Hu Chuanmin, Feng Lian, Lee Zhongping, Davis Curtiss O, Mannino Antonio, McClain Charles R, Franz Bryan A
University of South Florida, College of Marine Science, 140 Seventh Avenue, South, St. Petersburg, Florida 33701, USA.
Appl Opt. 2012 Sep 1;51(25):6045-62. doi: 10.1364/AO.51.006045.
Sensor design and mission planning for satellite ocean color measurements requires careful consideration of the signal dynamic range and sensitivity (specifically here signal-to-noise ratio or SNR) so that small changes of ocean properties (e.g., surface chlorophyll-a concentrations or Chl) can be quantified while most measurements are not saturated. Past and current sensors used different signal levels, formats, and conventions to specify these critical parameters, making it difficult to make cross-sensor comparisons or to establish standards for future sensor design. The goal of this study is to quantify these parameters under uniform conditions for widely used past and current sensors in order to provide a reference for the design of future ocean color radiometers. Using measurements from the Moderate Resolution Imaging Spectroradiometer onboard the Aqua satellite (MODISA) under various solar zenith angles (SZAs), typical (L(typical)) and maximum (L(max)) at-sensor radiances from the visible to the shortwave IR were determined. The L(typical) values at an SZA of 45° were used as constraints to calculate SNRs of 10 multiband sensors at the same L(typical) radiance input and 2 hyperspectral sensors at a similar radiance input. The calculations were based on clear-water scenes with an objective method of selecting pixels with minimal cross-pixel variations to assure target homogeneity. Among the widely used ocean color sensors that have routine global coverage, MODISA ocean bands (1 km) showed 2-4 times higher SNRs than the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) (1 km) and comparable SNRs to the Medium Resolution Imaging Spectrometer (MERIS)-RR (reduced resolution, 1.2 km), leading to different levels of precision in the retrieved Chl data product. MERIS-FR (full resolution, 300 m) showed SNRs lower than MODISA and MERIS-RR with the gain in spatial resolution. SNRs of all MODISA ocean bands and SeaWiFS bands (except the SeaWiFS near-IR bands) exceeded those from prelaunch sensor specifications after adjusting the input radiance to L(typical). The tabulated L(typical), L(max), and SNRs of the various multiband and hyperspectral sensors under the same or similar radiance input provide references to compare sensor performance in product precision and to help design future missions such as the Geostationary Coastal and Air Pollution Events (GEO-CAPE) mission and the Pre-Aerosol-Clouds-Ecosystems (PACE) mission currently being planned by the U.S. National Aeronautics and Space Administration (NASA).
用于卫星海洋颜色测量的传感器设计和任务规划需要仔细考虑信号动态范围和灵敏度（在此具体指信噪比或SNR），以便能够量化海洋属性的微小变化（例如，表层叶绿素a浓度或Chl），同时大多数测量不会饱和。过去和当前的传感器使用不同的信号水平、格式和惯例来指定这些关键参数，这使得进行跨传感器比较或为未来传感器设计建立标准变得困难。本研究的目标是在统一条件下对过去和当前广泛使用的传感器的这些参数进行量化，以便为未来海洋颜色辐射计的设计提供参考。利用搭载在Aqua卫星上的中分辨率成像光谱仪（MODISA）在不同太阳天顶角（SZA）下的测量数据，确定了从可见光到短波红外的典型（L(typical)）和最大（L(max)）传感器处辐亮度。以45°太阳天顶角下的L(typical)值作为约束条件，计算了10个多波段传感器在相同L(typical)辐亮度输入下以及2个高光谱传感器在类似辐亮度输入下的信噪比。计算基于清水场景，采用客观方法选择跨像素变化最小的像素以确保目标同质性。在具有常规全球覆盖的广泛使用的海洋颜色传感器中，MODISA海洋波段（1千米）的信噪比显示比宽视场海洋观测传感器（SeaWiFS，1千米）高2至4倍，与中分辨率成像光谱仪（MERIS）-RR（降低分辨率，1.2千米）相当，这导致在反演的Chl数据产品中具有不同的精度水平。MERIS-FR（全分辨率，300米）在空间分辨率提高的情况下，信噪比低于MODISA和MERIS-RR。在将输入辐亮度调整为L(typical)后，所有MODISA海洋波段和SeaWiFS波段（除SeaWiFS近红外波段外）的信噪比均超过了发射前传感器规格中的值。在相同或类似辐亮度输入下，各种多波段和高光谱传感器的L(typical)、L(max)和信噪比列表为比较产品精度中的传感器性能以及帮助设计未来任务（如美国国家航空航天局（NASA）目前正在规划的地球静止海岸和空气污染事件（GEO-CAPE）任务以及气溶胶-云-生态系统预研（PACE）任务）提供了参考。
