This paper examines the effectiveness of small star trackers for orbital estimation. Autonomous optical navigation has been used for some time to provide local estimates of orbital parameters during close approach to celestial bodies. These techniques have been used extensively on spacecraft dating back to the Voyager missions, but often rely on long exposures and large instrument apertures. Using a hyperbolic Mars approach as a reference mission, we present an EKF-based navigation filter suitable for nanosatellite missions. Observations of Mars and its moons allow the estimator to correct initial errors in both position and velocity. Our results show that nanosatellite-class star trackers can produce good quality navigation solutions with low position (< 300 m) and velocity (< 0.15 m/s) errors as the spacecraft approaches periapse.
ANGLE software for semiconductor detector efficiency calculations - long
existing and widely accepted tool in quantitative gamma spectrometry - has
been recently extended to scintillation NaI detectors. The extension features
in the latest edition (ANGLE 4) and it is briefly outlined. Discretization of
reference efficiency curve, meaning possibility of using ANGLE 4 for
particular gamma energies without constructing the complete reference
efficiency curve, is particularly emphasized. This yields both in enhanced
practicality and higher accuracy, while reducing the potential for systematic
errors. The present work is primarily focussed on experimental verification
of ANGLE 4 for NaI detectors. Two detectors (2 ? 2 and 3 x 3 inches) were
employed in the experiment. Commercially calibrated gamma sources (in the
forms of quasi point and cylinder) and homemade solutions (diluted from
calibrated ones) were measured at various distances from the detector(s),
ranging 0 cm to 50 cm. Energy range observed was 59 keV to 1408 keV.
Versatility of counting conditions, in terms of detectors and sources used,
gamma energies observed, source detector separations, etc., was aimed at
creating as large experimental evidence as possible for verification
purposes. Experimentally obtained efficiencies are compared with those
calculated by ANGLE 4. Very good agreement is obtained - well within the
experimental uncertainties - thus proving the reliability of the software.
Star trackers must be calibrated prior to flight so that they can make accurate measurements of star positions within the instrument field of view. This calibration is often performed in atmosphere and once the sensor is launched, it is not uncommon to observe a small shift in some of the calibration parameters. To maximize sensor performance, these parameter values must be corrected to better match the actual observa tions. In this paper, we explore the several practical strategies for on-orbit recalibration of star trackers. We consider both human-mediated batch-processing as well as autonomous, se quential algorithms. We base our results on orbital data from a number of Sinclair Interplanetary ST16 star trackers launched within the last year. Both approaches to on-orbit calibration can yield significant improvements both to sensor availability and geometric error.
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