Pre-Phase A Report - Lisa - Nasa

76 Chapter 3 Experiment Description
3.3.3 Autonomous star trackers
Each LISA payload will include four star tracker heads, mounted in pairs on the outside
of the thermal shield, one pair aligned with each of the two telescopes. Only two are
nominally operational – one on each telescope – the others serve as backups. Each has a
field of view of 22◦ ×16◦ and can provide attitude knowledge of about 2 arcsec in two axes
and 16 arcsec in their boresight roll axis. Star processing at an update rate of 1 Hz will
require a total of 16 MIPS of throughput for both operational trackers. This is provided
by a dedicated processor shared by the two optical heads. The processor unit must be
mounted remotely, away from the payload thermal shield. (An alternative option is to
use the payload processor instead of a dedicated processor, but this is not the current
baseline.)
The star trackers will be fully autonomous, and can tolerate direct Sunlight without
damage.
Field of view 22 ◦ ×16 ◦
Pitch, yaw accuracy 2arcsec ≈ 10 µrad
Roll accuracy 16 arcsec ≈ 80 µrad
Update rate 1Hz
Tracking rate 0.2 ◦ s −1 ≈ 3.5mrad/s
Number of stars tracked ≤ 50
Tracking sensitivity 7.5 mv
Guide stars in database 5650
Auxiliary stars in database 22600
Communications I/F 1553 or RS 422
Output format quaternions, Euler angles
Operating temperature − 30 ◦ to + 50 ◦ C
Radiation tolerance 1kJ/kg
Table 3.1 Star tracker specifications (per optical head).
The performance specifications of the star trackers are summarised in Table 3.1 . The
mass, power, and volume budgets are summarised in Table 3.2 . The data in these tables
is based on the system under development for the Oersted mission. Since the payload
star trackers are on the 1553 bus, they may also be accessed by the spacecraft computer
for coarse attitude control.
3-3-1999 9:33 Corrected version 2.08