Experimental SAR Research Articles

Similarity-enhanced target detection algorithm using polarimetric SAR images

Target detection and analysis using polarimetric synthetic aperture radar (PolSAR) images are currently of great interest in synthetic aperture radar (SAR) applications. For a complex target, the scattering characteristics are determined by different independent sub-scatterers and their interaction; therefore, the scattering characteristics should be described by a statistical method due to randomness and depolarization. Furthermore, the inherent speckle in SAR data must be reduced by spatial averaging at the expense of loss of spatial resolution. The polarimetric similarity parameter (PSP) is an effective parameter to analyse target characteristics. In order to describe a complex distributed target, two new methods for calculating PSP are proposed, namely Stokes matrix-based PSP (S-PSP) and multiple PolSAR similarity parameter (MPSP). The characteristics of a target can be described and extracted on the basis of the polarimetric similarity, and then the similarity-enhanced target detection methods using S-PSP and MPSP are implemented and demonstrated with German Aerospace Centre (DLR) experimental SAR L-band multiple temporal PolSAR images of Oberpfaffenhofen test site (DE), Germany. The results confirmed that the proposed methods are effective for detection and analysis of buildings in urban areas.

First Demonstration of Agriculture Height Retrieval With PolInSAR Airborne Data

A set of three quad-pol images acquired at the L-band in interferometric repeat-pass mode by the German Aerospace Center (DLR) with the Experimental SAR (E-SAR) system, in parallel with the AgriSAR2006 campaign, has been used to provide, for the first time with airborne data, a demonstration of the retrieval of vegetation height from agricultural crops by means of polarimetric SAR interferometry (PolInSAR)-based techniques. Despite the low frequency of the data, hence providing a weak response from the vegetation volume in contrast to the ground, accurate estimates of vegetation height at field level have been obtained over winter rape and maize fields. The same procedure does not yield valid estimates for wheat, barley, and sugar beet fields due to a mismatch with the physical model employed in the inversion and to the specific crop condition at the date of acquisition. These results show the value of the information provided by both interferometry and polarimetry for some agriculture monitoring practices.

Application of Subband Spectral Cancellation for SAR Narrow-Band Interference Suppression

Narrow-band interference (NBI) suppression is an important technique in the low-frequency synthetic aperture radar (SAR). NBI sources either intentional or unintentional can mask the SAR signals and cause image degradation. According to the different characteristics of the spectra between the NBI and the SAR signals, a new method using subband spectral cancellation is applied to suppress the NBI. In contrast to the conventional suppression method performed on the raw data, this letter deals with the focused SAR image. By subtracting different range subband spectra of the SAR image, the NBIs are obtained and cancelled. The proposed method does not need to detect and estimate the parameters of the NBI and is easy to put in practice. Its performance is tested on the real data acquired by an experimental SAR system at P-band.

First Bistatic Spaceborne SAR Experiments With TanDEM-X

TanDEM-X (TerraSAR-X Add-on for Digital Elevation Measurements) is a high-resolution interferometric mission with the main goal of providing a global and unprecedentedly accurate digital elevation model of the Earth surface by means of single-pass X-band synthetic aperture radar (SAR) interferometry. Despite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system has been mainly operated in pursuit monostatic mode. However, some pioneering bistatic SAR experiments with both satellites commanded in nonnominal modes have been conducted with the main purpose of validating the performance of both space and ground segments in very demanding scenarios. In particular, this letter reports about the first bistatic acquisition and the first single-pass interferometric (mono-/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focusing on the analysis of the experimental results. Even in the absence of essential synchronization and calibration information, bistatic images and interferograms with similar quality to pursuit monostatic have been obtained.

Low-frequency ultra-wideband synthetic aperture radar ground moving target imaging

A method for low-frequency ultra-wideband synthetic aperture radar (SAR) imaging of ground moving targets is proposed in this study. This method can deal with large range migration of the moving target. A first-order keystone transform is used to correct range walk (first-order range migration), and range curvature (second-order range migration) is compensated by using a second-order keystone transform. Both the first-order and second-order keystone transform parameters are independent of the motion parameters, so that this method can correct the range migration for all targets without knowing their velocity. The advantage of this method is that it can correct range curvature for all targets in one processing step, so that it is simple to implement. Simulation and experimental SAR data processing results are shown to demonstrate the validity of the proposed method.

