Polarimetric Radar System Research Articles

Clutter space‐time covariance matrix estimate based on multi‐polarised data

The problem of training sample insufficiency is frequently encountered in the space-time adaptive processing and significantly degrades the performance of radar target detection. In this study, the authors propose a clutter covariance matrix estimation algorithm using multi-polarised data in the polarimetric radar system, which can mitigate this problem with an enlarged training sample set. Based on the space-time signal model, they validate that the clutter snapshots for different polarisations share a common spectral structure theoretically. Then, the maximum likelihood estimations of clutter covariance matrixes with multi-polarised training samples are deduced under Gaussian and non-Gaussian statistic. Finally, the performance improvement of our method with limited training samples is demonstrated with simulated data.

A Simulation Study of Compact Polarimetry for Radar Retrieval of Soil Moisture

A compact polarimetric (CP) radar system requires fewer measurements than a fully polarimetric (FP) system, thus allowing added flexibility in radar system design. Previous studies have shown the potential of using compact polarimetry for radar remote sensing of soil moisture. This paper extends previous studies by considering a time series data cube retrieval algorithm and measurements in the presence of vegetation. Vegetation information is assumed to be provided by an ancillary data source in the retrieval process. The performance of an algorithm for reconstructing FP information from CP measurements of vegetated soil surfaces is also examined. The results of the study show that only a modest degradation in soil moisture retrieval performance occurs when compact-pol measurements are used in place of full-pol data.

Unsupervised polarimetric synthetic aperture radar classification of large-scale landslides caused by Wenchuan earthquake in hue-saturation-intensity color space

A simple and effective approach for unsupervised classification of large-scale landslides caused by the Wenchuan earthquake is developed. The data sets used were obtained by a high-resolution fully polarimetric airborne synthetic aperture radar system working at X-band. In the proposed approach, Pauli decomposition false-color RGB imagery is first transformed to the hue-saturation-intensity (HSI) color space. Then, a good combination of k-means clustering and HSI imagery in different channels is used stage-by-stage for automatic landslides extraction. Two typical case studies are presented to evaluate the feasibility of the proposed scheme. Our approach is an important contribution to the rapid assessment of landslide hazards.

Geophysical assessment of the hydraulic property of the fracture systems around Lake Nasser-Egypt: In sight of polarimetric borehole radar

Hydraulic property of the subsurface structures is a complicated mission. In this work, the polarimetric analysis for the measured dataset applied by the polarimetric borehole radar system in order to delineate the characteristics of subsurface fractures. Two different locations in USA and Egypt were selected to perform our investigation. The first polarimetric dataset has been acquired at Mirror Lake, USA which is well known as a standard site for testing the hydraulic properties of subsurface fractures (Sato et al., 1999). The results show the presence of nine fracture zones in one borehole FSE-1. The hydraulic properties were detected and the subsurface fractures were differentiated into four categories fracture zones after deriving the radar polarimetric analysis of alpha, entropy and anisotropy parameters at 30MHz frequency. The fracture zones at 24.75, 47.8 and 55.2m depths have the highest hydraulic transmissivity while the fracture zones at 28.5, 36.15m have the lowest hydraulic transmissivity. These results show a good consistency with the hydraulic permeability tracer test and the structures exist in the area.Similarly, we used the same technique to characterize the subsurface fracture systems detected by geoelectric and geomagnetic methods around Lake Nasser in Egypt using the previous results of Mirror Lake as a key guide. The results show a great correlation with detected structures prevailed in the sedimentary and basement rocks. These results illustrate an ideal explanation for the prevailed subsurface structures and the recharging of the main Nubian sandstone aquifer from Lake Nasser. Also, these results also show that the northeast fracture zone trends are most probably having the highest hydraulic transmissivity whereas the northwest fracture zones have the lowest one.The integration of surface geophysical measurements with the polarimetric borehole radar and the polarimetric analysis of its datasets introduce better understanding of the recharging mechanism between surface water and the subsurface aquifer and also can be used as clue for identifying the subsurface structures for different areas.

