# Error Sources of Interferometric Synthetic Aperture Radar Satellites
## 3 Systematic Errors
### Scatterer Movement (Ground Surface)
#### 3.3.2 Surface Movements
NISAR has apparently higher cycle-slicing limit because of their larger wavelengths, and this implies that they are relatively worse at detecting small motions. The swath width limit notably affects the actual size of the detectable area since it essentially symbolizes the size of the scan area of a swath. The plots display the x- and y-axes in log scale, so while the size of the region does not appear to change much, their true sizes are considerably different. The detectable area is about 15 km\({}^{2}\) for ERS-1 and about 330 km\({}^{2}\) for Sentinel-1. The subswath of the TerraSAR-X Stripmap mode is only 30 km, leading it to having the smallest detectable area of 7 km\({}^{2}\), while the TerraSAR-X ScanSAR mode has a detectable area of 155 km\({}^{2}\). NISAR has a much larger detectable area owing to its wide 240 km subswath. The detectable area for the NISAR S-band is 907 km\({}^{2}\), and for L-band it is 1828 km\({}^{2}\). The characteristics of each limit are arranged in Table 9 for the reader's information. The fourth column of Table 9 supplements the geophysical phenomenon that has the possibility of falling out of the detectable area according to [28; 46]. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Boundary Line** & **Restriction** & **Risks If Limit Exceeded** &
\begin{tabular}{l} **Phenomena That Might Fall** \\ **Out of Boundary Line** \\ \end{tabular} \\ \hline Upper gradient limit & Magnitude of surface movements & Decorrelation or & Volcano eruption, fault rupture, \\ Cycle-slicing limit & Magnitude of surface movements & Not detectable & catastrophic earthquake \\ Small-gradient limit & Magnitude of surface movements & Misinterpreted as other & Minor earthquake \\ Pixel-size limit & Spatial extent & Not detectable & Post-glacial rebound \\ Swath width limit & Spatial extent & Not detectable & Spalling in sidewalk pavement \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Five boundary lines of detectable area and their characteristics. Figure 5: Detectability for surface movements for (**a**) ERS-1, (**b**) Sentinel-1, (**c**) TerraSAR-X Striripmap mode, (**d**) TerraSAR-X ScanSAR mode, (**e**) NISAR S-band, and (**f**) NISAR L-band, modified from [28; 46]. The y-axis shows the magnitude of the surface movements in the range direction, which can be translated to phase gradients. The boundary lines for the magnitude of the surface movements include upper gradient limit, cycle-slicing limit, and small gradient limit, demonstrated in dashed light blue lines. There are three upper gradient limits for Sentinel-1. From top to bottom, they represent the limit of swath 1, swath 2, and swath 3, respectively. The cycle-slicing limit is defined as \(\lambda/20\), as mentioned in Equation (19). The x-axis shows the spatial scale from 1 m to \(10^{6}\) m. The spatial limitation is characterized by the pixel size limit and the swath width limit, demonstrated by dashed light-gray lines. There are three swath width limits for Sentinel-1. From left to right, they represent the case of using one subswath, two subswaths, and three subswaths, respectively. The light-yellow background color illustrates the detectable area.