The Analyzation of SAR Image Adaptive Sidelobe Reduction Algorithms and Experiments

Synthetic aperture radar is an active coherent high resolution imaging system. The classical imaging algorithms process the SAR received data by matched filter in two-dimensional frequency domain, and get reconstructed SAR images. In classical imaging algorithms, Fast Fourier Transform is used broadly. Because of the limited frequency domain support of the SAR systems, the system impulse responses in the range and azimuth directions are sinc functions. And the SAR images have great dynamic domain. The ratio of the returned power between the bright and the weak targets reached 50 dB, even higher. The sidelobe of a bright point target can easily obscure and violate the mainlobe of weaker points. So SAR images often demands sidelobe reduction. The conventional sidelobe reduction applies the fixed weighting functions on the whole aperture data, and the conventional weighting approach have to compromise between the degree of sidelobe reduction and imaging resolution. In this paper discusses two adaptively sidelobe reduction algorithms, and compared their processing results with conventional sidelobe reduction methods by the simulated data and the IECAS L\|SAR system raw data.

Three-Dimensional Imaging and Scattering Mechanism Estimation Over Urban Scenes Using Dual-Baseline Polarimetric InSAR Observations at L-Band

This paper introduces new polarimetric algorithms for generating 3-D images and estimating scattering mechanisms from polarimetric multibaseline (MB) interferometric synthetic aperture radar (SAR) measurements. First, an MB interferometric SAR signal model is generalized to the fully polarimetric configuration, establishing the notion of polarimetric reflectivity. Subsequently, polarimetric beamforming, Capon, and MUSIC methods that determine optimal polarization combinations for height estimation are developed. These new techniques allow for extracting the height of reflectors, the associated scattering mechanisms, and the polarimetric (pseudo)reflectivities. By means of polarimetric dual-baseline interferometric SAR observations of an urban environment, the performance of the conceived algorithms is examined in detail. Producing 3-D images of a building layover, the quality of the approaches is compared in terms of refined resolution and lowered side lobes. Furthermore, the scattering processes occurring in urban scenes are investigated thoroughly by analyzing the optimal reflection types. The algorithms are validated using dual-baseline polarimetric SAR interferometric data at L-band acquired by German Aerospace Center's experimental SAR system over Dresden city.

Correlation Coefficients Between Polarization Signatures for Evaluating Polarimetric Information Preservation

In order to evaluate quantitatively the preservation performance of polarimetric information for polarimetric synthetic aperture radar (PolSAR) speckle filters, this letter presents correlation coefficients between the original and the filtered polarimetric data based on the theory of polarization synthesis. The copolarized and the cross-polarized correlation coefficients are furthermore discussed in cases whether the transmitting and receiving radar antennas are characterized by the same or orthogonal polarization state. To show how these coefficients can be exploited in evaluating the preservation of polarimetric information, they are demonstrated with a German Aerospace Center (DLR) Experimental Synthetic Aperture Radar L-band full-polarized image of the Oberpfaffenhofen Test Site Area (DE), Germany, which was obtained on September 30, 2000. Results make evident that the correlation coefficients are efficient to describe quantitatively the performance in preserving polarimetric information for PolSAR speckle filters.

3-D Time-Domain SAR Imaging of a Forest Using Airborne Multibaseline Data at L- and P-Bands

In this paper, a time-domain back-projection based tomographic processing approach to a 3-D reconstruction grid is detailed, with the focusing in the third dimension being either modified versions of multilook standard beamforming, robust Capon beamforming, or multiple signal classification. The novel feature of the proposed approach compared to previous synthetic aperture radar (SAR) tomography approaches is that it allows for an approximation-free height-dependent calculation of the sample covariance matrix by exploiting the azimuth-focused data on the 3-D reconstruction grid. The method is applied to experimental multibaseline quad-pol SAR data at L- and P-bands acquired by German Aerospace Center's (DLR) E-SAR sensor: Tomographic images of a partially forested area, including a 3-D voxel plot that visualizes the very high level of detail of the tomographic image, are shown, and an analysis of the focusing performance is given for the full as well as reduced synthetic aperture in the normal direction.

