Radar Calibration Research Articles

A 77-GHz SiGe Integrated Six-Port Receiver Front-End for Angle-of-Arrival Detection

In this paper an integrated six-port receiver front-end for angle-of-arrival detection of 77-GHz signals is presented. Applications of the circuit are direction finding, automotive radar calibration, or high precision industrial radar. The measurement principle is based on passive superposition of two incident signals and power detection. The circuit features two input amplifiers, a broadband passive six-port network, and four power detectors. The integrated circuit has a power consumption of 95 mW with 5 V supply voltage. It is fabricated in a 200-GHz f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> SiGe bipolar technology and occupies only 1028 × 1128 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The circuit operates in a 3-dB bandwidth from 75 GHz to 84 GHz and has a responsivity of 152 kV/W at 80 GHz. A simple calibration method is proposed and all calibration parameters are calculated for different frequency values.

Doppler radar calibration system

The developed prototype has proved to be a useful instrument for the calibration of Doppler/range radars. The system includes innovative solutions for the generation of Doppler and distances with high precision. It is able to generate accurate Doppler shifts with a resolution higher than 4·10-3 Hz and speeds with an accuracy of around ±2.76 ppm - from 1000 independent measurements - at slant-ranges from 100 meters to 11 kilometers. It can be used with several types of radars in S- and X-band in large dynamic range from miliwatts to several hundreds of Watts. The system has proved very effective in radar calibrations and is being used for the improvement of accuracy and resolution of FM-CW radar systems.

지표면 영향을 고려한 삼각 전파 반사기의 RCS 분석

위성 SAR(Synthetic Aperture Radar)를 정확하게 보정하기 위해서는 검보정에 사용되는 삼각 전파 반사기(Trihedral Corner Reflector: TCR)의 RCS(Radar Cross Section)를 정확하게 계산해야 한다. 본 연구에서는 TCR이 설치된 지표면이 TCR의 RCS 값에 얼마나 영향을 미치는지를 알아보기 위해서 지표면에서의 반사파와 TCR 모서리에서의 회절파를 이론적으로 계산하여 검보정 사이트의 지표면 상태에 따른 RCS 변화를 분석하였다. 지표면 반사파를 구하기 위해 지표면에 대한 PO 반사 계수를 이용하였으며, PO 반사 계수는 지표면 상태 변수인 거칠기와 유전율에 대한 함수이다. 중심 주파수 9.65 GHz에서 지표면 위에 설치된 <TEX>$10{\lambda}$</TEX> 크기 TCR의 RCS 값은 공기 중의 TCR에 비해 0.46 dB 변화가 발생하였고, 이 변화는 지표면의 상태 및 TCR 크기에 따라 크게는 1.55 dB정도의 차이가 날 수 있다. 지표면 영향에 따른 TCR의 RCS 값은 지표면의 거칠기가 작고 유전율이 클수록 크며, 중심 주파수가 낮고 TCR의 크기가 작을수록 크게 발생하였다. The radar cross section(RCS) of a trihedral corner reflector(TCR) should be accurately computed when it is used as an external calibration target for a satellite synthetic aperture radar(SAR) calibration campaign. This paper presents the RCS analysis on a trihedral corner reflector which is installed on a calibration site, using the wave reflection from the rough surface and the wave diffraction from the TCR edges. The results in this paper show quantitatively the effect of the front surface on the RCS of a TCR. The difference of the RCS between a TCR in air and a TCR on a ground surface is computed by including the interaction term which consists of the edge diffraction from the TCR edges and the surface reflection from the front rough surface. The reflection coefficient of a randomly rough surface is a function of the surface roughness and dielectric constant of the surface. The RCS of <TEX>$10{\lambda}$</TEX> size TCR on a ground is 0.46 dB higher than TCR in air at 9.65 GHz, and this can reach at maximum 1.55 dB depending on a surface condition and TCR size. The effect of the front surface on the RCS of a TCR increases, as the surface roughness decreases, the soil moisture increases, and the size of TCR in wavelength decreases.

