Synthetic Aperture Radar Processing Algorithms Research Articles

High-Resolution Ice-Sounding Radar Measurements of Ice Thickness Over East Antarctic Ice Sheet as a Part of Chinese National Antarctic Research Expedition

This paper presents the ice thickness, fine resolution internal reflecting horizons (IRHs), and distinct bottom topography measurements of Chinese Kunlun Station and Grove Mountains, Antarctica, derived from sounding these glaciers with a high-resolution radar. To enable the development of next-generation ice-sheet models, we need information on IRHs, bottom topography, and basal conditions. To this end, we performed measurements with the progressively improved ice-sounding radar system, currently known as the high-resolution ice-sounding radar developed by the Key Laboratory of Electromagnetic Radiation and Sensing Technology of Institute of Electronics, Chinese Academy of Sciences, Beijing, China. We processed the collected data using focused synthetic aperture radar (SAR) algorithm named the modified range migration algorithm using curvelets and the modified nonlinear chirp scaling algorithm to improve radar sensitivity and reduce along-track surface clutter. Representative results from selected transects indicate that we successfully sounded 3-km-thick ice with a fine resolution of 0.75 m. In this paper, we provide a brief description of the radar system, discuss the focused SAR processing algorithms, and provide sample results to demonstrate the successful sounding of the ice sheet in Antarctica.

Generalized range migration algorithm for synthetic aperture radar image reconstruction of metasurface antenna measurements

Waveguide-fed dynamic metasurface antennas (DMAs) can be used in a variety of synthetic aperture radar (SAR) modalities for microwave and millimeter-wave imaging. The DMA consists of an electrically large array of resonant, dynamically reconfigurable metamaterial radiators, each excited by the fields of a guided wave. A given metamaterial element can be modeled as a polarizable dipole, with polarizability that equates to a complex weighting factor in the context of antenna array calculations. The DMA produces a sequence of diverse radiation patterns as a function of the weights, which can be rapidly varied by external control. The unconventional radiation patterns of the DMA, however, introduce added computational complexity for traditional SAR processing algorithms. One SAR reconstruction approach that has successfully been adapted for the static (motionless) DMA is the range migration algorithm (RMA). Here, we extend the RMA to scenarios in which the DMA is physically translated in one or two dimensions, deriving the RMA for each case and providing experimental demonstrations using a fabricated microstrip-based DMA. Excellent reconstruction quality is observed in all cases, verifying the efficacy of the proposed algorithms and demonstrating the imaging capabilities of the DMA in the synthetic aperture context. The combination of the DMA platform with efficient reconstruction algorithms can find applications in fields such as Earth observation, security screening, and autonomous vehicle navigation.

Imaging of Oil-Well Perforations Using UWB Synthetic Aperture Radar

Ultrawideband (UWB) synthetic aperture radar (SAR) as a tool for oil well perforation monitoring is experimentally investigated in this paper. Experiments are conducted on a lab prototype that emulates the small section of an oil well. Oil well perforations are scanned using the designed UWB radar system for different experimental conditions. High-resolution images in both down-range and cross-range are obtained by using UWB pulses and signal processing. High-quality images are constructed using SAR processing algorithm with the proposed modifications. Experiments are carried out for different perforation conditions such as open, partially clogged and fully clogged cases. It is shown by the experiments that the proposed radar technique and the data processing methods are capable of providing more revealing high-quality images. This approach can be applied effectively to monitor the oil well medium and detect well impairment locations.

Range Doppler and chirp scaling processing of synthetic aperture radar data using the fractional Fourier transform

Synthetic aperture radar (SAR) systems are used to form high-resolution images from radar backscatter signals. The fractional Fourier transform (FrFT), which is a generalised form of the well-known Fourier transform, has opened up the possibility of a new range of potentially promising and useful applications that involve the use and detection of chirp signals that include pattern recognition and SAR imaging. In this study a time variant problem associated with the use of the FrFT for SAR processing is addressed and a new algorithm is presented that resolves this problem. Two new FrFT-based SAR processing algorithms are presented namely the FrRDA and the eFrCSA that are shown to improve the well-established range-Doppler and chirp-scaling algorithms for SAR processing. The performance of the algorithms are assessed using simulated and real Radarsat-1 data sets. The results confirm that the FrFT-based SAR processing methods provide enhanced resolution yielding both lower side lobes effects and improved target detection.

SAR PROCESSING ALGORITHM WITH BUILT-IN GEOMETRIC CORRECTION

Synthetic aperture radar (SAR) systems onboard small aircrafts suffer from trajectory deviations and instabilities of antenna orientation. These kinds of motion errors lead to significant geometric distortions in SAR images. In order to correct the distortions, we propose a time-domain multi-look stripmap SAR processing algorithm with built-in geometric correction. In the algorithm, the azimuth reference functions and range migration curves are designed to produce SAR images directly on a correct rectangular grid on the ground plane. The proposed technique has been successfully tested by using a Ku-band airborne SAR system installed onboard light-weight aircraft.