ANALYSIS OF TEMPORAL POLARIZATION PHASE DIFFERENCE FOR MAJOR CROPS IN INDIA

A polarimetric radar system measures the complete scattering matrix of a target in the backscattered fleld that includes magnitudes of linearly polarized scattering amplitudes and the co- polarised and cross-polarised phase angles. Apart from backscattering intensity, the co-polarization phase difierence (CPD) calculated from polarimetric synthetic aperture radar (SAR) data produces important information about target physical, geometrical and dielectric properties. In the present work, the distribution of CPD in C-band polarimetric SAR data corresponding to major kharif and rabi crops (denoting the monsoon and the winter season) and other land cover features have been studied over Central State Farm, Hisar, Haryana. The probability density functions (PDF) of CPD have been compared with dominant scattering contributions from these targets as obtained from polarimetric target decompositions. The results show that crops and other land cover features show characteristic CPD distributions, which relates well with crop physical and geometrical properties. An intuition of the rate of growth and plant vigour is indicative from the temporal PDF pattern.

A MODIFIED METHOD FOR POLARIMETRIC SAR CALIBRATION ALGORITHM

Abstract. Present fully polarimetric synthetic aperture radar (SAR) systems often update calibration techniques to further enhance the accuracy to the polarimetric data. In this paper, we propose a modified method to estimate the value of crosstalk based on the corrected observed value. Since Ainsworth calibration algorithm firstly set the value of k to be one. And the value of k relates to the copolarization channel imbalance .We consider the effects of value of k and analyze it. Through comparison to crosstalk results between the stimulated parameters and the estimated parameters, we assume high co-polarization channel imbalance will be obviously to affect crosstalk results. Then, used covariance observation value of the initial value of k rewrites the model to solve related parameters. And crosstalk parameter is calculated by the same iterative method. To verify the effect of the modified calibration method, this letter compares the accuracy of the two methods using the simulated polarimetric SAR data and Chinese airborne X-band polarimetric SAR data. The results confirm that the modified method tends to get more accurate crosstalk results.

Partial Beam Blockage Correction Using Polarimetric Radar Measurements

Abstract A new method for mitigation of partial beam blockage that uses the consistency between reflectivity factor Z and specific differential phase KDP and their radial integrals in rain is presented. The immunity of differential phase ΦDP to partial beam blockage is utilized to estimate the bias of reflectivity factor caused by beam blockage. The algorithm is tested on dual-polarization radar data collected by the NCAR S-band polarimetric Dopper radar system (S-Pol) during the Southwest Monsoon Experiment/Terrain-Influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX) in June 2008 in Taiwan. Corrected reflectivity factors in the blocked sectors are compared with corresponding values deduced from a digital elevation model (DEM) to show the advantage of the suggested method in areas where obstacles such as high-rise buildings cause additional blockage that is not accounted for by DEM. The accuracy and robustness of the method is quantitatively evaluated using a series of radar volume scans obtained in three rainfall events.

Anisotropic diffusion for speckle filtering of polarimetric synthetic aperture radar imagery

The mission of speckle filtering is more intricate for a polarimetric synthetic aperture radar (SAR) system than for a single polarization SAR system. A new speckle filter that employs nonlinear anisotropic diffusion is proposed. The speckle filtering principles in a polarimetric SAR system are thoroughly investigated. Our study demonstrates that preservation of polarimetric properties and reduction of speckle level are very important clauses among those principles. Anisotropic diffusion outperforms window-based filters because it utilizes a diffusion tensor that can bias the direction of diffusion toward the orientation of interesting features. The procedure of diffusion is iterative, so it can lead to a gradual alleviation of speckle noise. The parameters for diffusion are flexible and adjustable, thus the filtering results are controllable according to the requirements of specific applications. Multidimensional and vector-valued model is combined with local statistics based on polarimetric SAR configuration. The experimental results on electromagnetics institute synthetic aperture radar (EMISAR) datasets show that the proposed filter has good performance in speckle reduction and preservation of polarimetric properties.

