# Potential of a SAR Small-Satellite Constellation for Rapid Monitoring of Flood Extent
## 4 Discussion  
In both cases 1 and 2, the observed flood extent in some instances was insufficient relative to the flood extent at each time. To investigate this issue, Figure 6 presents the captured flood extent with the constellation of \(T=56\), \(P=28\) in case 1. The red zone represents the observed extent, whose swath width is 30 km. Here, it is clear that the observed area did not always match the flood area--for example, at 5:29, 7:16, and 9:20--and this discrepancy caused insufficient data acquisition. This is generally attributable to two factors. The first factor is the suitability of satellite passes: satellites do not always pass over an appropriate location with respect to each area of interest, so they are sometimes unable to capture it well. This is related to the second factor, which is the limited range of SAR's incidence angles. The look direction of a SAR instrument cannot be adjusted in all ranges, and this restriction prevents flexible operation according to the target area. However, some discrepancy seems inevitable, particularly for satellite observations collected using onboard SAR. Satellite passes are a predefined condition in their orbit and cannot be easily changed according to the area of interest. About the incidence angle, in  
Figure 6: Observed flood area (the matched area of the red zone and the yellow–green–blue area) at 5:29, 6:00, 7:16, 7:47, and 9:20 on 13 July 2012 in the case of \(T=56\), \(P=28\). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline  & **T1** & **T8** & **T9** & **T12** & **T12** & **T20** & **T24** & **T28** & **T28** & **T40** & **T40** & **T56** & **T56** \\  & **(Large Sat.)** & **P4** & **P2** & **P6** & **P1** & **P10** & **P5** & **P12** & **P8** & **P4** & **P7** & **P20** & **P10** & **P8** & **P28** \\ \hline Case 1 & 0.00 & 0.50 & 0.39 & 0.55 & 0.42 & 0.62 & 0.54 & 0.63 & 0.61 & 0.62 & 0.70 & 0.78 & 0.55 & 0.76 & 0.81 \\ \hline Case 2 & 0.16 & 0.48 & 0.51 & 0.63 & 0.54 & 0.87 & 0.74 & 0.91 & 0.80 & 0.85 & 0.89 & 0.94 & 0.75 & 0.96 & 0.92 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The performance index of the capture ratio in case 1 and 2.  
addition to its physical restriction, the range of angles is also bounded by the accuracy of flood detection [23]. That is, an incidence angle that is too large or too small would disturb the interpretation of SAR images, resulting in reduced flood detection accuracy. Therefore, the beam angle should be maintained within a specific range to guarantee SAR observation data quality. One possible way to mitigate the technical challenges of SAR systems would be to expand the swath width. Currently, small satellites can only be equipped with antennae that are smaller than those used for large satellites, making it more difficult to achieve wider swaths. However, technological innovations are expected to overcome this issue: small satellites can go through more development cycles than larger, traditional satellites, and they present more opportunities for harnessing new technologies [12]. Another way to mitigate the technical challenges should be implemented in the operational phase, regardless of technological innovations. As shown in Figures 4 and 5, even low levels of acquisition can sometimes contribute to improving the extent of observed damage--for instance, at 5:54 in T40P20 of case 1 and at 17:31 in T24P8 of case 2. This implies that a suitable location of the observation target area relative to both the present situation and past observations is critical for better situational awareness, even if it is bounded by the restrictions mentioned above. Furthermore, this suggests that careful selection of the target area should be implemented by taking into account possible future flood situations, because the flood extent at each time does not appear prior to observation.