A High-Squint TOPS SAR Imaging Algorithm for Maneuvering Platforms Based on Joint Time-Doppler Deramp Without Subaperture

Full aperture imaging algorithm for highly squinted TOPS SAR

During the data acquisition of a squint terrain observation by progressive scan (TOPS) synthetic aperture radar (SAR), the steering of the antenna main beam increases the azimuth bandwidth and results in the azimuth signal aliasing in the Doppler domain. Besides, the range curvature and the Doppler frequency modulation (FM) rates after linear range walk correction are azimuth-variant for squint TOPS SAR. These problems may cause some difficulties for the SAR data processing. To deal with the problems, a new imaging algorithm for the squint TOPS SAR is introduced in this paper. After linear range walk correction, the azimuth preprocessing is implemented to achieve the two-dimensional frequency spectrum without aliasing. Then, utilizing a modified chirp scaling algorithm, we complete the range cell migration correction (RCMC) and range compression to the TOPS SAR data without subaperture. Finally, the frequency nonlinear chirp scaling (FNCS) is proposed to correct the variation of the FM rates and the azimuth signal is focused in the Doppler domain via the spectral analysis (SPECAN) method. Both simulation and real data results show the effectiveness of the proposed algorithm.

Terrain Observation by Progressive Scans (TOPS) synthetic aperture radar (SAR) and spotlight SAR are advanced SAR imaging modes for wide range swath and high resolution. In order to obtain a wider range coverage, azimuth multichannel is introduced in the literature. Since the azimuth bandwidth of beam steering SAR (BS-SAR; spotlight SAR, sliding spotlight SAR, or TOPS SAR) is much greater than that of a stripmap SAR, a signal reconstruction algorithm used for multichannel stripmap SAR may not be effective for multichannel BS-SAR. In this paper, a multichannel full-aperture azimuth processing algorithm is proposed for a BS-SAR. The key of this algorithm lies in the beam and the azimuth bandwidth compressions of multichannel signals in the Doppler-array and slow time-angle planes, respectively. Through compression processing, the beamwidth and the azimuth bandwidth are smaller than the available angle and equivalent pulse repeating frequency , respectively. Then, an improved post-Doppler STAP method is proposed to recover a 2-D spectrum. With the recovered signal, further processing can be utilized to focus the multichannel signal. Simulation and real data results show the effectiveness of the proposed algorithm.

For different applications with different requirements, many synthetic aperture radar (SAR) modes have been developed in the literature, such as, Terrain Observation by Progressive Scans (TOPS) SAR and sliding spotlight SAR. In this paper, we call TOPS SAR, sliding spotlight SAR, and spotlight SAR as beam steering SAR (BS-SAR for short). Comparing with stripmap SAR, BS-SAR can obtain a wide diversity of resolutions by increasing or reducing the azimuth synthetic time. Traditional polar formation algorithm (PFA) is an efficient algorithm which is mainly developed for spotlight SAR. The PFA has been validated to obtain well-focused results of raw data. In this paper, we extend the traditional PFA to process sliding spotlight SAR and TOPS SAR data, and we call it generalized PFA (GPFA). Comparing with the traditional PFA, GPFA contains a different azimuth deramping function and an additional azimuth scaling operation. The simulated and real data are used to validate the effectiveness of this method.

Abstract. Synthetic Aperture Radar (SAR) on the geosynchronous orbit has the advantage in remote sensing, with high resolution to provide a large coverage in a short visit time. the space-variant coverage and range-azimuth couple of GEO SAR are required to compensate, using the electrical beam steering, the irregular data acquisition can be convert to classical SAR modes, such as stripmap, spotlight, sliding spotlight, and terrain observation by progressive scans (TOPS) SAR, the dopper analysis of these work modes of SAR have been studied. Based on deramping processing and improved subaperture chirp scaling algorithm, a novel focusing approach is presented in this paper, whose performance is determined by the work mode, data acquisition and the deramping processing. Data of each mode can be focused by utilizing this presented approach with the information of subapterture centroid time and range rotation center. Simulation results are presented to validate the analysis of the proposed approach.

