<title>Cassini star tracking and identification architecture</title>

Abstract

The Cassini spacecraft will perform a detailed examination ofthe Saturnian system, including the release ofa probe to study Saturn's largest satellite, Titan. The star irackerforthe Cassini mission must provide accurate data during the entire flight including four years ofmeasurement in a harsh radiation environment. The star iracker will provide autonomous star identification over the entire celestial sphere using a 4,000 entry on-board star catalog. Three axis attitude reference will be determined by measurements oftwo to five stars in the tracker field ofview which will allow the gyroscopes to be powered offduring the cruise phase ofthe flight. When the gyros are operational, attitude updates will be provided.The Cassini star tracker consists ofa CCD based star camera, called the Stellar Reference Unit (SRU), which is being designed and built by Officine Galileo. The operation ofthe SRU, including functional modes, exposure times, and areas ofthe CCD to digitize is under the control ofthe Cassini Attitude and Articulation Conirol Subsystem (AACS) Flight Computer (AFC). The raw digital pixel data is transmitted from the SRU through a dedicated direct memory access (DMA) interface to the AFC memory for subsequent processing. All pixel processing and centroiding is performed within the AFC.Once the initial attitude has been determined, the AFC algorithms will choose which stars within the SRU field ofview to track in order to maintain attitude knowledge. The SRU will have a 15° field of view and will provide 60 .trad (3o)2-axis position measurement accuracy for stars of approximately 6.05 visual magnitude and brighter. The required 1 mrad (3 a) twist accuracy is provided by star separation. ThITRODUCTIONThe evolution ofthe Cassini star tracking and identification architecture has been shaped by many factors including the architecture ofthe AACS itseWand the challenging Cassini mission requirements. Tn addition, JPL's previous experience in developing and testing the ASTROS star tracker' has contributed to the selected approach.The AACS uses a single, centralized computer to perform all the attitude measurement, computation and control functions, including the processing ofthe raw pixel data from the SRU. This approach avoids the duplication ofcomputing resources which would be necessaiy should the star tracker be required to perform its own data processing. The star tracking and identification software, resident in the AFC, can be modified from the ground to compensate for changes in SRU performance or unanticipated changes in mission requirements. Flexibility, a very desirable trait for the long, complex Cassini mission, is afforded by this architecture. Finally, software developed by JPL for the ASTROS star tracker demonstrated that automatic star acquisition and high rate star measurements could be made using a window, or limited field of view readout of the CCD2. The star tracking algorithms being developed for Cassini are an extension of those developed during the ASTROS program. This paper is intended to pr...

<title>Cassini star tracking and identification algorithms, scene simulation, and testing</title>

“…It is represented by two polynomials, and takes into account position in the field of view as well as star color and optics barrel temperature: tan(a) = Pxi +Pa2Z +Px3Y +p4XT + pz5(r2) +p6x(r3) +p2,7x(r4) +px2 +Pz9XY (4) tan(/3) = Pyl +p22y +y3X +py4yr +p25(r2) + Py6y(7'3) +py7y(r4) +pi,sy2 +py9xy (5) where tan(a) and tan (3) coefficients-IEU uses pixels), r = x + r, and the Pz2, P92 terms contain the temperature dependence according to:…”