Radar Calibration Research Articles

A practical millimeter-wave radar calibration target

A simple rectangular-plate millimeter-wave radar calibration target gives RCS values ranging from hundreds to thousands of square meters. This enable tests over extended ranges and with equipment having modest output power. Precise mechanical alignment mechanisms and an optical sight are mandatory, due to the narrow beamwidth. Absorber collars provide an easy way of adjusting the actual RCS according to specific mission requirements, and effectively reduce edge-diffraction effects. The measured V-band calibration uncertainty was less than 0.5 dB, and the pointing resolution was better than 0.05/spl deg/.

The Sydney 2000 Olympic Games Forecast Demonstration Project: Forecasting, Observing Network Infrastructure, and Data Processing Issues

Abstract The Sydney 2000 Olympic Games World Weather Research Programme Forecast Demonstration Project (WWRP FDP) aimed to demonstrate the utility and impact of modern nowcast systems. The project focused on the use of radar processing systems and products for nowcasting, including severe weather. The forecast problems facing the Australian Bureau of Meteorology (BoM) on these short timescales during the FDP are briefly described. The observing system is then discussed and enhancements to the network that supported the Olympic Games forecast requirements and the WWRP FDP project are outlined. In particular, issues related to radar calibration and quality control are discussed in some detail. The paper concludes with a brief discussion on the observing system requirements to meet such modern nowcast systems, areas of further development, and impacts that the FDP had on BoM nowcasting systems. The need for end-to-end design of systems from data gathering, to analysis and product generation is emphasized.

Calibration of the TRMM Precipitation Radar Using the Active Radar Calibrator (ARC)

Calibration of the TRMM Precipitation Radar Using the Active Radar Calibrator (ARC)

Archival precipitation data set for the Mississippi River Basin: Algorithm development

The goals of the Global Energy and Water Cycle Experiment Continental‐Scale International Project (GCIP) point to the need for high‐resolution data sets on all elements of the land surface and atmospheric hydrologic cycle. A high‐resolution precipitation data set has been derived from radar reflectivity observations taken from the National Weather Service WSR‐88D radars in the continental U.S. The data set is available for a continuous five‐year period (1996–2000) at an hourly, 4 × 4 km2 resolution for the Mississippi River Basin. Development of the data set involved data management and quality control of input radar‐reflectivity, parameter estimation for radar‐reflectivity transformation, and product accumulation and quality control of the precipitation product. Quality control algorithms for the input radar‐reflectivity included procedures to deal with radar calibration differences, an especially important problem in developing a long‐term, continental‐scale data set for diverse hydroclimatological applications. Rainfall estimation was based on a Z‐R conversion algorithm that involved an optimization technique to determine the parameters for the transformation of radar‐reflectivity to rainfall. Rainfall accumulation involved integrating to hourly, 4 × 4 km2 resolution and then visually inspecting the final product. Some limitations of the algorithm are presented and suggestions are proposed for improving the development of a long‐term, large‐scale precipitation product. Initial comparisons of the radar‐based product with a rain gauge based product after a quality analysis of both products show good agreement in the Mississippi River Basin.

Rain Gauge and Disdrometer Measurements during the Keys Area Microphysics Project (KAMP)

Abstract Four impact disdrometers and 27 tipping bucket rain gauges were operated at 11 different sites during August and September 2001, as part of the Keys Area Microphysics Project. The rain gauge and disdrometer network was designed to study the range dependency of radar calibration and rainfall verification in tropical storms. The gauges were collocated at eight sites, while three to five gauge clusters were deployed at three sites. Four disdrometers were also collocated with rain gauges. Overall the experiment was quite successful, although some problems did occur including flooding of gauge loggers, vandalism, and excessive noise at disdrometer sites. Both a south-to-north and east-to-west rainfall gradient was observed, whereby the gauges on the western and northern sides of the Lower Keys recorded more rainfall. Considering the campaign-long rain accumulations, collocated gauges agreed well, with differences generally less than 2%, except for one gauge cluster where the rain accumulation differen...

Development of a Retrodirective PARC for ALOS/PALSAR Calibration

Polarimetric radar calibration is a procedure that corrects the polarization distortion of a measured scattering matrix by referring to the scattering matrix of a known target. The present paper describes the principle, design, manufacture and measurement results of a novel retrodirective polarimetric active radar calibrator (PARC). It accommodates both the depolarization characteristic by using dual-polarized antennas and retrodirectivity with the Van Atta array concept simultaneously. The PARC was designed for Phased Array L-band SAR (PALSAR) calibration based on the proposed principle. It consists of a 6/spl times/6-element antenna array with a 1-m-square aperture and four amplifiers with a 20-dB gain. The whole array is divided into four 3/spl times/3-element subarrays to form a two-dimensional (2-D) Van Atta array. Retrodirectivity extends the angular width, where the radar cross section exceeds 35 dBm/sup 2/, which is a preliminary design goal, to almost twice the width of a conventional array reflector of the same size. However, it should be noted that the present design needs at least four times as many amplifiers as a conventional fixed-beam array reflector to be capable of 2-D source tracking. A prototype model of the present retrodirective PARC is manufactured in the L-band to allow Advanced Land-Observation Satellite (ALOS)/PALSAR calibration. The results we obtained through measurement agree well with the theoretical predictions, and substantiate the premise behind the present design of the retrodirective PARC for polarimetric SAR calibration.

