Doppler Centroid Frequency Research Articles

Airborne SAR Moving Target Signatures and Imagery Based on LVD

This paper presents a new ground moving target imaging (GMTIm) algorithm for airborne synthetic aperture radar (SAR) based on a novel time-frequency representation (TFR), Lv's distribution (LVD). We first analyze generic moving target signatures for a multichannel SAR and then derive the analytical spectrum of a point target moving at a constant velocity by a polar format algorithm for SAR image formation. SAR motion deviation from a predetermined flight track is considered to facilitate airborne SAR applications. LVD, as a recently developed TFR for the analysis of multicomponent linear-frequency-modulated signal, is adopted to represent the target kinematic spectrum in the Doppler centroid frequency and chirp rate domain. As a result, the proposed SAR-GMTIm algorithm is capable of imaging multiple moving targets even when they are located at the same range resolution cell. Some practical issues such as imaging maneuvering targets and small/weak targets are discussed to enhance the applicability of the proposed algorithm. Simulation results with isotropic point moving targets are presented to validate the effectiveness and superiority of the proposed algorithm. Raw data collected by an airborne multichannel SAR are also used to verify the performance improvement made by the proposed algorithm.

Imaging Formation Algorithm of the Ground and Space-Borne Hybrid BiSAR Based on Parameters Estimation from Direct Signal

This paper proposes a novel image formation algorithm for the bistatic synthetic aperture radar (BiSAR) with the configuration of a noncooperative transmitter and a stationary receiver in which the traditional imaging algorithm failed because the necessary imaging parameters cannot be estimated from the limited information from the noncooperative data provider. In the new algorithm, the essential parameters for imaging, such as squint angle, Doppler centroid, and Doppler chirp-rate, will be estimated by full exploration of the recorded direct signal (direct signal is the echo from satellite to stationary receiver directly) from the transmitter. The Doppler chirp-rate is retrieved by modeling the peak phase of direct signal as a quadratic polynomial. The Doppler centroid frequency and the squint angle can be derived from the image contrast optimization. Then the range focusing, the range cell migration correction (RCMC), and the azimuth focusing are implemented by secondary range compression (SRC) and the range cell migration, respectively. At last, the proposed algorithm is validated by imaging of the BiSAR experiment configured with china YAOGAN 10 SAR as the transmitter and the receiver platform located on a building at a height of 109 m in Jiangsu province. The experiment image with geometric correction shows good accordance with local Google images.

A Study of Sea Surface Range-Resolved Doppler Spectra Using Numerically Simulated Low-Grazing-Angle Backscatter Data

A numerical study of sea surface range-resolved Doppler spectra using low-grazing-angle backscatter measurements is described. Backscattered fields as a function of frequency are computed using the method of moments (MOM) for a single realization of a 1-D oceanlike surface profile as the realization evolves in time. Transformation into the range-Doppler domain enables examination of properties of the resulting Doppler spectra (for both HH and VV polarizations) and their relationship to properties of the surface profile. In general, a strong correspondence between the “long-wave” orbital velocity of the surface projected along the radar line of sight and the Doppler centroid frequency is observed for visible portions of the surface, as well as some evidence of relationships between the “width” of the Doppler spectrum and variations of the projected velocity in time at a given range point. Evidence of similar relationships even in some shadowed portions of the surface is also provided. Doppler spectra from HH and VV polarizations are qualitatively similar, despite differences in total power levels, although the portion of shadowed surface points from which Doppler information is available is somewhat larger in VV polarization. A further examination is conducted using backscattered fields computed with a single-scattering method, which neglects shadowing and any multiple-scattering effects. The remarkable similarities observed between MOM and single-scattered Doppler spectra even in some shadowed portions of the surface suggest that non-line-of-sight propagation effects do not significantly influence Doppler properties in such regions.

ScanSAR interferometric processing using existing standard InSAR software for measuring large scale land deformation

Scanning synthetic aperture radar (ScanSAR) mode is an efficient way to map large scale geophysical phenomena at low cost. The work presented in this paper is dedicated to ScanSAR interferometric processing and its implementation by making full use of existing standard interferometric synthetic aperture radar (InSAR) software. We first discuss the properties of the ScanSAR signal and its phase-preserved focusing using the full aperture algorithm in terms of interferometry. Then a complete interferometric processing flow is proposed. The standard ScanSAR product is decoded subswath by subswath with burst gaps padded with zero-pulses, followed by a Doppler centroid frequency estimation for each subswath and a polynomial fit of all of the subswaths for the whole scene. The burst synchronization of the interferometric pair is then calculated, and only the synchronized pulses are kept for further interferometric processing. After the complex conjugate multiplication of the interferometric pair, the residual non-integer pulse repetition interval (PRI) part between adjacent bursts caused by zero padding is compensated by resampling using a sinc kernel. The subswath interferograms are then mosaicked, in which a method is proposed to remove the subswath discontinuities in the overlap area. Then the following interferometric processing goes back to the traditional stripmap processing flow. A processor written with C and Fortran languages and controlled by Perl scripts is developed to implement these algorithms and processing flow based on the JPL/Caltech Repeat Orbit Interferometry PACkage (ROI_PAC). Finally, we use the processor to process ScanSAR data from the Envisat and ALOS satellites and obtain large scale deformation maps in the radar line-of-sight (LOS) direction.

Parameter estimation of moving targets in the SAR system with a low PRF sampling rate

In the synthetic aperture radar (SAR) system with low pulse repetition frequency (PRF) sampling, it is difficult for the motion parameters estimation of the moving targets, because of the Doppler spectrum ambiguity and Doppler centroid frequency ambiguity of the echo signals. Considering that moving targets are sparsely distributed in the observed scene, their positions and velocities can be reconstructed by using the compressed sensing (CS) technique. In this paper, the range-walk correction are implemented by the Keystone transform and the sparse range-walk correction (SRWC), then the CS technique is proposed to reconstruct motion parameters by processing the azimuth signals of the moving targets. Experiments using the simulated and real data are performed, and the results confirm the validity of the proposed method.

