Ku-band system includes a rendezvous radar that skin-tracks satellites
or payloads in orbit to facilitate orbiter rendezvous with them.
For large payloads that must be carried into orbit one section at
a time, the orbiter will rendezvous with the payload segment currently
in orbit to add on the next section. The gimbaling of the Ku-band
antenna permits it to conduct a radar search for space hardware.
The Ku-band system is first given the general location of the space
hardware from the orbiter computer; then the antenna makes a spiral
scan of the area to pinpoint the target.
for space hardware may use a wide spiral scan of up to 60 degrees.
Objects may be detected by reflecting the radar beam off the surface
of the target (passive mode) or by using the radar to trigger a
transponder beacon on the target (active mode).
During a rendezvous
operation, the radar system is used as a sensor that provides target
angle and range information for updating the rendezvous navigation
data. The operation is similar to using the crewman optical alignment
sight or star trackers except that the radar provides target data
in addition to angle data. Angle tracking maintains appropriate
antenna pointing to keep the target within the antenna beam. Range
tracking is accomplished by electronically measuring the time between
a transmitted pulse and a return pulse from the target. The return
pulse may be reflected from a passive (skin-tracked) target or cooperative
target transmitter that is triggered by the radar-initiated pulse.
The latter provides a longer range capability.
can be accomplished in two ways: computer or manual designations
or automatic (auto) servo. During manually or computer-designated
tracking, the antenna beam angle is positioned by services external
to the Ku-band system. Computer tracking provides designated angle
data based on combined target and orbiter state vector information.
Manual tracking applies manually initiated rates to the antenna
control system from switches at the orbiter aft flight deck station.
tracking applies error rates to the antenna control system from
a receiving station that measures the target position relative to
the antenna beam center. This closed-loop servosystem, internal
to the Ku-band system, ignores external computer or manual designations.
is always automatic, although computer-designated ranges are applied
to the Ku-band system until automatic tracking is achieved. In the
automatic tracking mode, the Ku-band system provides actual antenna
angle, angle rate, range, and range rate data through an MDM for
rendezvous and proximity operations. Data routed to panel A2 represent
hard-wired azimuth, elevation, range and range rate information,
which is not processed by the GPCs.
radar system has four steering modes for locating and tracking a
target: crew-controlled; automatic; manual antenna steering, with
programmed spiral search routines for angular acquisition; and range
tracking (previously mentioned). In addition, there are two antenna
radar mode is used, the orbiter is normally maneuvered to an attitude
with the minus Z axis pointing at the GPC-calculated target location.
In all modes, therefore, the radar antenna is normally maneuvered
around the minus Z axis.
system provides for antenna steering modes, each with a different
combination of capabilities for acquiring and tracking a target:
GPC, GPC designate, auto track and manual slew. All are mutually
exclusive and crew selectable.
The GPC mode
is fully automatic in all phases of target acquisition. Two GPC
CRT commands are required before this mode can be initiated. One
CRT command enables target position data to be routed from the GN&C;
to the SM antenna management program through the intercomputer data
bus. The antenna management program converts the target position
to antenna pointing angles and estimated orbiter-to-target range
data. Another CRT command enables the antenna management program
to send designated antenna pointing and range data to the Ku-band
system through the payload 1 data bus and the payload forward 1
When the GPC
mode is selected, the antenna points to the commanded angles and
adjusts the ranging system to the specified range. If a receive
signal is detected, the automatic closed-loop system tracks the
target in angles and range and provides data for the SM and GN&C;
computers and panel displays. If the target is not detected, the
antenna is automatically commanded to search around the designated
point. The antenna is inertially stabilized during the search operation.
The GPC designate
mode provides the same designated pointing as the GPC mode without
angle search or angle-tracking capabilities. No closed-loop angle
tracking is provided. The designated angles are updated every two
seconds. Range search and tracking are automatic. The antenna may
be either inertially or body stabilized.
The auto tracking
mode begins with manual antenna control, including a manually initiated
search and inertial stabilization during the target acquisition
phase. Once the target is detected, automatic angle and range tracking
is initiated and manual control is inhibited until auto tracking
slew mode allows manual control of antenna movement with maximum
automatic range search. Once the target is detected, an automatic
range track is initiated, but angles are still under manual control.
No angle search is available in this mode.
mode involves a programmed antenna movement that causes the radar
beam to describe a spiral pattern starting at the designated angles.
The beam angle spirals out to a maximum of 30 degrees from the designated
angle. In the GPC steering mode, the variation of the maximum angle
of search from the designated point is inversely proportional to
designated range. The smallest search spirals outward to a maximum
of 6.2 degrees from the designated point for ranges from 145 to
300 nautical miles. At minimum ranges (zero to 8 nautical miles),
the spiral search is the maximum 30 degrees from the designated
point. If the antenna drive system detects but overshoots the target
during a spiral search, a miniscan program is automatically initiated
near the point of detection. The miniscan searches to a maximum
of 9 degrees from the starting point in one minute. In the auto
track steering mode, only the manually initiated main 30-degree
scan is available.
to angle search, the Ku-band system provides a range search. The
process includes electronically varying the timing within the range
system until it coincides with the time interval between the transmitted
and received radar pulses. Once the intervals coincide, radar tracking
is established and the range data output represents the range between
the target and the orbiter. The crew can read the range and range
rate values as panel and CRT parameters.
The crew is
provided with two controls associated with range. Transmitter power
output is automatically varied in proportion to range when a target
is being tracked to keep the return signal relatively constant,
regardless of range. If, however, the track is lost and the range
system begins to search, the transmitter may transmit maximum power
during the search. The crew can limit the maximum transmitter power
by using an aft station panel control. The other control is a CRT
command that limits the range search in GPC steering mode to 2,400
system has body and inertial antenna stabilization modes. When the
body is stabilized, the antenna beam remains in a fixed relationship
to the orbiter X, Y and Z axes during orbiter attitude changes.
When the system is inertially stabilized, the antenna beam remains
aligned to a point in inertial space, regardless of the orbiter
attitude changes. Both of these modes are effective only when the
radar angle-tracking system is not tracking a target. In the target
angle-tracking mode, the system aligns the antenna beam to the target,
and antenna movement is independent of orbiter attitude changes.
steering mode has a specific stabilization mode except the designate
mode. The designate mode selects either body or inertial stabilization
on the basis of a real-time GPC command (not available to the crew).
Since the designate mode provides range tracking only, inertial
stabilization is effective during target tracking.