Two Methods of Ambiguity Resolution in Pulse Doppler Weather Radars

A comparison is made of the performance of a weather Doppler radar with a staggered pulse repetition time and a radar with a random (but known) phase. As a standard for this comparison, the specifications of the forthcoming next generation weather radar (NEXRAD) are used. A statistical analysis of the spectral momentestimates for the staggered scheme is developed, and a theoretical expression for the signal-to-noise ratio due to recohering-filteringrecohering for the random phase radar is obtained. Algorithms for assignment of correct ranges to pertinent spectral moments for both techniques are presented

Sensors and Systems to Enhance Aviation Safety Against Weather Hazards

Weather-related factors are among major causes of aviation hazards, passenger discomfort, poor airline schedule-keeping, and poor operating economy. A variety of new high-technology electronic sensors and systems for aviation weather are being developed and installed across the US. The aviation weather monitoring system of the future will be centered around Doppler weather radars which offer the best combination of coverage, resolution, and agility for this purpose, and are able to detect and estimate the severity of atmospheric air motion, including wind shear and turbulence, directly. Three major new Doppler radar systems are pertinent: the Next Generation Weather Radar, the Terminal Doppler Weather Radar, and the Airport Surveillance Radar with a dedicated weather channel. Other relatively simple new instruments for aviation weather support include the Low Level Wind Shear Alert System, the Doppler Wind Profilers, the Automated Weather Observation System, and the Automated Surface Observation System. These systems are designed to perform higher level functions such as detection, characterization, and hazard potential estimation of aviation-significant weather phenomena, as well as their communication and display automatically. Together with conventional information sources and data from improved satellite imagers and sounders, the modern dedicated weather surveillance systems hold the potential of being powerful tools in enhancing the quality of aviation well into the next century

Practical Algorithms For Mean Velocity Estimation In Pulse Doppler Weather Radars Using A Small Number Of Samples

Doppler weather radars with fast scanning rates must estimate spectral moments based on a small number of echo samples. This paper concerns the estimation of mean Doppler velocity in a coherent radar using a short complex time series. Specific results are presented based on 16 samples. A wide range of signal-to-noise ratios are considered, and attention is given to ease of implementation. It is shown that FFT estimators fare poorly in low SNR and/or high spectrum-width situations. Several variants of a vector pulse-pair processor are postulated and an algorithm is developed for the resolution of phase angle ambiguity. This processor is found to be better than conventional processors at very low SNR values. A feasible approximation to the maximum entropy estimator is derived as well as a technique utilizing the maximization of the periodogram. It is found that a vector pulse-pair processor operating with four lags for clear air observation and a single lag (pulse-pair mode) for storm observation may be a good way to estimate Doppler velocities over the entire gamut of weather phenomena

Use of Copolar Correlation Coefficient for Probing Precipitation at Nearly Vertical Incidence

We present observations of the copolar correlation coefficient between horizontally and vertically polarized echoes Phv(O) 1. These were made with ground-based and airborne weather radars at nearly vertical incidence. A sharp decrease of I p,,,(O)I occurs at the bright band bottom, and is attributed to a varying mixture of hydrometeors with diverse shape, size, and thermodynamic phase. The largest contribution to decor-relation seems to come from wet aggregates; this is substantiated by consideration of two simple models. One consists of randomly oriented wet prolate spheroids, and the other considers an ensemble of distorted spheres. Prolates with axis ratios of 3 or distorted spheres with rms roughness equal to 15% of the diameter decrease the correlation to 0.8 at S band. At Ku band and for the size range encountered in the bright band, the decrease is a function of equivalent diameter because scattering is in the Mie regime. 1 ph,(o) 1 measurement at 13.8 GHz and from the aircraft are the first ever. Also, differential phase and differential reflectivity at a'10" off nadir are the first of its kind. These last two variables showed a distinct signature in the bright band. This is significant because it might lead to applications on airborne or spaceborne platforms