Igor R. Ivic

Whitening in Range to Improve Weather Radar Spectral Moment Estimates. Part II: Experimental Evaluation

Abstract Demonstration of a method for improved Doppler spectral moment estimation is made on NOAAs research and development Weather Surveillance Radar-1988 Doppler (WSR-88D) in Norman, Oklahoma. Time series data have been recorded using a commercial processor and digital receiver whereby the sampling frequency is several times larger than the reciprocal of the transmitted pulse width. The in-phase and quadrature-phase components of oversampled weather signals are used to estimate the first three spectral moments by suitably combining weighted averages in range with usual processing at fixed range locations. The weights are chosen in such a manner that the resulting signals become uncorrelated. Consequently, the variance of estimates decreases significantly as is verified by this experiment.

Signal Design to Suppress Coupling in the Polarimetric Phased Array Radar

AbstractExamined are two related modes of polarimetric signal transmission that reduce coupling between the orthogonal components of received signals. For the surveillance scan with large unambiguous range and the simultaneous mode of horizontal (H) and vertical (V) transmission, pulse-to-pulse coding is suggested. It relaxes conditions on cross-coupling isolation from about 45 to about 25 dB while preserving the unambiguous range of over 460 km. For application to systematic codes during Doppler data acquisition, time-multiplexed (back to back) H and V pulses are proposed. This approach also relaxes the cross-coupling isolation to about 25 dB. These theoretically predicted values agree with those obtained by emulating the two schemes using oversampled time series data.

Radial-Based Noise Power Estimation for Weather Radars

AbstractA radar antenna intercepts thermal radiation from various sources including the ground, the sun, the sky, precipitation, and man-made radiators. In the radar receiver, this external radiation produces noise that constructively adds to the receiver internal noise and results in the overall system noise. Consequently, the system noise power is dependent on the antenna position and needs to be estimated accurately. Inaccurate noise power measurements may lead to reduction of coverage if the noise power is overestimated or to radar data images cluttered by noise speckles if the noise power is underestimated. Moreover, when an erroneous noise power is used at low-to-moderate signal-to-noise ratios, estimators can produce biased meteorological variables. Therefore, to obtain the best quality of radar products, it is desirable to compute meteorological variables using the noise power measured at each antenna position. In this paper, an effective method is proposed to estimate the noise power in real time...

Multichannel Receiver Design, Instrumentation, and First Results at the National Weather Radar Testbed

When the National Weather Radar Testbed (NWRT) was installed in 2004, a single-channel digital receiver was implemented so that the radar could mimic typical Weather Surveillance Radar (WSR) version 1988 Doppler (WSR-88D) capability. This, however, left unused eight other channels, built into the antenna. This paper describes the hardware instrumentation of a recently completed project that digitizes the radar signals produced by these channels. The NWRT is the nations first phased array devoted to weather observations, and this testbed serves as an evaluation platform to test new hardware and signal processing concepts. The multichannel digital data will foster a new generation of adaptive/fast scanning techniques and space-antenna/interferometry measurements, which will then be used for improved weather forecasting via data assimilation. The multichannel receiver collects signals from the sum, azimuth-difference, elevation-difference, and five broad-beamed auxiliary channels. One of the major advantages of the NWRT is the capability to adaptively scan weather phenomena at a higher temporal resolution than is possible with the WSR-88D. Access to the auxiliary channels will enable clutter mitigation and advanced array processing for higher data quality with shorter dwell times. Potential benefits of higher quality and higher resolution data include: better understanding of storm dynamics and convective initiation; better detection of small-scale phenomena, including tornadoes and microbursts; and crossbeam wind, shear, and turbulence estimates. These items have the distinct possibility to ultimately render increased lead time for warnings and improved weather prediction. Finally, samples of recently collected data are presented in the results section of this paper.

