Calibration method of phase distortions for cross polarization channel of instantaneous polarization radar system

A novel polarimetric calibration method for new target property measurement radar system is presented. Its applica- tion in the real radar system is also discussed. The analysis indicates that instantaneous polarization radar （[PR） has inherent cross-polarization measurement error. The proposed method can effectively eliminate this error, and thus enhance the polarization scattering matrix （PSM） measurement precision. The phase error caused by digital receiver＇s direct IF sampling and mixing of two orthogonal polarization channels can be removed. Consequently, the inherent error of target polarization scattering measurement of the instantaneous polarization radar system is well revised. It has good reference value for further ploarimetric calibration and high practical application prospect.

Practical calibration method for instantaneous polarization radar systems utilizing a metal sphere

A practical calibration method is proposed for instantaneous polarization radar systems.The method only needs one measurement by using a metal sphere.The distortions of system and the actual polarization scattering matrix（PSM）of target can be obtained.First,an instantaneous polarization radar system is presented.The system can obtain PSM by a single pulse echo.The dual-polarization antenna can transmit and receive two orthogonal polarization waves.The multilayer micro-strip patch antenna is adopted for this kind of radar system.Second,based on the multi-port network theory,the operation and system errors of instantaneous polarization radar system are analyzed.By making assumption on the cross-talk factors of antenna,distortion matrices of R and Tare derived.Finally,the calibration method based on instantaneous polarization measurement is introduced.Simulation results show the performance of this calibration method.The values of calibrated PSM are in agreement with the actual ones after calibration.

The scattering mechanism of squall lines with C-Band dual polarization radar.Part Ⅱ:the mechanism of an abnormal ZDR echo in clear air based on the parameterization of turbulence deformation

In part I,the clear air echo in front of the squall line is caused by turbulence diffraction,which makes the Z_(DR) echo characteristics different from particle scattering.To study the turbulence deformation phenomenon that is affected by environmental wind,the turbulence-related method is used to analyze the characteristics of three-dimensional turbulence energy spectrum density,and the parametric model of turbulence integral length scale and environmental wind speed is established.The results show that the horizontal scale of turbulence is generally larger than the vertical scale.The turbulence is nearly isotropic in the horizontal direction,presenting a flat ellipsoid with the vertical orientation of the rotation axis when there is no horizontal wind or the horizontal velocity is small.When horizontal wind exists,the turbulence scale increases along the dominant wind direction.The turbulence scale is positively correlated with the wind speed.The power function is used to fit the relationships of turbulence integral length scale and horizontal wind speed,which obtains the best fitting effect,and the goodness of fit(GF)is above 0.99 in each direction.The deforming turbulence can cause 8-9 dB Z_(DR) anomalies in the echo of dual polarization radar,which the ratio of scales in the dominant wind and the vertical direction of deforming turbulence(L_(u)/L_(w))is around 4.3.The variation in Z_(DR) depends on the turbulence shape,orientation and the relative position between turbulence and radar.The shape of turbulence derived from radar detection results is consistent with that of the parametric model,which can provide a parametric scheme for turbulence research.The results reveal the mechanism of abnormal Z_(DR) echo caused by deforming turbulence.

STUDY ON RAINFALL MEASUREMENT AND RAINDROP SPECTRA WITH THE DIFFERENTIAL REFLECTIVITY Z_(DR)TECHNIQUE OF A DUAL LINEAR POLARIZATION RADAR

In this paper,it is shown that the differential reflectivity Z_(DR) is related to drop size distribution(DSD)and the spectral shape variation of different truncated diameters.Z_(DR) is a sensitive function to variation of DSD of the spectra.The effects of shape variation of DSD on rainfall can be deduced with a(Z_H,Z_(DR))dual-parameter technique,which is effective to improve the accuracy of rainfall measurement and is of the priority of identifying large particles of hydrometeors(especially hailstone)from the rain region.