Four-year result of external calibration for precipitation radar (PR) of the tropical rainfall measuring mission (TRMM) satellite

External calibration experiments using active radar calibrator (ARC) were conducted for the calibration of the Precipitation Radar (PR) of the Tropical Rainfall Measuring Mission (TRMM) satellite from 1998 to 2001. Three modes of ARC operation are used for the experiment: the ARC transmitter mode for the receiver system of PR, the ARC receiver mode for the transmitter of PR, and the transponder mode for the overall system of PR. The experiments were conducted several times a year. The results of the experiments show that the performance of the PR is close to the prelaunch performance and that PR has been working stably for four years.

Calibration of the MERIC full-polarimetric radar: theory and implementation

This paper describes the architecture and calibration design of the experimental ground based radar station MERIC. This full-polarimetric radar is conceived for the analysis and the recognition of non cooperative aircraft in flight. We carefully study how the full-polarimetric capability is obtained for a simultaneous transmit (simultaneous transmission of two linear FM with opposite slopes) radar system, using analogue deramping with a replica. The phase distortions of the signal propagating in the four polarimetric channels are carefully estimated. We define a phase calibrating method compatible with the outdoor measurements conditions and few constraints on reference targets. We show the phase accuracy obtained with the proposed calibration method on real measurements.

Fundamental experiment of a polarization‐rotating reflector of Van Atta array design for polarimetric calibration

Polarization‐rotating reflectors are important as a reference target for polarimetric calibration of a synthetic aperture radar. A polarization‐rotating retrodirective reflector of Van Atta array design has already been proposed by one of the authors. This paper describes the result of an experimental verification of the proposed polarization‐rotating retrodirective reflector. The experimental system consists of two subarrays each composed of 2 × 2 dual‐polarized elements for horizontal and vertical polarizations working at 2.4‐GHz frequency band. In the experiment, two subarrays are arranged side by side, and the respective orthogonal polarization ports of the subarrays are connected with lines of the same electrical length to form a one‐dimensional polarization‐rotating Van Atta array. To increase the radar cross section, amplifiers are inserted into the lines. The reflection pattern of the reflector is measured in a radio anechoic chamber by rotating the reflector around the axis perpendicular to the array normal direction. From the experiment, a wider angular response of the proposed reflector in the horizontal direction, which is provided by the retrodirective characteristics of the Van Atta array, than the conventional array reflector of the same element number as the proposed reflector is verified in addition to its polarization‐rotating capability introduced by the dual‐polarized antenna elements.

Radiometer‐based calibration of the relation between radar reflectivity and microwave attenuation

The use of higher and higher frequencies in telecommunication systems requires the investigation of the effects of troposphere on electromagnetic wave propagation, and particular attention must be paid to the attenuation induced by raindrops along the radio path. For this purpose the direct measurements approach, in many cases, is not feasible or convenient, and the use of indirect measuring technique is now a common practice. In this respect, meteorological radar operating at nonattenuating frequencies can be of great help owing to their ability in collecting data over wide areas in a short time. The drawback is the incomplete knowledge of the relation between radar reflectivity (Z) and the quantity of direct interest so that an external source of “calibration” is required. In this paper we show that radiometers are proper instruments to calibrate single parameter radar and specifically to derive, on event basis, the relation to convert Z into specific attenuation (α). For this purpose, brightness temperatures at 13 GHz collected by a radiometer along the 30.6° slant path toward the Olympus satellite, direct attenuation measurements, and radar reflectivity profiles have been jointly worked out. First, the choice of the average effective medium temperature to estimate attenuation from brightness measurements has been addressed. Second, the best α ‐Z relation on an event basis has been evaluated through a best fit procedure that forced the radar predicted attenuation to agree with radiometer “measured” ones. The results are very encouraging and confirm the possibility to make use of radiometers as an external source of radar calibration.