Focusing of Medium-Earth-Orbit SAR With Advanced Nonlinear Chirp Scaling Algorithm

The signal processing of the medium-Earth-orbit synthetic aperture radar (SAR) is more challenging than that of the current low-Earth-orbit SAR because the imaging geometry is more complicated, and the range and azimuth variances are more severe. This paper deals with these imaging problems in three aspects. First, an advanced hyperbolic range equation (AHRE) is proposed for the first time, which is more precise for a spaceborne SAR than the conventional hyperbolic range equation (CHRE). Second, the point target spectrum based on the AHRE is analytically derived, which is useful for developing efficient SAR processing algorithms. Third, the well-known nonlinear chirp scaling (NLCS) algorithm is modified according to this new spectrum, and the so-called AHRE-based advanced NLCS (A-NLCS) algorithm is established. The simulation results validate the correctness of our method for L-band SAR systems at altitudes from 1000 to 10 000 km with an azimuth resolution around 3 m. It is also shown that the A-NLCS algorithm has better performance than the CHRE-based algorithms in longer integration time cases. Therefore, we recommend the A-NLCS algorithm for a spaceborne SAR with a lower frequency, finer resolution, and higher satellite altitude.

Multichannel SAR With Reflector Antenna for High-Resolution Wide-Swath Imaging

To overcome the inherent limitation between high azimuth resolution and wide swath in the conventional one-channel synthetic aperture radar (SAR) system, a large receiving planner antenna, which is split into multiple subapertures in both azimuth and elevation, is usually adopted. However, such a system has high system implementation complexity and requires azimuth signal reconstruction under nonuniform sampling condition. To reduce the complexity of the SAR system and processing algorithm, a reflector antenna is adopted in the spaceborne multichannel SAR system presented in this letter for high-resolution wide-swath (HRWS) imaging. According to the special characteristics of reflector antenna, a modified single-phase-center (SPC) technique is presented. The system configuration and imaging scheme are described in detail. A typical spaceborne SAR example with a reflector antenna for HRWS imaging is designed, and its performances are analyzed. Simulation results denote that the presented SAR system shows satisfactory performances with lower data rate, system load, and implementation complexity.

Focusing Spaceborne/Airborne Hybrid Bistatic SAR Data Using Wavenumber-Domain Algorithm

This paper focuses on the bistatic synthetic aperture radar (SAR) data processing in a spaceborne/airborne hybrid bistatic configuration. Due to the extreme differences in platform velocities and slant ranges, the airborne system operates in the inverse sliding-spotlight mode, while the spaceborne system works in the sliding-spotlight mode to achieve a tradeoff between azimuth scene size and azimuth resolution. In this extreme bistatic configuration, our original bistatic formula shows a limitation of accurately describing the bistatic point-target reference spectrum, owing to the assumption of equal contributions of transmitter and receiver to the total Doppler spectrum. We extend our previous formula using the weighting operation where the weighting factor is the ratio of the azimuth time-bandwidth product (TBP) of the platform to the total azimuth TBP. In this paper, the bistatic-deformation and azimuth-dependent range-cell-migration terms were removed with phase multiplications performed blockwise in range-azimuth subsections. The remaining quasi-monostatic term shows the characteristic of the conventional monostatic SAR besides an additional azimuth-scaling term. For the monostatic characteristic, any precision monostatic SAR processing algorithms can handle. In this paper, we prefer the wavenumber-domain algorithm (also known as Omega-K), since it can accurately correct the range dependence of the range-azimuth coupling, as well as the azimuth-frequency dependence. For the azimuth-scaling term, an inverse scaled Fourier transformation is performed to correct it. Finally, a hybrid spaceborne/airborne simulation experiment is conducted to validate the proposed processing procedure.

Squint Spotlight SAR Raw Signal Simulation in the Frequency Domain Using Optical Principles

Synthetic aperture radar (SAR) distributed target scene raw signal simulation is an important tool for the study and test of SAR systems and processing algorithms, mission planning, and inversion algorithm design. In this paper, the squint spotlight SAR scene model is evaluated to achieve spotlight SAR raw signals in the 2-D frequency domain and the precision of the model is analyzed. It is known that the range migration phenomenon in the time domain is explained as the coupling between the range and azimuth in the 2-D frequency domain. To realize the coupling relation in the 2-D frequency, interpolation may be needed. However, interpolation is a time-consuming manipulation. For efficiency, some signal processing methods are employed to couple the range and azimuth frequencies. Those tricks are derived from some optical principles, which give us some novel thoughts. Therefore, the efficiency of the simulator is highly improved, which facilitates its application to the verification and test of the real-time processor.