Evaluation of Phase Coding to Mitigate Differential Reflectivity Bias in Polarimetric PAR

One of the main challenges to the use of phased array radar for weather observations is the implementation of dual polarization with acceptable levels of cross-polar fields induced by the antenna. For example, to achieve acceptable differential reflectivity (ZDR) bias (i.e., less than 0.1 dB) using simultaneous transmission and reception of horizontally and vertically polarized waves, the isolation between coaxial cross-polar and copolar beams needs to be in excess of 50 dB. This presents a formidable challenge because sufficient isolation cannot be achieved at an affordable price by antenna hardware alone. Hence, additional approaches are required to reduce ZDR bias due to cross-polar fields. One such approach is pulse-to-pulse phase coding of the transmitted waves. Herein, this approach is evaluated using theoretical analysis and simulated and time series data from a weather radar. The main criteria for evaluation are the bias and standard deviation of ZDR estimates. The results indicate that phase coding has the capability to significantly decrease ZDR bias without a substantial increase in the standard deviation of estimates.

On the Use of a Radial-Based Noise Power Estimation Technique to Improve Estimates of the Correlation Coefficient on Dual-Polarization Weather Radars

AbstractA weather surveillance radar antenna intercepts thermal radiation from various sources, including the ground, the sun, the sky, and precipitation. In the radar receiver, this external radiation produces noise that adds to the receiver internal noise and results in the system noise power varying with the antenna position. If these variations are not captured, they translate into erroneous signal powers because these are computed via subtraction of noise power measurements from the overall power estimates. This may lead to biased meteorological variables at low to moderate signal-to-noise ratios if those are computed using signal power estimates. In dual-polarization radars, this problem is even more pronounced, particularly for correlation coefficient estimates that use noise power measurements from both the horizontal and vertical channels. An alternative is to use estimators that eliminate the need for noise corrections but require sufficient correlation of signals in sample time, which limits th...

A demonstration of adaptive weather-surveillance capabilities on the national weather radar testbed phased-array radar

This paper describes the latest adaptive scanning capabilities of the National Weather Radar Testbed Phased-Array Radar located in Norman, OK. Focused observations, tailored observations, and the required scheduling algorithms are introduced, and their performance is illustrated with real-data examples. It is demonstrated that adaptive scanning for weather radars has the potential to reduce revisit times and to provide meteorological data that can aid in the forecasters warning-decision process.

Threshold Calculation for Coherent Detection in Dual-Polarization Weather Radars

It is customary to censor signals in conventional weather radar using estimates of signal-to-noise ratio (SNR) and/or magnitude of autocorrelation coefficient at lag one. Dual-polarized weather radar provides a pair of highly correlated signals from the two orthogonally polarized returns. A novel censoring technique, previously proposed, sums powers, autocorrelations, and correlation between signals in the two channels and compares the sum to a threshold. In this paper an efficient procedure for calculating such a threshold is proposed.

An Approach to Simulate the Effects of Antenna Patterns on Polarimetric Variable Estimates

AbstractOne of the main challenges of using phased array radar for weather observations is the implementation of dual polarization with acceptable errors of polarimetric variable estimates. This is because the differences between the copolar antenna patterns at the horizontal and vertical polarizations, as well as cross-polar fields, can introduce unacceptable measurement biases, as the main beam is electronically steered away from the principal planes. Because the sufficient cross-polar isolation is difficult to achieve by the phased array antenna hardware and because the copolar as well as cross-polar patterns inevitably vary with each beam position, it is crucial to properly evaluate errors of estimates due to radiation patterns. Herein, a method that combines the measured or simulated radiation patterns and simulated time series is introduced. The method is suited for phased array and parabolic antennas, and it allows for evaluation of radiation-pattern-induced polarimetric variable biases and standar...

Adaptive-Weather-Surveillance and Multifunction Capabilities of the National Weather Radar Testbed Phased Array Radar

The National Weather Radar Testbed (NWRT) is maintained and operated by NOAAs National Severe Storm Laboratory in Norman, OK, USA. It is a phased array radar (PAR) that was established to evaluate the potential to perform aircraft and weather surveillance with a single, multifunction radar. The NWRT PAR is also being used to demonstrate advanced weather-surveillance concepts that are well suited to the unique capabilities offered by phased arrays. This paper provides an overview of the adaptive-weather-surveillance and multifunction capabilities of this system.