Chen Shuguo, Song Qingjun, Ma Chaofei, Lin Mingsen, Liu Jianqiang, Hu Lianbo, Li Sicong, Xue Cheng
Opt Express. 2021 Apr 12;29(8):11712-11727. doi: 10.1364/OE.423108.
Accurate retrieval of biogeochemical components of the ocean at a global scale from space requires accurately calibrated top-of-atmosphere (TOA) radiance, which is usually achieved by deriving a vicarious gain coefficient (g-factor) through a process called system vicarious calibration (SVC). Currently, only two SVC sites, Marine Optical Buoy (MOBY) and BOUée pour l'acquiSition d'une Série Optique à Long termE (BOUSSOLE), are routinely operated to support the SVC process for all on-orbit ocean color satellite payloads. However, high-quality matchups between satellite observations and in situ measurements are rare because of the strict requirements of the SVC process. Meanwhile, a stable g-factor is usually computed by averaging sufficient gain measurements. Therefore, more SVC sites are required to derive a stable g-factor in a short duration, particularly for the initial calibration of newly launched satellite sensors. In this study, nearly twenty years of well-calibrated ocean color satellite data were used to calculate the mean and standard deviation of physical and optical properties of waters and the atmosphere in the South China Sea (SCS) to evaluate the feasibility of establishing a SVC site. A region was identified that meets all requirements that were used to evaluate the MOBY and BOUSSOLE sites. Two in situ measurements within this region were used to derive a g-factor for MODIS-Terra and MODIS-Aqua and were compared with the g-factor derived using MOBY data. The consistence of the two g-factors indicates that the identified region in the SCS could be a potential area for establishing a long-term moored SVC site.
要从太空在全球范围内准确反演海洋的生物地球化学组分，需要精确校准的大气层顶（TOA）辐射率，这通常是通过一个称为系统替代定标（SVC）的过程得出替代增益系数（g因子）来实现的。目前，仅有两个SVC站点，即海洋光学浮标（MOBY）和长期光学数据获取浮标（BOUSSOLE），在常规运行以支持所有在轨海洋颜色卫星有效载荷的SVC过程。然而，由于SVC过程的严格要求，卫星观测与现场测量之间高质量的匹配数据很少。同时，稳定的g因子通常是通过对足够数量的增益测量值进行平均来计算的。因此，需要更多的SVC站点在短时间内得出稳定的g因子，特别是对于新发射卫星传感器的初始定标。在本研究中，利用近二十年校准良好的海洋颜色卫星数据，计算了中国南海（SCS）水体和大气的物理及光学特性的均值和标准差，以评估建立一个SVC站点的可行性。确定了一个满足用于评估MOBY和BOUSSOLE站点的所有要求的区域。利用该区域内的两次现场测量得出了MODIS-Terra和MODIS-Aqua的g因子，并与利用MOBY数据得出的g因子进行了比较。两个g因子的一致性表明，在中国南海确定的区域可能是建立一个长期锚定SVC站点的潜在区域。
