1. Star Detection and Tracking Embedded Linux Hardware
Stellar Gyroscope: Visual Spacecraft Attitude Determination from Apparent Motion of Stars
Samir Rawashdeh, Marc Higginson-Rollins, Bill Danhauer, Trevor Fenwick, James E. Lumpp, Jr.
Space Systems Laboratory, Electrical and Computer Engineering, University of Kentucky
Star Correspondence
Overview
A Stellar Gyroscope is a star based attitude propagator that is capable of propagating a spacecraft’s attitude between camera frames by tracking the motion of the stars in the field of view. The Stellar Gyroscope developed at the University of Kentucky addresses a growing need in small satellites for attitude determination systems that are low cost and do not require a prohibitively large amount of volume and power.
Star Detection and Tracking
A single star affects multiple pixels on the sensor as can be seen in the figures below. Using the brightest pixel results in a reasonable estimate of the star location, however, sub-pixel resolution can be achieved when a normal distribution curve is fit over the data. Calculating the expected value as the star location utilizes information in several pixels and results in a more accurate estimate of the star location. This approach is often referred to as centroiding.
Random Sample Consensus (RANSAC) is an iterative method used in this work to solve the correspondence problem to estimate the relative attitude between photos.
The steps of RANSAC can be summarized as:
* Hypothesize: A Minimum Sample Set (MSS) is randomly selected as two stars in each frame and the rotation matrix is computed using only that randomly selected set.
* Test: The estimated rotation matrix is tested against the remaining stars. The stars that show consensus are counted towards the Consensus Set (CS).
* Iterate: RANSAC iterates between the above two steps until a random hypothesis finds “enough” consensus to some selected threshold.
A set of images of the Cassiopeia constellation. The stars appear to be panning and rotating in a way that indicates a unique attitude maneuver of the camera in 3 degrees of freedom.
S. Rawashdeh, W. C. Danhauer, J. E. Lumpp, “Design of a Stellar Gyroscope for Visual Attitude Propagation for Small Satellites”, 2012 IEEE Aerospace Conference, Big Sky, MT
Prospective Flights
A 5 megapixel CubeSat Camera Module is being developed at the Space Systems Laboratory for a variety of missions, including the planned KySat-2 mission. Also, the Stellar Gyroscope will be sensor on a commercial CubeSat ADCS system developed by SSBV Space and Ground Systems UK, and is an experiment on the United Kingdom’s TechDemoSat-1.
S. Rawashdeh, J. E. Lumpp, “Development of a Drift-Free Stellar Gyroscope”, 25th Annual AIAA/USU Conference on Small Satellites, 2011, Logan, UT
Attitude propagation is based on successfully performing star correspondence between camera frames. This is prone to several sources of errors where a star can be falsely paired with a different star in the next frame. Also, image noise can trigger false star detections, and can cause stars to become undetectable in some frames. Finally, as the camera sweeps the sky, tracked stars will leave the frame, and new stars will enter.
Embedded Linux Hardware
The camera is based on the Aptina MT9P031 Monochrome 5 megapixel CMOS sensor and a lens with a focal length of 16mm. This configuration results in a 15° by 20.2° field of view. The hardware is based on the open source embedded Linux Beagleboard project. A CubeSat camera module is being developed for generic imaging
Star field image overlaid by star detections in 5 consecutive frames, with a 3 degree rotation between each frame. This figure illustrates the tracking challenge where the data consists of reliable stars, false positives, and false negatives. Colors are adjusted for clarity
BeagleBoard-xM and 5MP Sensor Board
as well as stellar
photography
KySat-1, example of a 1-Unit CubeSat, a 10 cm cube
Simulation of Stellar Gyroscope payload onboard a satellite in Low Earth Orbit. Using Passive Magnetic Stabilization, the sensor has an unobstructed view of the sky 50% of the time.