Absolute Calibration of 94/95-GHz Radars Using Rain

Abstract Absolute calibration of cloud radars is very difficult. A new method is proposed for 94/95-GHz radars that exploits the fact that at this frequency, the radar reflectivity factor of rain measured at a range of 250 m is approximately constant at 19 dBZ for rain rates between 3 and 10 mm h–1, due to the combined effects of extinction and non-Rayleigh scattering. The standard deviation of around 1.5 dB is due to natural variations in the number concentration of drops and is consistent with the variation predicted from theory, but averaging over a number of different rain events over a month or more should be sufficient to reduce the calibration error to less than 1 dB. A thin layer of rainwater on the radomes of the 94-GHz radar at Chilbolton, in southern England, was found to cause a two-way attenuation of between 9 and 14 dB, but it is shown here that the technique may be successfully implemented by operating the radar at a low elevation angle and employing a shelter to keep it dry. Most 94-GHz cl...

Radar reflectivity calibration using differential propagation phase measurement

A method for calibrating radar reflectivity using polarization radar measurements in rain is described. Accurate calibration of radar reflectivity is essential for obtaining reliable rain rate estimation. In the case of polarization radar, rain rate can be independently estimated using power and phase measurements. Thus phase measurement can be estimated from power measurements. Comparison of the direct estimate of propagation phase (Φm) measurement, which is unaffected by absolute calibration of the radar system, with the estimated propagation phase (Φe) from power measurements is the basis for the calibration method. Polarization measurements such as reflectivity (Z), differential reflectivity (ZDR), and differential propagation phase (ΦDP) are sensitive to drop size distribution (DSD) and mean drop shape. It is important to devise a calibration technique relatively unperturbed by changes in DSD and drop shape. Statistical fluctuation in Φe is derived to estimate the accuracy of the calibration procedure, raindrop shape, and attenuation. The proposed method is applied for calibrating reflectivity measurements of the National Center for Atmospheric Research (NCAR) S‐band polarization radar (S‐Pol) in midlatitude, subtropical, and tropical rain events.

Radar rainfall error variance and its impact on radar rainfall calibration

The high degree of uncertainty in radar rainfall estimation is caused by the variability in the vertical profile of reflectivity, the errors in measurements of radar reflectivity, conversion of reflectivity to rainfall rate, and the error of using point rain gauge rainfall in representing mean-areal rainfall of a radar grid size in the radar rainfall calibration. Presented here is an approach that explicitly takes into account the variations in reliability of radar rainfall estimates associated with range from the radar in calibrating of Z– R relationship. The reliability of radar rainfall estimates is calculated from the proposed climatological variance of radar rainfall error model that accounts for rainfall intensity, nature of rainfall event and number of pulses averaged in obtaining a reflectivity pixel value. Modifications to the parametric and probability matching methods (PMM) are proposed to account for the reliability of the radar rainfall estimates at the gauge locations. Six months of reflectivity-rain gauge data from the Kurnell radar in Sydney, Australia are used to test the model. The result shows the improvements in accuracy of radar measurements of rainfall by incorporating the reliability of radar rainfall estimates in the PMM are about 10% and 5% compared with the conventional PMM and parametric Z– R relationship method, respectively.

Multi-Channel Multi-Variate Equalizer Design

The polarimetric calibration of synthetic aperture radar (SAR) imagery requires the equalization of a multiple-input, multiple-output (MIMO) distortion system. For wide-band, wide-angle SAR systems, the distortion is frequency and angle dependent and can be accurately modelled as a two-dimensional finite impulse response (FIR) filter bank. This paper presents a design algorithm, using a Grobner basis, to compute a FIR filter bank that exactly inverts the multi-channel distortion. The results presented for polynomial inversion of MIMO FIR systems hold for sampled data signals of arbitrary dimension.

Redundant space calibration of hexagonal and Y-shaped beamforming radars and interferometric radiometers

The calibration of large antenna arrays in the absence of a beamforming point source is a common problem in beamforming radars and interferometric radiometers. In this paper the fundamentals of the redundant space calibration (RSC) method for phase and amplitude are reviewed, pointing out the parallelism between the active and passive cases, the technique is then applied in a general and systematic way to two cases of interest: hexagonal planar and Y-shaped arrays, which are known to be the optimum periodic twodimensional configurations. In both cases, the system of equations is determined taking into account the available redundancies and symmetries of these structures. The performance of the RSC method is analysed in terms of the propagation of errors in reference phaselamplitude. The technique is then considered for two hexagonal array systems: the Turbulent Eddy Profiler (TEP), a volume-imaging radar of the lower atmosphere developed at the University of Massachusetts, and the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS), an L-band interferometric radiometer to be launched in 2005 in the Earth Explorer Opportunity Mission-Soil Moisture and Ocean Salinity Mission (SMOS) of the European Space Agency (ESA). In the TEP case the proposed scheme stabilizes an otherwise ill-posed inversion problem, and in the MIRAS case it presents an alternative calibration method to the noise-injection one, without any additional hardware required.

