Development and Performance Characterization Of Colour Star Trackers

Abstract

Star trackers provide an essential component to a satellite mission requiring high-precision and high-accuracy attitude measurements. A star tracker operates by taking pictures of the celestial sphere and attempting to identify the stars in the image using a combination of the geometric and brightness patterns. The star-positions in the image then determine the attitude of the sensor in the inertial frame. I propose extending the capability of star trackers by including the colour properties of the stars into the star identification process; hence, colour star tracking. Current generation star trackers exist in a variety of forms, with a variety of additional potential designs and operational algorithms proposed in the literature. However, they all share the common trait of using a combination of geometric and monochrome brightness derived patterns to identify stars. Including colour information with the geometric and brightness properties into the identification process represents a new branch in the field of star tracker design. The process of measuring colour also causes a reduction in the amount of light gathered by the sensor, decreasing the number of stars observed. The challenge in colour star tracking becomes establishing that the additional information provided by colour to star patterns is greater than the loss of observable stars due to the measurement process. While superficially brief, accomplishing it touches upon a wide range of topic areas. This includes most research developed for monochromatic star trackers including imaging hardware, optics, noise rejection, parameter estimation, signal detection, data mining, pattern matching, and astronomy. Additionally, using colour necessitates introducing the topics of stellar photometry, spectral filtering, and colour imaging. The approach to colour star tracker development, presented here, considers three aspects to the operation of the technology: colour measurement, star detection, and star pattern matching. In the measurement of colour analysis, a new set of estimation techniques are developed to estimate the colour and position of stars using colour-filter-array and trichroic prism cameras. Validation of the proposed techniques is achieved through a combination of laboratory and nigh-sky testing of hardware prototypes. The detection performance of the colour star tracker designs centres on a comparison with equivalent monochrome designs. By considering primitive detection algorithms, essentially raw thresholding, allows for a fair determination of the relative performance. Numerical simulations of potential designs examine the percentage of the celestial sphere where sufficient quantity of stars can be observed to yield identification. Finally, extending the results of the detection analysis allows for a determination of the ambiguity within observed star scenes. While not explicitly pattern matching, this analysis establishes a baseline for the performance to be expected from practical pattern matching algorithms. Together, the combined results establish the overall expected increase in performance of colour star tracking over equivalent monochrome designs. A critical goal of any star tracker design is to maximize the region of sky where the star tracker can successfully return an attitude solution. Additionally, the reliability of achieving correct attitude solutions must also be a factor. The work presented demonstrates that, given the correct design circumstances, colour star trackers can supersede their monochrome counterparts in these two aspects. Specifically by resolving formerly ambiguous scenes and increasing the total number of scenes that can yield a solution. As a consequence, colour measurement should now become a viable and explicit consideration in future star tracker design processes.