The vision-based navigation and localization system of China's "Yutu" lunar rover is controlled by a ground teleoperation center. A large-spacing traveling mode with approximately 6-10 m per site is adopted for the rover to maximize the driving distance of the lunar rover and improve the efficiency of remote control exploration. This results in a significant distance between adjacent navigation sites, and considerable rotation, translation, and scale changes in the captured images. Furthermore, the low overlap between images and the vast differences in regional shapes, combined with weak texture and illumination variations on the lunar surface, pose challenges to image feature matching among different sites. Currently, the "Yutu" lunar rover employs inertial measurements and visual matches among different sites for navigation and positioning. The ground teleoperation center adopts inertial measurements as initial poses and optimizes the poses with visual matches by bundle adjustment to obtain the final rover poses. However, due to the wide baseline and significant surface changes of images at different sites, manual assistance is often required to filter or select the correct matches, significantly affecting the efficiency of the ground teleoperation center. Therefore, improving the robustness of image feature matching between different sites to achieve automatic visual positioning is an urgent problem to be addressed.

To address the poor performance and low success rate of current image matching algorithms in wide-baseline lunar images with weak textures and illumination variations, we propose a global attention-based lunar image matching algorithm by the view synthesis. First, we utilize sparse feature matching methods to generate sparse pseudo-ground-truth disparities for the rectified stereo lunar images at the same site. Next, we finetune a stereo matching network with these disparities and perform 3D reconstruction for the lunar images at the same site. Then, we leverage inertial measurements among different sites to convert the original image into a new synthetic view for matching based on the scene depth, addressing the low overlap and large viewpoint changes among images of different sites. Additionally, we adopt a Transformer-based image matching network to improve matching performance in weak-texture scenes, and an outlier rejection method that considers plane degeneration in the post-processing stage. Finally, the matches are returned from the synthetic image to the original image, yielding the matches for wide-baseline lunar images at different sites.

We conduct experiments on the real lunar dataset from the "Yutu 2" lunar rover (referred to as the Moon dataset), which includes two parts. The first part is stereo images from five continuous stations (employed for stereo reconstruction), and the second is 12 sets of wide-baseline lunar images from adjacent sites (for wide-baseline image matching testing). In terms of lunar 3D reconstruction, we calculate the reconstruction error within different distance ranges, where the reconstruction network GwcNet (Moon) yields the best reconstruction accuracy and reconstruction details, as shown in Table 1 and Fig. 4. Meanwhile, Fig. 5 illustrates the synthetic images obtained from the view synthesis scheme based on the inertial measurements between sites and the scene depth, which solves the large rotation, translation, and scale changes between adjacent sites. For wide-baseline image matching, existing algorithms such as LoFTR and ASIFT have matching success rates of 33.33% and 16.67% respectively as shown in Table 2. Our DepthWarp-LoFTR algorithm achieves a matching success rate of 83.33%, significantly improving the matching success rate and accuracy of wide-baseline lunar images (Table 3). Additionally, this algorithm increases the matching success rate from 16.67% to 41.67% compared to the ASIFT algorithm. We present the matching results of different algorithms in Fig. 7, where DepthWarp-LoFTR obtains more consistent and denser matching results compared to other methods.

We propose a robust feature matching method DepthWarp-LoFTR for wide-baseline lunar images. For stereo images captured at the same site, the sparse disparities are generated through a sparse feature matching algorithm. These disparities serve as pseudo-ground truth to train the GwcNet network for 3D reconstruction of lunar images at the same site. To handle the wide baseline and low overlap of images from different sites, we propose a view synthesis algorithm based on scene depth and inertial prior poses. Image matching is performed on the synthesized current-site image and the next-site image to reduce the feature matching difficulty. For the feature matching stage, we adopt a Transformer-based LoFTR network, which significantly improves the success rate and accuracy of automatic matching. Our experimental results on real lunar datasets demonstrate that the proposed algorithm greatly improves the success rate of feature matching in complex lunar wide-baseline scenes. This lays a solid foundation for automatic visual positioning of the "Yutu 2" lunar rover and subsequent routine patrols of lunar rovers in China's fourth lunar exploration phase.