Precipitation Observations with NSSL’s X-band Polarimetric Radar during the SNOW-V10 Campaign

In support of SNOW-V10, the National Oceanic Administration/National Severe Storms Laboratory (NOAA/NSSL) mobile dual-polarized X-band (NO-XP) radar was deployed to Birch Bay State Park in Birch Bay, Washington from 3 January 2010 to 17 March 2010. In addition to being made available in real time for Science and NOWcasting of the Olympic Weather for Vancouver 2010 (SNOW-V10) operations, NO-XP data are used here to demonstrate the capabilities of easily deployable, polarimetric X-band radar systems, especially for regions where mountainous terrain results in partial beam blockage. A rainfall estimator based on specific attenuation is shown to mitigate the effects of partial beam blockage and provide potential improvement in rainfall estimation. The ability of polarimetric X-band radar to accurately detect melting layer (ML) height is also shown. A 16 h comparison of radar reflectivity (Z), differential reflectivity (Z DR), and correlation coefficient (ρhv) measurements from NO-XP with vertically pointing Micro Rain Radar observations indicates that the two instruments provide ML height evolution that exhibit consistent temporal trends. Since even slight changes in the ML height in regions of mountainous terrain might result in a change in precipitation type measured at the surface, this shows that horizontally extensive information on ML height fluctuations, such as provided by the NO-XP, is useful in determining short term changes in expected precipitation type. Finally, range-height indicator (RHI) scans of NO-XP Z, Z DR, and ρhv fields from SNOW-V10 are used to demonstrate the ability of polarimetric radar to diagnose microphysical processes (both above and below the ML) that otherwise remain unseen by conventional radar. Near-surface enhancements in Z DR are attributed to either differential sedimentation or the preferential evaporation of smaller drops. Immediately above the ML, regions of high Z, low Z DR, and high ρhv are believed to be associated with convective turrets containing heavily aggregated or rimed snow that supply water/ice mass that later result in enhanced regions of precipitation near the surface. Higher up, horizontally extensive regions of enhanced Z DR are attributed to rapid dendritic growth and the onset of snow aggregation, a feature that has been widely observed with both S band and C band radars.

An Empirical Assessment of Temporal Decorrelation Using the Uninhabited Aerial Vehicle Synthetic Aperture Radar over Forested Landscapes

We present an empirical assessment of the impact of temporal decorrelation on interferometric coherence measured over a forested landscape. A series of repeat-pass interferometric radar images with a zero spatial baseline were collected with UAVSAR (Uninhabited Aerial Vehicle Synthetic Aperture Radar), a fully polarimetric airborne L-band radar system. The dataset provided temporal separations of 45 minutes, 2, 7 and 9 days. Coincident airborne lidar and weather data were collected. We theoretically demonstrate that UAVSAR measurement accuracy enables accurate quantification of temporal decorrelation. Data analysis revealed precipitation events to be the main driver of temporal decorrelation over the acquisition period. The experiment also shows temporal decorrelation increases with canopy height, and this pattern was found consistent across forest types and polarization.

The design and development of a FMCW polarimetric imaging radar

A FMCW polarimetric imaging radar system has been successfully designed and developed at the Multimedia University, Malaysia. Upon completion, a course of controlled testing and experiments were conducted to verify the performance of the radar. The validity of the imaging was confirmed by running tests in the laboratory which extended to the field measurements. Polarimetric images have been produced and shown virtual imaging of the target This radar system may be useful and necessary for future sensor design and space technology development.

S-Band Polarimetric Radar Scattering Measurement from Different Random Rough Surface

This paper explores a new set of indoor radar scattering measurements system from terrain by reporting the results of an investigation involving measurements of the hemispherical pattern of the field scattered by a random soil surface. S-band polarimetric radar scattering measurements have been carried out on a random surfaces of different characterized. The measurements were performed by a 3.2-GHz fully polarimetric radar system with transmitter and receiver modules mounted on Semi-circular orbit. The acquired data were analyzed to determine the angular sensitivities of several attributes of the scattered field, including backscattering scattering coefficients of different polarization, different soil surface parameters. In order to further improve the system's precision, many independent samples were measured by turntable rotation, and show that the indoor radar scattering measurements system was effective and accurate for study the mechanism of random rough surface scattering.

Practical method to extract active distortions of radar systems

An efficient method is presented to extract the active distortions of polarimetric radar systems without the measurement of a point calibration target under field conditions. The active distortions are extracted using the signal returned from the antenna input. The backscattering coefficients of distributed targets measured using the proposed method agree quite well with results of numerical simulations and spaceborne SAR measurements.

