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Fine Guidance System: Mapping Strategy and CCD Design

Proposed field mapping and spectroscopy acquisition strategy, dwell scan algorithm, CCD design for drift rate control. Explore stellar distribution, guide star selection, and roll control for spacecraft system engineering.

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Fine Guidance System: Mapping Strategy and CCD Design

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  1. SNAP Fine Guidance Art Bradley/Spacecraft System Engineering Services Landis Markley/NASA/GSFC Nov. 16, 2001 Guidance & Control

  2. Observing strategy • An acquisition strategy is proposed to conduct field mapping and spectroscopy. • Field mapping is performed with a dwell scan algorithm • Simple flight code and command I/F • Primary eigenvector • Secondary eigenvector • Number of dwell points • Jerk pulse width • Constant rate interval • Primary eigenvector jerk pulse magnitude • Secondary eigenvector jerk pulse magnitude • Dwell time • Fine guidance CCD damps drift rate during shutter closure. Guidance & Control

  3. Stellar distribution • Star tables of candidate Guide Stars provided for planning purposes only. • Two 25 deg2 fields have been compiled with centers at 16h25m +57º and 4h30m -52º • Started with HST GSC. • Supplemented with latest Yale Bright Star Catalog & removed duplicates. • Supplemented with USNO A2.0 to get beyond 18mv & removed duplicates. Guidance & Control

  4. Stellar distribution NEP histograms Guidance & Control

  5. Stellar distribution NEP histograms Guidance & Control

  6. Stellar distribution SEP histograms Guidance & Control

  7. Stellar distribution SEP histograms. GSC population from 15.5 to 16mv not strong, should have used A2.0 in that region. Guidance & Control

  8. FGS Design – Area • 4 geometric arrangements were examined • 200x200 arcsec - one CCD • 400x400 arcsec - four CCD’s • 200x400 arcsec - two CCD’s column stacked in Declination • 400x200 arcsec - two CCD’s row stacked in Right Ascension Guidance & Control

  9. FGS Design – placement Guidance & Control

  10. FGS Design – Guide Star Probability • Determined with 100 arcsec increments in Right Ascension and Declination. • NEP mv range is 5.5 to 18.2 • SEP mv range is 6.1 to 17.6 Guidance & Control

  11. FGS Design – Guide Star Probability • Metric will be reduced for near neighbor’s and exclusion areas around CCD edge resulting from attitude Star Tracker error. Guidance & Control

  12. FGS Design – Histograms Guidance & Control

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  20. FGS Design – Roll Control • CCD’s placed on opposite ends of the focal plane can provide: • optional roll control • disturbance detection • Small focal plane X-Y disturbances will be greatly magnified in the roll axis. • This indicator (even if not used for control), could assist in post facto image analysis and on-board Loss-Of-Lock detection. Guidance & Control

  21. Recommendation • 200x400 arcsec CCD’s on opposite ends of the focal plane • Redundancy • Roll control •  95% Guide Star probability after star elimination. Guidance & Control

  22. Guidance & Control

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