Millimeter wave radar can measure distances, directions, and Doppler velocity for objects in harsh conditions such as fog. The 4D imaging radar with both vertical and horizontal data resembling an image can also measure objects' height. Previous studies have used 3D radars for ego-motion estimation. But few methods leveraged the rich data of imaging radars, and they usually omitted the mapping aspect which is affected by the radar multipath returns, thus leading to inferior odometry accuracy. This paper presents a real-time imaging radar inertial odometry and mapping method, iRIOM, based on the submap concept. To fend off moving objects and multipath reflections, the iteratively reweighted least squares method is used for getting the ego-velocity from a single scan. To measure the agreement between sparse non-repetitive radar scan points and submap points, the distribution-to-multi-distribution distance for matches is adopted. The ego-velocity, scan-to-submap matches are fused with the 6D inertial data by an iterative extended Kalman filter to get the platform's 3D position and orientation. A loop closure module is also developed to curb the odometry module's drift. To our knowledge, iRIOM based on the two modules is the first 4D radar inertial SLAM system. On our and third-party data, we show iRIOM's favorable odometry accuracy and mapping consistency against the FastLIO-SLAM and the EKFRIO. Also, the ablation study reveal the benefit of inertial data versus the constant velocity model, the scan-to-submap matching versus the scan-to-scans matching, and loop closure.

翻译：毫米波雷达能够在雾等恶劣环境中测量物体的距离、方向和多普勒速度。兼具垂直与水平信息、类似图像的四维成像雷达还可测量物体的高度。以往研究常使用三维雷达进行自运动估计，但鲜有方法充分利用成像雷达的丰富数据，且往往忽略受雷达多径反射影响的建图环节，导致里程计精度不足。本文提出一种基于子图概念的实时成像雷达惯性里程计与建图方法iRIOM。为规避运动物体与多径反射影响，采用迭代重加权最小二乘法从单次扫描中获取自速度；针对稀疏非重复雷达扫描点与子图点的一致性度量，采用匹配点的分布到多分布距离。通过迭代扩展卡尔曼滤波器将自速度、扫描-子图匹配结果与六维惯性数据融合，获取平台的三维位置与姿态。此外，设计回环检测模块以抑制里程计漂移。据我们所知，基于上述两模块的iRIOM是首个四维雷达惯性SLAM系统。在自有及第三方数据集上的实验表明，相较于FastLIO-SLAM与EKFRIO，iRIOM具有更优的里程计精度与建图一致性。消融研究进一步揭示了惯性数据（相对于恒速模型）、扫描-子图匹配（相对于扫描-扫描匹配）及回环检测的有效性。