Charles J. Wetterer

Attitude Determination from Light Curves

T HE unscented Kalman filter (UKF) [1,2] has been used extensively in spacecraft attitude determination based on standard in situ measurements using gyro-based models for attitude propagation [3,4] and, more recently, in orbit determination from groundbased angles and range measurements [5]. In the latter application, the filter is unable to track changes in attitude, because these changes affect the nonconservative forces, which are subtle as compared with the dominant gravitational forces. (i.e., becoming manifest in positional data over long time scales). Attitude changes must be modeled and tracked to properly account for the nongravitational forces experienced by the object. In thisNote,we describe for thefirst time the use of the UKF in attitude determination from ground-based brightness measurements (i.e., light curves), which are very sensitive to attitude variations. This novel application of the UKF allows for the possibility of more accurate and precise orbit determination by exploiting multi-data-type fusion, combining both astrometric and photometric observationswithin the samedata-reduction process [5].

Refining Space Object Radiation Pressure Modeling with Bidirectional Reflectance Distribution Functions

High-fidelity orbit propagation requires detailed knowledge of the solar radiation pressure on a space object. The solar radiation pressure depends not only on the space object’s shape and attitude, but also on the absorption and reflectance properties of each surface on the object. These properties are typically modeled in a simplistic fashion, but are here described by a surface bidirectional reflectance distribution function. Several analytic bidirectional reflectance distribution function models exist, and are typically complicated functions of illumination angle and material properties represented by parameters within the model. In general, the resulting calculation of the solar radiation pressure would require a time-consuming numerical integration. This might be impractical if multiple solar radiation pressure calculations are required for a variety of material properties in real time; for example, in a filter where the particular surface parameters are being estimated. This paper develops a method...

Photometric Attitude Estimation for Agile Space Objects with Shape Uncertainty

The problem of estimating attitude for actively maneuvering or passively rotating space objects with unknown mass properties/external torques and uncertain shape models is addressed. To account for agile space object maneuvers, angular rates are simply assumed to be random inputs (e.g., process noise), and model uncertainty is accounted for in a bias state with dynamics derived using first principles. Bayesian estimation approaches are used to estimate the resulting severely non-Gaussian and multimodal state distributions. Simulated results are given, conclusions regarding performance are made, and future work is outlined.