STARDUST 20 Nov 2003CERR - Shyam Bhaskaran1 of 10 STARDUST Project CRITICAL EVENTS READINESS REVIEW COMET P/WILD 2 ENCOUNTER Autonomous Nucleus Tracking Shyam Bhaskaran JPL: AM / PM LMA:MSA :00 AM - 05:00 PM PST 20 Nov 2003

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran2 of 10 Nucleus Tracking Rationale Delivery of spacecraft using ground based radio/optical navigation good to about 16 km crosstrack and km ( sec) downtrack (1 sigma) at the E - 18 hour point. –Sufficient to maintain spacecraft safety. –Not sufficient, however, to keep visual lock on comet for close approach science imaging. In particular: Crosstrack uncertainty not low enough to determine flyby plane for the mirror to sweep as comet goes by. Downtrack uncertainty too large to determine mirror angles needed to maintain lock through closest approach. Improved knowledge of target relative spacecraft trajectory in both crosstrack and downtrack directions is possible using images of the comet as spacecraft approaches it, but 40 minute round trip light time prevents ground-in-the-loop tracking. Onboard closed loop tracking only option

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran 3 of 10 Setup Camera FOV = 3.5x3.5 deg Rotating mirror reflects light path from s/c +X (mirror angle = 0 deg) to s/c -Z (mirror angle = 90 deg) to s/c -X (mirror angle = 180 deg), From angles 0 to 18 deg, the light path also goes through a periscope to see around the front Whipple shields –Periscope incurs additional light attenuation –Between about 6 and 16 deg, split light path also results in image doubling

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran4 of 10 Nucleus Tracking Procedure Onboard tracking initiated at E-30 minutes. Algorithm initialized with nominal values for the following: –Initial target-relative state (position and velocity), and associated covariance, at E- 30 minutes, based on results from ground-based navigation using E-12 hour opnav –Camera characteristics. –Predicts for camera pointing for all picture opportunities based on nominal comet- relative trajectory. Images taken at second intervals. As each image is taken, –Image is processed to locate nucleus center-of-figure (centroiding). –Pixel/line values of observed centroid differenced with predicted center location to form residuals. At E-15 or E-10 min, all images processed with Kalman filter to get updated state information. Following this, state update is performed after every image is processesed. At E-11 min or E-6 min, latest state information used to determine s/c roll angle to align s/c X-Z plane with flyby plane Inertial pointing predicts are decomposed by ACS into s/c attitude, mirror angles and rates to control mirror. Closed-loop tracking terminated several minutes past encounter.

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran5 of 10 Nucleus Tracking Verification Basic algorithm successfully used for DS1 Borrelly flyby Stardust specific software tested successfully at Annefrank encounter last year –Primary differences with Wild km vs 300 km flyby distance Annefrank images started at mirror angle of 20 deg (so no periscope images were used) vs 0 deg for Wild 2 Due to fortuitous out-of-plane errors being small, roll was never exercised at Annefrank No dust environment or coma at Annefrank Numerous Monte Carlo simulations performed on unix workstation –Simulates realistic images, ACS gyro drift errors, state errors –Does not simulate attitude excursions due to dust impacts STL testing at Lockheed Martin –Uses fully integrated software with all s/c subsystems –Limited number of simulations performed

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran6 of 10 Annefrank Tracking

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran7 of 10 Tracking Imaging Sequence Large uncertainty in TOF value –+/- 5 min (3 sigma) if approach opnavs successful and dynamic correlations hold –+/- 9 min (3 sigma) assumes no improvement from opnavs 2 image sequences designed to accommodate each of these cases At 2 days prior to encounter, the decision would be made as to which sequence to implement The nominal sequence will be chosen under the following conditions: –Detection of Wild 2 from spacecraft shows it to be within reasonable tolerances from the predicted location –Ephemeris fits using combined ground and opnav data consistent –Ephemeris fits using different comet dynamic models show consistent results

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran8 of 10 Tracking Image Sequence (cont.) Nominal sequence –E-30 m to E-5 m : 1 frame/30 s –E-5 m to E+5 m: 1 frame/10 s –E+5 m to E+8 m: 1 frame/60 s –Filter updates started at E-10 m –Roll maneuver at E-6 m Expanded sequence –E-35 m to E-7 m: 1 frame/30 s –E-7 m to E+7 m: 1 frame/15 s –E+7 m to E+10 m: 1 frame/30 s –Filter updates start at E-18 m –Roll maneuver at E-11 m Number of frames limited by total buffer limit of 72 frames

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran 9 of 10 Monte Carlo Simulation Results - Nominal Overall probability of obtaining images within 2000 km: 99.5% Average # of successful images within 2000 km: 47

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran10 of 10 Interface Files with Lockheed Martin

STARDUST 20 Nov 2003CERR - Shyam Bhaskaran11 of 10 Issues/Concerns Need data through periscope to determine throughput loss for exposure settings –Planned image through periscope in early December Image doubling –Separation of images (about 20 pixels maximum) roughly the same as expected size of the nucleus –Separation also about the size of spacecraft deadband motion –Not expected to cause serious problems –Currently not simulated, either in Monte Carlo runs or Lockheed Martin STL runs –Plans to simulate this effect in the very near future No open ISAs No 1st time events