In this paper, a novel non-local filtering method based on fringe compensation using multi-frequency data is proposed. Firstly, the low-frequency phase is unwrapped and converted to reference frequency according to the frequency ratio to estimate the basic phase of the fundamental terrain, then the basic phase is removed from the high-frequency noisy phase, and non-local filtering is performed on the residual phase. Finally, the filtered phase is obtained by compensating the removed basic phase. Simulation and real data experiments are carried out to verify the effectiveness of the proposed method.

N-SAR-SG is a Chinese multi-functional integrated experimental airborne SAR system developed by Nanjing Research Institute of Electronic Technology. It supports up to 6 frequency bands (P, L, S, C, X and Ku), which can operate with multi-band, -polarimetry, -channel, -mode, and - baseline simultaneously. To certificate the systems, as well as new concepts, new applications, and new technologies in microwave remote sensing, we carried out a series of flight experiments. Based on the dataset, some interesting processing results are obtained such as dual band tree height precise inversion, ground elevation measurement enhancement with the combination of cross-track and along-track interferometry, terrain Classification based on joint frequency-interference-polarization SAR information. These novel experiments not only prove the progressiveness of N-SAR-SG, but also inspire some potential development directions of SAR techniques.

Polarimetric synthetic aperture radar (SAR) tomography (Pol-TomoSAR) can be used for global forest digital terrain model (DTM) and canopy height model (CHM) mapping with high spatial and temporal resolution at low economic cost. However, the performance of DTM and CHM inversion in current Pol-TomoSAR methods is often compromised when the ground-to-volume ratio (GVR) is low, which usually happens in complicated terrain where large negative slope angles are commonly present, or in dense tropical forest where the ground visibility is low due to strong attenuation by the dense vegetation layer. In this work, a novel method for the separation of ground and volume scattering, aiming at robust and accurate DTM and CHM inversion in dense tropical forest and complicated terrain, is proposed. By fully exploiting multibaseline polarimetric SAR data, the proposed method can perform a more effective separation of ground and volume scattering. Subsequently, by applying the SAR tomography technology on the separated ground and volume scattering, the proposed method can retrieve accurate DTM and CHM information even at low GVR areas. Numerical experiments conducted on both simulated data and P-band airborne F-SAR data show that, compared to the most commonly used two-component algebraic synthesis method, the proposed one has a much better performance in terms of separation of ground and volume scattering. Furthermore, the inversed DTM and CHM present better agreement with light detection and ranging (LiDAR) measurements, especially in large negative slope angle terrain where the GVR is rather low.

Future SAR missions may have a great demand for high-resolution wide-swath imaging. Digital beamforming with SCAN-ON-RECEIVE (SCORE) in elevation is a powerful technique, which is characterized by using a narrow and high gain receive beam to follow the echoes backscattered from the illuminated areas timely. Generally, it is beneficial to increase the pulse duration and antenna size to improve the signal-to-noise (SNR) ratio. However, the pulse extension loss (PEL) would also be more significant, especially in topographic variation situation. This paper focuses on PEL compensation in presence of terrain height variations. With assistance of the external DEM, the numerical representation of actual DoA is established first, and an innovative SCORE approach is proposed by applying appropriate time varying weights to each subband of the echo. This approach can achieve better radiometric quality. Theoretical analysis and simulation results are also presented to validate the effectiveness of the proposed scheme.

Multichannel parameter estimation is a key technology for azimuth multichannel terrain observation by progressive scans synthetic aperture radar (MC TOPS SAR) systems. Normally, for the traditional multichannel parameter estimation algorithms, the mismatch parameters are concluded as phase error, baseline error, and Doppler centroid. The beam rotation speed error is ignored. However, for MC TOPS SAR, beam rotation speed error will degrade the reconstruction performance of the Doppler spectrum. Moreover, only part of the mismatch parameters can be estimated for the traditional algorithms. To address those problems, a joint parameters estimation algorithm for TOPS SAR based on spatial time cross correlation coefficient (STCCC) is proposed. Combined with phase wrapping and the least-squares method (LSM), the algorithm is capable of realizing a joint parameters estimation of channel consistency error, azimuth baseline error, Doppler centroid frequency, and beam rotation speed estimation without iteration. The real data processing result shows that the method can effectively realize MC TOPS SAR data error estimation.

To respond the demand of variable scientific applications, such as difficult topography mapping, terrain classification, biomass estimation, and high resolution wide-swath (HRWS) observation, we presented an experimental multi-channel multi-mode polarimetric airborne SAR system named ‘Nriet-SAR’ or ‘N-SAR’ (SAR of Nanjing Research Institute of Electronic Technology) in 2017. As a scalable SAR system, we expand its X-Band architecture to a multi-Band configuration, intending for foliage/building penetration, change detection, littoral surveillance so on. To distinguish it from its ancestor, the latest evolved multi-band payload is entitled as N-SAR Second Generation (N-SAR-SG). In this paper, we attempt to provide a conceptual introduction of the new features, instrument design and the data acquisition capability of N-SAR-SG. Then a series of flight experiments are casted, either under way or in the schedule of the next few years.

