<title>Modifications of a mosaic focal plane star tracker for scanning application</title>

Abstract

Modifications ofa mosaic focal planestar tracker for scanning applicationMark A. Radovich, Jr.Ball Aerospace Systems Division, Boulder, Colorado 80306ABSTRACTThe Ball Aerospace Systems Division CT-63 1 Star Tracker is a miniaturized version of the previous CT-601/621 units;volume, power and weight have been reduced. The fundamental algorithms for driving a CCD and processing its output aresofiware driven. This architecture lends itseifto other applications, one ofwhich is a Star Scanner.The CT-632 Star Scanner is a CT-63 1 Star Tracker, modified to operate on a platform spinning at rates at least two ordersof magnitude greater than nominal tracker rates. Time delay integration (TDI) is used to accumulate star image signal as itmoves across the CCD. The image is processed by standard star tracker algorithms to determine star centroids and intensities.Output data, continually updated as stars leave the field of view, consists of X and Y angles and latency time for the sixteenbrightest stars, and diagnostic data. Commands range from entire reprogram ability to simple mode commands. Sensitivityranges from 0 MI to 3.8 Mj.1. TRACKER MODIFICATIONSThe architecture of the CT-600 series of trackers adapts to multiple interfaces and applications. Simple software changestailor operational protocols and algorithms to meet mission specific requirements. Applying the well known concept of TDI tothe CT-63 1 star tracker has increased its ability to track moving stars by two orders ofmagnitude with no significant impact tothe error budget. With several CT-600 units delivered, the scanner takes advantage of the mature design, proven algorithmsand software flexibility. Only the method of exposure (integration) has been modified; the pixel charge is transferred at therotation rate. By design, the star image is blurred no greater than a pixel, guaranteeing that the star image remains near thecenter of the track block, so it doesn't compromise tracking ability or the algorithms used to compute the centroid. Fig. 1 is apicture ofthe CT-63 1 Star Tracker/CT-632 Star Scanner.During fabrication, the detector rows are carefully aligned to roll pads, located on the bottom of the star tracker, adjacentto two of the three mounting pads. To achieve optimum performance, the tracker boresight must be mounted perpendicular tothe spin axis, and the roll pads aligned to the plane perpendicular to the spin axis. Scanner spin axis rotation is shown in Fig.2. This orientation ensures that stars move parallel to the rows and in the proper direction.Although the CT-632 operates as a scanner on power-up, it retains standard tracker features, and by command, will returnto the tracker mode of operation. Thus, for applications with rotation rates less than or equal to 2.5