The scattering mechanism of squall lines with C-Band dual polarization radar.Part Ⅰ:echo characteristics and particles phase recognition

Squall line is a kind of common mesoscale disaster weather.At present,there are few studies on the elaborate detection of squall line by dual polarization radar.With the dual polarization upgrade of weather radar network,we need to study the relationship between squall line echoes of base data and polarization data to reveal new echo phenomena and formation mechanisms.The relationship between radar parameters and atmospheric physical processes also need to be examined.Based on the NUIST CDP radar,a squall line in the Yangtze and Huaihe River basin that occurred from July 30 to 31,2014 is analyzed.The results show that polarization parameters have obvious advantages in the characteristics analysis of size,phase state,shape and orientation of the water condensate particles.The phase states of water condensate particles in convection cell can be distinguished through comparative discussion.Several phase states exist in the squall line,including small,medium and large raindrops,melting hails,dry hails and ice crystal particles and the Z_(DR)column can be used to identify the location of the main updraft.In addition,the polarization parameters are more sensitive to the melting layer.The gust front is presented as a narrow linear echo in Z affected by strong turbulence.It is an obvious velocity convergence line in V and approximately 0.70 inρ_(HV).The Z_(DR)can be used as a criterion to distinguish the horizontal and vertical scale of turbulence.The deforming turbulence,which is affected by environmental airflow,will cause an abnormally high Z_(DR)in the gust front and a negative Z_(DR)before and after the gust front.The variation of Z_(DR)depends on the turbulence arrangement,orientation and relative position between turbulence and radar.These dual polarization parameter characteristics offer insights into understanding the structure and evolution of the squall line.

STUDY ON APPLICATIONS OF C-BAND DUAL LINEAR POLARIZATION RADAR IN METEOROLOGY

Theoretical study and development of a dual linear polarization weather radar in China are briefly presented. Also discussed are the potential uses of the new radar system in improving the accuracy of areal rainfall measure- ments and analysing the spacial structure of storms and distribution of hydrometecrs in clouds based on the radar observational data from the field experimcnts during the summers of 1987—1989. The results indicate that a C-band dual polarization weather radar, after considering the microwave attenuation correction, may be employed to quantitatively measure rainfall and to monitor heavy rain and flood events and becomes an impor- rant means to study storm structure.

Statistics-based Optimization of the Polarimetric Radar Hydrometeor Classification Algorithm and Its Application for a Squall Line in South China

A modified hydrometeor classification algorithm （HCA） is developed in this study for Chinese polarimetric radars. This algorithm is based on the U.S. operational HCA. Meanwhile, the methodology of statistics-based optimization is proposed including calibration checking, datasets selection, membership functions modification, computation thresholds modification, and effect verification. Zhuhai radar, the first operational polarimetric radar in South China, applies these procedures. The systematic bias of calibration is corrected, the reliability of radar measurements deteriorates when the signal-to-noise ratio is low, and correlation coefficient within the melting layer is usually lower than that of the U.S. WSR-88D radar. Through modification based on statistical analysis of polarimetric variables, the localized HCA especially for Zhuhai is obtained, and it performs well over a one-month test through comparison with sounding and surface observations. The algorithm is then utilized for analysis of a squall line process on 11 May 2014 and is found to provide reasonable details with respect to horizontal and vertical structures, and the HCA results---especially in the mixed rain-hail region--can reflect the life cycle of the squall line. In addition, the kinematic and microphysical processes of cloud evolution and the differences between radar- detected hail and surface observations are also analyzed. The results of this study provide evidence for the improvement of this HCA developed specifically for China.

Multiple interferences suppression with space-polarization null-decoupling for polarimetric array

The adaptive digital beamforming technique in the space-polarization domain suppresses the interference with forming the coupling nulls of space and polarization domain.When there is the interference in mainlobe,it will cause serious mainlobe distortion,that the target detection suffers from.To overcome this problem and make radar cope with the complex multiple interferences scenarios,we propose a multiple mainlobe and/or sidelobe interferences suppression method for dual polarization array radar.Specifically,the proposed method consists of a signal preprocessing based on the proposed angle estimation with degree of polarization(DoP),and a filtering criterion based on the proposed linear constraint.The signal preprocessing provides the accurate estimated parameters of the interference,which contributes to the criterion for null-decoupling in the space-polarization domain of mainlobe.The proposed method can reduce the mainlobe distortion in the space-polarization domain while suppressing the multiple mainlobe and/or sidelobe interferences.The effectiveness of the proposed method is verified by simulations.

