Improved Radiometric and Spatial Capabilities of the Coastal Zone Imager Onboard Chinese HY-1C Satellite for Inland Lakes

The coastal zone imager (CZI) onboard HY-1C satellite provides a new data source to monitor the lake environments. Here, we provided a preliminary evaluation for the applications of CZI on inland lakes and a comparison with the <italic>in situ</italic>, Landsat-8 operational land imager (OLI), and Sentinel-2 multispectral instrument (MSI) measurements. First, the in-orbit signal-to-noise ratios (SNRs) were estimated based on homogenous ocean pixels. SNRs of CZI reached ~200:1 in the visible bands and ~150:1 at the near-infrared bands, which are comparable with the OLI and slightly higher than those of the MSI. Then, the performance of 6SV and fast line-of-sight atmospheric analysis of hypercubes (FLAASH) models on the retrievals of remote sensing reflectance (<inline-formula> <tex-math notation="LaTeX">$R_{\mathrm {rs}}$ </tex-math></inline-formula>) from CZI measurements was evaluated. 6SV-derived <inline-formula> <tex-math notation="LaTeX">$R_{\mathrm {rs}}$ </tex-math></inline-formula> showed higher accuracy than that of FLAASH, validated by the synchronous <italic>in situ</italic> (<inline-formula> <tex-math notation="LaTeX">$R^{2} \simeq 0.50$ </tex-math></inline-formula>, absolute percent difference (APD) <inline-formula> <tex-math notation="LaTeX">$\simeq ~20$ </tex-math></inline-formula>%) and OLI-derived <inline-formula> <tex-math notation="LaTeX">$R_{\mathrm {rs}}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$R^{2} \simeq ~0.75$ </tex-math></inline-formula>, APD <inline-formula> <tex-math notation="LaTeX">$\simeq ~5$ </tex-math></inline-formula>%). Finally, the abilities of CZI to observe cyanobacterial bloom, suspended particular matter (SPM), and chlorophyll-<inline-formula> <tex-math notation="LaTeX">$a$ </tex-math></inline-formula> (Chl<inline-formula> <tex-math notation="LaTeX">$a$ </tex-math></inline-formula>) were assessed. CZI-derived, the area of cyanobacterial bloom and SPM, showed good agreements with the results yielded by the OLI and MSI data on May 24, 2019, in Lake Taihu (<inline-formula> <tex-math notation="LaTeX">$R^{2} = 0.68$ </tex-math></inline-formula>, root-mean-square error (RMSE) = 9.68 mg/L, APD = 13.52% for SPM). While CZI only has four wide bands, Chl<inline-formula> <tex-math notation="LaTeX">$a$ </tex-math></inline-formula> derived by CZI using an empirical algorithm was relatively consistent with MSI-derived values. CZI demonstrated a decent performance in monitoring the environments of large lakes, and it is expected to add the bands for atmospheric correction and further works in the small-medium lakes in the future.

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