## 3 Methodology
### Monitoring Method of Glacier Flow Velocity in Wide-Area POT
#### 3.1.1 Data Preprocessing  
The measurement accuracy of the POT technique depends on the SAR data resolution and the co-registration accuracy of the primary and secondary images. SAR data resolution is determined by satellite sensors and observation modes. In order to improve the accuracy of data registration, this paper uses a three-level registration strategy to register the primary and secondary images of each map data set [29]. 1. Coarse registration is based on orbit parameters. The initial offsets of the points to be registered in the primary image and the corresponding points in the secondary image are calculated by using the orbit parameters for automatic registration. Affected by the accuracy of the orbit parameters, the coarse registration accuracy is generally about 10 to 30 pixels. 2. Pixel-level registration is achieved by utilizing window matching techniques based on coarse registration. A template window of a certain size is set in the primary image, while a larger search window is established in the auxiliary image, containing a sub-window of the same size as the template window. The cross-correlation coefficient of the two scene images serves as the matching quality evaluation metric, and a matching quality evaluation coefficient matrix is computed row by row and column by column. The pixel at the peak of this matrix is considered the central image point of the optimal search window, representing the registration point in the auxiliary image. The offset is calculated from the difference between the registration point's coordinates and the template window's center coordinates in the primary image. The offset of the center pixel of the template window essentially represents the average offset of all pixels within the window. Variations in pixel offset within the window can also affect the accuracy of the results. For large glaciers, local displacement \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Orbital**} & \multirow{2}{*}{**Satellite**} & \multirow{2}{*}{**Satellite**} & \multirow{2}{*}{**Band of Wave**} & \multirow{2}{*}{**Imaging Mode**} & \multirow{2}{*}{**Mode of Polarization**} & \multirow{2}{*}{**Direction of Orbit**} & \multirow{2}{*}{**Sheet**} & \multirow{2}{*}{**Azimuth Angle (\(\lx@over{\lx@over{\lx@over{\lx@over{\lx@over{\lx@par}{ }}}}}\))**} & \multicolumn{2}{c}{**Azimuth Angle (\(\lx@over{\lx@over{\lx@over{\lx@par}{ }}}\))**} & \multirow{2}{*}{**Image Size**} & \multirow{2}{*}{**Time**} & \multirow{2}{*}{**Number of Images**} \\  & & & & & & & & & & **(Days)** & **(Scene)** \\ \hline \multirow{11}{*}{17} & Sentinel- & \multirow{1}{*}{C} & \multirow{1}{*}{HW TOPS SLC} & & 248 & \(-32.5\) & & & & & 29 \\  & & & & 253 & \(-35.7\) & & & & & 28 \\  & & & 259 & \(-39.4\) & & & & & 29 \\  & & & 264 & \(-42.3\) & & \(13.88\times\) & & 2021\(-\)28 & & 28 \\  & & & & 262 & \(-42.3\) & & \(2.32\) & December & & 58 \\  & & & & 257 & \(-38.3\) & & & 2021 & & 58 \\ \hline \multirow{11}{*}{174} & Sentinel- & \multirow{1}{*}{C} & \multirow{1}{*}{HW TOPS SLC} & & 248 & \(-32.5\) & & & & & 29 \\  & & & 253 & \(-35.7\) & & & & & 28 \\  & & & & 259 & \(-39.4\) & & & & 29 \\  & & & & 264 & \(-42.3\) & & \(13.88\times\) & & 2021\(-\)21 & & 29 \\  & & & & 262 & \(-42.3\) & & \(2.32\) & December & & 58 \\  & & & & & 257 & \(-38.3\) & & & 2021 & & 58 \\  & & & & 252 & \(-34.9\) & & & & & 58 \\ \hline \end{tabular}
\end{table} 
Table 1: Main parameters of the Sentinel-1 data set used (time: year/month/day). rates are typically not significant due to internal forces, allowing for the use of larger windows to estimate the offsets. However, excessively large windows may reduce the fineness of the results. Therefore, under the condition of minimal noise in glacier areas, selecting the smallest possible window can effectively preserve the detailed displacement of the glacier surface, facilitating result analysis. The proposed method achieves pixel-level registration accuracy. 3.