Oil spill detection by a support vector machine based on polarization decomposition characteristics

Marine oil spills have caused major threats to marine environment over the past few years.The early detection of the oil spill is of great significance for the prevention and control of marine disasters.At present,remote sensing is one of the major approaches for monitoring the oil spill.Full polarization synthetic aperture radarc SAR data are employed to extract polarization decomposition parameters including entropy（H） and reflection entropy（A）.The characteristic spectrum of the entropy and reflection entropy combination has analyzed and the polarization characteristic spectrum of the oil spill has developed to support remote sensing of the oil spill.The findings show that the information extracted from（1-A）×（1-H） and（1-H）×A parameters is relatively evident effects.The results of extraction of the oil spill information based on H×A parameter are relatively not good.The combination of the two has something to do with H and A values.In general,when H〉0.7,A value is relatively small.Here,the extraction of the oil spill information using（1-A）×（1-H） and（1-H）×A parameters obtains evident effects.Whichever combined parameter is adopted,oil well data would cause certain false alarm to the extraction of the oil spill information.In particular the false alarm of the extracted oil spill information based on（1-A）×（1-H） is relatively high,while the false alarm based on（1-A）×H and（1-H）×A parameters is relatively small,but an image noise is relatively big.The oil spill detection employing polarization characteristic spectrum support vector machine can effectively identify the oil spill information with more accuracy than that of the detection method based on single polarization feature.

Signal processing method of a novel polarized array radar seeker

This paper proposes a novel polarized radar seeker based on the polarized antenna array. A fully polarized signal processing method for the proposed radar seeker is studied under the environments with electromagnetic interferences. A dual polarized antenna array is employed to transmit and receive the radar signals. The instantaneous polarization signal processing technique is used to detect and recognize the targets. The direction of arrival （DOA） of the target is measured through the spatial spectrum with high resolution for the polarized array radar seeker system. The fully polarized signal model of the polarized array radar seeker is formulated and a specific signal processing algorithm is expounded. The theoretical research and numerical simulation results demonstrate that the proposed radar seeker has good performances in target detection and electronic warfare. The research results can provide an effective technical approach to develop and research the new generation radar seeker.

STUDY ON MONOPULSE ANGLE MEASUREMENT METHOD BASED ON VIRTUAL POLARIZATION MATCHING

A monopulse angle measurement method for polarization array radar is studied in this paper.The receiving signal model is established and then a monopulse angle measurement method based on virtual polarization matching is proposed.To analyze the estimation performance,the Cramer-Rao Lower Bound(CRLB)of angle estimation is derived.Both theoretical analysis and simulation show that:firstly,the proposed method is superior to the traditional angle measurement methods based on the single polarization.Secondly,the performance of the new method is unrelated to the echo polarization.Thirdly,angle estimation of this method is asymptotically optimal.The results show that this method has great potential to be used in polarization array radar.

Utilization of Specific Attenuation for Rainfall Estimation in Southern China

This study uses rain gauge observations to assess the performance of different radar estimators R(ZH),R(KDP)and R(A)in estimating precipitation based on the observations of an S-band polarimetric radar over southern China during a typical convective storm and an extremely severe typhoon,i.e.,Typhoon Manghkut.These radar estimators were derived from observations of a local autonomous particle size and velocity(Parsivel)unit(APU)disdrometer.A key parameter,alpha(α),which is the ratio of specific attenuation A to specific differential phase KDP with three fixed values(α=0.015 dB deg^(-1),α=0.0185 dB deg^(-1)andα=0.03 dB deg^(-1))was examined to test the sensitivity of the R(A)rain retrievals.The results show that:(1)All radar estimators can capture the spatio-temporal patterns of two precipitation events,R(A)withα=0.0185 dB deg^(-1)is well correlated with gauge measurement via higher Pearson’s correlation coefficient(CC)of 0.87,lower relative bias(RB)of 16%,and lower root mean square error(RMSE)of 17.09 mm in the convective storm while it underestimates the typhoon event with RB of 35%;(2)R(A)withα=0.03 dB deg^(-1)shows the best statistical scores with the highest CC(0.92),lowest RB(7%)and RMSE(25.74 mm)corresponding to Typhoon Manghkut;(3)R(A)estimates are more efficient in mitigating the impact of partial beam blockage.The results indicate thatαis remarkably influenced by the variation of drop size distribution.Thus,more work is needed to establish an automated and optimizedαfor the R(A)relation during different rainfall events over different regions.