Science Requirements. Two proposed projects: MAXIM and Stellar Imager are used to provide requirements on a generic star tracker.The instruments will be located at the L2 Lagrange point and require a stable reference at the level of 30 micro arc-seconds.This level of tracking must be held stable for hours to days. Instruments consist of Objective and Detector spacecraft separated by up to hundreds of kilometers..
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1. Super Star Tracker Introduction to the ISAL Study
H. John Wood
8 February 2002
Revised 15 February 2002
3. Derived Requirements A laser beacon is attached to the objective s/c which appears as a star as seen from the detector s/c.
“Telescope Assembly” involves flying the two s/c into alignment with the target.
“Coarse Track” involves using the beacon and precision information on the positions of the two s/c in the inertial frame of reference to center the beacon in the super tracker FOV.
“Fine Lock” uses the laser beacon and the inertial information on the detector s/c in a feedback loop to guide the “Telescope ensemble” on the target at the required 30 micro are sec level.
4. Trade Studies
Six options were considered by the science team
Of the six options, four were looked at in detail in the ISAL study:
1. Similar to the Hubble Space Telescope Fine Guidance Sensors (FGS)
2. Similar to Gravity Probe-B (GP-B)
3. Similar to GP-B but with accelerometers
4. Similar to StarLight (Kilometric Optical Gyro w/ 4 km perimeter Sagnac Effect)
5. Option 2 Initial Design Concept Both s/c have state-of-the-art star trackers (currently 1 arc-sec with 5 degree FOV)
The objective s/c has a laser beacon with milliwatt output – beam divergence is ~ 1 arc min
The detector s/c has a 30 cm aperture Schmidt-Cassegrain telescope with a 4-fold field detector for tracking of the beacon at the micro-arc sec level
The FOV of the beacon tracker is ~15 arc sec
Detector s/c has gyros with micro-arc sec capability
6. Option 2 Concept Con’t The tracking loop operates on “angles only” measurements by the gyros and the beacon telescope
Control involves either angle or translation of the detector s/c
The study provides flow diagrams showing how the tracking measurements are made and how they feed into the s/c guiding loop
7. Additional Concepts A ”Science Mode Control Loop” was developed using input from the science instrument in addition to the beacon and gyro angles
A “Beacon/Gyro Mode Control Loop” using only beacon input and gyro angles was developed using rolls and translations of the detector s/c only
This mode did not use information from the science instrument
8. Feasibility of the Concepts The beacon tracker can be done with today’s technology and is discussed in the Optics Chapter by Dennis Evans
The gyros are based on Gravity Probe B and have a roadmap to production
9. Additional Engineering The signal to noise ratio of the beacon tracer was studied by Eric Young to verify the optical design developed by Dennis Evans
The diffraction limited performance in object space of the 30cm diameter telescope/quad tracker looking at a mwatt-level laser at a distance of 100km is 45 µarcsec
This is more than adequate performance
The thermal evaluation by Wes Ousley showed that extreme structural stability is required between the attitude sensor and the instrument
Either extremely low CTE material would have to be developed or temperature control to fractions of a milli Kelvin would be necessary
The ISAL engineering team was generally pleased that the instrument concepts derived could be developed using existing and near-future technologies
No unobtainable technologies were required and those currently beyond state-of-the-art had credible roadmaps to fruition
Advancements in technologies discussed could change which option would be pursued