Development and On-Orbit Results of the SSTL Low Cost Commercial Star Tracker

Abstract

[Abstract] This paper will present Surrey Satellite Technology Limited’s low cost approach to designing a star tracker employing maximum use of commercial off the shelf components and a compact development cycle. Details of the design, analysis and on-ground characterization will be presented along with in-orbit performance results. The SSTL Altair HB star tracker has been flown on Bilsat-1 and Beijing-1 and will fly on CFESat and the five-satellite constellation, RapidEye as the primary spacecraft attitude sensor. I. Overview HE commercial star tracker developed by Surrey Satellite Technology Limited (SSTL) is currently in operation on the Bilsat-1 and Beijing-1 remote sensing satellites and is scheduled to fly on CFESat and the RapidEye constellation in the coming year. The SSTL star tracker, known as the Altair HB, was developed as an alternative low cost, high accuracy, spacecraft attitude determination and control sensor. Initially, this paper will overview the instrument hardware and validation performed during the qualification of a flight ready star tracker for use on Earth imaging satellites. Design details will be presented, including a brief discussion on SSTL’s approach to using commercial off the shelf (COTS) components to achieve a high level of performance whilst making the star tracker highly cost effective. Additionally, the star tracker attitude determination software and its totally autonomous Lost In Space (LIS) initialization algorithm will be explained, including an overview of the star catalogue used and how full sky visibility and robust tracking is achieved. The high accuracy attitude solutions generated by the star tracker make it an ideal sensor to integrate into the closed loop control of a 3-axis stabilized spacecraft. How the attitude estimates of the Altair HB are integrated with the spacecraft’s on-board control system, examples of the on-orbit performance from the Bilsat-1 and Beijing-1 satellites will be given. This paper is intended to present to the space navigation community a viable alternative for a low cost attitude determination instrument that does not sacrifice performance and has proven in-orbit heritage. II. Hardware and Specifications The SSTL Altair HB star tracker is the next generation Star Tracker from SSTL and was born out of the initial work on Star Trackers done by SSTL for the remote sensing satellites PoSAT-1 and UoSAT-12 1 . Its hardware modules have been constructed using commercial off the shelf parts, and consist of two electronics modules and a camera head with baffle. The processor module is primarily responsible for calculating the star tracker derived attitude and rates using images of star fields as an input, while the control module generates the signals needed to drive the camera head and format the data for the processor module. Together, the modules coordinate all the data exchanged between the star tracker and the external spacecraft components, including the on-board control computers and the global positioning system receiver. The camera head is mounted externally on the spacecraft and is attached to a light-limiting baffle, designed to prevent unwanted reflections and light sources from entering the camera’s field of view (FOV). Each of these individual components and an overall summary of the performance achieved by the star tracker are described in more detail in the following sections.