Research On The Projection Direction Of The Solar Rotation Axis For Astronomical Autonomous Navigation

Astronomical autonomous navigation has gradually become a research focus in the field of deep space exploration due to its advantages of complete autonomy,no accumulation of navigation errors over time,high reliability,and the ability to improve the on-orbit survivability of deep space probes.Astronomical autonomous navigation requires the use of different astronomical measurement information,such as the relative angle between celestial bodies obtained by celestial sensors,the distance estimated by the arrival time of pulse signals obtained by X-ray sensors and the relative doppler velocity obtained by spectrometers.These astronomical measurement information can constitute three navigation methods of angle measurement,distance measurement and speed measurement,which is an important basis for the realization of astronomical autonomous navigation.In the observation of the sun,a new study shows that the probe can use four spectrometers to measure the Doppler velocities of two sets of points on the edge of the solar disk that are perpendicular to each other and pass through the center of the solar disk and use the Doppler velocity difference between two sets of points to model and calculate the observation angle of the solar rotation axis,and use it as a new type of measurement information for astronomical autonomous navigation.However,the method of obtaining measurement information based on spectrometer velocity is not only affected by instability caused by solar spectral distortion,but also brings extra load to the probe,thereby affecting the performance of astronomical navigation.Therefore,it is of great significance to improve the acquisition and calculation methods of measurement information to improve the performance of astronomical navigation systems.In order to explore new ways of acquiring and calculating measurement information,this paper expands the processing method of astronomical measurement information,and proposes a method that does not rely on spectrometers to obtain the solar spectrum,but obtains the solar image through cameras and processes the solar image to calculate the projection angle of the solar rotation axis.The main work of the paper is as follows:(1)Feature extraction methods for sunspots and coronal holes in solar images are studied.Extracting features such as sunspots and coronal holes can eliminate the influence of solar activities to a certain extent,which is the premise to realize the calculation of the projection angle of the solar rotation axis.First,a series of preprocessing are performed on the sun images,such as Gaussian denoising,Sobel operator edge detection,Hough transform detection of the center and radius of the solar disk,and correction of the dark area on the edge of the sun images,so as to eliminate the influence of noise and other interference factors on solar image feature extraction.Then,the methods of threshold segmentation,mathematical morphology,and region growth are used to segment the sunspots and coronal holes in the solar images to achieve feature extraction.The experimental results achieved the successful extraction of sunspots and coronal holes.(2)A calculation method of the projection angle of the solar rotation axis based on the optical flow method and solar feature extraction is proposed.In order to calculate the projection angle of the solar rotation axis,this paper applied the HS optical flow method and the LK optical flow method to the solar images to calculate the solar surface velocity field,and the validity of the velocity field calculation is verified by the image structure similarity index.Considering that solar activity has a great influence on the velocity field of the solar surface,the velocity field of surface features such as sunspots or coronal holes is selected to calculate the projection angle of the solar rotation axis.The experimental results show that the LK optical flow method is more efficient than the HS optical flow method.The LK optical flow method can obtain the effective and continuous projection angle of the solar rotation axis by tracking the umbra of the sunspot.