Spatial Resolution of Bistatic Synthetic Aperture Radar: Impact of Acquisition Geometry on Imaging Performance

This paper analyzes the spatial resolution of bistatic synthetic aperture radar (SAR) in general hybrid configurations, such as air- and spaceborne systems moving along independent trajectories. The gradient method is utilized to point out the effects of the acquisition geometry, namely, position and velocity of both the transmitter and the receiver, on image resolution. This general approach is applied to different realizations of bistatic SAR, such as low-Earth-orbit monostatic-bistatic SAR, spaceborne-airborne bistatic SAR, and a bistatic system consisting of a high-altitude long-endurance illuminator and lower altitude airborne receivers. The main features of the method are then put in evidence, including the derivation of analytical tools to individuate adequate relative geometries for achieving satisfactory resolutions. A comparison to the other proposed techniques for computing the spatial resolution of bistatic SAR is also reported in order to highlight some peculiarities of all presented methodologies. Finally, the good agreement between the image resolution results achieved by recently carried out bistatic SAR experiments and the ones derived by the gradient method strengthens the potentialities of the proposed approach.

A novel super-resolution method of PolSAR images based on target decomposition and polarimetric spatial correlation

The polarimetric synthetic aperture radar (PolSAR) is becoming more and more popular in remote-sensing research areas. However, due to system limitations, such as bandwidth of the signal and the physical dimension of antennas, the resolution of PolSAR images cannot be compared with those of optical remote-sensing images. Super-resolution processing of PolSAR images is usually desired for PolSAR image applications, such as image interpretation and target detection. Usually, in a PolSAR image, each resolution contains several different scattering mechanisms. If these mechanisms can be allocated to different parts within one resolution cell, details of the images can be enhanced, which that means the resolution of the images is improved. In this article, a novel super-resolution algorithm for PolSAR images is proposed, in which polarimetric target decomposition and polarimetric spatial correlation are both taken into consideration. The super-resolution method, based on polarimetric spatial correlation (SRPSC), can make full use of the polarimetric spatial correlation to allocate different scattering mechanisms of PolSAR images. The advantage of SRPSC is that the phase information can be preserved in the processed PolSAR images. The proposed methods are demonstrated with the German Aerospace Center (DLR) Experimental SAR (E-SAR) L-band full polarized images of the Oberpfaffenhofen Test Site Area in Germany, obtained on 30 September 2000. The experimental results of the SRPSC confirms the effectiveness of the proposed methods.1

New Approach for SAR Imaging of Ground Moving Targets Based on a Keystone Transform

We propose here a new approach for synthetic aperture radar (SAR) imaging of ground moving targets. The unique characteristic of this approach is that range curvature (i.e., quadratic range migration) can be corrected by a simple processing step. A keystone transform is used to correct range walk (i.e., linear range migration) for all targets without knowing their velocities, and the range curvature is corrected in the range-Doppler domain. The advantage of this approach is that it is simple to implement and can correct range curvature for all targets in one processing step, so that it is computationally efficient. Simulation and experimental SAR data processing results are presented to demonstrate the validity of the proposed approach.

