# Presentation: 863627

## SNAP Fine Guidance

		Art Bradley/Spacecraft System Engineering Services
		Landis Markley/NASA/GSFC
Nov. 16, 2001 

- 		Art Bradley/Spacecraft System Engineering Services
- 		Landis Markley/NASA/GSFC
- Nov. 16, 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.

- 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.

## 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.

- 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.

## Stellar distribution NEP histograms

## Stellar distribution NEP histograms

## Stellar distribution SEP histograms

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

- 	GSC population from 15.5 to 16mv not strong, should have used A2.0 in that region.

## 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

- 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

## FGS Design – placement

## 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

- 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

|  | NEP | SEP |
| --- | --- | --- |
| 200x200 asec | 95.77% | 97% |
| 400x400 asec | 99.995% | 100% |
| 200x400 asec | 99.8% | 99.9% |
| 400x200 asec | 99.8% | 99.3% |

## 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.

- Metric will be reduced for near neighbor’s and exclusion areas around CCD edge resulting from attitude Star Tracker error.

## FGS Design – Histograms

## Slide 13

## Slide 14

## Slide 15

## Slide 16

## Slide 17

## Slide 18

## Slide 19

## 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.

- 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.

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

- 200x400 arcsec CCD’s on opposite ends of the focal plane
- Redundancy
- Roll control
-  95% Guide Star probability after star elimination.