Optical Alignment Sight
optical alignment sight is used if inertial measurement unit alignment
is in error by more than 1.4 degrees, rendering the star tracker
unable to acquire and track stars. The COAS 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 COAS is
mounted at the commander's station so the crew can check for proper
attitude orientation during ascent and deorbit thrusting periods.
For on-orbit operations, the COAS at the commander's station is
removed and installed next to the aft flight deck overhead right
minus Z window.
The COAS is
an optical device with a reticle projected on a combining glass
that is focused on infinity. The reticle consists of 10-degree-wide
vertical and horizontal cross hairs with 1-degree marks and an elevation
scale on the right side of minus 10 to 31.5 degrees. A light bulb
with variable brightness illuminates the reticle. The COAS requires
115-volt ac power for reticle illumination. The COAS is 9.5 by 6
by 4.3 inches and weighs 2.5 pounds.
the COAS at the aft flight station, the flight crew member must
manually maneuver the orbiter until the selected star is in the
field of view. The crew member maneuvers the orbiter so that the
star crosses the center of the reticle. At the instant of the crossing,
the crew member makes a mark by depressing the most convenient att
ref push button; the three att ref push buttons are located on panels
F6, F8 and A6. At the time of the mark, software stores the gimbal
angles of the three IMUs. This process can be repeated if the accuracy
of the star's centering is in doubt. When the crew member feels
a good mark has been taken, the software is notified to accept it.
Good marks for two stars are required for an IMU alignment. The
separation between the two stars should be between 60 and 120 degrees.
the star being sighted and the COAS's location and mounting relationship
in the orbiter, software can determine a line-of-sight vector from
the COAS to the star in an inertial coordinate system. Line-of-sight
vectors to two stars define the attitude of the orbiter in inertial
space. This attitude can be compared to the attitude defined by
the IMUs and can be realigned to the more correct orientation by
the COAS sightings if the IMUs are in error.
mounting relative to the navigation base on which the IMUs are mounted
is calibrated before launch. The constants are stored in software,
and COAS line-of-sight vectors are based on known relationships
between the COAS line of sight and the navigation base.
COAS can also
be used to visually track targets during proximity operations or
to visually verify tracking of the correct star by the minus Z star
COAS data processing
is accomplished in the star tracker SOP. This SOP accepts and stores
crew inputs on COAS location, star identification or calibration
mode; accepts marks; computes and stores the line-of-sight vectors;
enables IMU alignment when two marks have been accepted; and computes,
updates and provides display data.