This paper presents four imaging approaches for terrain observation by progressive scans (TOPS)-mode SAR focusing, all of which take advantage of modified chirp scaling (CS) algorithms according to echo properties in TOPS. After adopting different azimuth preprocessing steps to resolve the aliased Doppler spectra, the resulting TOPS raw data can be handled by a classic CS processor. Unfortunately, the resulting images would appear back folding, if only a standard stripmap processor is adopted. To avoid or resolve azimuth back folding in the final focused TOPS SAR images, four modified CS algorithms are presented according to special properties of the slow time-frequency diagram in TOPS. Imaging results on point targets validate the presented imaging approaches.

The novel displaced phase center antenna (DPCA) azimuth multichannel technique is well suited to improve the coarser azimuth resolution of the terrain observation by progressive scans (TOPS) mode for a fixed total receive antenna length. However, both the azimuth multichannel nonuniform sampling and the aliased Doppler spectrum in TOPS raise the azimuth data processing difficulty. This letter proposes an innovative full-aperture processing approach, which adds an azimuth derotation step for each azimuth channel before azimuth multichannel data reconstruction and utilizes the prefiltering operation in the two-step focusing technique to resolve both azimuth ambiguity problems caused by the DPCA technique and azimuth beam progressive scanning. In addition to presenting the proposed azimuth preprocessing approach, the impact of the approach on the azimuth ambiguity-to-signal ratio is analyzed in detail. Simulation results validate the proposed multichannel processing approach for the TOPS mode.

This paper presents a time-domain synthetic aperture radar (SAR) processor designed for Sentinel-l Terrain Observation by Progressive Scans (TOPS) monostatic data. The processor focuses Interferometric Wide (IW) swath and Extra Wide (EW) swath data on a selected region-of-interest (ROI) and consists of the following main stages: decoding of Level-0 data, selection of the relevant burst and pulses for the targeted ROI, range compression, azimuth frequency unfolding and resampling, and image focusing by a fast subaperture-based version of the back-projection algorithm. The performances of the developed processor are assessed on datasets acquired in IW and EW imaging modes. Such a time-domain processor can be regarded as a first step towards a geometry/frequency-agnostic SAR processing kernel for future monostatic/multi static spaceborne SAR missions.

TOPS (Terrain Observation by Progressive Scans) mode can cover large azimuthal scenes in a short time through beam scanning. Aiming at airborne FMCW SAR system, the imaging issues of airborne TOPS mode are studied. By analyzing the characteristics of echo data in this mode, the original frequency scaling algorithm (FSA) has fan-shaped deformation caused by the inconsistency of azimuthal sample interval at different ranges during azimuth scaling, thus an improved frequency scaling algorithm which exploit baseband azimuth scaling method as azimuth process is proposed in this paper to solve the problem. The validity of the proposed algorithm is verified by simulation and equivalent measured data.

Synthetic Aperture Radar (SAR) echo simulation is an effective tool of the radar system design, which enables us to validate imaging algorithms and evaluate the performance of SAR systems before actual use. The increasing demand of higher SAR resolution makes the SAR simulation more and more complicated. The traditional simulation methods are not precise enough so that a new simulation method applicable to most complicated targets is needed. This paper demonstrates a novel simulation method based on the theory of electromagnetic scattering which simulates the received echo data through taking use of the backscattered electric field. Our simulation method can be used to simulate a complicated target in SAR simulation. This method opens a door to conducting SAR echo simulation with theory of signal and system and the electromagnetic scattering taken into consideration, though a few improvements are needed to increase the performance of the SAR simulation.

This paper presents an efficient phase preserving processor for the focusing of data acquired in sliding spotlight, TOPS (Terrain Observation by Progressive Scans) and ScanSAR imaging modes. They share in common a linear variation of the Doppler centroid along the azimuth dimension, which is due to a steering of the antenna (either mechanically or electronically) throughout the data take for the first two modes, and due to the burst mode in the ScanSAR case. Existing approaches for the azimuth processing can become inefficient due to the additional processing to overcome the folding in the focused domain. In this paper an azimuth scaling approach is presented to perform the azimuth processing, whose kernel is the same for all three modes. Data acquired by TerraSAR-X in sliding spotlight, TOPS and ScanSAR modes are used to validate the developed algorithm.

