Hongjun Song

Arc FMCW SAR and Applications in Ground Monitoring

As a novel mode of ground-based synthetic aperture radar (GB-SAR), Arc frequency-modulated continuous wave (FMCW) SAR is rarely discussed in the literature up to now. Compared with the conventional rail GB-SAR system, the synthetic aperture is formed by the rotation of antennas, which can scan a wide azimuth extent. Due to its convenience and potential in SAR applications, the Institute of Electronics, Chinese Academy Sciences, carried out a series of Arc FMCW SAR experiments in March 2013, and a lot of results were obtained. In this paper, we discuss the signal processing of Arc FMCW SAR and its first results in change detection and interferometric SAR. The components of the Arc FMCW SAR system are first described, and then, we focus on its signal processing, where the signal model and its properties are investigated; two focusing algorithms are developed for different purposes. The effectiveness of the algorithms is validated by both simulation and real data experiments. We also exhibit the applications of Arc FMCW SAR in landslide detection and digital-elevation-model extraction for the first time. Results show its huge potential in ground-based applications.

An Accurate Range Model Based on the Fourth-Order Doppler Parameters for Geosynchronous SAR

This letter presents an accurate range model, named as Doppler parameter range model (DRM), to perform signal processing for geosynchronous synthetic aperture radar (Geo-SAR). Besides that a credible approach, defined as accurate coordinate numeric model (ACNM), is proposed to simulate the raw echo signals. To improve the accuracy of DRM and ACNM, the rotating Earth is modeled as an ellipsoid and the eccentricity of the satellites orbit is analyzed. The superiority of DRM that it is more precise than hyperbolic range equation (HRE) and advanced hyperbolic range equation (AHRE), presented before, is illuminated. The point target 2-D frequency spectrum of DRM is derived. Finally, the simulation results validate that DRM and ACNM has a good effect on an L-Band Geo-SAR system with an azimuth resolution around 5 m.

A Novel High-Order Range Model and Imaging Approach for High-Resolution LEO SAR

High-resolution spaceborne synthetic aperture radar (SAR) mainly poses two challenges to signal processing. The first challenge involves the signal model, where a precise range equation of spaceborne SAR should be considered as the conventional hyperbolic range equation fails to precisely describe the range history in the high-resolution case. The second challenge is an efficient focusing algorithm since the existing SAR processors are inaccurate or inefficient for high-resolution spaceborne SAR. Therefore, in this paper, a novel fourth-order polynomial range equation based on Doppler parameters is proposed, and the method for parameter determination is also addressed. Compared with conventional range equations, the presented one is more accurate and concise for low-earth-orbit SAR so that a higher azimuth resolution can be achieved. Based on the range model, a 2-D spectrum is derived, and an extended range Doppler domain algorithm for SAR image formation in the sliding spotlight mode is also developed. Additionally, we carried out several simulations to validate the presented approach. Results demonstrate high performances of the focusing algorithm as well as the range equation.

The Accurate Fourth-Order Doppler Parameter Calculation and Analysis for Geosynchronous SAR

This paper presents a new approach to calculate the 1{ 4th order Doppler parameters for Geosynchronous Synthetic Aperture Radar (Geo-SAR). To get accurate calculation results, the Earth is modeled as an ellipsoid and the relative motion between the sensor in an elliptical orbit and the rotating Earth is analyzed. The J2, J3 and J4 orbital perturbation items and attitude steering are analyzed. Ignoring the perturbation force would produce errors of the Doppler parameters for spaceborne SAR because it can in∞uence the six orbital elements. Since the Doppler parameters are related to the antenna beam pointing directions and in∞uenced by attitude of SAR platform, the calculation results before and after attitude steering are shown. Furthermore, the Doppler parameter properties during the whole orbital periods of Geo- SAR are compared with those of Low-Earth-Orbital SAR (Leo-SAR). Finally, the efiects on Doppler parameters stemmed from the radar beam pointing accuracy are analyzed.

Modified frequency scaling processing for FMCW SAR

In this paper, we present a modified frequency scaling processing method for FMCW SAR data. Compared to the conventional frequency scaling algorithm for FMCW SAR, an analytical slant model is employed so that it can work well in all conditions for FMCW SAR. Firstly, the algorithm model is deduced in detail and its implementation is given, and then a simulation experiment is carried on to evaluate its efficiency. Finally, real data of FMCW SAR validate the proposed algorithm.