Polarimetric Interferometric Eigenvalue Similarity Parameter and Its Application in Target Detection

Polarimetric synthetic aperture radar (SAR) interferometry (PolInSAR) combines SAR polarimetry and SAR interferometry and is much more sensitive to the distribution of orientated scatterers compared with polarimetric or interferometric data alone. The polarimetric similarity parameter is an efficient parameter to analyze target characteristics using the similarity between a target and the canonical target. In this letter, the polarimetric interferometric eigenvalue similarity parameter (PIESP) is proposed based on the similarity between two polarimetric SAR images obtained by two interferometric antennas. The PIESP is defined by the eigenvalues of two polarimetric coherence matrices in the PolInSAR system, and the eigenvalues of polarimetric coherence matrix are independent on the target orientation angle; therefore, the PIESP is rotation invariant. PolInSAR systems use two antennas to measure the same ground area with slightly different image geometry. Thus, the PIESP can be used to distinguish the target based on coherence and similarity. Then, the target detection method using the PIESP is implemented with the DLR experimental SAR L-band full polarized image of the Oberpfaffenhofen test site of Germany obtained on September 30, 2000. The results confirmed that the proposed model is accurate and effective for the detection and the analysis of buildings in urban areas.

Bistatic Forward-Looking SAR: Results of a Spaceborne–Airborne Experiment

Forward-looking radar imaging continues to gain in significance due to a variety of convenient applications, like landing assistance for aircraft in poor visibility conditions. Synthetic aperture radar (SAR) techniques are typically used to achieve a high azimuth resolution, but conventional monostatic SAR is not applicable in forward direction because of azimuth ambiguities and poor Doppler resolution. To improve the Doppler resolution and to avoid azimuth ambiguities, bistatic SAR configurations can be used to obtain high-resolution radar images. This is demonstrated for the first time in a spaceborne-airborne SAR experiment by using TerraSAR-X as the illuminator and the Phased Array Multifunctional Imaging Radar as the receiver. For convenience, the receiver's SAR antenna was mounted on the aircraft's loading ramp and looked backward. Due to identical image properties and the same challenges for forward- and backward-looking sensors, this configuration also demonstrates the feasibility of forward-looking bistatic SAR. This letter describes the experimental setup, analyzes the performance, and presents the imaging results.

ALGAE: A Fast Algebraic Estimation of Interferogram Phase Offsets in Space-Varying Geometries

This paper deals with the estimation of terrain topography from multipass synthetic aperture radar (SAR) interferometry (InSAR), focusing on the case where variation of the system geometry within the imaged swath is relevant. A typical case is represented by airborne multipass interferometric campaigns where, due to the closeness between the radar sensor and the targets, the incidence-angle sensitivity undergoes a dramatic increase with respect to the spaceborne case, resulting in a high spatial variability of the normal baselines. The space-varying nature of the system geometry gives rise to a major issue in multipass InSAR analyses in that it prevents from compensating for the presence of interferogram phase offsets by simply phase locking the data stack to a reference point, therefore hindering the retrieval of terrain topography. To cope with this issue properly, we propose a novel approach that exploits the algebraic properties of the problem. Such an approach allows casting the problem in terms of identification of a null-space component for terrain topography after which both topography and the interferogram phase offsets are quickly obtained without exploiting calibration points. Experimental results are shown based on a P-band data set acquired by the Experimental SAR (E-SAR) airborne system, operated by the German Aerospace Center (DLR), in the framework of the European Space Agency (ESA) campaign BIOSAR 2008.

Nonuniform Spatial Sampling in a Ground-Based Noise SAR

Nonuniform Spatial Sampling in a Ground-Based Noise SARThe paper presents an idea of nonuniform spatial sampling applied to a noise synthetic aperture radar. In certain cases it is desirable to limit the number of spatial (along-track) domain samples acquired in a SAR radar because of external constraints on sampling frequency or on the overall number of samples - e.g. in order to economy on time or power consumed. Lowering number of samples taken may, however, lead to spatial aliasing and incorrect reconstruction of the image. Nonuniform sampling allows to reduce the aliasing effect and reconstruct the image better. This technique can be applied with standard reconstruction methods, but it works best together with Compressive Sensing reconstruction algorithms. The idea will be verified with an experimental noise SAR built at ISE PW.

Bistatic Noise SAR Experiment with a Non-Cooperative Illuminator

Bistatic Noise SAR Experiment with a Non-Cooperative IlluminatorThe paper describes the results of a conception-stage experiment with a ground-based bistatic noise SAR (Synthetic Aperture Radar) demonstrator. Its aim was to research the ability of a simple Commercial-Off-The-Shelf (COTS) build system to provide a bistatic SAR image using non-cooperative illuminator. The noise signal used in the experiment is similar to a signal used in many transmission systems such as DVB-T that can be employed in passive bistatic radars. The paper presents the system setup, details of the measurement campaign, signal processing and the results of SAR imaging.

