Online calibration for star trackers

Star trackers are perhaps the most accurate means of measuring a spacecraft's orientation in space and are becoming a popular sensing instrument for attitude determination systems amongst conventional larger satellites as well as micro satellites. In order to produce and maintain high fidelity measurements, the systematic effects of lens distortion and possible sensor alterations due to environmental changes and instrument aging must all be accounted for through calibration, both on the ground and on orbit. In this study, a calibration method is presented to account for errors in star camera parameters, namely the focal length, bore sight offset, higher order radial distortion terms and the tip and tilt of the detector array in relation to the lens arrangement. This method does not depend on a costly high-precision lab setup; instead it simply employs the star camera images and a star catalogue to calibrate the instrument given reasonable initial estimates. This allows for a reduction in pre-mission calibration requirements and is feasible for an online implementation, allowing the star tracker to calibrate itself through out its life-cycle.

Online calibration for star trackers

Star trackers are perhaps the most accurate means of measuring a spacecraft's orientation in space and are becoming a popular sensing instrument for attitude determination systems amongst conventional larger satellites as well as micro satellites. In order to produce and maintain high fidelity measurements, the systematic effects of lens distortion and possible sensor alterations due to environmental changes and instrument aging must all be accounted for through calibration, both on the ground and on orbit. In this study, a calibration method is presented to account for errors in star camera parameters, namely the focal length, bore sight offset, higher order radial distortion terms and the tip and tilt of the detector array in relation to the lens arrangement. This method does not depend on a costly high-precision lab setup; instead it simply employs the star camera images and a star catalogue to calibrate the instrument given reasonable initial estimates. This allows for a reduction in pre-mission calibration requirements and is feasible for an online implementation, allowing the star tracker to calibrate itself through out its life-cycle.

On-Orbit Calibration of Installation Matrix between Remote Sensing Camera and Star Camera Based on Vector Angle Invariance

To achieve photogrammetry without ground control points (GCPs), the precise measurement of the exterior orientation elements for the remote sensing camera is particularly important. Currently, the satellites are equipped with a GPS receiver, so that the accuracy of the line elements of the exterior orientation elements could reach centimeter-level. Furthermore, the high-precision angle elements of the exterior orientation elements could be obtained through a star camera which provides the direction reference in the inertial coordinate system and star images. Due to the stress release during the launch and the changes of the thermal environment, the installation matrix is variable and needs to be recalibrated. Hence, we estimate the cosine angle vector invariance of a remote sensing camera and star camera which are independent of attitude, and then we deal with long-term on-orbit data by using batch processing to realize the accurate calibration of the installation matrix. This method not only removes the coupling of attitude and installation matrix, but also reduces the conversion error of multiple coordinate systems. Finally, the geo-positioning accuracy in planimetry is remarkably higher than the conventional method in the simulation results.

Attitude determination of system coordinate gradiometer with respect to inertial space

The article represents the algorithm of attitude determination in gradiometer coordinate system with respect to inertial space. The problem can be solved in two steps. The first step is to determine the values of matrix transformation from celestial system (ICRF) to star camera coordinate system (SSRF) using observations star. The second step is to determine the values of matrix transformation from star camera coordinate system (SSRF) to gradiometer coordinate system (GRF). This problem is solved through mounting sensor systems on board of a satellite. Due to the mission GOCE three star cameras are mounted there. The matrix of transformation from star camera coordinate system (SSRF) to gradiometer coordinate system (GRF) is determined for every star camera. The values of transformation matrix are represented in file of data AUX_EGG_DB. Processing star camera’s (star cameras’) observations include the following steps