An Improved Doppler Parameter Estimator for Synthetic Aperture Radar

To achieve a high quality synthetic aperture radar (SAR) image, the motion caused by the radar and/or the moving target needs be compensated for coherent processing, and this requires the accurate estimation of the Doppler parameters of the received signals. A new, ap- proximate, maximum likelihood (ML) estimator in the time domain is obtained recently to jointly estimate the Doppler parameters, and it is shown to have much better performance than that of an existing approximate ML estimator in the literature, for the values of the Doppler parameters of interest. The efiects of the parameter estimation errors on the SAR system performance are analyzed. By revealing the relationship between the range/azimuth resolution of SAR imaging and the estimation accuracy of the Doppler parameters, we show that our new estimator can be applied in SAR imaging to improve the image quality. 1. INTRODUCTION With the reputation of high resolution and impressive quality of image, synthetic aperture radar (SAR) has played an important role in cartography, oceanography, and numerous military applica- tions (1). Since the signal energy from a point target is spread in range and azimuth, the purpose of SAR focussing is to collect this dispersed energy into a single pixel in the output image. The optimum focusing of the SAR data is a space-variant and two-dimensional operation, which makes SAR processing a challenge. The most popular SAR processing algorithm is the Range-Doppler technique (2) and its variations; see (3) and (4). The method is e-cient, and in principle, solves the problems of azimuth focussing and range cell migration correction. These imaging algorithms require the accurate estimation of the Doppler parameters, namely, the centroid Doppler frequency and the frequency rate, to perform coherent processing. Our main goal here is to estimate accu- rately the Doppler parameters to compensate for the motion caused by the radar or the moving targets. This compensation is challenging and important for moving targets, where the motion is non-cooperative as in inverse SAR (ISAR) (5). Without correct motion compensation, the im- age quality may be degraded in several ways, such as shifting, distortion, defocusing and so on. The traditional estimation for the centroid Doppler frequency and the frequency rate is usually per- formed separately for simplicity by using clutter-locking and autofocus technologies, respectively (6). However, this leads to error propagation, and thus, the optimal estimator is to jointly implement clutter-locking and autofocus. The conventional joint estimation in the frequency domain (7) is a two-dimensional, nonlinear search. Unfortunately, there is still no exact, closed-form solution for solving the general nonlinear programming problem, and the search complexity is high. In this paper, we present a new, approximate, maximum likelihood (ML) estimator in the time domain. It shows that for the range of values of interest of the Doppler parameters (6), our estimator leads to a better performance than that of the only other existing approximate, ML estimator in the time domain, i.e., the Djuric-Kay (DK) estimator (8).

Trajectory deviations in airborne SAR: analysis and compensation

This paper concerns the analysis and compensation of trajectory deviations in airborne synthetic aperture radar (SAR) systems. Analysis of the received data spectrum is carried out with respect to the system geometry in the presence of linear, sinusoidal, and general aircraft displacements. This shows that trajectory deviations generally produce spectral replicas along the azimuth frequency that strongly impair the quality of the focused image. Based on the derived model, we explain the rationale of the motion compensation (MOCO) strategy that must be applied at the SAR processing stage in order to limit the resolution loss. To this end aberration terms are separated into range space invariant and variant components. The former can be accounted for either in a preprocessing step or efficiently at range compression stage. The latter needs a prior accommodation of range migration effect. We design the procedure for efficient inclusion of the MOCO within a high precision scaled FT based SAR processing algorithm. Finally, we present results on simulated data aimed at validating the whole analysis and the proposed procedure.

ERS-1 processing chain development and product support by the EODC science team, the algorithm development facility and the calibration and validation facility

Abstract The Earth Observation Centre (EODC), located at Farnborough, has been developed to serve the needs of users of data from Earth remote sensing missions, beginning with the European Space Agency's ERS-1 spacecraft. To this end, the facilities of the ESA Processing and Archiving Facility (PAF) for ERS-1 reside within the EODC. With the exception of the core Synthetic Aperture Radar processing algorithms, the majority of the scientific processing chains for exploitation of data from ERS-1 at the EODC were designed and specified by a Science Team, consisting of specialists from UK universities, industry, and government research institutes. These designs were implemented by an industrial consortium. The Science Team was supported by an Algorithm Development Facility (ADF) during the processing chain development. The Science Team currently provide operational support using the EODC Calibration and Validation Facility (CVF). This consists of the verification, calibration, validation, and continuous asses...

Benchmarking A Massively ParallelComputer By A Synthetic Aperture RadarProcessing Algorithm

Benchmarking A Massively ParallelComputer By A Synthetic Aperture RadarProcessing Algorithm