A Novel Graph-Guided Global Bundle Block Adjustment of OSIRIS-REx Laser Altimeter Data for Topographic Mapping of Asteroid Bennu

During the Orbital B Phase mission of the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx), the OSIRIS-REx Laser Altimeter (OLA) instrument scanned asteroid Bennu and obtained hundreds of point cloud frames, making a meticulous 3-D surface modeling of Bennu realizable. However, these acquired point clouds suffer from problems such as inaccurate pose information, low overlap ratios, uneven densities, and outliers. To create a precise 3-D shape model via topographical mapping using OLA data, we present an optimized connection graph-guided global bundle block adjustment (BA) method with variable weights capable of constructing a complete 3-D model of the asteroid via a straightforward global adjustment of offsets. Specifically, by using an improved Fast Double-channel Aggregated Feature Transform (iFDAFT), 3-D keypoint extraction and their correspondences can be determined with a subpixel level of matching accuracy. Then, a graph of the shortest path algorithm is constructed to establish the optimized connections of all the OLA point clouds. Afterward, the global bundle BA method with variable weights is introduced to minimize the keypoint matches simultaneously, reducing the offsets between the overlapping point clouds. Finally, the shape model of asteroid Bennu was constructed using the finely adjusted point clouds, with RMSE as small as 0.0743 m compared with the reference shape model, reaching approximately the limit of the instrument range errors (i.e., 3 cm) of observation data. Results of the case study on asteroid Bennu demonstrated that the proposed method could improve the accuracy of point cloud registration and meet the application requirements.