LiDAR sensors have the advantage of being able to generate high-resolution imaging quickly during both day and night; however, their performance is severely limited in adverse weather conditions such as snow, rain, and dense fog. Consequently, many researchers are actively working to overcome these limitations by applying sensor fusion with radar and optical cameras to LiDAR. While studies on the denoising of point clouds acquired by LiDAR in adverse weather have been conducted recently, the results are still insufficient for application to autonomous vehicles because of speed and accuracy performance limitations. Therefore, we propose a new intensity-based filter that differs from the existing distance-based filter, which limits the speed. The proposed method showed overwhelming performance advantages in terms of both speed and accuracy by removing only snow particles while leaving important environmental features. The intensity criteria for snow removal were derived based on an analysis of the properties of laser light and snow particles.

Although Light Detection and Ranging (LiDAR) is a sensor type for autonomous vehicles, it is recognized as an essential tool in various fields, such as drones, Unmanned Surface Vehicles (USVs), Unmanned Ground Vehicles (UGVs), and surveillance facilities. This is because LiDAR has advantages including a high resolution, 360-degree sensing capabilities, and consistent performance during the day and at night. However, LiDAR has the fatal disadvantage of performing poorly in snow, rain, and fog. Therefore, studies are attempting to remove noise points caused by snow, rain, and fog to ensure that LiDAR performs well even under extreme weather conditions. These studies began in approximately 2020, coinciding with accelerated research on autonomous driving, and they are still being actively pursued today. In particular, with the development of artificial intelligence technology in addition to the existing conventional methods, various approaches are being developed, but papers that thoroughly analyze and organize this research have not yet been published. Accordingly, in this study, we aim to comprehensively review the studies that have attempted to remove LiDAR sensor noise under extreme weather conditions through various methods. This review discusses the latest LiDAR denoising algorithms, with a particular focus on techniques implemented under snowy conditions. These algorithms can be categorized into five types: distance-based, intensity-based, fusion-based, learning-based, and other methods. In addition, this paper covers extreme weather datasets and related perception studies.

This paper proposes a follower-stabilizing strategy for leader-waypoint-follower nonholonomic vehicle formation control. The follower-stabilizing strategy consists of the follower-stabilizing area and the follower's velocity correction term. When the follower crosses the waypoint generated by the leader, its steering is susceptible to oscillation. To overcome this issue, the follower-stabilizing area around the waypoint is proposed such that the follower imitates the leader's heading angle as it enters the follower-stabilizing area. At the same time, the velocity correction term drives the follower to track the waypoint stably. The follower-stabilizing strategy is validated by various simulations, which confirm that the follower-stabilizing area effectively reduces steering oscillation and that the velocity correction term verifies stable waypoint tracking. Additionally, GPS and AHRS-based small-scale vehicles are built to demonstrate the proposed method by experiment. The CTRA model-based Kalman filter is designed for GPS and AHRS to estimate the agents' states, such as the heading angle and linear velocities.

Owing to the recent advances in the field of deep-learning-based approaches, state-of-the-art performance has been achieved for optical flow estimation. However, non-deep-learning-based improvement in the optical flow estimation performance is still required because many platforms, such as small UGVs and drones, involve constraints that make mounting GPUs difficult. Thus, in this study, we do not apply deep learning methods; rather, we improve the accuracy of the optical flow pipeline by optimizing only its fundamental parameters. The optical flow estimation performance is influenced by the number of coarse-to-fine estimation pyramids, the filter applied to each pyramid level, the window size, and the graduated nonconvexity (GNC) step number. No significant differences from the previous research were achieved by optimizing the parameters heuristically because they have already been optimized. Therefore, we decided to change the filter applied in each pyramid, which is the most important factor in determining the optical flow estimation performance. As a result of verification, the optical flow with the median filter did not show good optical flow estimation performance due to oversmoothing of the image boundary, and the optical flow with the weighted median filter proposed to overcome this drawback could not well address the complexity of the equation and deal with the large computation cost. The proposed Hampel filter shows better performance by minimizing the loss of the original image in the image smoothing process and reduced computational complexity compared to the weighted median filter. The principle of the Hampel filter is similar to that of the median filter, except that if the reference pixel is statistically close to the pixel median in the window, it works by preserving the original pixel value. This is usually used for filtering outliers in a 1D signal but has never been applied for 2D image filters. This study expanded the application of the existing Hampel filter to include 2D images. Compared to the conventional median filter, this filter demonstrates relatively good performance, as it can considerably reduce the loss in the image data and improve the excessive edge smoothing effect such that it can better identify motion. The performance-related improvements achieved by this research were verified using the KITTI Vision Benchmark Suite, and the simulation results demonstrate that the Hampel filter can replace the existing filter and that it has strengths related to multi-object detection, background recognition, and the identification of moving objects at the edges of images.

Studies on autonomous driving have started to focus on snowy environments, and studies to acquire data and remove noise and pixels caused by snowfall in such environments are in progress. However, research to determine the necessary weather information for the control of unmanned platforms by sensing the degree of snowfall in real time has not yet been conducted. Therefore, in this study, we attempted to determine snowfall information for autonomous driving control in snowy weather conditions. To this end, snowfall data were acquired by LiDAR sensors in various snowy areas in South Korea, Sweden, and Denmark. Snow, which was extracted using a snow removal filter (the LIOR filter that we previously developed), was newly classified and defined based on the extracted number of snow particles, the actual snowfall total, and the weather forecast at the time. Finally, we developed an algorithm that extracts only snow in real time and then provides snowfall information to an autonomous driving system. This algorithm is expected to have a similar effect to that of actual controllers in promoting driving safety in real-time weather conditions.