# High-Temporal-Resolution High-Spatial-Resolution Spaceborne SAR Based on Continuously Varying PRF
## 7 Conclusions  
For high-temporal-resolution and high-spatial-resolution spaceborne SAR, a novel concept based on a sliding spotlight mode called CVPRF has been proposed. This technique offers the potential to overcome the limitation due to conflicts between the receive window and 'blind ranges'. With parameters similar to TerraSAR-X, one minute repeat-observation interval images with 0.25 m resolution and 5 km swath in both the azimuth and ground range can be achieved by the CVPRF technique. With higher orbit height and higher squint angle, images with repeat-observation intervals varying from tens of seconds to several minutes can be available. Moreover, with the proposed CVPRF technique, large-area applications will also be supported, e.g., comprehensive information analysis of targets and higher quality SAR images. Furthermore, the design of the sequence of the PRF, as well as the performance prediction of the RASR, has been provided. Based on the high-order range equation model, an efficient imaging algorithm for the CVPRF technique has been developed, where resampling processing is introduced to solve the non-uniform sampling problem. In addition, accurate focusing is achieved by a high-order imaging algorithm. Simulation results have been provided to verify the improvement in terms of the effectiveness of echo acquisition and the accuracy of the proposed imaging algorithm. For a compromise and practical strategy, PRFs would change discretely according to the precision of hardware. User requirements always push the development of new technologies. As requirement increases, techniques of varying PRF would become a trend for Spaceborne SAR and difficulties on hardware design would be overcome in the future.  
Figure 17: Focused result by the proposed algorithm.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Range**} & \multicolumn{2}{c}{**Azimuth**} \\ \hline  & \(\rho_{r}\) & **PSLR** & **ISLR** & \(\rho_{a}\) & **PSLR** & **ISLR** \\  & **(m)** & **(dB)** & **(dB)** & **(m)** & **(dB)** & **(dB)** \\ \hline
1 & 0.226 & \(-13.057\) & \(-9.649\) & 0.210 & \(-13.254\) & \(-10.635\) \\
2 & 0.226 & \(-13.052\) & \(-9.646\) & 0.211 & \(-13.256\) & \(-10.640\) \\
3 & 0.226 & \(-13.050\) & \(-9.647\) & 0.212 & \(-13.260\) & \(-10.650\) \\
4 & 0.226 & \(-13.051\) & \(-9.656\) & 0.208 & \(-13.258\) & \(-10.617\) \\
5 & 0.226 & \(-13.071\) & \(-9.638\) & 0.209 & \(-13.256\) & \(-10.625\) \\
6 & 0.226 & \(-13.057\) & \(-9.652\) & 0.210 & \(-13.254\) & \(-10.633\) \\
7 & 0.226 & \(-13.061\) & \(-9.656\) & 0.206 & \(-13.254\) & \(-10.611\) \\
8 & 0.226 & \(-13.064\) & \(-9.657\) & 0.207 & \(-13.256\) & \(-10.612\) \\
9 & 0.227 & \(-13.057\) & \(-9.656\) & 0.208 & \(-13.254\) & \(-10.613\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Performance analysis of point targets.  
**Acknowledgments:** This work was supported by the National Natural Science Foundation of China (Grant No. 61628101) and the Innovation Foundation of Aerospace Science and Technology of Shanghai (Grant No. SAST2016029).  
**Author Contributions:** The work presented here was performed in collaboration among all authors. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] defined the research theme, and [PERSON] and [PERSON] designed the methods and simulations, conducted the laboratory experiments, analyzed the data, interpreted the results and wrote the paper. All authors have contributed to and approved the manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.