# A Novel Spaceborne Sliding Spotlight Range Sweep Synthetic Aperture Radar: System and Imaging
## 1 Introduction  
The Spaceborne Synthetic Aperture Radar (SAR) has dozens of operation modes, such as the stripmap mode, ScanSAR mode, TOPS mode, spotlight mode and the sliding spotlight mode, to image regions of interest (ROI) with variant combination of resolutions and swaths [1; 2; 3; 4; 5; 6; 7; 8]. Despite the differences of these modes, they have one thing in common: their Beam Illumination Strip (BIS) (except for the spotlight mode) are all along the satellite ground-track direction [9; 10; 11; 12; 13]. As a result, for most current SAR satellites that move in near-polar orbits, their BISs are all nearly along north-south directions, whatever modes they operate in. In this case, if a ROI is tilted with respect to the satellite orbit and has a span wider than the cross-track swath of the BIS as shown in Figure 1a, more strips should be used to fully cover the tilted ROI. The seismic fault is a typical kind of such tilted ROI. To implement full coverage, one strategy is to use the ScanSAR mode or TOPS mode by dividing the whole Data Acquisition Period (DAP) into multiple bursts for multiple sub-strips by sacrificing the azimuth resolution. Another strategy is to use multiple orbits of observation by sacrificing the DAP. Though a recent Staggered SAR mode can image a much wider swath without the resolution scarification or DAP prolongation [14; 15; 16], it is, along with the former two strategies, to induce large amount of invalid data from disinterested regions such as those marked as Region E in Figure 1a. In sum, all these methods have inevitable drawbacks and thus are not the best solutions for the problem of imaging a tilted-with-orbit ROI in high resolutions and with a high data utilization efficiency. To solve this problem, a Spaceborne Stripmap Range Sweep SAR (SS-RSSAR) has been proposed in a previous paper to generate a ROI-matched BIS by continuously sweeping the beam in elevation [17]. However, restricted by its fixed azimuth antenna pointing, the SS-RSSAR can only image a ROI in a stripmap-level azimuth resolution. In this paper a new Spaceborne Sliding Spotlight Range Sweep SAR (SSS-RSSAR) is proposed to achieve a higher azimuth resolution than the SS-RSSAR by not only continuously sweeping the beam in range for the ROI tracking, but also in azimuth to achieve an enlarged synthetic aperture for an improved azimuth resolution as shown in Figure 1b. Two major aspects of the SSS-RSSAR are to be focused in this paper: the system design and the imaging technique. The former part determines how the echo from the ROI should be collected and the latter part discusses how this echo should be focused. They serve as the basis for the new SSS-RSSAR theory. Similar to the SS-RSSAR mode, the SSS-RSSAR's Central Beam Pointing (CBP), which denotes the beam pointing at the central data acquisition time, is arranged to be perpendicular to the BIS for a good balance between the imaging swath and the signal-to-noise ratio [17]. In this case, the SSS-RSSAR is likely to operate with a large squint angle if the ROI is highly squinted with respect to the satellite orbit. As the radar antenna cannot receive while transmitting [18], the SSS-RSSAR is likely to suffer from the problem of transmission blockage due to the serious slant range variation caused by the continuous Two Dimensional (2D) beam steering. Aiming at overcoming this problem, a new Continuous Varying Pulse Interval (CVPI) technique is employed by the SSS-RSSAR to counteract the slant range variation by continuously adjusting the Pulse Intervals (PIs) based on the instant Central Beam Slant Range (CBSR), defined as the slant range from the SAR antenna center to the ground beam pointing. The core strategy of the CVPI technique is to locate the echo from the ROI at the central part of every PI. In this case, the space that can be used for the non-blocked echo receiving can be maximized.