## 3 Method
### Waveform Retracking Method
#### 3.2.3 Non-Specular Echo Processing
However, when the satellite nadir is close to the shoreline, the annulus measurement footprint covers more pure sea surface, while the circular measurement footprint entirely covers the coastal 'pollution', which means that the distortion of the waveform is located around the leading edge. More steady and effective information on the trailing edge is retained within a certain range offshore. Therefore, an effective trailing edge search method is proposed. The effective part of the trailing edge is identified by the power slope between each gate and the fixed gate to reduce the influence of waveform distortion on waveform retracking. For determining the width of distorted waveform, [PERSON] [21] gives some suggestions to avoid influencing the precision of fitting, however, to retrieve the coastal data as possible, this article takes a more radical way and the choice of effective trailing edge reaching parameters are empirical values based on the statistics of real HY-2B altimeter waveform. The details of the effective trailing edge are shown in Figure 8.  
First, we confirm the power-changing trend of the entire trailing edge through linear fitting. If the power-changing trend of the entire trailing edge is essentially rising and the parameter C is zero, or the second reference gate is close to the lowest power gate of the trailing edge, the trailing edge is processed as follows: if the trend of the trailing edge is rising, the total trailing edge is not processed and weighted as the accurate leading edge; if the second reference gate is close to the lowest power gate of the trailing edge, the range of the effective trailing edge is from the first reference gate to the second reference gate. Except as previously discussed, the trailing edge is processed as follows:  
Depending on the linear-fitting power, the distortions of more than 15 gates with a power higher than the value of the linear-fitting power are removed from the range of the effective trailing edge. The distorted trailing edge will deviate from the fitting line to form a 'convex' or 'concave', as shown in Figure 9. According to the number of distortions, there are 'convex' (Figure 9a) or 'concaves' (Figure 9b). Based on the above processing, the lowest power gate of the trailing edge is used as the fixed gate to calculate the power slope at each gate of the trailing edge. According to the gate distance threshold, the trailing edge containing the nine gates with the largest power slope is divided into several parts. The widest part is the effective trailing edge. When the power difference between the lowest power gate and some trailing edge gates is smaller than 0.0025, the headmost is selected as the fixed gate to ensure that the searching range of effective trailing edge is the absolutely decreasing trend. The gate distance threshold is 12. Then, the effective trailing edge is linearly fit and the power of linear power adding a 0.5 times trailing edge oscillation is taken to modify the noise gates. The noise gates are those with a power beyond linear power adding a 1.5 times trailing edge oscillation, and the trailing edge oscillation is the average of the absolute power difference between the top and the bottom of each peak that the effective trailing edge consists of. The modified effective trailing edge is shown in Figure 5.0  
Figure 5.1 Flow diagram of effective trailing edge search. Figure 5.2 _Cont_. Once the range of the effective trailing edge does not include the range of the 100 th-128 th gates, the availability of the trailing edge tail is checked. If the ten-gate average values of the 100 th-128 th gates are decreasing in order and the least, the 100 th-128 th gates are taken as the supplement for the effective trailing edge. * Weighting and fitting  
Weighting the gates depends on the process of the leading edge and trailing edge. Since an accurate a priori value of the mispointing angle is unavailable for the distorted coastal waveform, four-parameter fitting is adopted based on the WLS (WLS4).