An Extended Kalman Filter Framework for Polarimetric X-Band Weather Radar Data Processing

Abstract The different quantities measured by dual-polarization radar systems are closely linked to each other. An extended Kalman filter framework is proposed in order to make use of constraints on individual radar observables that are induced by these relations. This new approach simultaneously estimates the specific differential phase on propagation Kdp, the attenuation-corrected reflectivity at horizontal polarization Zh, and the attenuation-corrected differential reflectivity Zdr, as well as the differential phase shift on backscatter δhυ. In a simulation experiment it is found that Kdp and δhυ can be retrieved with higher accuracy and spatial resolution than existing estimators that solely rely on a smoothed measurement of the differential phase shift Ψdp. Attenuation-corrected Zh was retrieved with an accuracy similar to standard algorithms, but improvements were found for attenuation-corrected Zdr. In addition, the algorithm can be used for radar calibration by comparing the directly retrieved differential phase shift on propagation Φdp with the accumulated Kdp estimates. The extended Kalman filter estimation scheme was applied to data collected with an X-band polarimetric radar in the Swiss Alps in 2010. Radome attenuation appears to be significant (up to 5 dB) in moderate to intense rain events and hence needs to be corrected in order to have reliable quantitative precipitation estimates. Measurements corrected for radome and propagation attenuation were converted into rain-rate R with a newly developed relation between R, Kdp, and Zdr. The good agreement between rain-rate values inferred from ground observations and from the radar measurements confirms the reliability of the proposed radar processing technique.

Long‐term monitoring of French polarimetric radar data quality and evaluation of several polarimetric quantitative precipitation estimators in ideal conditions for operational implementation at C‐band

AbstractThis article presents the evaluation of several polarimetric quantitative precipitation estimation (Pol‐QPE) algorithm candidates for operational implementation in the Météo‐France polarimetric weather radars (nine C‐band and one S‐band out of a total of 24 radars in 2011). Since the performance of the algorithms depends on both the quality of the polarimetric variables and the applied polarimetric processing, the article first describes the current polarimetric processing chain and then provides a comprehensive analysis of the quality of the polarimetric variables.Three types of QPE algorithms have been studied at C‐band in ideal conditions (short ranges, no partial beam blocking, no vertical profile of reflectivity effects):(i) algorithms based on simple Z‐R relationships with and without attenuation correction using the differential phase Φdp,(ii) an algorithm based on reflectivity (Zh) and differential reflectivity (Zdr), and(iii) algorithms based on specific differential phase (Kdp).Evaluation against rain‐gauges, performed hourly, confirms the superiority of polarimetric algorithms as repeatedly reported in the literature. It also shows that the benefits brought by algorithms based on power variables, Zh and Zdr, are critically dependent upon the capability to correctly calibrate them. Long‐term monitoring shows that, with the current radar systems and operational calibration procedures, a stability of ±0.4 dB is achieved on Zdr. In that context, QPE algorithms based on Kdp, which are considered to be immune to partial beam blocking, calibration, attenuation, etc., are preferred for operational use until improvements are made to the calibration of Zdr. To this end, a hybrid algorithm based on attenuation‐corrected Zh for low rain rates and Kdp for higher rain rates has been designed and tested Copyright © 2012 Royal Meteorological Society

Reference Target Correction Based on Point-Target SAR Simulation

The backscattering from man-made point targets like passive corner reflectors and active transponders is often used as a radiometric calibration standard for synthetic aperture radar (SAR) calibration. As new systems emerge and the demand for more accurate systems increases, it becomes necessary to better understand the effects of real or imperfect targets on the radiometric calibration results. Therefore, a point-target SAR simulator is presented which models the complete external radiometric calibration process. It incorporates a number of target properties like frequency response, transponder internal calibration strategies, noise, and interference signals, and it takes the instrument SAR mode settings into consideration. Thereby, the relevant target backscatter variation as observed in the processed SAR image with respect to an ideal or any other target can be determined. The simulation results are relevant during the design process of a new target as well as during the actual calibration of a SAR system. Based on these point-target simulations, correction coefficients can be stated for each target and SAR mode, therefore decreasing the remaining radiometric point-target uncertainties. The quantitative examples in this paper show that these corrections can influence the absolute radiometric calibration by more than 1 dB.

Analysis and Correction of Time Delay Active Radar Calibrator Imaging for High-resolution SAR

延时有源定标器是合成孔径雷达(Synthetic Aperture Radar, SAR)图像质量评估和辐射定标的关键设备，有源定标器延时技术的采用可以降低本地杂波的影响，提高测量精度。但其点目标成像质量不可避免地会受到延时的影响，尤其在高分辨率条件下更为严重。该文建立了高分辨率条件下延时对有源定标器点目标成像质量影响分析模型，定量地分析了不同延时对有源定标器点目标响应峰值功率、主瓣3 dB宽度、峰值旁瓣比和积分旁瓣比的影响程度，通过研究提出了一种延时影响的校正方法，仿真实验验证了该方法的正确性。

