Star tracker is one of the most promising optical attitude measurement devices and it is widely used in spacecraft for its high accuracy. However, how to realize and verify such an accuracy remains a crucial but unsolved issue until now. The authenticity of the accuracy measurement method of a star tracker will eventually determine the satellite performance. A new and robust accuracy measurement method for a star tracker based on the direct astronomical observation is proposed here. In comparison with the conventional method with simulated stars, this method utilizes real navigation stars as observation targets which makes the measurement results more authoritative and authentic. Transformations between different coordinate systems are conducted on the account of the precision movements of the Earth, and the error curves of directional vectors are obtained along the three axes. Based on error analysis and accuracy definitions, a three-axis accuracy evaluation criterion has been proposed in this paper, which could determine pointing and rolling accuracy of a star tracker directly. Experimental measurements confirm that this method is effective and convenient to implement. Such a measurement environment is close to the in-orbit conditions and it can satisfy the stringent requirement for high-accuracy star trackers.

/pdf/an-accuracy-measurement-method-for-star-trackers-based-on-2zrduvrf41.pdf

An accuracy measurement method for star trackers based on direct astronomic observation

A calibration method for binocular stereo vision sensor with short-baseline based on 3D flexible control field

Noise suppression algorithm of short-wave infrared star image for daytime star sensor

Calibration of visual model for space manipulator with a hybrid LM–GA algorithm

The star sensor is a prerequisite navigation device for a spacecraft. The on-orbit calibration is an essential guarantee for its operation performance. However, traditional calibration methods rely on ground information and are invalid without priori information. The uncertain on-orbit parameters will eventually influence the performance of guidance navigation and control system. In this paper, a novel calibration method without priori information for on-orbit star sensors is proposed. Firstly, the simplified back propagation neural network is designed for focal length and main point estimation along with system property evaluation, called coarse calibration. Then the unscented Kalman filter is adopted for the precise calibration of all parameters, including focal length, main point and distortion. The proposed method benefits from self-initialization and no attitude or preinstalled sensor parameter is required. Precise star sensor parameter estimation can be achieved without priori information, which is a significant improvement for on-orbit devices. Simulations and experiments results demonstrate that the calibration is easy for operation with high accuracy and robustness. The proposed method can satisfy the stringent requirement for most star sensors.

On-orbit calibration for star sensors without priori information.

Under highly dynamic conditions, the star-spots on the image sensor of a star tracker move across many pixels during the exposure time, which will reduce star detection sensitivity and increase star location errors. However, this kind of effect can be compensated well by setting an appropriate exposure time. This paper focuses on how exposure time affects the star tracker under highly dynamic conditions and how to determine the most appropriate exposure time for this case. Firstly, the effect of exposure time on star detection sensitivity is analyzed by establishing the dynamic star-spot imaging model. Then the star location error is deduced based on the error analysis of the sub-pixel centroiding algorithm. Combining these analyses, the effect of exposure time on attitude accuracy is finally determined. Some simulations are carried out to validate these effects, and the results show that there are different optimal exposure times for different angular velocities of a star tracker with a given configuration. In addition, the results of night sky experiments using a real star tracker agree with the simulation results. The summarized regularities in this paper should prove helpful in the system design and dynamic performance evaluation of the highly dynamic star trackers.

/pdf/exposure-time-optimization-for-highly-dynamic-star-trackers-4r2xkv1gmk.pdf

Exposure Time Optimization for Highly Dynamic Star Trackers

The invention discloses a start point centroid error compensation method in a high dynamic situation. The method comprises the following steps: A, calculating angular velocities of three axes, performing least square fit on the angular velocities of the three axes respectively and performing kalman filter on fitting results; B, approximately expressing the angular velocities of the three axes in exposure time by using a first-order polynomial, and substituting angle information on rotation of the three axes obtained by integrating the angular velocities into a rotation matrix of a star sensor; C, performing second-order Taylor expansion approximation on the rotation matrix, and performing perspective projection transformation to obtain a movement track coordinate expression of star point spots on an image sensor; D, performing second-order Taylor expansion approximation on the star point track coordinate expression, solving a star point centroid coordinate referring to a centroid method, and calculating star point centroid error compensation. According to the method, the star point positioning accuracy in the high dynamic situation of the star sensor is improved, the processing algorithm is simpler, the compensation calculation is accurate, and the method has better application prospect in the high dynamic situation.

Start point centroid error compensation method in high dynamic situation

The invention discloses a code-type sun sensor error modeling and calibration method. The method includes the steps of carrying out analysis on error factors of a code-type sun sensor, building a code-type sun sensor error compensation model which contains fine code signal processing algorithm errors and structural errors, and utilizing a calibration system to carry out calibration on model parameters in the error compensation model. Through adoption of the code-type sun sensor error modeling and calibration method, the measuring accuracy of the code-type sun sensor can be effectively improved.

Code-type sun sensor error modeling and calibration method

Error factors in the encoded sun sensor (ESS) are analyzed and simulated. Based on the analysis results, an ESS error compensation model containing structural errors and fine-code algorithm errors is established, and the corresponding calibration method for model parameters is proposed. As external parameters, installation deviation between ESS and calibration equipment are introduced to the ESS calibration model, so that the model parameters can be calibrated accurately. The experimental results show that within plus/minus 60 degree of incident angle, the ESS measurement accuracy after compensation is three times higher on average than that before compensation.

Error modeling and calibration for encoded sun sensors.

Star sensor calibration based on integrated modelling with intrinsic and extrinsic parameters

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