Chlorophyll-a (Chl-a) mass concentration is a primary indicator for water color retrieval. In the visible wavelength band, compared to the contribution of atmospheric radiance, the contribution of water radiance constitutes only a small fraction of the total radiance received by the remote sensing sensor at the top of the atmosphere. Therefore, there is a high demand for the radiance detection accuracy of the remote sensors, which requires accurate on-orbit radiometric calibration before the data can be applied. The large field-of-view remote sensor, the directional polarimetric camera (DPC), is not designed with an on-board calibration and cross-calibration system. With the development of remote sensing technology, site calibration has become a common method for alternative calibration of satellite remote sensors in orbit. We use the ocean scene to carry out the on-orbit calibration of the visible band and verify the water color remote sensing products before and after the calibration. This proves that Rayleigh scattering calibration based on the ocean scene can improve the authenticity and accuracy of the water color remote sensing products of the DPC.

Guided by this concept, to expand the application scope of the Carbon Monitoring in Terrestrial Ecosystems Satellite (TECIS)’s DPC, we initially perform Rayleigh scattering calibration on the DPC for ocean scenes from the CM Satellite, screening according to surface atmospheric conditions to obtain calibration sample points. Subsequently, we estimate the top-of-atmosphere apparent reflectance of the DPC in the blue and green bands, comparing it with the measured reflectance of the DPC to derive the radiometric calibration factor A. Finally, we validate the accuracy of the radiometric calibration. Then, combining the MODIS aerosol optical thickness product with the look-up table method, we estimate the atmospheric range radiation for atmospheric correction. The DPC bands at 443, 490, and 565 nm, pre- and post-radiometric calibration, are used as inputs to the Chl-a retrieval algorithm. The inversion region is selected from data detected in the northwest Australia ocean region on August 22, 2022, with authenticity testing conducted using Moderate Resolution Spectroradiometer (MODIS) water color retrieval data from the same region and date.

We obtain results from Rayleigh scattering calibration, atmospheric correction, and validation of retrieved Chl-a mass concentrations: 1) At ocean calibration sites, we verify the accuracy of radiometric calibration in the blue-green bands of the DPC (443, 490, 565, 670 nm bands). Results indicate that the radiometric calibration factors for each band are close to 1, with correlation coefficients (R²) above 0.9, root-mean-square errors (RMSE) below 2%, and mean absolute errors (MAE) below 0.02, suggesting minimal dispersion in these calibration results. Rayleigh scattering calibrations are validated using desert and polar calibrations, with radiative calibration factors for the ocean scene deviating about 3% from those for snow/ice and desert scenarios (Figs. 3-4 and Tables 6-8). 2) Utilizing the 6SV atmospheric radiative transfer model and combining it with the MODIS aerosol optical thickness product along with other observational and atmospheric environmental parameters, we performed atmospheric corrections for the study area. Results show that the corrected surface reflectance is generally lower than the apparent reflectance before correction, with the Surface Reflectance Product MOD09GA verifying relatively with an error of less than 25% in each band (Fig. 5 and Table 9). 3) Employing the OC3 Chl-a retrieval algorithm, we test the authenticity of water color remote sensing products before and after calibration. Results reveal that the R² for Chl-a mass concentrations measured by the post-calibration DPC is higher at 0.7720, with a lower RMSE at 0.0578 and MAE at 0.0457 (Fig. 6 and Table 10).

In this study, we establish a water color retrieval algorithm suitable for the DPC. To enhance the retrieval accuracy of water color remote sensing products, we employ a Rayleigh calibration method combining multiple ocean scenes for rapid on-orbit calibration tests in the visible wavelength band before applying water color products. We also conduct authenticity tests on water color remote sensing products pre- and post-adjustment of calibration coefficients. The consistency between calibration results and measurements is good, demonstrating minimal dispersion in calibration outcomes, thereby affirming the effectiveness and reliability of this calibration method. We also use MODIS aerosol optical thickness products and look-up tables to carry out atmospheric correction, and the results show that the atmospheric correction results are good, basically eliminating the influence of the atmosphere. Finally, we perform water color product retrieval and authenticity tests on water color remote sensing products pre- and post-calibration coefficient adjustment, confirming that Rayleigh scattering calibration based on ocean scenes significantly improves the authenticity and accuracy of DPC water color remote sensing products. Test outcomes substantiate a notable rise in the correlation between Chl-a mass concentrations measured by the post-calibration DPC and MODIS data.