A Novel Spaceborne Sliding Spotlight Range Sweep Synthetic Aperture Radar: System and Imaging

remote sensing Article A Novel Spaceborne Sliding Spotlight Range Sweep Synthetic Aperture Radar: System and Imaging Yan Wang 1 , Jingwen Li 2 , Jian Yang 1, * and Bing Sun 2 1 2 * Department of Electronic Engineering, Tsinghua University, Beijing 100084, China; School of Electronic Information Engineering, Beihang University, Beijing 100191, China; (J.L.); (B.S.) Correspondence: Academic Editors: Zhong Lu, Zhenhong Li and Prasad S. Thenkabail Received: 10 May 2017; Accepted: 26 July 2017; Published: 31 July 2017 Abstract: In this paper, a new Spaceborne Sliding Spotlight Range Sweep Synthetic Aperture Radar (SSS-RSSAR) is proposed to generate a high-resolution image of a Region of Interest (ROI) tilted with respect to the satellite track. Comparing to the traditional Spaceborne Sliding Spotlight Synthetic Aperture Radar (SSS-SAR), the SSS-RSSAR is superior in contributing to less data amount, lighter computational load and hence higher observation efficiency. Unlike the Spaceborne Stripmap Range Sweep Synthetic Aperture Radar (SS-RSSAR) proposed in a previous paper, the SSS-RSSAR not only continuously sweeps the beam in range for the ROI tracking, but also in azimuth to enlarge the synthetic aperture for an improved azimuth resolution. Two aspects of the SSS-RSSAR are focused: system and imaging. For the system part, a Continuous Varying Pulse Interval (CVPI) technique is proposed to avoid the transmission blockage problem by non-uniformly adjusting the pulse intervals based on the geometry. For the imaging part, a Modified Polar Format Algorithm (MPFA) is proposed to accommodate the original polar format algorithm to the echo received with the CVPI technique. Moreover, an integrate system parameter design flow for the SSS-RSSAR is also suggested. The presented approach is evaluated by exploiting the point target simulations. Keywords: SSS-RSSAR; CVPI; MPFA; system parameter design 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 [1–8]. 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 [9–13]. 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 [14–16], 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. Remote Sens. 2017, 9, 783; doi:10.3390/rs9080783 www.mdpi.com/journal/remotesensing Remote Sens. 2017, 9, 783 2 of 24 Remote Sens. 1a. 2017,In9,sum, 783 all these methods have inevitable drawbacks and thus are not the best solutions 2 of 24 in Figure for the problem of imaging a tilted-with-orbit ROI in high resolutions and with a high data utilization efficiency. 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. (a) (b) Figure1.1.Data Dataacquisition acquisitionof: of: (a) (a)conventional conventional spaceborne spaceborne Synthetic Synthetic Aperture Aperture Radar Radar (SAR); (SAR); and and (b) (b) Figure SpaceborneSliding SlidingSpotlight SpotlightRange RangeSweep SweepSynthetic SyntheticAperture ApertureRadar Radar(SSS-RSSAR). (SSS-RSSAR). Spaceborne To solve this problem, a Spaceborne Stripmap Range Sweep SAR (SS-RSSAR) has been To solve this problem, a Spaceborne Stripmap Range Sweep SAR (SS-RSSAR) has been proposed in proposed in a previous paper to generate a ROI-matched BIS by continuously sweeping the beam in a previous paper to generate a ROI-matched BIS by continuously sweeping the beam in elevation [17]. elevation [17]. However, restricted by its fixed azimuth antenna pointing, the SS-RSSAR can only However, restricted by its fixed azimuth antenna pointing, the SS-RSSAR can only image a ROI in a image a ROI in a stripmap-level azimuth resolution. In this paper a new Spaceborne Sliding stripmap-level azimuth resolution. In this paper a new Spaceborne Sliding Spotlight Range Sweep Spotlight Range Sweep SAR (SSS-RSSAR) is proposed to achieve a higher azimuth resolution than SAR (SSS-RSSAR) is proposed to achieve a higher azimuth resolution than the SS-RSSAR by not only the SS-RSSAR by not only continuously sweeping the beam in range for the ROI tracking, but also in continuously sweeping the beam in range for the ROI tracking, but also in azimuth to achieve an azimuth to achieve an enlarged synthetic aperture for an improved azimuth resolution as shown in enlarged synthetic aperture for an improved azimuth resolution as shown in Figure 1b. Two major Figure 1b. Two major aspects of the SSS-RSSAR are to be focused in this paper: the system design aspects of the SSS-RSSAR are to be focused in this paper: the system design and the imaging technique. and the imaging technique. The former part determines how the echo from the ROI should be The former part determines how the echo from the ROI should be collected and the latter part discusses collected and the latter part discusses how this echo should be focused. They serve as the basis for how this echo should be focused. They serve as the basis for the new SSS-RSSAR theory. the new SSS-RSSAR theory. Similar to the SS-RSSAR mode, the SSS-RSSAR’s Central Beam Pointing (CBP), which denotes the Similar to the SS-RSSAR mode, the SSS-RSSAR’s Central Beam Pointing (CBP), which denotes beam pointing at the central data acquisition time, is arranged to be perpendicular to the BIS for a good the beam pointing at the central data acquisition time, is arranged to be perpendicular to the BIS for balance between the imaging swath and the signal-to-noise ratio [17]. In this case, the SSS-RSSAR is a good balance between the imaging swath and the signal-to-noise ratio [17]. In this case, the likely to operate with a large squint angle if the ROI is highly squinted with respect to the satellite orbit. SSS-RS (...truncated)