Research On Three-Dimensional Terrain Recovery For Descent Images Of Lunar Lander Based On SFS

The Lunar exploration is one of the major technology projects in Chinese history.High-precision topographic information of the landing area will play an important role in reaching the target of the second stage of Lunar exploration and ensuring the safety of the Lunar Lander.The three-dimensional terrain recovery results of traditional methods are obtained after image matching.Due to the low reflectance and the uniform material of the lunar surface,the texture information is lacking,we can't get enough accurate feature points for terrain recovery using image matching techniques.Therefore,there are important theoretical and practical significance to propose a new terrain recovery algorithm oriented to the characteristics of the lunar surface to supplement the shortage of traditional recovery by stereo matching.We carry out research on SFS(shape from shading),propose a SFS recovery algorithm based on feature edge gradient constraints for descent images,take the tie points obtained by matching between two images as the control points to constraint the SFS recovery results,convert the SFS relative terrain into a absolute coordinate system.Then we test the three-dimensional recovery results in a simulated lunar environment.The main research contents of the paper include the following aspects:1.In order to ensure the image matching accuracy and reduce the time-consuming of matching process,an improved SIFT(Scale-Invariant Feature the Transform)matching algorithm is proposed after the detailed analysis of the characteristics of lander descent images.The experiment results show that the improved algorithm has a stable matching accuracy for descent images on different height intervals,and the algorithm efficiency is greatly improved when a big change of coverage area happens between the matching images.2.Coordinate transformation process between the position of the Sun,the Earth and the Moon is designed after analysis of the relationship between the different celestial coordinate system.And then a new illumination direction estimation algorithm is proposed for the descent images based on JPL(Jet Propulsion Laboratory)precision ephemeris.The experiment results show that the algorithm does not affected by the imaging effect;the results are reliable and have a high accuracy.3.To inhibit the generation of false edge,a new feature edge convolution mask is put forward,in which the weight of the inclined edge is increased.Edge extraction results show that the inhibition of the false edge with the new mask works better in the case of similar number effective edge.4.A regularized irradiance equation is obtained after derivation of formula of least squares edge fitting and design of evolution rules for gradient ratio factor.The results show that this method fitting the edge well,which can also solve the gradient ratio factor effectively.5.The.irradiance equation under the perspective projection is derived under the analysis of the descent images.The gradient ratio factor is taken to restrict the irradiance equation,and then the irradiance equation is solved by linear approximation.The three-dimensional recovery results of SFS are restricted by matched control points.An experiment in simulated lunar environment is carried out.The results show that the accuracy of our algorithm is greatly improved compared with other SFS algorithms.