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时间同步：
一、统一时钟源
1、假设各传感器都有自己的内部时钟，但由于每个时钟的钟漂不一样，导致各时钟之间存在时间差，所以需要统一各传感器的时间源。
2、一般采用GPS时间作为统一时间源，通过GPS给各个传感器提供基准时间，各传感器根据提供的基准时间校准各自的时钟时间。
3、各传感器根据已经校准后的各自时间为各自独立采集的数据加上时间戳信息，来做到所有传感器时间戳同步。
二、硬件同步
由于每种传感器的采样频率不一致，如lidar通常为10Hz，camera通常为25/30Hz，不同传感器之间的数据传输还存在一定的延迟，那么
可以通过寻找相邻时间戳的方法找到最近邻帧，但如果两个时间戳相差较大，且传感器或障碍物又在运动，那么最终会得到较大的同步误
差。这个情况可以采用硬件同步触发的方法来缓解查找时间戳造成的误差现象。如何实现激光雷达和相机的时间同步呢？也就是在激光雷
达和相机的时间源都统一到GPS时间后，如何保证他们同步触发采集呢？是以PPS信号为触发信号，实现激光雷达和相机均在PPS信号上
升沿的时候采集数据，且打上各自时钟的时间戳。由于PPS的频率只有1Hz，所以通常需要一个设备把PPS信号转发为任意频率、但是跟
PPS信号同相位的方波，这样就可以控制相机的采集频率了
三、软同步
当两传感器的采集时刻不一致，但又想得到同一时刻下两传感器的结果，就可以使用软件同步的方式。目前常采用的软件同步方式为内插
外推法，此方法适用于传感器间帧率不存在倍数关系的情况，或者传感器有帧率不稳定的情况。主要利用两个传感器帧上的时间标签，计
算出时间差，然后通过包含有运动信息的目标帧与时间差结合，对帧中每一个目标的位置进行推算，推算出新的帧时各个目标的位置，并
于原有的两帧之间建立新的帧。
多传感器时间同步图解
Multi-camera calibration using one or more calibration patterns
Calibration of Asynchronous Camera Networks: CALICO
Infrastructure-based Multi-Camera Calibration using Radial Projections.
Infrastructure-Based Calibration of a Multi-Camera Rig
Leveraging Image-based Localization for Infrastructure-based Calibration of a Multi-camera Rig.
Deep Learning for Camera Calibration and Beyond: A Survey
An Extrinsic Calibration Tool for Radar, Camera and Lidar
Joint camera intrinsic and lidar-camera extrinsic calibration.2022
Unified Intrinsic and Extrinsic Camera and LiDAR Calibration under Uncertainties（ICRA2020）
Automatic online calibration of cameras and lasers. RSS2013
Automatic targetless extrinsic calibration of a 3d lidar and camera by maximizing mutual information. AAAI2012.
Online camera-lidar calibration with sensor semantic information. ICRA2020
Crlf: Automatic calibration and refinement based on line feature for lidar and camera in road scenes.2021
3d lidar-camera extrinsic calibration using an arbitrary trihedron.2013
Automatic extrinsic calibration of vision and lidar by maximizing mutual information.2015
Automatic calibration of lidar and camera images using normalized mutual information.2012
Multi-FEAT: Multi-Feature Edge AlignmenT for Targetless Camera-LiDAR Calibration.2022
A Novel Calibration Method between a Camera and a 3D LiDAR with Infrared Images（ICRA2020）
Automatic extrinsic calibration between a camera and a 3D Lidar using 3D point and plane correspondences
Online LiDAR-Camera Extrinsic Parameters Self-checking
An optimization-based IMU-Lidar-Camera Co-calibration method
Online Multi Camera-IMU Calibration
Spatiotemporal Calibration of 3D mm-Wavelength Radar-Camera Pairs
General, Single-shot, Target-less, and Automatic LiDAR-Camera Extrinsic Calibration Toolbox
A kalman filter-based algorithm for IMU-camera calibration: Observability analysis and performance evaluation
Spatio-temporal initialization for IMU to camera registration
Optimization based IMU camera calibration
An optimization-based IMU-Lidar-Camera Co-calibration method
Online Multi Camera-IMU Calibration
Spatiotemporal Calibration of 3D mm-Wavelength Radar-Camera Pairs
Integrating millimeter wave radar with a monocular vision sensor for on-road obstacle detection applications
Targetless Rotational Auto-Calibration of Radar and Camera for Intelligent Transportation Systems
A novel method of spatial calibration for camera and 2D radar based on registration
Radar and vision sensors calibration for outdoor 3D reconstruction
An optimization-based IMU-Lidar-Camera Co-calibration method
Online Multi Camera-IMU Calibration
Spatiotemporal Calibration of 3D mm-Wavelength Radar-Camera Pairs
Extrinsic and Temporal Calibration of Automotive Radar and 3D LiDAR
Online extrinsic calibration based on per-sensor ego-motion using dual quaternions. 2021
Online temporal calibration for monocular visual-inertial systems. IROS 2018
Rggnet: Tolerance aware lidarcamera online calibration with geometric deep learning and generative model. 2020
Online Camera-LiDAR Calibration with Sensor Semantic Information. ICRA 2020.
An Extrinsic Calibration Tool for Radar, Camera and Lidar
Extrinsic 6DoF Calibration of a Radar – LiDAR – Camera System Enhanced by Radar Cross Section Estimates Evaluation
激光雷达-IMU标定
Deepcalib: A deep learning approach for automatic intrinsic calibration of wide field-of-view cameras. 2018
Calibration of fish eye camera using entrance pupil
Croon: Automatic multi-lidar calibration and refinement method in road scene. 2022
Extrinsic multi-sensor calibration for mobile robots using the gauss-helmert model. IROS 2017
A general approach to spatiotemporal calibration in multisensor systems. 2016
Unified temporal and spatial calibration for multi-sensor systems.2013.
Real-time temporal and rotational calibration of heterogeneous sensors using motion correlation analysis
Regnet: Multimodal sensor registration using deep neural networks. 2017
Calibnet: Geometrically supervised extrinsic calibration using 3d spatial transformer networks. IROS 2018
Extending kalibr: Calibrating the extrinsics of multiple IMUs and of individual axes. ICRA 2016
Automatic online calibration of cameras and lasers
Extrinsic Calibration of an Eye-In-Hand 2D LiDAR Sensor in Unstructured Environments Using ICP.
Efficient Extrinsic Calibration of Multi-Sensor 3D LiDAR Systems forAutonomous Vehicles using Static Objects Information
Target-free Extrinsic Calibration of a 3D-Lidar and an IMU
An optimization-based IMU/Lidar/Camera Cocalibration method
OpenCalib
Autoware
ApolloCalib
kalibr
Autoware Calib
LivoxCalib
MatlabCalib
PJLabCalib
Teri4Calib
使用3D-3D点对应进行LiDAR相机标定
相机-LiDAR标定手册
ROS相机激光雷达标定包
基于全景基础设施的多摄像机和3D激光雷达标定
双激光雷达联合标定
T-FAC:多线激光雷达的无目标自动校准 [code]