Game theoretic design for polarimetric MIMO radar target detection

Polarimetric radar systems allow the flexibility of transmitting arbitrarily polarized waveforms that match the scattering profiles of the target. Since different types of targets have varying profiles, the advantages of a polarimetric radar system can fully be exploited only when the type of target is accurately estimated. However, accurate estimation requires a significant amount of training data, which can be expensive. We propose a polarimetric design scheme for distributed multiple input multiple output (MIMO) radar target detection. We formulate the selection of transmit polarizations using a game theoretic framework by examining the impact of all possible transmit schemes on the detection performance with different available target profiles (see also Gogineni and Nehorai, 2011 [1]). This approach does not require training data, and we show a significant performance improvement due to the polarimetric design. Other radar design problems can also be solved using this game theoretic approach.

Reconfigurable digital receiver design and application for instantaneous polarimetric measurement

This paper presents the development of a reconfigurable receiver to undertake challenging signal processing tasks for a novel polarimetric radar system. The field-programmable gate arrays (FPGAs)-based digital receiver samples incoming signals at intermediate frequency (IF) and processes signals digitally instead of using conventional analog approaches. It offers more robust system stability and avoids unnecessary multichannel calibrations of analog circuits for a full polarimetric radar. Two kinds of dual-orthogonal signals together with corresponding processing algorithms have been investigated; the digital implementation architectures for all algorithms are then presented. Processing algorithms implemented in FPGA chips can be reconfigured adaptively regarding to different transmitted waveforms without modification of hardware. The successful development of such reconfigurable receiver extends our radar capacity and thus yields tremendous experimental flexibility for atmospheric remote sensing and polarimetric studies of ground-based targets.

An Internal Calibration Scheme for Polarimetric Synthetic Aperture Radar System

Two important aspects of internal calibration of polarimetric synthetic aperture radar are discussed, i.e., system gain calibration and individual transmit/receive module (TRM) calibration. System gain variation is measured, utilizing the internal calibration loop. A dual-channel TRM gain and phase calibration is carried out using orthogonal phase coding, where a signal of individual TRM is phase encoded according to a set of orthogonal codes in order to be separated from the composite calibration signal of all TRMs. Calibration results are developed both theoretically and through simulation in the case of TRM failure. Error introduced by phase-shifter toggling inaccuracy is discussed. A cross talk model is used to investigate the effect of imperfect isolation between the two polarization channels of each TRM on the calibration error.

Target Recognition by Means of Polarimetric ISAR Images

Automatic target recognition (ATR) is generally the reason why inverse synthetic aperture radar (ISAR) imaging systems are employed. Moreover, the use of fully polarimetric radar systems in radar imaging applications such as SAR and ISAR has enhanced both image quality and classification capabilities. The authors propose a novel technique for ATR using polarimetric ISAR (Pol-ISAR) images. The proposed method is based on a model matching approach. Results are obtained that show the effectiveness of such a technique.

Improved Estimators of Faraday Rotation in Spaceborne Polarimetric SAR Data

Spaceborne polarimetric synthetic aperture radar systems operating at lower frequencies, such as P-band, are significantly affected by Faraday rotation (FR). A new set of FR estimators is derived from the off-diagonal terms in the measured covariance matrix of a distributed target. These estimators have a phase ambiguity of period , instead of /2 as for the published estimators, and this ambiguity can be completely resolved for arbitrarily large values of FR using total electron content maps derived from Global Navigation Satellite System measurements. Simulations show that one of the new estimators has particularly high resistance to system noise and channel amplitude imbalance but greater sensitivity to channel phase imbalance than the published estimators. Hence, the expected values of residual system distortion after calibration may affect the choice of estimator.

C-Band Polarimetric Backscattering Signatures of Newly Formed Sea Ice During Fall Freeze-Up

A study of the polarimetric backscattering response of newly formed sea ice types under a large assortment of surface coverage was conducted using a ship-based C-band polarimetric radar system. Polarimetric backscattering results and physical data for 40 stations during the fall freeze-up of 2003, 2006, and 2007 are presented. Analysis of the copolarized correlation coefficient showed its sensitivity to both sea ice thickness and surface coverage and resulted in a statistically significant separation of ice thickness into two regimes: ice less than 6 cm thick and ice greater than 8 cm thick. A case study quantified the backscatter of a layer of snow infiltrated frost flowers on new sea ice, showing that the presence of the old frost flowers can enhance the backscatter by more than 6 dB. Finally, a statistical analysis of a series of temporal-spatial measurements over a visually homogeneous frost-flower-covered ice floe identified temperature as a significant, but not exclusive, factor in the backscattering measurements.

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.