Scan Beam Landing System
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.
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.
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
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
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
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.
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.
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.
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.
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.
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.