Michele Galletti

Bias in Copolar Correlation Coefficient Caused by Antenna Radiation Patterns

We present a theoretical study of the bias in the copolar correlation coefficient caused by cross-polar radiation patterns and by unmatched horizontal and vertical copolar radiation patterns. The analysis of the bias induced by cross-polarization radiation is carried out for both modes of operation of polarimetric radars, designated as the simultaneous transmission and reception of horizontally and vertically polarized waves and the alternate transmission of horizontally and vertically polarized waves, respectively. The bias caused by unmatched horizontal and vertical copolar radiation patterns as a function of slight differences in pointing angles and beamwidths is also analyzed. In well-designed weather radars, for the purpose of hydrometeor classification, the overall acceptable bias in the copolar correlation coefficient should be less than about 0.01. The levels of cross-to-copolar gain ratios for acceptable performance are indicated. Ultimately, pointing angle and beamwidth tolerances are indicated for horizontal and vertical copolar antenna patterns.

Measurement and Characterization of Entropy and Degree of Polarization of Weather Radar Targets

To date, few polarimetric weather radars have exhibited the capability to measure full scattering matrices. In contrast, in the synthetic aperture radar (SAR) community, considerable experience has been gained in dealing with complete scattering matrices and their statistical behavior. This paper aims to place weather radar parameters in a wider context in order to exploit more general concepts like target decomposition theorems and polarization basis transformations. Entropy, which is a fully polarimetric variable derived from the Cloude-Pottier decomposition, and the degree of polarization, which is derived from Wolfs coherence matrix, are the subject of this paper. The theoretical analysis carried out in the first part is checked against fully polarimetric data from POLDIRAD, which is the german aerospace center research weather radar. The entropy and the degree of polarization are compared with the copolar correlation coefficient in order to understand whether they can add value to radar meteorological investigations. Because the degree of polarization is available to conventional dual-polarization coherent systems, it is important to assess its potential for operational use.

Concept design of a near-space radar for tsunami detection

Off-shore detection of tsunami waves is a critical component of an effective tsunami warning system (TWS). Even more critical is the off-shore detection of local tsunamis, namely tsunamis that strike coastal areas within minutes from the triggering quake. In this paper we propose a new concept for tsunami detection. NESTRAD (Near-Space Tsunami Radar) consists of a real aperture radar accommodated inside a stationary stratospheric airship providing continuous monitoring of tsunamigenic oceanic trenches.

CLEAN technique in strip-map SAR for high-quality imaging

The maximum obtainable resolution of a strip-map synthetic aperture radar (SAR) system can be retained by simply avoiding weighting, or tapering, data samples in the along-track compression process. However, this will lead to hazardous artifacts caused by strong sidelobes of the corresponding adjacent scatterers whose interference might severely weaken the desired targets or even introduce false targets. On the other hand, some residual artifacts, even after tapering process, may still deteriorate the quality (contrast) of the SAR image. These issues can be remedied by applying the so-called CLEAN technique, which can mitigate these ill-effects in strip-map SAR imagery while maintaining the maximum resolution. This, indeed, is carried out as a post processing step, i.e., after the azimuth compression is accomplished, in the SAR system. The objective of this paper is to extend the CLEAN technique to strip-map SAR system to produce high-quality images with a very good along-track resolution. The algorithm is then applied to data from a ground-based circular SAR (CSAR) system to verify its implementation as well as this new application of the CLEAN technique.

Fully polarimetric analysis of weather radar signatures

In this work the concept of depolarization response, namely the degree of polarization as a function of transmit polarization state, is investigated. Application examples are shown in the field of radar meteorology, namely for hydrometeor identification with fully polarimetric weather radar signatures. Data are from POLDIRAD, DLR research weather radar.

Degree of polarization: theory and applications for weather radar at LDR mode

We present theoretical and experimental results concerning a polarimetric variable available at LDR mode: the Degree of Polarization. It is demonstrated that it possesses attractive properties in terms of robustness with respect to antenna cross-channel coupling and to propagation due to coherent cross-polarization upon forward scattering.

Polarimetric phased array weather radar: Concepts for polarimetric calibration

The National Severe Storms Laboratory (NOAA-NSSL) and the Advanced Radar Research Center at the University of Oklahoma (OU-ARRC) are presently involved in the design, development and construction of a cylindrical polarimetric phased array radar (CPPAR) to demonstrate polarimetric capabilities for weather sensing within the Multi-function Phased Array Radar (MPAR) project. The bias in radar meteorological polarimetric variables is dependent on a number of parameters like cross-to-copolar gain ratio, integrated cross-polarization level, cross-to-copolar phase, copolar beam-width mismatch and beam pointing angle mismatch [1–2]. In the first part we present a preliminary system design for CPPAR, in the second part we present a procedure for polarimetric calibration using a target of opportunity, namely rain. Polarimetric calibration such as this one is extremely important for polarimetric phased arrays, since calibration needs be done on a beam by beam basis.

CPPAR - cylindrical polarimetric phased array radar system design

The National Severe Storms Laboratory (NOAA-NSSL) and the Advanced Radar Research Center at the University of Oklahoma (OU-ARRC) are presently involved in the design, development and construction of a cylindrical polarimetric phased array radar (CPPAR) to demonstrate polarimetric capabilities for weather sensing within the Multi-function Phased Array Radar (MPAR) project. In this paper we present preliminary system design of CPPAR.

Degree of polarization for weather radars

Future operational weather radars are likely to implement hybrid polarization, an operating mode that involves transmitting 45deg slant polarization and receiving the horizontal and vertical components of the backscattered field. In this work, the degree of polarization at slant send is theoretically considered and experimentally evaluated from fully polarimetric signatures in order to assess its potential for use in next generation operational weather radars.

RELAX-based autofocus algorithm for high-resolution strip-map SAR

This paper addresses the non-iterative quality phase gradient autofocus (QPGA) technique which was originally proposed to remove one-dimensional phase errors in spotlight-mode synthetic aperture radar (SAR) imagery. By enriching the source pool, the method is modified in a way suitable for autofocus in stripmap-mode SAR system with the advantage of being independent of any priori assumptions. Unlike the QPGA the potential candidates, i.e., dominant scatterers located along azimuth in each specific range bin, are automatically selected by exploiting the one-dimensional RELAX algorithm. Furthermore, RELAX is capable of estimating the size of blur window which is, in fact, associated with the Doppler spread of signal spectrum. The corresponding model includes four parameters i.e., complex amplitude, delay, Doppler center and spectral width. The proposed method has been applied to data extracted by a ground-based rotating coherent Doppler radar operating in strip-mapping mode SAR, with the aim of high-resolution clutter detection.