Star position

Abstract: A method of attitude measurement for an artificial satellite (100) utilizes one or more star trackers (12) together with an earth sensor (30). Periodic updates of satellite orbital information, either propagated onboard or from a ground station are combined with earth and star position coordinate data to provide a continuous and accurate measurement of the spacecraft body 3-axis attitude. The method can be used for ground-based attitude determination or onboard closed loop control systems.

Abstract: The grid method is an all-sky star identification approach which tolerates high values of position and stellar magnitude noise compared to its state-of-the-art counterparts. Nevertheless, its performance is affected by sensor’s radiometric bias and large values of position noise standard deviation (>1 pixel). In this paper, some optimization approaches are exploited to make the grid algorithm more robust against the above hazards. First, a statistical analysis of stars’ population with respect to the stellar magnitudes is performed to define radiometric clusters which make the grid algorithm less sensitive to radiometric calibration bias. Second, by optimizing an objective function defined by the product of robustness factor and grid resolution, the optimal grid cell size is achieved to make the algorithm more robust against noisy sky field. Finally, using Bayesian decision theory, a method is developed for computing optimal static threshold for miss/correct matches classification. The proposed algorithm has been tested on a camera (with 14.6◦ ×14.6◦ field of view and 512×512 pixels resolution) against harsh conditions of both star position and stellar magnitude uncertainty. Sensor identification probability of 99.8% against 2 pixels position noise standard deviation has been obtained.

Abstract: A novel star identification network (RPNet) based on representation learning is proposed in this paper. Unlike other pattern-based stars identification algorithms, the RPNet does not require the creation of an elaborate pattern, nor does it need to search among patterns. Instead, a star pattern generator (SPG) in the RPNet helps in finding the best pattern that can distinguish different stars clearly. A star pattern classifier (SPC) in the RPNet is utilized to recognize the pattern generated before. The simulations show that the RPNet is extremely robust toward star position noise, star magnitude noise, and false stars. The performance on simulation images outperforms almost all other pattern-based stars identification algorithms. On average, it achieves an identification rate of 99.23% in simulated star images. The identification rate on real star images is higher than 98%. Moreover, the algorithm achieves this performance with lesser memory and faster speed compared to polygon algorithms.

Abstract: The Advanced Land Observing Satellite (ALOS) is required to achieve stringent attitude determination accuracy (3.0×10 -4 deg on-board and 1.4×10 -4 deg ground-based), position determination accuracy (1m ground-based), and attitude stability (3.9 × 10 -4 deg/5sec) in order to provide precise geometric accuracy for high-resolution images without ground control points. It is designed to yield the geolocation determination accuracy of 6m from attitude and position estimates and that of 3m with an additional high-bandwidth measurement. Presented in this paper are ALOS's platform and ground systems technologies developed for achieving the attitude determination accuracy and the position determination accuracy. They include a precision star tracker, optimal attitude estimation algorithms (real-time and off-line), an alignment change reduction, a jitter sensor, a precision GPS receiver, and a ground-based position estimation algorithm. The star tracker provides the best star position accuracy (random error: 9.0arcsec, and bias error: 0.74arcsec). The on-board attitude determination algorithm estimates attitude quaternion by applying an extended Kalman filter. The off-line attitude estimation introduced an extended-Kalman-filter-based smoother. To minimize the alignment change, the sensors are placed on the optical bench subject to precise temperature control. The jitter sensor provides precise angular information (0.010arcsec) from 2Hz to 500Hz and extends the attitude determination bandwidth. The dual-frequency GPS receiver capable of measuring pseudoranges and carrier phases allows the ground-based position determination with sub-meter accuracy.