Viet Thuy Vu

Detection of Moving Targets by Focusing in UWB SAR—Theory and Experimental Results

Moving-target detection in ultrawideband (UWB) synthetic aperture radar (SAR) is associated with long integration time and must accommodate azimuth focusing for reliable detection. This paper presents the theory on detection of moving targets by focusing and experimental results on single-channel SAR data aimed at evaluating the detection performance. The results with respect to both simulated and real data show that the ability to detect moving targets increases significantly when applying the proposed detection technique. The improvement in signal-to-clutter noise ratio, which is a basic requisite for evaluating the performance, reaches approximately 20 dB, using only single-channel SAR data. This gain will be preserved for the case of multichannel SAR data. The reference system for this study is the airborne UWB low-frequency SAR Coherent All RAdio BAnd Sensing II.

An Impulse Response Function for Evaluation of UWB SAR Imaging

Based on analysis of a point target imaged by different synthetic aperture radar (SAR) systems, the commonly used impulse response function in SAR Imaging (IRF-SAR)-a two-dimensional (2-D) sinc function-is shown to be inappropriate for ultrawideband-ultrawidebeam (UWB) SAR systems utilizing a large fractional signal bandwidth and a wide antenna beamwidth. As a consequence, the applications of the 2-D sinc function such as image quality measurements and spatial resolution estimations are limited to narrowband-narrowbeam (NB) SAR systems exploiting a small fractional signal bandwidth and a narrow antenna beamwidth. In this paper, a more general IRF-SAR, which aims at UWB SAR systems, is derived with an assumption of flat two-dimensional (2-D) Fourier transform (FT) of a SAR image and called IRF-USAR. However, the derived IRF-USAR is also valid for NB SAR systems.

RFI Suppression in Ultrawideband SAR Using an Adaptive Line Enhancer

In this letter, we propose an approach to suppress radio-frequency interference (RFI) in ultrawideband (UWB) low-frequency synthetic aperture radar (SAR). According to the proposal, RFI is suppressed by using an adaptive line enhancer controlled by the normalized least mean square algorithm. The approach is tested successfully on real UWB low-frequency SAR data. In order to keep the computational burden down, possible ways to integrate the RFI suppression approach into SAR imaging algorithms are also suggested.

Suppression of Clutter in Multichannel SAR GMTI

In this paper, the results of moving-target detection in multichannel UHF-band synthetic aperture radar (SAR) data are shown. The clutter suppression is done using finite-impulse response (FIR) filtering of multichannel SAR in combination with a two-stage fast-backprojection algorithm to focus the moving target using relative speed. The FIR filter coefficients are chosen with the use of space-time adaptive processing filtering. Two parameters are used for target focusing, target speed in range and in azimuth. When the target is focused, both speed parameters of the target are found. In the experimental results, two channels were used in order to suppress clutter. In the resulting SAR images, it is obvious that very strong scatterers and the forest areas have been suppressed in comparison to the moving target in the image scene. The gain obtained can be measured using signal-to-clutter-and-noise-ratio gain, which is about 19 dB. Another way to measure the signal processing gain is the ability to suppress the strongest reflecting object in the SAR scene. The gain of the target in relation to this object is 25 dB. This shows that using UHF-band SAR ground moving target indication (GMTI) for suppressing forest and increasing the target signal can work.

Moving Target Relative Speed Estimation and Refocusing in Synthetic Aperture Radar Images

In this paper, a method for moving target relative speed estimation and refocusing based on synthetic aperture radar (SAR) images is derived and tested in simulation and on real data with good results. Furthermore, an approach on how to combine the estimation method with the refocusing method is introduced. The estimation is based on a chirp estimator that operates in the SAR image and the refocusing of the moving target is performed locally using subimages. Focusing of the moving target is achieved in the frequency domain by phase compensation, and therefore makes it even possible to handle large range cell migration in the SAR subimages. The proposed approach is tested in a simulation and also on real ultrawideband (UWB) SAR data with very good results. The estimation method works especially well in connection with low frequency (LF) UWB SAR, where the clutter is well focused and the phase of the smeared moving target signal becomes less distorted. The main limitation of the approach is target accelerations where the distortion increases with the integration time.

Fast Time-Domain Algorithms for UWB Bistatic SAR Processing

Two fast time-domain algorithms are introduced for ultrawideband-ultrawidebeam (UWB) bistatic synthetic aperture radar (SAR) processing; they are bistatic fast backprojection (BiFBP) and bistatic fast factorized backprojection (BiFFBP). Both algorithms process radar echoes on a subaperture and subimage basis in order to minimize processing time. They are shown to work with any configuration of bistatic SAR. They also own time-domain characteristics, which are essential for UWB radar signal processing. BiFBP and BiFFBP are experimented successfully on the CARABAS-II simulated data.

A Comparison between Fast Factorized Backprojection and Frequency-Domain Algorithms in UWB Lowfrequency SAR

Two frequency-domain algorithms chirp scaling (CS) with the advantage of simplification and range migration (RM) with the advantage of accuracy are candidates for a comparative study to the time-domain algorithm Fast factorized backprojection (FFBP) with reference to a UWB system are presented in this paper. The comparison is based on UWB SAR image quality measurements such as spatial resolution, Integrated Sidelobe Ratio (ISLR), peak sidelobe ratio (PSLR) and processing time connected to computational cost. The simulated SAR data, which is used in this study, is based on the parameters of the airborne UWB low frequency CARABAS-II system.

Definition on SAR image quality measurements for UWB SAR

Analyses in this study show that measurements under currently used definitions on SAR image quality measurement may be unsuitable for UWB SAR. The main objective of this paper is therefore to propose a definition based on the shape of a single point target in a SAR image which is more suitable for UWB SAR. We use both real and simulated data based on the airborne UWB low frequency SAR CARABAS-II in experiments. The time-domain algorithm Global Backprojection (GBP) is selected for the image formation in this study.

Moving Target Detection by Forcusing for Frequency Domain Algorithms in UWB Low Frequency SAR

Moving target detection at low radar frequencies is associated with long integration time and has to handle azimuth focusing for reliable detection. The detection by focusing in which time-domain is the basis for the focusing approach has been proposed and successfully experimented in reality. The main objective of this paper is to apply and evaluate the detection method for frequency-domain algorithms. Range Migration Algorithm (RM) is chosen as a candidate for this study due to the accuracy of the algorithm with very wide integration angles. The simulated SAR data, which is used in this study, is based on the parameters of a airborne UWB low frequency system such as CARABASII.

Fast backprojection algorithm for UWB bistatic SAR

The paper introduces an algorithm for Ultrawideband Ultrawidebeam (UWB) bistatic Synthetic Aperture Radar (SAR). The algorithm works in time-domain and therefore inherits time-domain characteristics such as unlimited scene size, local processing and manageable motion compensation. The proposed algorithm is not limited by any configuration of bistatic SAR. The algorithm processes the UWB bistatic SAR data on a subaperture and subimage basis. This means, instead of backprojecting directly the SAR data to a ground image plane, the algorithm handles the data in two stages: beam forming and local backprojection. The algorithm is named Bistatic Fast Backprojection (BiFBP) and has been tested successfully with the simulated UWB bistatic SAR data.