Wideband Synthetic Aperture Radar Research Articles

New Insights into Wideband Synthetic Aperture Radar Interferometry

New Insights into Wideband Synthetic Aperture Radar Interferometry

Compensation for the ionospheric dispersion effect on spaceborne P-band full-polarimetric wideband SAR

ABSTRACT Spaceborne P-band full-polarimetric wideband synthetic aperture radar (SAR) will be affected by ionospheric phase dispersion and polarimetric dispersion. An improved contrast optimization autofocus (COA) method using four polarization channel data is proposed to compensate for both phase dispersion and polarimetric dispersion. The basis and steps of the method are given. Its performance is analysed by typical target simulation under clutter background and compared with traditional COA method for single channel. The residual quadratic phase error (QPE) of single channel compensated by traditional method may exceed 45° focusing threshold, while the proposed method can reduce the error to less than 10°. Due to the lack of spaceborne P-band SAR data, we simulate the effect of ionosphere on P-band SAR based on ALOS2 PALSAR2 data and verify the proposed method. The simulation result shows the proposed method is effective and superior to traditional method.

위성탑재용 광대역 영상레이다의 빔 분산 및 고도 변화에 의한 거리 방향 모호성 성능 개선 방안

In a wideband radar, a true time delay line (TTDL) is used to compensate for the propagation delay that has a different value according to the operational frequency. This study analyzes the antenna pattern characteristics of wideband synthetic aperture radar (SAR) that does not require any TTDL. In addition, it defines an antenna pattern to compensate for beam dispersive effects and a method to improve the range ambiguity to signal ratio (RAR). Considering the altitude change that occurs in a SAR satellite, this paper proposes a method to optimize the operational parameters and consequently enhance the RAR performance. The proposed method uses altitude-dependent equations to calculate the operational parameters; hence it can be easily applied to the real operation of a SAR satellite. The RAR analysis for operational altitude variation of 35 km shows that an improvement of approximately 2.1-dB, 0.9-dB, and 2.1-dB is achieved at the stripmap, wide-swath, and high-resolution modes respectively. The RAR is improved to comply with the requirement in the stripmap mode, whereas the wide-swath and high-resolution modes have a sufficient margin.

A Method for Detection of Flat Walls in Through-the-Wall SAR Imaging

Through-the-wall imaging using wideband synthetic aperture radar (SAR) is a powerful tool that enables seeing through visually opaque walls by providing high-resolution images of objects behind the walls. Previous works focus on detection of static and moving point scatterers behind the walls and methods for enhancing the image by removing the effect of transmission through the walls. Besides imaging of hidden objects, detection of walls and large flat surfaces provides complete map of buildings’ interiors and enables better path planning and treat assessment in rescue and military operations. In the standard high-resolution SAR processing, walls and objects with large surface are imaged as discrete points instead of solid lines and as a result, without prior knowledge about the imaging area, walls may be interpreted as few discrete closely spaced targets. In this letter, a method is presented to discriminate walls and objects with large flat surfaces from other objects. In this approach, instead of focusing the synthetic radar beam on a point on the wall surface, the beam is focused at the location of the image of transmitter with respect to the wall surface considering only specular reflections. This is done by assuming there exists a wall at a distance from the transmitter with a known orientation. This results in an image in polar format in which locations of peaks determine the distance and orientation of the actual wall surfaces inside the imaging area. The method is applied to the measured SAR data and the results exhibit the capability of the method in detection of walls in real scenarios.

An Accelerated Algorithm Based on GO-PO/PTD and CWMFSM for EM Scattering From the Ship Over a Sea Surface and SAR Image Formation

This article proposes an accelerated ray-tracing algorithm to improve the computational efficiency of composite scattering from the 3-D ship target located on a random sea surface. The ray tracing is accelerated by using a new neighbor search technique, with a new 0-1 transformation rule and arrangement of octree nodes. It can reduce the code complexity and improve the computational efficiency. Combined with this technique, an accelerated algorithm based on the geometrical optics, physical optics with physical theory of diffraction (GO-PO/PTD) and capillary wave modification facet scattering model (CWMFSM) is further established. This is meaningful for the simulation of scattered echoes for wideband synthetic aperture radar (SAR), enabling SAR images to be formed using an imaging algorithm. The effectiveness of the proposed method was verified by comparing the simulation results with those produced by the FEKO-based multilevel fast multipole method (MLFMM). Further confirmation of its accuracy was obtained by comparing the simulated images with the measured ship image.

