偏振激光雷达探测大气—水体光学参数廓线

激光雷达在上层水体垂直廓线的遥感中展现出巨大优势。本文研制了一套高垂直分辨率的实时探测偏振激光雷达,提出了一种基于偏振激光雷达回波信号的反演算法,采用Fernald理论和多次散射原理反演非均匀大气—水体的衰减和退偏光学产品,以高效稳定地处理偏振激光雷达实验数据。展示了一个中国内陆水体激光雷达探测实例,观测到了两次气溶胶积聚现象和一次水体浑浊现象。对实验数据的分析表明,退偏比主要由前向多次散射和后向单次散射产生的退偏两部分组成。当多次散射强度较大时,退偏比的变化主要取决于多次前向散射退偏;反之,则主要依赖于单次后向散射退偏。

Detecting atmospheric-water optical property profiles with a polarized lidar

The ocean covers more than 71% of the Earth. Studies on vast oceans are of great significance for resource utilization and climate change. Several methods have been employed to detect the interior of the ocean. In-situ methods can accurately obtain marine information but their efficiency is limited. Ocean color remote sensing can collect global data. However, the limited information about the depth and dependence on natural light restrict its applications. Acoustics are widely used for seawater profiling, but they can only work under water due to the high loss in air-water interface. LiDAR is an effective method used to deal with seawater profiling with few limitations of platforms and natural light, in which its applicable coverage is from the water surface to the depth with several tens of meters.
A polarized lidar with high vertical resolution (approximately 0.225 m in the water and 0.3 m in the atmosphere) and real-time detecting capability was developed to detect the profile information of atmospheric-water particulates. The laser was linearly-polarized to obtain the polarized information about the water column. Two refractive telescopes were used to collect the backscattering light from the water. Two polarizers were set in front of the telescopes, which only transmitted return signals that were co-polarized or cross-polarized with the laser. A retrieval algorithm based on polarized lidar returns was proposed to obtain atmospheric-water attenuation and depolarized optical products, such as, extinction coefficient (atmosphere), diffuse attenuation coefficient (water), depolarized ratio (atmosphere and water), forward depolarized coefficient (water), and backward depolarized ratio (water).
An experimental sample from the inland water in Xiakou Reservoir, China during the night between April 6 and 7, 2017 was presented to discuss the physical meanings and scientific values of the optical products. The retrieved optical products presented the variation of atmospheric aerosols and water turbidity during the night. For the clear atmosphere, the depolarized ratio was approximately 0.01 and the extinction coefficient was approximately 0.08 m^-1. However, they became 0.06 and 0.02 m^-1, respectively, when the atmosphere was interrupted by several aerosols. For the water column, the diffuse attenuation coefficient varied from 0.6 m^-1 to 0.4 m^-1, depolarized ratio was from 0.6 to 0.4, and forward depolarized coefficient was from 0.06 m^-1 to 0.02 m^-1 when water turbidity became constantly clear after the rainfall. The analysis showed that depolarized ratio can be divided into depolarizations caused by forward multiple scattering and backward single scattering. Diffuse attenuation coefficient can be employed to describe the intensity of multiple scattering and is related to the depolarization caused by forward multiple scattering. Furthermore, depolarized ratio depends on the depolarization caused by forward multiple scattering when multiple scattering is strong. Otherwise, depolarized ratio depends on the depolarization caused by backscattering.
A polarized lidar was developed to detect atmospheric-water particulate profile information. A retrieval algorithm based on polarized lidar returns was proposed to obtain atmospheric-water attenuation and depolarized optical products. An experimental sample from the inland water in Xiakou Reservoir, China was presented. High-range-resolution optical products were obtained, and the variations of aerosols and water were observed. The analysis presented that depolarized ratio can be divided into depolarizations caused by forward multiple scattering and backward single scattering. Furthermore, depolarized ratio depends on the depolarization caused by forward multiple scattering when multiple scattering is strong. Otherwise, depolarized ratio depends on the depolarization caused by backscattering.