-
文中选择443 nm、486.1 nm与532 nm三个典型激光波长对激光雷达回波信号进行对比分析。三个典型波长中,443 nm是水色遥感的常用典型波长,在最清洁的大洋水中穿透力较好;486.1 nm波长在大洋水中具有较好的穿透能力,而且该波长对应一条太阳夫朗和费暗线,可以有效降低太阳背景光对激光雷达探测性能的影响;532 nm是激光技术最成熟的蓝绿波长,在沿岸海水比蓝光有更好的穿透能力。
使用2009~2019年的Argo全球海域叶绿素a剖面数据,将数据进行质量控制与经纬度1°×1°的空间网格化处理,得到全球海域1 m垂直分辨率的叶绿素a浓度剖面。虽然Argo叶绿素a数据不足以覆盖全部海域,但仍可以选择典型海域进行统计分析。根据连续10年的全球Argo叶绿素a浓度剖面数据得到的地中海(4°W~42°E, 32°N~46°N,共8 824个剖面)、印度洋(65°E~110°E, 30°S~10°N,共3 417个剖面)、南大洋(180°W~180°E, 80°S~40°S,共15 838个剖面)与太平洋(170°E~120°W, 30°S~40°N,共2 000个剖面)四个海区的平均叶绿素a浓度剖面如 图7所示。可以看出:南大洋海域平均叶绿素a峰值浓度较高、峰值深度较浅,太平洋峰值深度最深、峰值浓度较小。
根据 表1所列的激光雷达系统参数以及所需的大气光学参数(1976年美国标准大气模型)、水体光学参数(由叶绿素a剖面计算得到)等,通过公式(7)所示的激光雷达方程计算得到无背景光情况下的1 s累加回波光子数,以1个回波光子作为阈值确定单光子探测深度, 图8为地中海(a)、印度洋(b)、南大洋(c)、太平洋(d)四个海区在三个典型波长下的激光雷达回波光子数剖面,横向虚线为各个波长的单光子探测深度,0 m处尖峰为模拟的海面强反射信号。
Input parameters Value Laser wavelength/nm 443, 486.1, 532 Repetition rates/Hz 20 Pulse energy/mJ 200 Pulse width/ns 10 Laser linewidth/nm 0.1 Laser divergence/mrad 0.2 Telescope diameter/m 1.2 Field of view/mrad 0.3 Receiving spectrum width/nm 0.2 Optical efficiency 0.6 Orbital height/km 550 Solar spectral irradiance/W·m −2·um −1 205 Sea surface wind speed/m·s −1 5 Solar altitude angle/(°) 60 Range resolution/m 1 Table 1. Parameters for simulation of spaceborne oceanographic lidar
Figure 8. Photon number of lidar echo at different wavelengths in four sea areas (the transverse dotted line is the detection depth corresponding to single photon echo)
由于太平洋海域水体最清澈,叶绿素a浓度较小且峰值深度较深,回波信号衰减较慢,探测深度最深,486.1 nm与443 nm探测深度约为120 m,532 nm探测深度较浅,约为80 m;地中海和印度洋叶绿素a浓度高于太平洋且峰值深度较浅,两海域探测深度结果类似,486.1 nm与443 nm探测深度约为100 m,532 nm探测深度约为75 m;南大洋叶绿素a浓度较大且峰值深度较浅,回波信号衰减较快,各个波长的探测能力都相对较弱,探测深度约为65 m,同时也能看出在较为浑浊的海域,532 nm的信号与其他两个波长更加接近,486.1 nm比443 nm衰减的更慢,探测深度最深。图中0 m处尖峰为模拟的海面强反射信号。
在上述回波光子数曲线中加入背景光子数,模拟经采集后得到的光子计数值,并计算信噪比。以太平洋海域为例的光子计数值剖面与信噪比剖面如 图9所示。
从 图9中可以看出532 nm波长采集光子计数值、信噪比都低于486.1 nm、443 nm波长。若以SNR = 2作为最大探测深度判据,486.1 nm、443 nm约能达到90~100 m深度,532 nm只能探测到60 m。
Estimation of chlorophyll profile detection capability of spaceborne oceanographic lidar
doi: 10.3788/IRLA20200164
- Received Date: 2020-10-15
- Rev Recd Date: 2020-11-20
- Available Online: 2021-02-07
- Publish Date: 2021-02-07
-
Key words:
- spaceborne oceanographic lidar /
- simulation system /
- Chl-a /
- detection depth
Abstract: In order to evaluate and analyze performance of spaceborne oceanographic lidar for global ocean optical properties detection, a simulation system for spaceborne oceanographic lidar was developed based on lidar equation and the results of Monte Carlo simulation model. The lidar simulation system consisted of three modules, forward simulation, data inversion and error analysis, which could simulate the whole process of laser emission, transmission and detection. According to the given lidar parameters, the detection signals of 443 nm, 486.1 nm and 532 nm in four typical areas, Mediterranean Sea, Indian Ocean, Southern Ocean and Pacific Ocean, were simulated. The results show that the detection depths of 443 nm and 486 nm are approximately the same and deeper than that of 532 nm. For the given lidar parameters, the detection depths of 486.1 nm wavelength in the Pacific Ocean and the Southern Ocean are 120 m and 70 m, respectively, and the detection depth in the Mediterranean Sea and the Indian Ocean is about 100 m. The detection depths of chlorophyll-a concentration in the above sea areas are about 80 m, 50 m and 70 m, respectively.