Use of Ground Clutter to Monitor Polarimetric Radar Calibration

Abstract It is suggested that urban ground clutter can have a role in monitoring calibration of reflectivity factor ZH and differential reflectivity ZDR on polarimetric radars. The median and average values of these variables are considered. Analysis of data from 1 month of cold season in Germany (X-band radar) and 3.5 hot days in Oklahoma (S-band radar) is presented. In the presence of up to moderate rain or snow a reflectivity threshold suffices for separating significant clutter from precipitation observed with an X-band radar. The same threshold was suitable on observations with an S-band radar in Oklahoma because heavy precipitation was not present. The tests suggest the scheme is worthy considering for operational monitoring of ZH as its median values at both locations were within the quantization interval of 0.5 dB. Environmental factors that can influence reflectivities from clutter are examined. The effects on ZDR can be significant. These are quantified in the data and possible uses for calibration and monitoring radar status are indicated.

Scheme of dynamic polarimetric calibration

Proposed is a new scheme of frequency-domain dynamic polarimetric calibration by designing a polarimetric active radar calibrator (PARC) with two rotatable antennas. The proposed scheme can eliminate clutter and the error due to a set of measurements in the traditional static calibration scheme. The calibration results in the proposed scheme are not affected by the initial angles of the PARC antennas. The calibration accuracy of the proposed method is better than the static calibration methods, for example, the target spinor calibration and the full polarimetric calibration technique. This result was verified by simulations under various conditions.

Mapping Bragg Scatter with a Polarimetric WSR-88D

Abstract Using a polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D) radar to distinguish Bragg scatterers from insects and birds in an optically clear atmosphere has the potential to provide information on convective boundary layer depth. Measured median differential reflectivities ZDR of Bragg scatterers lie between −0.08 and 0.06 dB, which supports the hypothesis that the intrinsic ZDR of Bragg scatters is 0 dB. Thus, the intrinsic 0 dB of Bragg scatter can be used for verifying of ZDR radar calibration. Measured copolar correlation coefficients ρhv have distributions peaked at about 0.998–1.0. If insects and birds are spatially separated from Bragg scatterers, the dual-polarization capability of the WSR-88D allows distinguishing echoes from these two types of scatterers since ZDR from biota is significantly larger than 0 dB. In mixtures of Bragg and biota scatter, polarimetric spectral analysis shows differences in portions of the H and V spectra where birds and insects could be contaminating echoes from Bragg scatterers. The values of ZDR ≈ 0 and ρhv ≈ 1 that characterize Bragg scatterers allow coherent summation of signals from the H and V receiver channels and restoration of measurement capability lost as a result of splitting power into H and V channels. Further enhancements to data collection and signal processing allow power measurement, with a standard deviation of about 1 dB, of weak echoes from Bragg scatterers having equivalent reflectivity factors of about −28 dBZ at distance of 10 km from the radar. This level of reflectivity corresponds to a refractive index structure parameter of about 4 × 10−15 m−2/3, a typical magnitude found in maritime air.

Feasibility of using radar differential Doppler velocity and dual-frequency ratio for sizing particles in thick ice clouds

[1] Measurements from ground-based collocated Ka- and W-band vertically pointing Doppler radars were used to evaluate the differential Doppler velocity (DDV) approach for retrieving a size parameter of the aggregate particle distributions in ice clouds. This approach was compared to a more traditional method based on the dual-frequency reflectivity ratio (DFR) using case study observations in different clouds. Because of measurement errors and other uncertainties, meaningful DDV-based retrievals were generally available for the size slope parameter interval of 9 cm−1 < Λ < 25 cm−1. The DFR were generally available for particle populations with Λ up to about 45 cm−1. Expected retrieval errors in the Λ interval between 9 cm−1 and 25 cm−1 were about 40% for the DFR-based estimates and about a factor of 2 larger for the DDV method. Errors increase for noisier measurements. Comparisons of the DDV- and DFR- inferred values of Λ when both retrievals were available revealed their general consistency with a relative standard deviation between results being within retrieval uncertainties. While the DFR approach appears to be more accurate, it requires a 0 dB constraint near cloud tops, which mitigates uncertainties in absolute radar calibrations and differing attenuation paths. The DDV approach generally does not require such a constraint if radar beams are perfectly aligned in vertical (which might not be exactly a case during some observations). Given this, DDV measurements may potentially allow ice particle sizing in situations when DFR constraining is not effective or available (e.g., in precipitating clouds and in clouds with substantial amounts of supercooled water).

