# Airborne GNSS Reflectometry for Water Body Detection
## 6 Data Analysis
### Water Body Edge Localization  
The proposed radar technique is applied for localizing the edges of the detected water body surfaces along the satellites footprints traces. Google Earth does not provide up-to-date map images. In this regard, the Google Earth images and the experimentation were obtained with one year of difference (Google Earth dates its imagery data to September 2019). We select in our study the water body surfaces that can be clearly observed using both IGN maps and Google Earth. We can assume that the water level was the same between the 2 dates. We assess the accuracy of the proposed automatic edge localization technique using manual edge localization on Google Earth. We report in Table 3 the parameters of the detected water body surfaces by satellite PRN 5 for 100 s of an area along the trajectory. We report the approximate size (in meters) for each of the mentioned surfaces. We also report the difference in localization between the automatic and manual approach (in meters) for the starting and ending edges of the detected water body surfaces. These values can be positive or negative. In addition, we record the total localization distance error (in meters), i.e., the total absolute offset for the start and end edges of each water body as well as the automatic and manual detection length (in meters), i.e., the length covered by the satellite footprints trace along the water body surface using the proposed radar technique and Google Earth, respectively. We observe in Table 3, perfect edge localization (i.e., zero total distance error) for the lake by the proposed radar technique as compared to Google Earth. The localization of the river edges are less accurate especially for the start of the river but clearly better than that of wetland and stream. In this regard, edge localization requires precise measurements. The localization accuracy is affected by the characteristics of the water body surface (type, shape, size, etc. ), as well as the nature of the landforms surrounding the water body at its edges. We can clearly observe from the Google Earth images that the stream reported in Table 3 is small in size ( 4.56 m width), muddy, and surrounded by vegetation. It is worth noting that this stream reported the lowest localization accuracy among all the water body surfaces localized along the traces of the three satellites footprints during the whole trajectory. Other streams are localized more accurately. We apply the aforementioned quantitative analysis for the detected water body surfaces by the traces of the three satellites' footprints along the whole trajectory. We report in Table 4 the number of detections of each water body type as well as the percentage of perfect edge localizations (i.e., whenever the total localization distance error is 0). We also record the mean distance localization error (in meters) which is the absolute value of the offset in meters between the manual and the automatic edge localization. Finally, we record the localization difference standard deviation (in meters) which represents the standard deviation of the difference in the starting and ending edges. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Water Body** & **Wetland** & **Lake** & **River** & **Stream** \\ \hline Approximate size (m) & \(40\times 48\) & \(125\times 141\) & \(W=22\) & \(W=4.56\) \\ \hline Localization diff—Start (m) & \(+3.45\) & 0 & \(+0.82\) & \(+4.9\) \\ \hline Localization diff—End (m) & 0 & 0 & 0 & \(+1.47\) \\ \hline Distance error—Total (m) & 3.45 & 0 & 0.82 & 6.37 \\ \hline Automatic detection length (m) & 52.21 & 171.3 & 24.57 & 10.93 \\ \hline Manual detection length (m) & 48.76 & 171.3 & 23.75 & 4.56 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters of the detected water body surfaces by satellite PRN 5 trace for 100 s corresponding to an area located between ( 50.888837\({}^{\circ}\)N, 1.869701\({}^{\circ}\)E) and ( 50.887512\({}^{\circ}\)N, 1.880296\({}^{\circ}\)E) along the trajectory.