Profiles of Low-Level Stratus Cloud Microphysics Deduced from Ground-Based Measurements

The microwave radiometer‐derived cloud liquid water path (LWP) and a profile of radar reflectivity are used to derive a profile of cloud liquid water content (LWC). Two methods (M1 and M2) have been developed for inferring the profile of cloud-droplet effective radius (re) in liquid phase or liquid dominant mixed phase stratocumulus clouds. The M1-inferred re profile is proportional to a previously derived layer-mean re and to the ratio of the radar reflectivity to the integrated radar reflectivity. This algorithm is independent of the radar calibration and is applicable to overcast low-level stratus clouds that occur during the day because it is dependent on solar transmission observations. In order to extend the retrieval algorithm to a wider range of conditions, a second method is described that uses an empirical relationship between effective radius and radar reflectivity based on theory and the results of M1. Sensitivity studies show that the surface-retrieved re is more sensitive to the variation of radar reflectivity when the radar reflectivity is large, and the uncertainties of retrieved re related to the assumed vertically constant cloud-droplet number concentration and shape of the size distribution are about 9% and 2%, respectively. For validation, a total of 10 h of aircraft data and 36 h of surface data were collected over the Atmospheric Radiation Measurement (ARM) program’s Southern Great Plains (SGP) site during the March 2000 cloud intensive observational period (IOP). More detailed comparisons in two cases quantify the agreement between the aircraft data and the surface retrievals. When the temporal averages of the two datasets increase from 1 min to 30 min, the means and standard deviations of differences between the two datasets decrease from 22.5% 6 84% to 1.3% 6 42.6% and their corresponding correlation coefficients increase from 0.47 to 0.8 for LWC; and decrease from 24.8% 6 36.4% to 23.3% 6 22.5% with increased coefficients from 0.64 to 0.94 for re (both M1 and M2). The agreement between the aircraft and surface data in the 30-min averages suggests that the two platforms are capable of characterizing the cloud microphysics over this temporal scale. On average, the surface retrievals are unbiased relative to the aircraft in situ measurements. However, when only the 1-min averaged aircraft data within 3 km of the surface site were selected, the means and standard deviations of differences between the two datasets are larger (23.4% 6 113% for LWC and 28.3% 6 60.7% for re) and their correlation coefficients are smaller (0.32 for LWC and 0.3 for re) than those from all 1-min samples. This result suggests that restricting the comparison to the samples better matched in space and time between the surface and aircraft data does not result in a better comparison.

Improved Polarimetric Calibration for Atmospheric Radars

Polarization properties of radar waves that are scattered from atmospheric objects are of great interest in meteorological studies. However, polarimetric radar measurements are often not sufficiently accurate for retrieving physical properties of targets. To compensate for errors, radar polarimetric calibration is applied. Typical calibrations are performed based on measurements of point targets with known scattering matrices located in the boresight of the antenna. Such calibration takes into account the polarization state of the antenna pattern only at one point. Since radar measurements of atmospheric phenomena involve distributed targets that fill the full antenna beam, point target radar calibrations are inadequate for meteorological studies. This paper explains in detail the effects of the complete antenna patterns on weather echoes. It is shown that the conventional polarimetric calibration can be significantly improved by incorporating light-rain (<20 dBZ) zenith-pointing measurements into the calibration procedure. As a result, the sensitivity of cross-polar measurements can be improved by 7 dB on average. Also it is shown that the bias in co-cross-polar correlation coefficient can be reduced.

Accuracy of radar rainfall estimates for streamflow simulation

The aim of this work is to analyse the impact of errors in radar rainfall estimates on rainfall-runoff modelling. This issue is addressed through application of radar rainfall estimates for continuous, lumped rainfall-runoff modelling of the Brue catchment, a mid-sized basin in South-West England, over a two and a half-year period. The study is focused on radar rainfall errors associated with range-related bias due to the vertical profile of reflectivity and with mean-field bias due to systematic errors in the radar calibration and biased reflectivity-to-rainrate relationship. Streamflow is simulated through a conceptual hydrological model based on mean areal rainfall estimates obtained by using various radar rainfall processing scenarios. These simulations are evaluated and compared with corresponding streamflow simulations from a dense raingauge network. The comparisons show that radar errors may preclude the use of unadjusted radar estimates for runoff modelling. Radar rainfall adjustment significantly improves model results with simulation efficiency increasing up to 30% after adjustment. Comparison of radar-driven simulations with observed discharge data reveals a simulation efficiency of 0.75 for the lowest radar scan (adjusted), whereas simulation efficiencies are lower for higher radar scans. The results reveal the critical importance of using radar rainfall estimates as close as possible to the ground and the considerable impact that effects of vertical variability of reflectivity have on runoff simulation.

Radar Calibration: Some Simple Approaches

Radar Calibration: Some Simple Approaches

RADAR CALIBRATION

Abstract The proper calibration of weather radars has been at the heart of the problem of accurate reflectivity measurements for more than five decades. This paper summarizes a number of methods that have been used previously and others that areworthy of trial.