. Above 9,000 feet, the scale will be pegged. At 1,500 feet, the raw data recorder indicator changes scale. The RA off flag will appear if there is a loss of power, loss of lock, data good-bad or after three communications faults.

Because there are only two radar altimeters on board the orbiter, the altitude data from the two units are averaged in redundancy management when the radar altimeter is used for the autoland mode.

Each radar altimeter receiver/transmitter measures 3.13 inches high, 7.41 inches long and 3.83 inches wide and weighs 4.5 pounds.

The radar altimeter contractor is Honeywell Inc., Minneapolis, Minn.