Multichannel Wideband Synthetic Aperture Radar for Ice Sheet Remote Sensing: Development and the First Deployment in Antarctica

We developed a multichannel wideband synthetic aperture radar (SAR) that operates over a frequency range of 190–450 MHz for measurements over the ice sheets in Antarctica and Greenland. The antenna-array, which consists of eight elements housed in a certified external structure for a BASLER aircraft, was used for measurements during the 2013–2014 Antarctic field season. We performed measurements with this system in conjunction with two ultra-wideband radars operating over a frequency range of 2–8 GHz and 12–18 GHz on Siple Coast ice streams in West Antarctica during December 2013 and January 2014. We sounded ice thicker than 2 km with a signal-to-noise ratio (SNR) of more than 20 dB in an area with two-way ice loss of about 27 dB/km. The same system also simultaneously mapped near-surface internal layers with submeter resolution from the ice-surface to a depth of about 1100 for 1200 m thick ice. In this paper, we provide a detailed overview of the radar instrumentation and signal processing algorithms and present a few sample results. The radar will be operated over a frequency range of 150–550 MHz with a 24-element antenna-array for wide-ranging measurements over the Greenland and Antarctic ice sheets, starting around August 2015.

Multichromatic Analysis of Satellite Wideband SAR Data

The multichromatic analysis (MCA) can be applied to interferometric pairs of synthetic aperture radar (SAR) images processed at range subbands and consists of exploring the phase trend of each pixel as a function of the different central carrier frequencies. The phase of stable scatterers linearly evolves with the subband central frequency, with a slope proportional to the absolute electromagnetic path difference that can be estimated and used for both phase unwrapping and height computation. MCA has been theoretically evaluated and tested on airborne wideband SAR data, appearing optimally suited for the new generation of satellite sensors, which operate with larger bandwidths than previously available instruments, generally limited to few tens of megahertzs. In this letter, we illustrate MCA application to satellite SAR data acquired in spotlight mode over the Uluru monolith in Australia. The topographic measurements derived through MCA on the monolith are compared with those provided by a high-resolution digital elevation model from optical stereo imagery. The theoretical parametric model describing the MCA performances according to the processing parameters is also validated.

The Radiometric Measurement Quantity for SAR Images

A synthetic aperture radar (SAR) system measures among other quantities the terrain radar reflectivity. After image calibration, the pixel intensities are commonly expressed in terms of radar cross sections (RCSs) (for point targets) or as backscatter coefficients (for distributed targets), which are directly related. This paper argues that pixel intensities are not generally proportional to RCS or derived physical quantities. This paper further proposes to replace the inaccurate term RCS by equivalent RCS as the radiometric measurement quantity for SAR images. The measurement procedure as such remains unchanged. Two problems are seen with using RCS as the radiometric measurement quantity for point targets. First, the RCS is frequency and/or angle dependent for many targets, whereas a SAR system operates not at a single set but through a range of frequencies and aspect angles. This begs the question of which RCS within this 2-D range should actually be taken as the measurement result. Second, the pixel intensities seen in SAR images are the output of a filtering process, which is affected by the complex transfer function of the point target. RCS, on the other hand, does not depend on phase. The equivalent RCS expresses the reflectivity in terms of the RCS of an equivalent conducting sphere which would result in the same pixel intensity as the one observed if the sphere were to replace the actual target in the scene. This distinction is crucial in defining the measurand for radiometric SAR measurements in principal, and hence, it is important for radiometric SAR calibration. It is furthermore of particular practical importance for current and emerging wideband, high-resolution, and high-accuracy SAR systems.

Nonuniform Manual Scanning for Rapid Microwave Nondestructive Evaluation Imaging

Wideband synthetic aperture radar (SAR) technique is a robust imaging tool for microwave and millimeter-wave imaging such as nondestructive evaluation applications. In this paper, we present an alternative method to conventional raster scanning involving manually selected and nonuniformly distributed measurement positions, enabling the production of complete SAR images-potentially using only a fraction of the conventionally required measured data. The user is kept informed throughout the scanning process by a stream of real-time SAR images. Finally, data reconstruction algorithms are used offline to produce high-quality images with considerably lower background noise and image artifacts as compared to the real-time images. We also introduce a novel reconstruction method that uses the components of the SAR algorithm to advantageously exploit the inherent spatial information contained in the data, resulting in a superior data reconstruction and final SAR image. This paper presents the measurement methodology along with the images obtained from three different specimens of increasing geometrical complexity.