An Investigation of the Process Stability of RF SiP Made of DuPont 943 and 9K7

RF packaging is one of the most challenging topics in LTCC technology. Today LTCC is particularly capable for advanced packages and systems-in-package because of its electrical, functional, thermomechanical properties as well as its excellent long-term stability and reliability. LTCC combines the potential for miniaturization, low loss handling of high frequencies up to 110 GHz and offers the opportunity to integrate additional features. Therefore it has to go through various manufacturing steps and several refirings without any performance degradation or loss of dimensional accuracy. This paper discusses the impact of thermal post processing on RF characteristics and geometrical properties of LTCC. Ceramic substrates with radar front ends, calibration structures and other test vehicles made of Du Pont Green Tape® 943 and 9k7 were cofired following the recommended conditions and refired several times in order to investigate and compare the influence of the postfiring. The flatness, dimensions and RF performance of the ceramics up to 110 GHz were evaluated and compared.

Estimation of Specific Differential Phase and Differential Backscatter Phase From Polarimetric Weather Radar Measurements of Rain

The estimation of the specific differential phase particularly in the presence of the differential backscatter phase and nonuniform propagation paths is a long-standing goal in weather radar polarimetry. Furthermore, a reliable estimator of the differential backscatter phase for precipitation measurements has not been proposed yet, although it contains valuable information about the presence of non-Rayleigh scattering and the microphysics of precipitation. In this letter, we introduce a novel method to estimate the specific differential phase and the differential backscatter phase from polarimetric weather radar measurements of rain. This estimation does pay off particularly at the X-band where the scattering regime is non-Rayleigh already in moderate rain. In this case, the differential backscatter phase is useful as an additional weather radar observable with manifold applications such as rain rate estimation, microphysical retrieval, and operational radar calibration. The novel method also provides an improved range resolution of the specific differential phase compared to conventional estimators. We illustrate the estimation of the differential phases with data from the Delft University of Technology's polarimetric X-band International Research Centre for Telecommunications and Radar drizzle radar.

Calibration of Spaceborne CTLR Compact Polarimetric Low-Frequency SAR Using Mixed Radar Calibrators

Spaceborne synthetic aperture radar (SAR) systems operating at lower frequencies, such as P-band, are significantly affected by Faraday rotation (FR) effects. A novel algorithm for calibrating the circular-transmit-and-linear-receive (CTLR) mode spaceborne compact polarimetric SAR using mixed calibrators is proposed, which is able to correct precisely both FR and radar system errors (i.e., channel imbalance and crosstalk). Six sets of mixed calibrators, consisting of both passive calibrators and polarimetric active radar calibrators (PARCs), are investigated. Theoretical analysis and simulations demonstrate that the optimal calibration scheme combines four polarimetric selective mixed calibrators, including two gridded trihedrals and two PARCs, together with total-electron-content measurements by the Global Navigation Satellite System system.

Marine X-band weather radar data calibration

Application of weather radar data in urban hydrology is evolving and radar data is now applied for both modelling, analysis, and real time control purposes. In these contexts, it is all-important that the radar data is well calibrated and adjusted in order to obtain valid quantitative precipitation estimates. This paper presents some of the challenges in small marine X-band radar calibration by comparing three calibration procedures for assessing the relationship between radar and rain gauge data. Validation shows similar results for precipitation volumes but more diverse results on peak rain intensities.

CloudSat as a Global Radar Calibrator

Abstract The calibration of the CloudSat spaceborne cloud radar has been thoroughly assessed using very accurate internal link budgets before launch, comparisons with predicted ocean surface backscatter at 94 GHz, direct comparisons with airborne cloud radars, and statistical comparisons with ground-based cloud radars at different locations of the world. It is believed that the calibration of CloudSat is accurate to within 0.5–1 dB. In the present paper it is shown that an approach similar to that used for the statistical comparisons with ground-based radars can now be adopted the other way around to calibrate other ground-based or airborne radars against CloudSat and/or to detect anomalies in long time series of ground-based radar measurements, provided that the calibration of CloudSat is followed up closely (which is the case). The power of using CloudSat as a global radar calibrator is demonstrated using the Atmospheric Radiation Measurement cloud radar data taken at Barrow, Alaska, the cloud radar data from the Cabauw site, Netherlands, and airborne Doppler cloud radar measurements taken along the CloudSat track in the Arctic by the Radar System Airborne (RASTA) cloud radar installed in the French ATR-42 aircraft for the first time. It is found that the Barrow radar data in 2008 are calibrated too high by 9.8 dB, while the Cabauw radar data in 2008 are calibrated too low by 8.0 dB. The calibration of the RASTA airborne cloud radar using direct comparisons with CloudSat agrees well with the expected gains and losses resulting from the change in configuration that required verification of the RASTA calibration.

