snap fine guidance art bradley/ spacecraft system engineering services

16 November 2001

Guidance & Control Bradley/Markley p1

SuperNova/Acceleration Probe

SNAP Fine Guidance

Art Bradley/Spacecraft System Engineering Services

Landis Markley/NASA/GSFC

Nov. 16, 2001

16 November 2001



• 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.

16 November 2001



• 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.

16 November 2001



• Stellar distribution NEP histograms

16 November 2001



• Stellar distribution SEP histograms

16 November 2001



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

16 November 2001



• 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

16 November 2001



• FGS Design – placement

16 November 2001



• 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 17.6NEP SEP

200x200 asec

95.77% 97%

400x400 asec

99.995% 100%

200x400 asec

99.8% 99.9%

400x200 asec

99.8% 99.3%

16 November 2001



• 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.

16 November 2001



• FGS Design – Histograms

16 November 2001



16 November 2001



• 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.

16 November 2001



• Recommendation 200x400 arcsec CCD’s on opposite

ends of the focal plane Redundancy Roll control 95% Guide Star probability after

star elimination.

16 November 2001



snap fine guidance art bradley/ spacecraft system engineering services

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