A simple and effective method for removing residual reflected skylight in above-water remote sensing reflectance measurements

摘要

In situ remote sensing reflectance (R-rs) measured using the above-water approach usually suffers from residual reflected skylight (Delta), and thus its applicability is limited. In this study, we proposed a simple method, which only required the in situ R-rs spectrum itself as input, to further remove the Delta effects in the in situ R-rs spectrum. The performance of the proposed method was evaluated using both simulation data and in situ R-rs spectra measured using a radiance sensor with a black tube (Delta-free in situ R-rs). The results showed that the proposed method outperformed other existing methods and can be applied to various types of waters. For example, from the two simulation experiments, it was seen that the proposed method can provide accurate R-rs spectra with a mean absolute percentage error (MAPE) = 6% even if they are measured under various sky conditions and wind speeds (using Hydrolight) and can estimate accurate Delta values with MAPE = 5% (using a simple bio-optical model). From a comparison analysis using Delta-free in situ R-rs data, the proposed method also showed the best performance with the smallest MAPE of 45% (MAPE ranged from 52% to 260% for other existing methods). In addition, the proposed method can improve the quality of R-rs spectra collected under various sky conditions (clear, scattered clouds and overcast), with an increased proportion of higher quality assurance scores. Two showcases indicate that the Delta corrections should be carried out for all above water approach-measured R-rs spectra before they are applied to develop water quality estimation algorithms or to evaluate the performances of atmospheric correction algorithms.