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
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
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.
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.
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.
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.
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.
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.