# Data-Free Area Detection and Evaluation for Marine Satellite Data Products
## 4 Discussion
### Spatial and Temporal Distribution of Marine Coverage Objects Contained in Data-Free Areas  
The month-to-month variations of ocean objects in the data-free areas of each sensor have interesting trends (Figure 13). The missing amount of clean water is positively related to the missing amount of clouds, whereas the missing amount of turbid water has a significant inverse relationship. The missing amount of ocean coverage objects other than clouds in the data-free area of OLCI is much smaller than those of GOCI and MODIS. The number of missing elements does not vary much per month and tends to be stable. This phenomenon can be explained by the spatial and temporal distribution characteristics of typical marine coverage objects. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Factor** & **GOCI** & **MODIS** & **OLCI** \\ \hline Standard & 3.15 & 3.05 & 2.12 \\ Deviation (\%) & & & \\ Information Entropy (bit) & 15.51 & 13.93 & 10.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Spatiotemporal continuity indicators. A typical altocumulus translucidus (Figure 14) is also known as a fish-scale cloud. It is a large-range regular cloud with large gaps between clouds, and the clouds have a fish scale-like arrangement [64]. As light can pass through the gaps, optical remote sensing also collects the remote sensing information of the sea surface corresponding to this gap. Such information is missing in GOCI and MODIS at this location (Figure 14b,e, respectively). The reason is that, depending on the ability of the selected cloud detection method, the normal ocean color information may be removed as a mask for cloud pixels [2, 42]. A large amount of clean water information is lost. The normal ocean color information loss phenomenon occurs not only in the gaps between clouds but also at the edges of clouds. The irregular state appears as a missing internal fragment shape and a missing shape of edge patches. Usually, the edges of clouds tend to lose more ocean color information of cloud-free image pixels compared with the gaps between fish-scale clouds. By contrast, OLCI only misses the blocky cloud regions, and no anomalous missing information is observed. The normal ocean color information around the area is preserved, and the data-free area of OLCI manifests a fish-scale shape at this location (Figure 14h). High outbreaks of green tide algae occur every summer, particularly in June and July (Figure 10c). The Yellow Sea is one of the areas of such an outbreak, and the phenomenon mainly occurs in the northward direction of the shallow shoals of the northern Jiangsu Province and the northeastern direction of the Jiaodong Peninsula. However, an analysis of GOCI and MODIS indicates that information is missing for the areas with high algal bloom (Figure 15). The performance status of green algae in the data-free areas is different for the three products (Figure 15). Initially, the green tide algae in the ocean appear as small patches that float sporadically. Then, under the action of surface currents and summer monsoons, they continue to drift and grow. Finally, they gather into \"stripes\" and large patches corresponding to a green tide disaster [65]. The green tide algae in the data-free areas of GOCI and MODIS not only have spatial distributions appearing as circular spots and strips but also a buffer-like shape with missing pixels around them (Figure 15b,e, respectively). Similar to the missing cloud-free pixels around clouds, a missing algae-free pixel is apparent around green tide algae. The data-free areas containing green tide algae have spatially larger circular spots or appear to be connected with thick-striped distributions. As opposed to the missing information in GOCI and MODIS, the missing information in OLCI is only in the central area of the green tide algae with a high algal concentration and high phytoplankton contribution, and the missing pixels are scattered (i.e., not aggregated). Thus, it shows a sporadic distribution in space (Figure 15h).