Perceptual aliasing and weak textures pose significant challenges to the task of place recognition, hindering the performance of Simultaneous Localization and Mapping (SLAM) systems. This paper presents a novel model, called UMF (standing for Unifying Local and Global Multimodal Features) that 1) leverages multi-modality by cross-attention blocks between vision and LiDAR features, and 2) includes a re-ranking stage that re-orders based on local feature matching the top-k candidates retrieved using a global representation. Our experiments, particularly on sequences captured on a planetary-analogous environment, show that UMF outperforms significantly previous baselines in those challenging aliased environments. Since our work aims to enhance the reliability of SLAM in all situations, we also explore its performance on the widely used RobotCar dataset, for broader applicability. Code and models are available at https://github.com/DLR-RM/UMF.

Many robotic systems rely on visual sensing to accomplish simultaneously the tasks of state estimation, mapping, and path planning. One one hand, the usage of camera sensors represents a power-efficient and lightweight option for solving this problem. On the other hand, these tasks pose requirements on the quality of the visual input (e.g. number of tracked features for Visual Odometry) that are often in contrast to the optimal viewpoint planning for local mapping and obstacle avoidance. Dealing with this constraint is actively researched in the field of perception-aware planning. The approaches delivered by this field mostly concern Micro air vehicles (MAVs), but could be applied to a larger group of robotic systems. We propose a perception-aware trajectory planner for a class of robotic systems that can orient their cameras independently from their direction of travel. By using motion primitives, our planner does not require differentiable models for motion and perception objectives. We evaluate our method in simulation, showing increased capabilities in localization-aware motions around obstacles, and demonstrate its run-time capability on a real planetary rover. The code is released publicly under github.com/DLR-RM/palp.

Future planetary missions will rely on rovers that can autonomously explore and navigate in unstructured environments. An essential element is the ability to recognize places that were already visited or mapped. In this work, we leverage the ability of stereo cameras to provide both visual and depth information, guiding the search and validation of loop closures from a multi-modal perspective. We propose to augment submaps that are created by aggregating stereo point clouds, with visual keyframes. Point clouds matches are found by comparing CSHOT descriptors and validated by clustering, while visual matches are established by comparing keyframes using Bag-of-Words (BoW) and ORB descriptors. The relative transformations resulting from both keyframe and point cloud matches are then fused to provide pose constraints between submaps in our graph-based SLAM framework. Using the LRU rover, we performed several tests in both an indoor laboratory environment as well as a challenging planetary analog environment on Mount Etna, Italy, consisting of areas where either keyframes or point clouds alone failed to provide adequate matches demonstrating the benefit of the proposed multi-modal approach.

The ability to recognize previously mapped locations is an essential feature for autonomous systems. Unstructured planetary-like environments pose a major challenge to these systems due to the similarity of the terrain. As a result, the ambiguity of the visual appearance makes state-of-the-art visual place recognition approaches less effective than in urban or man-made environments. This paper presents a method to solve the loop closure problem using only spatial information. The key idea is to use a novel continuous and probabilistic representations of terrain elevation maps. Given 3D point clouds of the environment, the proposed approach exploits Gaussian Process (GP) regression with linear operators to generate continuous gradient maps of the terrain elevation information. Traditional image registration techniques are then used to search for potential matches. Loop closures are verified by leveraging both the spatial characteristic of the elevation maps (SE (2) registration) and the probabilistic nature of the GP representation. A submap-based localization and mapping framework is used to demonstrate the validity of the proposed approach. The performance of this pipeline is evaluated and benchmarked using real data from a rover that is equipped with a stereo camera and navigates in challenging, unstructured planetary-like environments in Morocco and on Mt. Etna.

The inherent scale ambiguity in monocular vision is a well known issue that forces the integration of other sensory sources to obtain metric references. However, 2D or 3D LiDARs and RGB-D sensors, while guaranteeing metrological accuracy, impose a non negligible burden both in terms of computational load and power requirements limiting the feasibility of being implemented on small exploration vehicles. This paper presents a scale aware monocular Visual Odometry framework that fuses range data from a laser altimeter in order to recover and maintain a correct metric scale. The proposed Visual Odometry method consists of a keyframe based tracking and mapping algorithm using optical flow where range data serves as a scale constraint on a keyframe to keyframe basis. An optimization backend based on iSAM2 is employed in order to refine the trajectory and map estimates eliminating the scale drift without the need of performing loop closures. We demonstrate that our algorithm can obtain very similar performances to state of the art stereo visual SLAM and RGB-D methods.