An Accurate and Efficient Extended Scene Simulator for FMCW SAR With Static and Moving Targets

Conventional frequency-domain frequency-modulation continuous-wave (FMCW) synthetic aperture radar (SAR) raw data simulators (RDSs) can only handle the case of static targets while the scene with moving targets cannot be accommodated. In FMCW SAR, the motion of the radar and moving targets during the pulse duration must be considered, thus posing a challenge for the FMCW SAR RDS. An accurate point target reference spectrum that considers this motion is first derived in the literature. Based on the spectrum, we design an accurate and efficient FMCW SAR RDS. Compared with its counterparts, it can precisely simulate the raw data of moving targets even in the case of high resolution and high squint. Both point target and extended scene simulations are performed to validate the simulator, and its efficiency is also analyzed. Results show its outstanding performance for generating the FMCW SAR raw data of extended scenes with static and moving targets.

A Modification to the Complex-Valued MRF Modeling Filter of Interferometric SAR Phase

This letter focuses on a modified complex-valued Markov random field (CMRF) modeling filter to improve the performance in the filtering of the synthetic aperture radar interferometric phase. In the reference CMRF modeling phase map filter, the CMRF model was employed to update residues and their neighbors. By this, the residues were reduced, and phase jumps were preserved simultaneously. However, residue reduction was still insufficient. Furthermore, incorrect model parameter estimation leads to wrong filtering in some blocks. To solve the two aforementioned problems, we propose a modified CMRF modeling filter, where the original interferogram is divided into overlapped blocks. In addition, the adaptive weighted neighbor values are used to estimate the CMRF model parameters. Both simulated and real data experiments are performed to validate this method.

A patch-based filter for InSAR interferograms

In this paper, an adaptive patch-based filter for Synthetic aperture Radar (SAR) Interferometric phase in the Fourier transform complex domain is proposed. The proposed filter fully utilizes the properties of locally planar terrain phase noise model so that the noise can be suppressed according to the local level of noise. Moreover, the denoising performance of the proposed filter is further improved by an adaptive selection strategy for patch size. The experimental results show that the proposed method delivers state-of-the-art denoising performance in terms of objective criteria.

A spectral diversity based approach for DEM reconstruction

In this letter, an approach based on spectral diversity and conventional Interferometric Synthetic Aperture Radar (InSAR) for Digital Elevation Model (DEM) reconstruction is proposed. Conventional InSAR technique often fails to correctly reconstruct the DEM due to the presence of discontinuities and/or interferometric noise. With the assistance of the spectral diversity method, the ill-posed phase-unwrapping can be regularized. Thus the capabilities of the conventional InSAR technique for DEM reconstruction can be notably improved. Comparing to the multi-channel InSAR technique, the proposed method requires less SAR images. Comparing to the multifrequency technique, the performance of the proposed method depends on the perpendicular or effective baseline and the ratio of the radars carry frequency to the bandwidth between the central frequencies of the two sub-looks; while the multifrequency technique depends more on the range bandwidth. Thus, the proposed method is more flexible than the multifrequency technique. The effectiveness of the proposed algorithm is validated by experimental results using real data.

Special MISO-SAR and MIMO-SAR Modes for Bidirectional Imaging

The paper proposes special multiple-input single-output synthetic aperture radar (MISO-SAR) and multiple-input multiple-output SAR (MIMO-SAR) for bidirectional imaging, which can simultaneously illuminate two areas from different directions in azimuth. For the proposed MISO-SAR, two subpulses with the same carrier frequency and phase coding are transmitted with different azimuth directions by switching the phase coefficients in the transmit modules, and echoes corresponding to the subpulses are received by the main lobe and the first grating lobe of the whole antenna. To suppress mutual interference, the two subpulses are transmitted with different range-frequency bands, and their echoes are demodulated and recorded in different channels in the proposed MIMO-SAR. This paper presents the system design of these modes and analyzes their azimuth ambiguity to signal ratio (AASR). Besides, simulation results on points are carried out to validate the proposed bidirectional imaging modes.