A ground moving target usually shifts its initial azimuth location in the synthetic aperture radar (SAR) image due to the radial motion. To address this problem, a new azimuth relocation method is proposed for a multichannel SAR in this letter. Based on the signal analysis in the range-compressed domain, it is found that the zeroth-order term of the interferometric phase without coregistration is determined by the moving target's initial azimuth location. A new azimuth relocation method utilizing the noncoregistrated interferometric phase is then proposed. First, the moving target is detected after clutter suppression in the range-compressed domain. Then, the interferometric phase along the slow time without coregistration can be extracted for parameter estimation via the least-squares method. Compared with the existing radial velocity-based relocation methods, the proposed method has higher accuracy, and there is no azimuth location ambiguity problem even when the target has a large radial velocity. Finally, both the simulated and real data are provided to demonstrate the effectiveness of the proposed method.

Much research from open literature shows that polarization diversity can provide another dimension which may be exploited to improve the performance in ground moving target indication (GMTI), compared with space-time adaptive processing (STAP). In this article, we report the multichannel synthetic aperture radar (SAR)-based GMTI (SAR-GMTI) experiment and its preliminary results with a N-SAR system which is an airborne quadrature-polarimetric (quad-pol) radar system. First, the joint polarization-space adaptive processing (JPolSAP) is performed in the image level, but two suboptimal versions of JPolSAP, where the polarimetric matched filter (PMF) vector and the full-one vector are exploited to substitute for the polarimetric steering vector, respectively, are evaluated since the polarimetric steering vector of the moving target is unknown precisely in practical applications. Then, considering the computational complexity and lack of secondary data in a inhomogeneous environment due to high degrees of freedom of the JPolSAP processor, two cascade processors are evaluated, including the polarization enhancement that uses PMF and noncoherence integration detection (NCID) technique. Furthermore, we utilize the polarization information to accomplish SAR terrain classification, and subsequently secondary data from the same scattering type clutter can be obtained for clutter suppression under the guidance of polarization classification results as a priori knowledge. Finally, the experimental results demonstrate that: 1) the suboptimal JPolSAP processor with PMF steering vector can effectively enhance GMTI performance about 13 dB (or even up to 5 dB) relative to the worst (or best) single-polarization (S-pol) processor case and has the best robustness compared with the one with full-one steering vector; 2) polarization enhancement using PMF also obtains a good output gain of polarization filter, especially for quad-pol enhancement, which gains up to 2-3 dB with respect to the best output of S-pol processor, and the NCID technique can obtain good performance of moving-target detection; and 3) under the guidance of polarization classification results, the capability of clutter suppression can improve even up to 15 dB with respect to the one without classification.

A straightforward and efficient method is presented in detail to extract the relative polarimetric distortion parameters (PDPs) of polarimetric radar systems including polarimetric synthetic aperture radar (PolSAR). By using one trihedral and two dihedrals and ignoring high order terms of channel crosstalk, the PDPs of a PolSAR system can be estimated straightforwardly and efficiently without any iterative procedure, complicated matrix operations, or assumptions of targets. Most importantly, the applicability issues are analyzed to obtain the accurate values of the PDPs. The sensitivity analysis of target orientation angle (TOA, a rotation about the radar line of sight) error is theoretically analyzed and experimentally studied. It appears that the impacts of the TOA error on the channel imbalances and channel crosstalk estimation are relatively independent. Based on simulation and practical N-SAR (a Chinese airborne SAR system) experiments, the results show that our proposed principle has an efficient performance.

The Alternating Bistatic Multi-Polarized mode (ABMP mode) is a new airborne SAR/InSAR operational mode which combines the polarimetric mode and the alternating bistatic mode. The pulse repetition frequency (PRF) of the ABMP mode is four times than the need of monostatic single-polarized mode based on two transmit-receive SAR radars of N-SAR (SAR of Nanjing Research Institute of Electronic Technology) system. In this paper, we show the experimental results to validate the efficiency of the ABMP mode. By using the ABMP mode, the monostatic and bistatic polarimetric SAR images can be simultaneously collected. Hence, we mainly demonstrate the monostatic and bistatic SAR, SAR interferometry (InSAR) and polarimetric SAR interferometry (Pol-InSAR) using N-SAR data acquired by the ABMP mode. We also firstly demonstrate the preliminary result of TomoSAR using N-SAR data. Moreover, the main test site lies in Shanxi province in the northwest of China.

In recent years we are interested in several brand new fields of SAR radar such as multichannel High Resolution and Wide Swath (HRWS) SAR, multi-baseline interferometric SAR (InSAR), multi-subband SAR, PolSAR and PolInSAR and so on. We believe that these new theories of SAR radar are valuable for scientific applications and we try to develop a new experimental airborne SAR system which is able to comply new requirements and scalable for the rapid development of modern SAR radar under the name `Nriet-SAR' or `N-SAR' (SAR of Nanjing Research Institute of Electronic Technology). In this paper we try to describe the general system design features and instrument design overview and the capabilities of N-SAR to give an overall conceptual description. Then a series of flight experiments which will be carried out in the next few years are shown.