Study of Effects of Flat Surface Assumption to Synthetic Aperture Radar Time-domain Algorithms Imaging Quality

时域算法具有精确成像和易于运动补偿等突出优点，在超宽带合成孔径雷达(Ultra Wide Band Synthetic Aperture Radar, UWB SAR)成像领域应用前景广阔。当使用时域成像算法对起伏地形成像时，可引入平地假设来简化成像几何模型，但平地假设会引入几何误差，进而影响成像质量。该文研究了平地假设对时域算法成像质量的影响，指出其影响在直线孔径条件下表现为位置偏移，在非直线孔径条件下还将造成目标散焦。同时推导得到了目标位置偏移量的表达式，给出了目标聚焦时非直线孔径最大偏移和地表高程起伏程度应满足的定量条件。最后采用时域算法中的后向投影(Back Projection, BP)算法处理仿真数据，处理结果验证了该文相关结论。

Bistatic Synthetic Aperture Radar Imaging Using UltraNarrowband Continuous Waveforms

We consider synthetic aperture radar (SAR) imaging using ultra-narrowband continuous waveforms (CW). Due to the high Doppler resolution of CW signals, we refer to this imaging modality as Doppler Synthetic Aperture Radar (DSAR). We present a novel model and an image formation method for the bistatic DSAR for arbitrary imaging geometries. Our bistatic DSAR model is formed by correlating the translated version of the received signal with a scaled or frequencyshifted version of the transmitted CW signal over a finite time window. High frequency analysis of the resulting model shows that the correlated signal is the projections of the scene reflectivity onto the bistatic iso-Doppler curves. We next use microlocal techniques to develop a filtered-backprojection (FBP) type image reconstruction method. The FBP inversion results in backprojection of the correlated signal onto the bistatic iso- Doppler curves as opposed to the bistatic iso-range curves, performed in the traditional wideband SAR imaging. We show that our method takes advantage of the velocity, as well as the acceleration of the antennas in certain directions to form a high resolution SAR image. Our bistatic DSAR imaging method is applicable for arbitrary flight trajectories, nonflat topography, and can accommodate system related parameters. We present resolution analysis and extensive numerical experiments to demonstrate the performance of our imaging method.

A new signal model for a wideband synthetic aperture imaging sensor

A new signal model was developed to replace the traditional one that is based on the go-stop-go assumptions (the go-stop-go model for short) for a wideband synthetic aperture imaging sensor. Using the new model, we traced the time line of the transmitted signal with a given pulse width, round trip time delay of the signal between the sensor and target, and receiving signal with a fixed pulse duration. The spatial displacement of the senor along the time line was calculated. Then, we obtained the two-dimensional spectrum of an individual target and utilized the two-dimensional joint stationary phase method to solve the spectrum. Incorporating the spectrum with available imaging algorithms, we were able to focus targets in a scene. The performance of the model was investigated by analyzing simulated wideband synthetic aperture radar and synthetic aperture sonar data. In the simulations, the imaging algorithms with the new or go-stop-go model were implemented and outputs compared. Results were very promising.

Channel error correction for wideband sar—A joint multiple subpulses processing method

A novel method, referred to as joint multiple subpulses processing, is developed to calibrate the nonideal transfer function of radio frequency front-end and I/Q imbalance in quadrature modulate/ demodulate systems simultaneously, which frequently occur in wideband Synthetic Aperture Radar (SAR) systems. Based on the time-frequency relation of the chirp signal and the analyses of the channel errors in wideband SAR, joint multiple subpulses processing method is adopted to separate the image frequency component due to the I/Q channel error. Then, the complete description of the channel error is acquired for building the correction function, which is used to correct the radar raw echo in frequency domain. The validity and capability of this method are demonstrated by the experiments of the channel error correction on the high resolution SAR system with the effective bandwidth of 500 MHz.

Ice-sheet bed 3-D tomography

AbstractInformation on bed topography and basal conditions is essential to developing the next-generation ice-sheet models needed to generate a more accurate estimate of ice-sheet contribution to sea-level rise. Synthetic aperture radar (SAR) images of the ice–bed can be analyzed to obtain information on bed topography and basal conditions. We developed a wideband SAR, which was used during July 2005 to perform measurements over a series of tracks between the GISP2 and GRIP cores near Summit Camp, Greenland. The wideband SAR included an eight-element receive-antenna array with multiple-phase centers. We applied the MUltiple SIgnal Classification (MUSIC) algorithm, which estimates direction of arrival signals, to single-pass multichannel data collected as part of this experiment to obtain fine-resolution bed topography. This information is used for producing fine-resolution estimates of bed topography over a large swath of 1600 m, with a 25 m posting and a relative accuracy of approximately 10 m. The algorithm-derived estimate of ice thickness is within 10 m of the GRIP ice-core length. Data collected on two parallel tracks separated by 500 m and a perpendicular track are compared and found to have difference standard deviations of 9.1 and 10.3 m for the parallel and perpendicular tracks, respectively.

