# Presentation: 532326

## Super Star Tracker

**Introduction to the ISAL Study**
**H. John Wood**
**8 February 2002**
**Revised 15 February 2002**

## 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.**

## 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.**

## 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)**

## 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**

## 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  **

## 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**

## 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**

## 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**

## Conclusion

**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**