A New Method of Zero-Doppler Centroid Control in GEO SAR

Yaw steering is a method of Doppler centroid compensation performed by attitude steering of the satellite platform. However, the conventional yaw steering method does not work well as the satellite orbit height becomes higher, such as with the geosynchronous Earth orbit synthetic aperture radar (GEO SAR) which has 36 000-km orbit height. This letter primarily analyzes the Doppler properties of the GEO SAR in the satellite local coordinate systems, and gives the calculation formulas of the Doppler centroid frequency and instantaneous Doppler bandwidth. Based on the vector analysis, a new calculation method is proposed for the 2-D attitude steering angles in GEO SAR. The new method, considering Earth's rotation and elliptical orbit effects, can accurately compensate the Doppler centroid to zero. Then, according to the 2-D attitude steering angles and the characteristics of GEO SAR, the 2-D phase scan method is put forward to carry out the highly accurate compensation of Doppler centroid, which can avoid the difficulty of rotating and stabilizing the GEO SAR system. The simulations validate the conclusion in this letter.

Research on Elliptic Orbit Total Zero Doppler Steering

星载合成孔径雷达(SAR)数据处理中，多普勒特性对于方位向性能以及成像精度都有直接的影响。基于轨道动力学理论，在椭圆轨道情况下，该文导出了同时考虑偏航控制和俯仰控制的多普勒中心频率解析表达式，分别分析了偏航控制和俯仰控制对多普勒中心的影响，提出一种基于椭圆轨道的全零多普勒方法，并用TerraSAR-X轨道参数进行了数值仿真，仿真结果表明，使用该方法后，多普勒中心频率被减小到不超过5 Hz，比传统的偏航导引方法缩小了约一百倍，比基于瞬时圆轨道的全零多普勒方法缩小了5倍，表明了该方法的有效性。

Estimation of doppler centroid frequency in spaceborne ScanSAR

Doppler centroid frequency is an essential parameter in the imaging processing of the Scanning mode Synthetic Aperture Radar (ScanSAR). Inaccurate Doppler centroid frequency will result in ghost images in imaging result. In this letter, the principle and algorithms of Doppler centroid frequency estimation are introduced. Then the echo data of ScanSAR system is analyzed. Based on the algorithms of energy balancing and correlation Doppler estimator in the estimation of Doppler centroid frequency in strip mode SAR, an improved method for Doppler centroid frequency estimation in ScanSAR is proposed. The method has improved the accuracy of Doppler centroid frequency estimation in ScanSAR by zero padding between burst data. Finally, the proposed method is validated with the processing of ENVIronment SATellite Advanced Synthetic Aperture Radar (ENVISAT ASAR) wide swath raw data.

Analysis and Focusing of Bistatic Airborne SAR Data

Bistatic synthetic aperture radar (SAR) processing requires precise knowledge about geometrical parameters of the flown bistatic constellation, whereas estimation of these parameters is even more important than in the monostatic case. As it is impossible to separate the individual semimonostatic parameters from the bistatic raw data, the searched parameters are derived from the tracks of the moving platforms. For this reason, global positioning system (GPS) and inertial navigation system (INS) position and velocity data of transmitter and receiver are combined in an optimal way by a Kalman filter approach. As a consequence of model-based interpolation, we can easily determine varying parameters like Doppler centroid frequency, azimuth velocity, and the bistatic parameters a2 and a0 referring to the illuminated scene at each time instant. Accurately determined position and velocity of the transmitter and receiver enable a first-order bistatic motion compensation (MC), which is also described in this paper. In verifying our approach, GPS/INS tracks and raw data of a bistatic airborne SAR experiment flown in 2003 were used, where the data were provided by Forschungsgesellschaft fu umlr Angewandte Naturwissenschaften e.V., Wachtberg, Germany. The focusing was done by a scaled inverse Fourier transformation processor for nearly parallel trajectories for transmitter and receiver. This paper focuses on analyzing the trajectories and antenna beams of bistatic missions to obtain accurate processing parameters and MC functions.

Geocoding SAR interferograms by least squares adjustment

A geocoding model for Synthetic Aperture Radar (SAR) interferometry based on a least-squares adjustment combining interferometric phase, range, Doppler centroid frequency, flight path and control point data is proposed. The complete mathematical framework for the computation of object space coordinates without approximations is presented. It provides a way to an efficient implementation of the algorithm for geocoding the pixels of an interferogram. The method is preferably applicable to spaceborne dual-pass interferometry, and independent of the orbit configuration. An accuracy analysis of object point positioning is conducted and results of geocoding an ERS tandem interferogram are shown.

Blind deconvolution for Doppler centroid estimation in high frequency SAR

For high-quality SAR (synthetic aperture radar) processing, the Doppler centroid frequency is needed. However, SAR data are sampled along the azimuth direction at the pulse repetition frequency (PRF); the estimation of the Doppler centroid frequency by means of spectral analysis techniques may produce ambiguous results due to aliases. The mathematical expression of the residual error that occurs when SAR data are focused with an incorrect alias of the PRF is thus derived. Then, a blind deconvolution technique is used to estimate the actual PRF replica from the focused image. Squinted X-band data, corresponding to those that will be generated by the SIR-C mission, have been generated from the JPL-AirSAR L- and C-band data by means of an inversion of the focusing process. Even if the real data may show differences with respect to the simulated data, the blind deconvolution method appears to be more precise and robust than the other conventional techniques tested. >