In this paper, we present a narrow-band interference (NBI) and wide-band interference (WBI) mitigation algorithm based on the deep residual network (ResNet). First, the short-time Fourier transform (STFT) is utilized to characterize the interference-corrupted echo in the time-frequency domain. Then, the interference detection model is built by the classical convolutional neural network (CNN) framework to identify whether the echoes exist interference signal component. Furthermore, the time-frequency feature of the target signal is extracted and reconstructed by utilizing the ResNet. Finally, the inverse time-frequency Fourier transform (ISTFT) is utilized to transform the time-frequency spectrum of the recovered signal into the time domain. The effectiveness of the interference mitigation algorithm is verified on the simulation and measured SAR data of the terrain observation by progressive scans (TOPS) mode. Moreover, the performance comparison with the notch filtering and eigensubspace filtering demonstrates the superiority of the proposed interference mitigation algorithm.

Very high requirement of co-registration accuracy better than 0.001 pixels for Sentinel-1 TOPS (Terrain Observation by Progressive Scan) mode presents a great challenge for application of SAR (synthetic aperture radar) interferometry in Earth Observation. The state-of-the-art techniques have demonstrated that the low coherence scenarios and abrupt loss of coherence between two arbitrary acquisitions are main sources of error to degrade the performance of TOPS time-series co-registration. In this paper, we present a modification to overcome both limitations through the coherence enhancement. The motive behind this is to improve the quality of observations before co-registration and meanwhile avoiding the coherence loss caused by fast decorrelation. To this end, principal components analysis based spatio-temporal filtering is first used to remove the artifacts in burst interferograms over strong noise areas. Rather than heuristically choosing a sub-set of interferograms as a small baseline technique does, we use Dijkstra's shortest path algorithm under graph theory framework to maximize the coherence of a sub-set interferograms. The performance of presented method against the state-of-the-art techniques is fully evaluated by synthetic data and a Sentinel-1A stack over a low coherence scene in Indonesia. Comprehensive comparisons demonstrate 9%–17% uncertainty reduction of time-series co-registration when applying our method.

This paper presents an efficient phase preserving processor for TOPS (Terrain Observation by Progressive Scans) imaging mode. TOPS has been proposed as a new wide swath imaging mode, which solves the problems of scalloping and azimuth-varying ambiguities introduced by the conventional ScanSAR mode by means of steering the antenna along the azimuth direction. An algorithm based on ECS (Extended Chirp Scaling) is proposed, which uses sub-apertures and a new azimuth scaling step. The proposed solution is also efficient in the sense that it allows selecting the final azimuth image spacing by means of azimuth scaling, hence easing the forthcoming mosaicking of the different sub swaths. Simulations with point targets are used to validate the processor.

Marine pollution is one of issues that is widely discussed such as marine debris, chemical contamination, and oil spill. It has many impacts on the environment. Remote sensing helps detect the presence of an oil spill via satellite platform. One commonly used satellite is Synthetic-Aperture Radar (SAR) imagery, which can see oil spills even in cloudy conditions. However, the black and white appearance of the radar sometimes makes it difficult to distinguish between an oil spill and look alike. Therefore, this study aims to determine the polarization combination in the Sentinel-1 SAR imagery. Sentinel-1 Single Look Complex (SLC) is used as input and processes through various stages to obtain Coherence Vertical-Vertical (VV) and Vertical-Horizontal (VH) results. There are four steps for processing the data; convert the data to backscattering coefficient from SLC data, stacking, Terrain Observation by Progressive Scan (TOPS) coherence, and building the robust combinations. The polarization in the Sentinel-1 was calculated to determine artificial bands for the correct combination, namely Divide (VV/VH) as band 1, Average ((VV+VH)/2) as band 2, Multiple (VVxVH) as band 3, Add (VV+VH) as band 4, and Subtract (VV-VH) as band 5. Furthermore, the determination of the band used in the polarization composite display is based on the intensity value. As a result, the recommendation for Sentinel-1 polarization is obtained, which involves bands 2, 3, and 5.