# Envisat RA-2 Individual Echoes: A Unique Dataset for a Better Understanding of Inland Water
## 4 Selected Case Studies
### Prek Tool Fra  
Prek Tool (13.1\({}^{\circ}\)N, 103.6\({}^{\circ}\)E) is on the NW corner of the famous Cambodian lake, Tonle Sap. The area undergoes an annual inundation cycle as the lake rises ~7 m from May to peak flooding in November. There are seven IE recorded passes, spanning May-December 2007, recording water rise from minimum to maximum. There are also three published works [22; 23; 24] with extensive in situ, MODIS, and InSAR observations in exactly the same area, and in the same time period. [PERSON] et al. [22] include a graph of 18-Hz Envisat RA-2 water levels (see their Figure 5). They attribute this data coming from EAPRS Laboratory and [PERSON] (personal communication). The quoted measurements corresponding to our seven IE data records are reported in Table 2.  
The IEs behavior shows how the specular features (easily identified by the echo power) are in accordance with the water levels from Table 2. In general, satellite passes show a nearly continuous quasi-specular and flat surface level, but echo amplitudes are very different. In the May, July, and August data, the peak echo power is in the 50-70 dB range, and the peak in MSC is <0.5. There are specular echoes, but intermittent and unremarkable. In the subsequent four passes, September, October, November, and December, the peak echo power increases by 30 dB, and the peak in MSC is >0.8. According to [PERSON] et al. [22], which refers to Benveniste, water level rose from the 6.7 m to 9.1 m level between August and September. That rise appears to have inundated the entire area covered by the IE tracks. Figure 19 (upper panel) is the radargram related to the pass on September 12, which shows an almost continuous 80 dB echo strength at nadir. The strongest echo is indicated by the dashed vertical line. The waveform for this echo is plotted on the bottom panel of Figure 19. This is the \begin{table}
\begin{tabular}{c c} \hline \hline
**Date** & **Tonle Sap Level (m) (Ref. ESA)** \\ \hline
30 May 2007 & 3.8 \\
4 July 2007 & 5.1 \\
8 August 2007 & 6.7 \\
12 September 2007 & 9.1 \\
17 October 2007 & 10.4 \\
21 November 2007 & 10.3 \\
26 December 2007 & 7.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Tonle Sap levels (m) in [PERSON] et al. [22] corresponding to the seven IE passages available for this study. Figure 18: Doppler velocity from River Semiliki; dashed line is the theoretical Doppler. Different colors are used to distinguish between the different satellite passages. waveform corresponding to the strongest IE over the 128 range bins (here offset by 64). The intent is to show the narrow range of the specular echoes, with peaks of 80 and 100 dB over a background of about 50 dB. The coherent burst integration is 20 dB greater than incoherent integration, proving benefit of coherent integration, applicable in the case of high coherence between IEs. Three variants of the waveform are shown. The waveform of an individual echo peaks at 80 dB with a noisy floor at off nadir ranges. The incoherent burst integrated power is also 80 dB. In the range bins before the first contact the noise floor level is about 50 dB. The coherent burst integration peak is 100 dB, a 20 dB gain from coherent integration. All these results are consistent and expected for specular echoes. The two-bin range estimation method (described in Section 3.2.3) was applied to the entire length, using coherent bursts of 100 echoes. For each range estimate a corresponding coherence was calculated by Equation (8). A threshold of 0.7 was set on the MSC, in order to select the best measurements and reject noisier measurements. Figure 20 shows the water level (relative to an arbitrary reference level) for the four final dates. The gaps are locations where coherence was below threshold. There is a consistently repeated 1 cm/km trend over the four dates. It was suggested that the trend reflects a geoidal height variation ([PERSON], private communication), estimating a down-slope 3 cm/km based on the EGM2008 and EGM1996 geoidal deformation models.