# High-Temporal-Resolution High-Spatial-Resolution Spaceborne SAR Based on Continuously Varying PRF
## 1 Introduction
The first factor is a high-resolution implementation mode, which would be achieved through illumination of the same ground patch for the duration of the coherent time. To satisfy the demands for high-resolution SAR in many civilian applications, the sliding spotlight mode and the staring spotlight mode have been employed in \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sensor** & **Operation** & **Resolution** & **Swath** & **Squint Angle** \\ \hline Seasat & 1978 & 25 m(A) \(\times\) 25 m(R) & 100 km(R) & – \\ Radarsat-2 & 2007–Present & 3 m(A) \(\times\) 3 m(R) & 20 km(R) & – \\ TerraSAR-X & 2007–Present & 1 m(A) \(\times\) 1 m(R) & 5 km(A) \(\times\) 10 km(R) & 2.2\({}^{\circ}\) \\ TanDEM-X & 2010–Present & 1 m(A) \(\times\) 1 m(R) & 5 km(A) \(\times\) 10 km(R) & 2.2\({}^{\circ}\) \\ Sentinel-1a & 2013–Present & 5 m(A) \(\times\) 5 m(R) & 80 km(R) & – \\ ALOS-2 & 2014–Present & 1 m(A) \(\times\) 3 m(R) & 25 km(A) \(\times\) 25 km(R) & 3.5\({}^{\circ}\) \\ TerraSAR-X NG & 2018+ & 0.25 m(A) \(\times\) 0.25 m(R) & 5 km(A) \(\times\) 5 km(R) & more than 5\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Typical SAR systems. TerraSAR-X. By focusing the antenna beam to a fixed point, the synthetic aperture length for every scattering point is increased, resulting in a higher resolution. Secondly, repeat-observation intervals varying from tens of seconds to several minutes require high squint illumination capability. Consider a planar antenna system similar to TerraSAR-X, as specified in Table 2. The squint angle as a function of illumination time for different look angles is represented in Figure 1, where the required squint angle for one minute repeat-observation interval increases along with a decreasing look angles. For instance, when the look angle is chosen as 35.0\({}^{\circ}\), the first image would be achieved with squint angle between 23.5\({}^{\circ}\) and 17.3\({}^{\circ}\), and the second image would be achieved with squint angle between \(-17.3^{\circ}\) and \(-23.5^{\circ}\). Squint angle from 17.3\({}^{\circ}\) to \(-17.3^{\circ}\) is the gap for the repeat-observation interval of 61.6 s.  
The squint angle as a function of illumination time for different orbit height is also represented in Figure 2, where the required squint angle decreases along with an increasing orbit height. For instance, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Units** \\ \hline Orbit height & 514 & km \\ Eccentricity & 0.00018 & - \\ Inclination & 97.45 & \({}^{\circ}\) \\ Longitude of ascend note & 92.39 & \({}^{\circ}\) \\ Argument of perigee & 91.28 & \({}^{\circ}\) \\ Carrier frequency & 9.65 & GHz \\ Bandwidth & 1.2 & GHz \\ Look angle & 35 & \({}^{\circ}\) \\ Antenna length & 4.8 & m \\ Antenna height & 0.8 & m \\ Azimuth resolution & 0.25 & m \\ Azimuth swath & 5.0 & km \\ Range swath & 5.0 & km \\ \hline \hline \end{tabular}
\end{table} 
Table 2: List of simulation parameters. Figure 1: Squint angle as a function of illumination time for different look angles. The illumination time is defined as the time relative to the moment of the minimum slant range. The required squint angle is 23.5\({}^{\circ}\) to ensure one minute repeat-observation interval images with a 0.25 m resolution and 5 km swath in both azimuth and range, when the look angle is chosen as 35.0\({}^{\circ}\) (red line). Black line represents the synthetic aperture process for images. with an orbit height of 1000 km, images with repeat-observation intervals varying from tens of seconds to 4 min will be available when the squint angle is higher than 32.1\({}^{\circ}\). In general, the maximum of repeat-observation intervals mainly depends on orbit height and squint angle.