The FUSE Observer's Guide
Appendix B: Spacecraft Information
Version 2.0, May 10, 2000

WARNING!!! Some information in this document may be dated!!

This appendix was formed from material cut out of the FUSE Observer's Guide, ver. 1.2. It contains extra discussion that was deemed "optional" for Cycle 2 observers and beyond, but may still be of interest. Some of this information has been duplicated and updated in the FUSE Observer's Guide, Version 2.0. For Any material shown here and in Version 2.0, the information in the main document is more current and should be used.

Outdated material in this appendix will be updated only as time permits.

Appendix B. The Spacecraft

The FUSE spacecraft bus is a modified version of the Explorer platform, which was used to support the Extreme Ultraviolet Explorer (EUVE) mission. The FUSE spacecraft also includes heritage from the X-ray Timing Explorer (XTE) and Tropical Rainfall Measurement Mission (TRMM) missions in the attitude control system and communications. The spacecraft was built by Orbital Sciences Corporation.

B.1 Mechanical System

The spacecraft bus is approximately 0.9 meters tall and 1.8 meters in diameter, with a mass of 580 kg. The mechanical subsystem consists of the primary spacecraft structure, the solar arrays, and mechanisms. The structure is an aluminum trapezoidal frame that interfaces with the instrument through three flexure mounts. The only mechanisms in the spacecraft are the solar array hinge and its actuator release system and the solar array drives. Each solar array panel uses an array drive to orient the panel to the sun. The drives are actuated only during major slews so that the pointing stability is not disturbed during observations.

B.2 Command & Data Handling (C&DH) System

The spacecraft C&DH system provides the hardware and software necessary to:

receive, validate and distribute commands
collect, format and transmit telemetry data to the ground
maintain and distribute a spacecraft clock
store and execute time-tagged commands
provide onboard data storage

Commands for the instrument subsystems (such as the Focal Plane Assemblies (FPAs) and the Fine Error Sensors (FESes), etc.) are forwarded directly to the Instrument Data System (IDS).

The C&DH processor is a 80386 microprocessor with 1 MByte of radiation-hardened, single-event-upset immune, random access memory for program execution. The bulk memory card provides 240 MBytes of data storage (of which slightly over half is reserved for the storage of scientific data).

B.3 Attitude Control System (ACS)

The ACS subsystem consists of two 3-axis Ring Laser Gyro inertial reference units, two 3-axis magnetometers, two coarse sun-sensor units, three magnetic torquer bars, and four reaction wheel assemblies. FUSE does not employ any star-trackers. Safehold software is resident in the ACS; if the ACS detects that the pointing of the satellite violates the beta angle restrictions, then the spacecraft will autonomously be commanded into a safehold mode. The beta angle is the angle measured from the anti-solar direction to the boresight and is nominally restricted to values of 15-105°. See the discussion of observing constraints elsewhere in this document.

The ACS provides autonomous control of the satellite and maintains pointing control to < 2° in coarse pointing mode (without information from the instrument Fine Error Sensor (FES)), and controls pointing jitter to < 0.5 arcsec (1 sigma) with a reference position measurement provided by the FES. The ACS enables the satellite to slew at a rate of up to 4° per minute, so even 90° slews can be completed during an occultation period. After a large slew (i.e., when going from one target to another), the uncertainty in boresight pointing will typically be 0.02*TSLEW arcmin, where TSLEW is the slew time in minutes. Short slews, which are typically performed during the target acquisition peakup sequence, are expected to be accurate to ~0.1 arcsecond. The predicted (corrected) gyro drift rate of the satellite is approximately 30 milliarcsec/sec. The IDS, however, provides reference position measurements to the ACS (measured quaternions from observations of guide stars by the FES) and allows the ACS to correct the pointing and keep the target in the chosen spectrograph slit with a jitter of < 0.5 arcsec (rms) over one orbit.

B.4 Power Subsystem

The power system for FUSE consists of two 40 Amp-hour batteries, two solar array panels, and associated electronics. The batteries employ a standard 22-cell NiCd arrangement. The batteries provide enough power for FUSE to operate normally during the 35 minute eclipse period every orbit. The solar arrays use Gallium Arsenide (GaAs) cells to provide 520 Watts of orbital-average-load power.

B.5 RF Communications Subsystem

The communications system consists of two omni-directional low-gain antennas and two S-band transponders with associated diplexers. The system interfaces with the central electronics unit of the C&DH. The system receives uplinked commands at 2 kbps and can downlink telemetry at a variety of data rates (up to 1 Mbps). The two omni-antennas are located on opposite sides of the spacecraft bus and provide almost spherical coverage.

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