Fig. 1. SC descriptor diagram

Fig. 2. Field angle schematic diagram of Livox-Mid70 solid-state LiDAR and Velodyne VLP-16 mechanical radar

Fig. 3. Comparison of SC descriptor of the submap and current frame that forms a loopback. (a) SC descriptor of the submap; (b) SC descriptor of current frame

Fig. 4. Comparison of SC descriptor between current frame and historical frame. (a) SC descriptor of submap; (b) binary descriptor of current frame; (c) submap SC descriptor obtained with Fig.4 (b) as the mask; (d) SC descriptor of current frame

Fig. 5. Pre-registration schematic diagram of two frame point clouds forming a loopback

Fig. 6. Registration results obtained by CFB-ICP

Fig. 7. Flow chart of the loopback algorithm

Fig. 8. Location and equipment of campus data collection. (a) Satellite map of the data collection area, where the five-pointed star is the starting point and the ending point of data collection process, and the rectangular frame is the loopback area; (b) handheld experimental platform

Fig. 9. Comparison of the global map under FR-IOSB dataset between the original FAST-LIO2 algorithm and after adding the loopback algorithm. (a) FAST-LIO2; (b) FAST-LIO2 + loopback algorithm

Fig. 10. Comparison of the local map under FR-IOSB dataset between the original FAST-LIO2 algorithm and after adding the loopback algorithm. (a) (d) scene 1; (b) (e) scene 2; (c) (f) scene 3

Fig. 11. Comparison of trajectories of each original algorithm in campus dataset with adding the loopback algorithm

Fig. 12. Three-dimensional trajectories of each algorithm in campus data set. (a) Before and after FAST-LIO2 adding loopback; (b) before and after FASTER-LIO adding loopback

Fig. 13. Comparison of the global map under campus dataset between the original FAST-LIO2 algorithm and after adding the loopback algorithm. (a) FAST-LIO2; (b) FAST-LIO2+loopback algorithm

Fig. 14. Comparison of the local map under campus dataset between the original FAST-LIO2 algorithm and after adding the loopback algorithm. (a) (d) scene 1; (b) (e) scene 2; (c) (f) scene 3