Range Velocity Component Research Articles

Slow-Moving Ground Target Imaging Using Vortex Synthetic Aperture Radar

Abstract A vortex synthetic aperture radar (SAR) can obtain more target information when combined with orbital angular momentum (OAM). However, ground-moving targets can cause image defocusing owing to their azimuthal velocity component and imaging position offset issues due to their range velocity component. However, more information on moving targets can be acquired for analysis using vortex SAR. This study first established an imaging model to extract moving target information from vortex echo signals. The target Doppler parameters were then estimated, and an improved range-Doppler algorithm was applied to compensate for the Bessel term and azimuthal phase term. A new azimuth-matched filter was designed based on the Doppler parameters to achieve focused imaging of moving targets. The simulation results verified the effectiveness of the proposed algorithm, and its performance was compared with that of plane-wave SAR. Vortex SAR offered better target quality parameters and azimuth resolution for the same synthetic aperture length. The proposed algorithm can effectively improve the azimuthal imaging of moving targets in low-tomoderate OAM modes.

A Theory of Multiaperture Along-Track Interferometric Synthetic Aperture Radar

In this letter, a novel theory is proposed to measure the velocity vector of moving targets such as ocean currents by multiaperture along-track interferometric synthetic aperture radar (MA-ATI SAR), utilizing the conventional dual-antenna ATI SAR data. In this system, the range component of the velocity vector is first computed with the conventional ATI SAR algorithm by correlating the two sets of complex images produced through the fore and aft antennas. For estimating the direction of the velocity vector, multilook/sublook processing is applied to two sets of range-compressed azimuth raw data. Now, there are four sets of complex subimages, two of which are forward-looking and the others are backward-looking. Then, from these subimage pairs, the two sublook interferograms of the corresponding directions are produced. The direction of the moving targets can be estimated either from the forward- and backward-looking azimuth velocity components, or directly from the interferometric phases. With this direction and the range velocity component, the velocity vectors of ocean currents and also moving hard targets on land and sea can be estimated.

Doppler Frequency Estimation of Point Targets in the Single-Channel SAR Image by Linear Least Squares

This paper presents a method and results for the estimation of residual Doppler frequency, and consequently the range velocity component of point targets in single-channel synthetic aperture radar (SAR) focused single-look complex (SLC) data. It is still a challenging task to precisely retrieve the radial velocity of small and slow-moving objects, which requires an approach providing precise estimates from only a limited number of samples within a few range bins. The proposed method utilizes linear least squares, along with the estimation of signal parameters via rotational invariance techniques (ESPRIT) algorithm, to provide optimum estimates from sets of azimuth subsamples that have different azimuth temporal distances. The ratio of estimated Doppler frequency to root-mean square error (RMSE) is suggested for determining a critical threshold, optimally selecting a number of azimuth subsample sets to be involved in the estimation. The proposed method was applied to TerraSAR-X and KOMPSAT-5 X-band SAR SLC data for on-land and coastal sea estimation, with speed-controlled, truck-mounted corner reflectors and ships, respectively. The results demonstrate its performance of the method, with percent errors of less than 5%, in retrieved range velocity for both on-land and in the sea. It is also robust, even for weak targets with low peak-to-sidelobe ratios (PSLRs) and signal-to-clutter ratios (RCSs). Since the characteristics of targets and clutter on land and in the sea are different, it is recommended that the method is applied separately with different thresholds. The limitations of the approach are also discussed.

MOVING-TARGET VELOCITY ESTIMATION IN A COMPLEX-VALUED SAR IMAGERY

In the ground moving target indication with synthetic aperture radar (SAR) community, algorithms used to estimate the velocity of a detected moving target are important because they are relative to the topics about refocusing and azimuth displacement correction. The velocity is regarded as a vector with two components, one in azimuth and one in range direction, and new algorithms aiming at estimating the two components are proposed and verifled. The range velocity estimator transforms a detected patch containing a moving target to range Doppler domain by using the 1-D fast Fourier Transform in each range bin to achieve its range Doppler locus. The slope of the range Doppler locus is computed by using the Radon Transform on the range Doppler plane and the range velocity component is worked out according to radar system parameters and the slope value. Two estimators are proposed to compute the azimuth velocity component. One is based on symmetric defocusing in Doppler domain, the other is based on phase gradient in wave-number domain. Experiments conflrm the efiectiveness of the estimators by using simulated and fleld data.

Ground moving target indication and parameters estimation using a dual-frequency synthetic aperture radar

A new scheme is presented to estimate the range and azimuth velocity components of a detected moving target by using a dual-frequency synthetic aperture radar (SAR). It consists of a moving target detector, a range velocity estimator, and an azimuth velocity estimator. In this scheme, two original SAR images are achieved from the returns first, and then processed by a symmetric defocusing filter pair (SDFP) to produce two defocused images. By comparing the sharpness of the two defocused images, the moving targets are indicated and isolated form each original SAR image. For a selected moving target, its range velocity component is estimated by using a Doppler ambiguity solver and a stepped approximation-and-comparison algorithm. After range velocity compensated, the target in the patch is concentrated in less range bins, and its azimuth velocity component is estimated by using an SDFP bank. Finally, the moving target is refocused and its azimuth displacement caused by range velocity component is corrected. The effectiveness of the proposed scheme is confirmed by the experiments with the field and simulated data.