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For additional, detailed information on the
Spitzer Science
Instruments and usage of the Observatory,
please also see the Spitzer Observer's Manual
Overview of the Spitzer Pointing Control System (PCS)
The pointing control subsystem (PCS) includes the hardware and flight
software necessary for precision telescope pointing, stabilization,
slewing, tracking, and safe mode functions. The PCS performed the initial
attitude acquisition of the spacecraft following launch vehicle
separation. It provides periodic boresight calibration for the
telescope. The PCS provides the capability for both rapid large angle
slews and small maneuvers to place and reposition science targets within
the science instrument apertures; it maintains the solar array orientation
toward the Sun; and it points the high gain antenna toward Earth for
downlink. The PCS also contains Wide Angle Sun Sensors which act as a
second check on the spacecraft orientation to ensure that the hard
pointing constraints are not violated. High level fault protection will
place the Observatory in a safe mode if a violation is detected.
The PCS is a celestial-inertial, three-axis stabilized control system. A
high performance star tracker/inertial reference unit (ST/IRU) package
provides attitude determination and reconstruction capabilities. On-board
pointing commands and variables use the J2000 coordinate system. Reference
to the J2000 celestial sphere is implemented within the star tracker (ST)
through autonomous identification of stars carried in an on-board catalog
of 87,000 Tycho stars down to 9th visual magnitude. [Note: The on-board
catalog actually uses the ICRS coordinate system, so in fact the on-board
pointing system is really ICRS. However, the differences between ICRS and
J2000 are so small (less than or equal to 120 mas) that no conversions are
made and Spitzer is effectively considered to use the J2000 coordinate
system.]
The ST is used to point an instrument boresight to a desired location on
the sky with an initial accuracy of at least 0.5 arcsec (1 sigma radial).
The ST field of view is 5° x 5°, which ensures that Spitzer can
point to any part of the sky and have the star tracker meet its pointing
requirements. Typically 40 stars are used simultaneously. The gyros
provide pointing stability when not using the star tracker as a pointing
reference; the pointing drift derived from the gyros is <3 mas/sec over 8
hours. The drift rate when using IRU-only mode should generally be better
than 1 mas/sec over 200 sec.
All telescope pointing is defined and calibrated relative to redundant
pointing control reference sensors (PCRSs) located in the focal plane.
During the course of the mission, the PCRS is periodically (about every 12
hours) used to calibrate the telescope-to-star tracker boresight alignment
that may drift due to thermo-mechanical effects. Each PCRS detector is a
Si PIN photodiode array divided into two 4x4 subarrays for redundancy.
Each pixel is 250 microns square, with a plate scale of 10 arcsec per
pixel. The PCRS calibration measures the star position with an accuracy
of 0.1 arcsec (1 sigma per axis), and is sensitive down to 10th visual
magnitude at a wavelength of 550 nm.
Spitzer also has Wide Angle Sun Sensors which measure the Sun's position
with respect to the spacecraft. These sensors were used during initial
attitude acquisition after launch, as well as for Sun avoidance, fault
protection, and safe mode during the mission. Each wide-angle Sun sensor
provides a field of view of 2 pi sr with an accuracy of +/-0.1° at
null. They are placed at the top and the bottom of the Sun shield to
maximize the coverage, with their boresights aligned to the spacecraft
Z-axis.
Four reaction wheels provide the primary control actuation for all modes
of operation. They are mounted in a pyramid orientation about the X-axis;
each canted at 30 degrees towards the X-axis. Over time, angular momentum
accumulates in the reaction wheels, due primarily to the small offset
between the center of mass and the center of (radiation) pressure. Unlike
an observatory in low Earth orbit, which can dump this momentum
magnetically, Spitzer has a "Reaction Control System" (RCS) which uses
nitrogen thrusters to provide the reaction wheel momentum unloading
capability; opportunities to dump momentum are scheduled during routine
downlinks. The nitrogen supply is sized to accommodate a mission lifetime
in excess of 5 years.
On-orbit measurements show that the PCS is capable of slewing the
telescope 180° in 900 sec, 1° in 60 sec, and 1 arcmin in 6 sec,
while maintaining its inertial pointing knowledge. These times include the
acceleration and deceleration of the telescope but do not include the time
it takes for the PCS to stabilize after the slew has completed. The
pointing system has several operating modes, and the AOTs are designed to
use the pointing mode most appropriate for each observing mode. Settling
time varies with operating mode and slew magnitude. For IRU-only slews,
slews less than 30 arcmin settle to within 0.2 arcsec rms within 10
seconds. (Settling may take longer in some cases.) The AOTs make use of
on-board slew completion and stabilization indicators to proceed with the
observation as soon after a slew as is possible. Note that the time
required for small slews, dithers, offsets, settling, etc. within an AOR
is considered part of the observation. Time estimates provided through
Spot account for these transparently and accurately. The best available
pointing system models are incorporated into Spot. However, note that
Spot estimates may change if, e.g., Spitzer's slew rate changes.
Go on to more information on the PCS.
For additional, detailed information on the
Spitzer Science
Instruments and usage of the Observatory,
please also see the Spitzer Observer's Manual
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