## 5 Discussion  
The evaluation and correction of the CZI OOB effect in this paper was performed by adopting the approach proposed by Reference [14], whose effort was dedicated for the visible bands of major satellite ocean color sensors of SeaWiFS (Sea-Viewing Wide Field-of-View Sensor), MODIS, and MERIS) characterized by a narrow band width. They found that for turbid coastal and inland waters, OOB responses of the above three sensors were insignificant, which is consistent with our CZI results for turbid Case 2 waters. For the Case 1 waters, they found obvious OOB response for SeaWiFS in 555 nm, VIIRS in 551 nm, and MODIS in 412 nm, whereas our results indicate significant OOB response in the CZI green band. The differences between our and their results can be attributed to the spectral response function characteristics of various sensors. It is also noted that in order to effectively evaluate the OOB effect on the ocean optical signal and in-water retrieval, our OOB analysis was based on the physical quantity of remote sensing reflectance, which was the sensor-observed signal just above the sea surface without atmosphere interference (as [PERSON] [14] did), rather than the top-of-atmosphere (TOA) radiance which is measured at the satellite altitude and dominated by the atmosphere contribution.  
The sensor's OOB response may have had a negative impact on the quantitative ocean color retrieval, which needs careful consideration. For example, for the Chla retrieval, which is usually based on the blue-green band ratio algorithm [24], the algorithm accuracy was proportional to the difference between the uncertainties of the remote sensing reflectance in the blue and green bands [25]. In addition, a 3% OOB response would lead to the positive bias in the derived Chla for the Case 1 water [26]. Our results indicate that the CZI green band had significant OOB response in the oligotrophic Case 1 water, and thus the OOB correction was highly recommended for the quantitative ocean color retrieval, especially for the Chla estimation.  
Figure 6: Estimation of the OOB correction factor for the CZI green band in Case 1 water from band ratios of CZI remote sensing reflectance.  
Different from the dedicated ocean color sensors for open ocean monitoring, which are characterized by multiple bands, narrow band width (typically 10-20 nm), and low spatial resolution (~1 km), the CZI onboard HY-1C was designed for coastal zone monitoring with a major focus on land monitoring. This kind of sensor is characterized by the limited number of bands, wide band width, and relatively high spatial resolution (compared with ocean color data). Importantly, in the near future we will see more similar sensors in orbit. In 2019, satellite HY-1D will be launched, carrying the same CZI as that onboard HY-1C. After 2020, HY-1E and HY-1F satellites will send another two CZIs into space, whose spatial resolution will be increased (from 50 m) to ~5 m. From these satellite images, quantitative ocean information with fine spatial structure (at meter scale) can be expected. In order to achieve this goal, the comprehensive prelaunch optical characterization, especially the OOB evaluation, is highly important for developing quantitative ocean products, as demonstrated here.