A Ground Validation Network for the Global Precipitation Measurement Mission

Abstract A prototype Validation Network (VN) is currently operating as part of the Ground Validation System for NASA’s Global Precipitation Measurement (GPM) mission. The VN supports precipitation retrieval algorithm development in the GPM prelaunch era. Postlaunch, the VN will be used to validate GPM spacecraft instrument measurements and retrieved precipitation data products. The period of record for the VN prototype starts on 8 August 2006 and runs to the present day. The VN database includes spacecraft data from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and coincident ground radar (GR) data from operational meteorological networks in the United States, Australia, Korea, and the Kwajalein Atoll in the Marshall Islands. Satellite and ground radar data products are collected whenever the PR satellite track crosses within 200 km of a VN ground radar, and these data are stored permanently in the VN database. VN products are generated from coincident PR and GR observations when a significant rain event occurs. The VN algorithm matches PR and GR radar data (including retrieved precipitation data in the case of the PR) by calculating averages of PR reflectivity (both raw and attenuation corrected) and rain rate, and GR reflectivity at the geometric intersection of the PR rays with the individual GR elevation sweeps. The algorithm thus averages the minimum PR and GR sample volumes needed to “matchup” the spatially coincident PR and GR data types. The result of this technique is a set of vertical profiles for a given rainfall event, with coincident PR and GR samples matched at specified heights throughout the profile. VN data can be used to validate satellite measurements and to track ground radar calibration over time. A comparison of matched TRMM PR and GR radar reflectivity factor data found a remarkably small difference between the PR and GR radar reflectivity factor averaged over this period of record in stratiform and convective rain cases when samples were taken from high in the atmosphere. A significant difference in PR and GR reflectivity was found in convective cases, particularly in convective samples from the lower part of the atmosphere. In this case, the mean difference between PR and corrected GR reflectivity was −1.88 dBZ. The PR–GR bias was found to increase with the amount of PR attenuation correction applied, with the PR–GR bias reaching −3.07 dBZ in cases where the attenuation correction applied is &amp;gt;6 dBZ. Additional analysis indicated that the version 6 TRMM PR retrieval algorithm underestimates rainfall in case of convective rain in the lower part of the atmosphere by 30%–40%.

Quality Control and Calibration of the Dual-Polarization Radar at Kwajalein, RMI

Abstract The dual-polarization weather radar on the Kwajalein Atoll in the Republic of the Marshall Islands (KPOL) is one of the only full-time (24/7) operational S-band dual-polarimetric (DP) radars in the tropics. Through the use of KPOL DP and disdrometer measurements from Kwajalein, quality control (QC) and reflectivity calibration techniques were developed and adapted for use. Data studies in light rain show that KPOL DP measurements are of sufficient quality for these applications. While the methodology for the development of such applications is well documented, the tuning of specific algorithms to the particular regime and observed raindrop size distributions requires a comprehensive testing and adjustment period. Presented are algorithm descriptions and results from five case studies in which QC and absolute reflectivity calibration were performed and assessed. Also described is a unique approach for calibrating the differential reflectivity field when vertically pointing observations are not available. Results show the following: 1) DP-based QC provides superior results compared to the legacy Tropical Rainfall Measuring Mission (TRMM) QC algorithm (based on height and reflectivity thresholds), and 2) absolute reflectivity calibration can be performed using observations of light rain via a published differential phase–based integration technique; results are within ±1 dB compared to independent measurements. Future extension of these algorithms to upgraded Weather Surveillance Radar-1988 Doppler (WSR-88D) polarization diverse radars will benefit National Aeronautics and Space Administration’s (NASA’s) Precipitation Measurement Missions (PMM) validation programs.

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.

Extraction of Water Information based on RADARSAT SAR and Landsat ETM+

Abstract As an important resource, water has a far-reaching impact on various economic activities. In agriculture, water controls the distribution of crops. In this paper, RADARSAT SAR and Landsat ETM+ imagery were used for the extraction of water information. First, radar image was preprocessed by the radar image calibration and filtering. K-T transformation was applied to Landsat ETM+ data for wetness index and NDVI was derived. Finally, the two classification results are compared. The results show that the complementary information of RADARSAT SAR and Landsat ETM+ image can quickly and accurately extract the water information.