Characterization of Chromatic Effects in Small Star Trackers

This paper explores how star color affects the design, calibration, and operation of star trackers. We develop a series of tools that characterize the chromatic performance of a star tracker and suggest how this information can be used in the design (e.g., selection of detector, lens, and catalog stars), and calibration (e.g., focusing, and camera calibration) of a star tracker to maximize sensor performance. We also present a simple per-star focal length correction aimed at minimizing arc length errors due to chromatic effects. Evaluating these corrections with on-orbit data from six different star trackers shows these corrections typically provide improvements of 3 − 15% in the arc-length RMSE ratio. The tools and corrections illustrate the many ways that engineers must account for color in the design of high performance star trackers

Ground Testing Strategies for Verifying the Slew Rate Tolerance of Star Trackers

The performance of a star tracker is largely based on the availability of its attitude solution. Several methods exist to assess star tracker availability under both static and dynamic imaging conditions. However, these methods typically make various idealizations that can limit the accuracy of these results. This study aims to increase the fidelity of star tracker availability modeling by accounting for the effects of detection logic and pixel saturation on star detection. We achieve this by developing an analytical model for the focal plane intensity distribution of a star in the presence of sensor slew. Using the developed model, we examine the effects of slew rate on star detection using simulations and lab tests. The developed approach allows us to determine the maximum slew rate for which a star of a given stellar magnitude can still be detected. This information can then be used to describe the availability of a star tracker attitude solution as a function of slew rate, both spatially, across the entire celestial sphere, or locally, along a specified orientation track

Calibration Techniques for Low-Cost Star Trackers

This study presents a series of cost-e_ective strategies for calibrating star trackers for microsatellite missions. We examine three such strategies that focus on the calibration of the imaging detector, geometric lab calibration, and optical calibration due to lens aberrations. Procedures are developed that emphasize speed of implementation, and accuracy, while trying to minimize manual setup procedures. Preliminary results show that employing existing camera calibration techniques reduces the variation in pixel sensitivity by approximately 10%, averaged across each pixel color given the use of a color imager. Although not substantial, this reduction in pixel variation helps preserve the Gaussian illumination pattern of imaged stars, aiding in correct centroid location. Results pertaining to the lab calibration show accurate star placement, in angular terms, to 4:3_10�3rads across most of the _eld of view. This provides an accurate, low-cost, variable solution for characterizing sensor performance; speci_cally pattern matching techniques. Finally, we present some initial results from lens aberration characterization. Using a Gaussian model of the star image shape gives trends consistent with astigmatism and _eld curvature aberrations. Together, these calibrations represent tools that aim to improve both development and manufacture of modern microsatellite star trackers

The Things You Can’t Ignore: Evolving a Sub-Arcsecond Star Tracker

The feasibility of a star tracker with 1 arc-second accuracy and 1 deg/sec slew capability is explored, using the ST-16 design as a starting point. Whole-sky simulation is used to determine the impact of optical design on the star tracker availability. The current optical calibration process is examined, and shown to be inadequate for the desired performance. Additional factors that become important at high accuracy are examined. These are detector calibration, temperature variation, chromatic aberration, stellar aberration and proper motion. Plausible methods of dealing with each of these are presented

Microsatellite Star Tracker Baffles: Validation and Testing

In this paper, we examine the challenges of ground-based stray light testing from a microsatellite perspective. We consider some of the historical approaches to simulation and laboratory testing and propose strategies for ground validation that require only modest investment in test facilities. The star tracker or instrument is characterized in the lab without any baffle, using a novel technique to subtract out reflections internal to the test chamber. From the resulting data a simulation element is produced, and placed at the exit of the baffle in a non-sequential ray tracing analysis. This hybrid experimental/simulation approach makes attenuation predictions that span about nine orders of magnitude, and is effective for predicting solar exclusion angles for devices that observe bright stellar targets (i.e., magnitude six or less)

Towards Star Tracker Only Attitude Estimation

Star trackers can provide full information about satellite attitude information from a single sensor. In this paper, we examine the feasibility of designing attitude determination systems using only star trackers. Star trackers can provide direct inertial attitude estimates without the need for sensor fusion, but current sensors are not robust enough to provide effective attitude estimates in all mission scenarios. Specific technical capabilities must be developed before star tracker only schemes could be practical. To this end, we discuss the performance, robustness, and survivability requirements that would be demanded of a star tracker only system and illustrate important developmental milestones delivered by the recently developed S3S star tracker. To illustrate the star tracker only approach, we present a case study showing variant designs for the CanX-4/5 satellites

Custom Optics vs Modified COTS for Small Spacecraft : The Build vs. Rebuild Decision

This paper provides a comparative case study detailing parallel experiences with both a modified commercial off-theshelf lens and a custom lens for small satellite applications. These lens designs were employed in two successive hardware generations of the ST-16 star tracker. The two designs are compared experimentally, measuring effective aperture, photometric efficiency, point-spread function and thermal stability. We show that opting for custom optical design can effectively remedy deficiencies in performance but often at the expense of technical and budgetary risks

Success by 1000 Improvements: Flight Qualification of the ST-16 Star Tracker

The first launch of a pair of 90 gram Sinclair Interplanetary ST-16 star trackers was in November, 2013 on-board the Skybox Imaging SkySat-1 satellite. The sensor performance — as captured by the sensors’ availability, accuracy, and bad-match rate — fell significantly below expectations. This paper explores the flight qualification campaign undertaken by the sensor developers to bring the sensors back to their intended level of performance. No single fix was sufficient and many small incremental improvements were necessary for success. We discuss the fault diagnosis procedures employed by the team and highlight some of the key improvements to star detection, star measurement, rate estimation, and catalog generation algorithms. Presently the ST-16 sensors on Skysat-1 are reporting availability of around 98% and cross-axis accuracies of roughly 10 arcseconds over an entire orbit in a nominal Earth-observing attitude