2013.B.2.3 Development of a Robust, Low-Cost, CubeSat Star Tracker

Author(s)

Alex Erlank (1)

Stellenbosch University, South Africa

Session

B.2

Keywords

star tracker, miniature sensors, technology demonstration, ADCS, ESL

Abstract

A star tracker is an essential part of any interplanetary craft’s ADCS. Unfortunately, while star trackers have been getting progressively smaller and more energy efficient, there is (at the time of writing) no CubeSat compatible star tracker that has been flown in space. Due to its complexity, a star tracker typically requires a team of people and several years to develop. Fortunately, Stellenbosch University’s Electronic Systems Laboratory has already developed several miniature satellite components. These include sun and horizon sensors and an ARM-based onboard computer for CubeSats, as well as star trackers for SUNSAT and FEDSAT microsatellites. The purpose of this paper is to describe how the lessons learned and subsystems developed for these products are allowing the rapid development of a CubeSat star tracker.

The star tracker being developed has a FOV of 50×26 degrees and a limiting magnitude of 3.8. The very wide field of view allows a much smaller star catalogue to be stored and allows very fast catalogue searches. A wide FOV is particularly well suited for interplanetary travel as the Earth’s horizon entering the FOV is no longer a problem. The star tracker can perform either a lost-in-space calculation, which requires searching through the whole catalogue, or an assisted match, which performs a search on a reduced catalogue based on a given rough attitude estimate. Either match can be performed faster than 1Hz on a 48MHz ARM processor. The matching algorithm is a modified version of the Geometric Voting Algorithm. Either individual star vectors or an inertial quaternion can be output to enable attitude determination or interplanetary navigation. The performance of the algorithm has been tested in MATLAB on simulated star images from all parts of the sky. The lost-in-space algorithm successfully matched at least three stars in 93% of the images, and the assisted match algorithm matched 98.5% of the images.

A completed engineering model has been tested on real and simulated stars. The hardware is composed of three small PCBs, each measuring 45x33mm, stacked behind one another. The test results prove that an accuracy of better than 0.03 degrees in bore sight pointing can be achieved on real sky images. The final hardware will consume less than 0.3W, weigh under 100g and be small enough that two fully contained star trackers will fit well within 0.5U volume. A fully functional CubeSat-compatible star tracker will be ready for flight by the end of 2013.