Bistatic SAR Experiments With PAMIR and TerraSAR-X—Setup, Processing, and Image Results

The spatial separation of the transmitter and the receiver in bistatic synthetic aperture radar (SAR) enables a variety of data acquisition geometries to achieve benefits like the increased information content of bistatic SAR data. In the case of hybrid bistatic SAR constellations where the transmitter is spaceborne and the receiver is onboard an aircraft, one has to deal with a huge discrepancy between platform velocities. This paper presents bistatic spaceborne/airborne SAR experiments, where the radar satellite TerraSAR-X is used as a transmitter and the airborne SAR sensor Phased Array Multifunctional Imaging Radar (PAMIR) of the Fraunhofer Institute for High Frequency Physics and Radar Techniques (FHR) is used as a receiver. Both sensors are equipped with phased-array antennas, which offer the possibility of beam steering and could be used for the first time for the “double sliding spotlight mode.” In this mode, the space- and airborne sensors operate with different sliding factors (ratio between footprint and platform velocity). The performance of two different experiments is analyzed, and the novel double sliding spotlight mode is presented. This paper describes the experimental setups, the synchronization system, and the data acquisition. The image results were processed by a modified backprojection algorithm and a frequency-domain algorithm. The analysis of the final bistatic images comprises the spatial resolution and the scattering behavior of selected objects. Parts of the bistatic SAR images are compared with the corresponding monostatic images of PAMIR and TerraSAR-X. It will be shown that hybrid bistatic SAR is a worthwhile and helpful addition to current monostatic SAR.

Bistatic SAR experiment with the Ingara imaging radar

Synthetic aperture radar (SAR) operation in a bistatic configuration not only offers various advantages over its now well-established monostatic counterpart but also poses various challenges. As part of a research programme into the potential benefits and challenges of bistatic SAR, the Ingara fully polarimetric X-band airborne imaging radar system, developed and operated by the Australian Defence Science and Technology Organisation, was upgraded to conduct experimental SAR data collections in a bistatic geometry. Initial trials of the new bistatic SAR system were conducted between December 2007 and April 2008: these involved operation of the existing airborne radar in a fine-resolution (600 MHz bandwidth) circular spotlight-SAR mode, in conjunction with a newly developed fully polarimetric stationary ground-based bistatic receiver. These experimental trials produced a set of fully polarimetric simultaneously collected monostatic and bistatic SAR data, collected over a wide range of bistatic angles. Results from a preliminary analysis of the data have been encouraging: focussed fine-resolution imagery has been obtained, indicating the successful maintenance of synchronisation and phase stability between the independent airborne and ground-based systems. Furthermore, interferometric coherence has been demonstrated between single-pass simultaneously collected monostatic and bistatic images from the airborne and ground-based receivers, and between repeat-pass bistatic images from the ground-based receiver collected some 100 min apart. This study gives an overview of the Ingara bistatic SAR system, discusses the experiments and data processing and presents initial experimental results.

Robust moving targets detection and velocity estimation using multi-channel and multi-look SAR images

For the sake of the robustness to ground moving targets indication (GMTI) for synthetic aperture radar (SAR) in heterogeneous environment, a new approach using multi-channel and multi-look SAR images is presented. This approach has been shown to attain improved suppression of monostatic clutter over its counterpart (the multi-channel and single-look algorithm). In this approach, the joint data vector is utilized which has robustness to image coregistration and deleterious system errors in practice. Consequently, a series of radial velocity estimation approaches are presented. The estimation accuracy can be improved by the multi-look algorithms. Experimental airborne SAR data illustrates the effectiveness of the proposed GMTI schemes. It is shown that, compared with conventional single-look algorithms, the multi-look approach can obtain better clutter cancellation performance and more accurate velocity estimation.