Optimum relative speed discretisation for detection of moving objects in wide band SAR

Here, Ground moving target indication (GMTI) using synthetic aperture radar (SAR) is considered. SAR GMTI requires that relative speed between the target and the SAR platform is included in the detection algorithm. A separation between the true relative speed and the relative speed used in the SAR process will cause unfocused targets, and decrease detectability. Blind hypotheses of relative speeds are used in the detection phase of moving targets in SAR. The step size between the hypotheses (or discretisation step) in relative speed involves a trade off between the number of hypotheses to test and detectability. A large number of tests will increase detectability but will also increase computation load and vice versa. The relevance of relative speed increases as the azimuth integration time gets larger. Long integration time is associated with low signature moving target detection in strong clutter environments, or for SAR GMTI at low frequencies. The optimum discretisation of normalised relative speed for moving target detection has been determined. The optimum discretisation is derived from the moving target impulse response. Use of optimum discretisation reduces the computation burden in SAR GMTI and secures the detectability.

Accuracy of Differential Shift Estimation by Correlation and Split-Bandwidth Interferometry for Wideband and Delta-k SAR Systems

Estimation of differential shift of image elements between two synthetic aperture radar (SAR) images is the basis for many applications, like digital elevation model generation or ground motion mapping. The shift measurement can be done nonambiguously on the macro scale at an accuracy depending on the range resolution of the system or on the micro scale by employing interferometric methods. The latter suffers from phase cycle ambiguities and requires phase unwrapping. Modern wideband high-resolution SAR systems boast resolutions as small as a few tens of a wavelength. If sufficiently many samples are used for macro-scale shift estimation, the accuracy can be increased to a small fraction of a resolution cell and even in the order of a wavelength. Then, accurate absolute ranging becomes precise enough to support phase unwrapping or even make it obsolete. This letter establishes a few fundamental equations on the accuracy bounds of shift estimation accuracy for several algorithms: coherent speckle correlation, incoherent speckle correlation, split-band interferometry, a multifrequency approach, and correlation of point scatterers in clutter. It is shown that the performance of split-band interferometry is close to the Crame/spl acute/r-Rao bound for a broad variety of bandwidth ratios. Based on these findings, Delta-k systems are proposed to best take advantage of the available radar bandwidth.

Detection of moving targets in wideband SAR

A likelihood ratio is proposed for moving target detection in a wideband (WB) synthetic aperture radar (SAR) system. WB is defined here as any systems having a large fractional bandwidth, i.e., an ultra wide frequency band combined with a wide antenna beam. The developed method combines time-domain fast backprojection SAR processing methods with moving target detection using space-time processing. The proposed method reduces computational load when sets of relative speeds can be tested using the same clutter-suppressed subaperture beams. The proposed method is tested on narrowband radar data.

A Wide-Band Approach to the Absolute Phase Retrieval in SAR Interferometry

Because of possible multiple solutions allowed, the unwrapping of interferometric fringe patterns in the spatial domain is an ill-posed problem which needs some a priori knowledge of the ground morphology for the solution of ambiguities. This is especially true for interferometric SAR (Synthetic Aperture Radar) data. In this paper we propose a different approach to InSAR processing for retrieving the height of ground points independently from each other, unlike most conventional phase unwrapping procedures, which operate in the spatial domain. The basic idea is to repeat raw data focusing by using range sub-bands centered at different frequencies, in order to find a point history of the interferometric phase variation vs. frequency. We introduce the general framework of the method together with considerations on the theoretical limits of applicability, then we report results of our simulations related to a wide-band SAR system. We show that, under certain conditions, height values can be retrieved over a network of coherent and strong scatterers, even when enclosed into low-coherence areas.

Analysis and processing of ultra wide-band SAR imagery for buried landmine detection

Experimental and theoretical results are presented for ultra wide-band (UWB) synthetic aperture radar (SAR) signatures of buried anti-tank and anti-personnel mines. Such are characterized by resonance-like peaks as well as valleys, across the 50-1200 MHz bandwidth considered. Consequently, frequency subbanding is used to highlight one target over another, of application to discriminating targets (mines) from clutter.

Resolution limits of ultra wideband synthetic aperture radar using a rectangular aperture for FFT processing

Formulas are derived for synthetic aperture radar range and cross-range resolution without restrictions on percentage bandwidth or integration angle. Restriction to rectangular apertures allows efficient FFT processing and weighting for sidelobe control. The optimum square aperture (equal resolution) and rectangular aperture (minimum resolution area) are derived.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>