# Evaluation of Coastal Sea Level Offshore Hong Kong from Jason-2 Altimetry
## 6 Discussion  
There is a long-standing debate in the coastal altimetry community concerning the choice of the best retracking algorithms and still today, no consensus has emerged. From the results of this study, we consider that regarding the complexity of the corresponding radar echoes associated with the variety of existing coastal land surfaces, the choice of the retracker should be specific to the coastal area considered. We have however to keep in mind that this option does not suit for regional or global applications because we need homogeneous data sets, which would be difficult to obtain if one switches from one retracker to another as a function of the oceanic region considered. This would certainly produce bias in the estimated sea level.  
Although the results obtained in this study with ALES in the HK coastal area are already encouraging, we believe that they could be further improved. For example, one of the most useful advances in altimeter waveform processing is the Singular Value Decomposition (SVD) approach proposed by [PERSON] (2009). The idea of SVD is to eliminate the waveform components which are relative to the minor singular value, since those components probably arise from system noises rather than real ocean signals. Near the coast, the waveforms can be very different, so only the \"Brown-like\" waveforms were singled out (according to the \"wf_class\" parameter in the PISTACH product) to establish the waveform matrix. Results for cycle #100 are shown here as an arbitrary example. For cycle #100, there are 420 waveforms considered as ocean waveforms offshore HK, 342 of which are Brown-like. Therefore it is sufficient to carry out SVD processing. We tested two typical thresholds: 90% and 80%.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Choice** & **ALES** & **MLE3** & **MLE4** \\ \hline Standard choice (Table 4) & 5.12 & 4.82 & 4.88 \\ composite wet delay & 5.02 & 4.80 & 5.01 \\ FES 2004 ocean tide & 4.87 & 4.76 & 4.82 \\ FES 2012 ocean tide & 4.89 & 4.56 & 4.88 \\ No DAC applied & 4.84 & 4.56 & 4.76 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Residual SLA, standard deviation w.r.t. tide gauge-based sea level (cm), for case 1, after 3-month smoothing, with different geophysical corrections choices.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ocean Tide Solution** & **GOT 4.8** & **FES 2004** & **FES 2012** \\ \hline Correlation & 0.91 & 0.89 & 0.95 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Correlations of different tide solutions with respect to the tide gauge tide height.  
The 20-Hz epoch noise characteristics are provided in Table 10 (the 1-Hz noise is roughly similar to the 20-Hz one). The measurements are split into two regions: far from the coast (shoreline distance >10 km) and near the coast (shoreline distance \(\leq\)10 km). For both regions, the epoch noises are significantly improved when using the SVD. The present study is somewhat preliminary since we have not computed nor analyzed the SLA performances after the SVD processing, but this strategy looks like a serious option to consider in the future for coastal altimetry processing.