The star tracker
system is part of the orbiter's navigation system. Its two units
are located just forward and to the left of the commander's plus
X window in a well outside the pressurized crew compartment-an extension
of the navigation base on which the IMUs are mounted. The star trackers
are slightly inclined off the vehicle's negative Y and negative
Z axes, for which they are named. The star trackers are used to
align the IMUs on board the orbiter as well as to track targets
and provide line-of-sight vectors for rendezvous calculations.
the IMUs is required approximately every 12 hours to correct IMU
drift, within one to two hours before major on-orbit thrusting duration
or after a crewman optical alignment sight IMU alignment. IMU alignment
is accomplished by using the star trackers to measure the line-of-sight
vector to at least two stars. With this information, the GPC calculates
the orientation between these stars and the orbiter to define the
orbiter's attitude. A comparison of this attitude with the attitude
measured by the IMU provides the correction factor necessary to
null the IMU error.
The GPC memory
contains inertial information for 50 stars chosen for their brightness
and their ability to provide complete sky coverage.
The star trackers
are oriented so that the optical axis of the negative Z star tracker
is pointed approximately along the negative Z axis of the orbiter
and the optical axis of the negative Y star tracker is pointed approximately
along the negative Y axis of the orbiter. Since the navigation base
provides the mount for the IMUs and star trackers, the star tracker
line of sight is referenced to the navigation base and the orbiter
coordinate system; thus, the GPC knows where the star tracker is
pointed and its orientation with respect to the IMUs.
Each star tracker
has a door to protect it during ascent and entry. The doors are
opened on orbit to permit use of the star trackers.
To enable the
star tracker doors to open, the star tracker power minus Y and minus
Z switches on panel O6 must be positioned to on. The star tracker
door control sys 1 and sys 2 switches on panel O6 control one three-phase
ac motor on each door. Positioning the sys 1 switch to open controls
motor control logic and drives the minus Y and minus Z star tracker
door by electromechanical actuators to the open position. Limit
switches stop the motors when the doors are open and control a talkback
indicator above the sys 1 switch. The talkback indicator indicates
when the doors are open. Setting the sys 2 switch to open controls
a redundant ac motor and electromechanical actuators to open the
minus Y and minus Z star tracker door, and limit switches stop the
motors when the doors are open and control the talkback indicator
above the sys 2 switch in the same manner as for system 1. Positioning
the sys 1 switch to close drives the minus Y and minus Z door closed;
the talkback indicator above the switch indicates cl. The door opening
or closing time with two motors is six seconds; with one motor,
it is 12 seconds. Setting the sys 2 switch to close drives the system
2 motors and closes the minus Y and minus Z door; the talkback indicator
above the switch indicates cl . The indicators indicate barberpole
when a door is between open or closed. The off position of the sys
1 or 2 switch removes power from the corresponding motor control
between the inertial attitudes defined by the star tracker and the
IMU is processed by software and results in IMU torquing angles.
If the IMU gimbals are physically torqued or the matrix defining
its orientation is recomputed, the effects of the IMU gyro drift
are removed and the IMU is restored to its inertial attitude. If
the IMU alignment is in error by more than 1.4 degrees, the star
tracker is unable to acquire and track stars. In this case, the
crewman optical alignment sight must be used to realign the IMUs
to within 1.4 degrees; the star trackers can then be used to realign
the IMUs more precisely. The star tracker cannot be used if the
IMU alignment error is greater than 1.4 degrees because the angles
the star tracker is given for searching are based on current knowledge
of the orbiter attitude, which is based on IMU gimbal angles. If
that attitude is greatly in error, the star tracker may acquire
and track the wrong star.
to aligning the IMUs, the star trackers can be used to provide angular
data from the orbiter to a target. This capability can be used during
rendezvous or proximity operations with a target satellite.
The star tracker
includes a light shade assembly and an electronics assembly mounted
on top of the navigation base. The light shade assembly defines
the tracker field of view (10 degrees square). Its shutter mechanism
may be opened manually by the crew using an entry on the cathode
ray tube display, or it can be opened and closed automatically by
a bright object sensor or target suppress software. The bright object
sensor reacts before a bright object, such as the sun or moon, can
damage the star tracker (the sensor has a larger field of view than
the star tracker shutter). The target suppress software reacts to
a broad light source (such as the sunlit Earth), which may not trip
the bright object sensor but could produce overall illumination
large enough to cause photo currents larger than desired.
assembly contains an image dissector tube mounted on the underside
of the navigation base. The star tracker itself does not move-the
field of view is scanned electronically. The star tracker may be
commanded to scan the entire field of view or a smaller offset field
of view (1 degree square) about a point defined by horizontal and
vertical offsets. An object is tracked when the proper intensity
and the correct location are sensed. Star tracker outputs are the
horizontal and vertical position within the field of view of the
object being tracked and its intensity.
There is no
redundancy management for the star tracker assemblies; they operate
independently, and either can do the whole task. They can be operated
either separately or concurrently.
The star tracker
subsystem operating program, or SOP, supports the modes that are
commanded manually: self-test, star track, target track, break track
and term/idle. Self-test consists of software and hardware tests.
In the star track mode, the star tracker does an offset scan search
for the star, acquires it and tracks it. The star may be selected
by the flight crew or GPC; in either case, field-of-view and occultation
checks are made. Target track is the same as star track, but the
flight crew must specify the target and its threshold. Break track
forces the star tracker to stop tracking a star and to perform a
search scan from the current location to track the next star it
acquires. In the term/idle mode, the star tracker continues its
operation, but all star tracker software processing ceases.
the star tracker SOP maintains the star table. When a star tracker
has acquired and tracked a star and the data has passed software
checks, the star identification, time tag and line-of-sight vector
are stored. The identification and time elapsed since time tag are
displayed in the star table. When two or three stars are in the
table, the angular difference between their line-of-sight vectors
is displayed. The difference between the star tracker and star catalog
angular differences is displayed as an error. The star tracker SOP
selects line-of-sight vectors of two stars in the star table for
IMU alignment and outputs an align ena discrete. Align
ena signifies that the star data meets certain criteria that
allows the repositioning of the IMU inertial platforms. The software
selects the star pair whose angular difference is closest to 90
degrees or the pair whose elapsed time of entry into the table is
less than 60 minutes. The flight crew may manually override the
SOP selection or clear the table if desired.
The SOP also
determines and displays star tracker status.
for the star trackers is Ball Brothers, Boulder, Colo.