Cross-polar Signals Research Articles

Development of a Multiple-Field-of-View Multiple-Scattering Polarization Lidar System at 355nm for Cloud Measurements

We developed a multiple-filed-of view multiple-scattering polarization lidar (MFMSPL) at 355nm to study microphysics of optically thick clouds and understand multiple scattering effects on space lidar measurements (e.g., EarthCARE). This system is based on depolarization Mie lidar technique; it uses a compact receiver module including a small telescope, polarizer, and detectors to measure co-polar and cross-polar backscatter signals, separately. The MFMSPL system has all the 10 measurement channels. The MFMSPL uses the five receiver modules with FOV of 10mrad tilted with different zenith angles (θ); it measures on-beam signals (θ=0mrad) as well as four off-beam signals (θ=10,20,30,40mrad). This MFMSPL system was based on previously developed 532nm MFMSPL system, however, we made the system more stable, robust, and compact by integrating co-polar and cross-polar channels in the receiver module than the previous system. In addition, we improved the dynamic range of the measurement in nighttime by synergy use of analog and photon counting measurements. In the conference, we present the MFMSPL system, data analysis method including calibration method, and observation results.

Orthogonal Channel Coding for Simultaneous Co- and Cross-Polarization Measurements

Abstract Dual-polarization weather radars typically measure the radar reflectivity at more than one polarization state for transmission and reception. Historically, dual-polarization radars have been operated at copolar and cross-polar states defined with respect to the transmit polarization states. Recently, based on the improved understanding of the propagation properties of electromagnetic waves in precipitation media, the simultaneous transmit and receive (STAR) mode has become common to simplify the hardware. In the STAR mode of operation, horizontal and vertical polarization states are transmitted simultaneously and samples of both horizontal and vertical copolar returns are obtained. A drawback of the current implementation of STAR mode is its inability to measure parameters obtained from cross-polar signals such as linear depolarization ratio (LDR). In this paper, a technique to obtain cross-polar signals with STAR mode waveform is presented. In this technique, the horizontally and vertically polarized transmit waveforms are coded with orthogonal phase sequences. The performance of the phase-coded waveform is determined by the properties of the phase codes. This orthogonal phase coding technique is implemented in the Colorado State University–University of Chicago–Illinois State Water Survey (CSU–CHILL) radar. This paper outlines the methodology and presents the performance of the cross-polar and copolar parameter estimation based on the simulation as well as data collected from the CSU–CHILL radar.

Distinction between rain and ice depolarisation by calculation of differential attenuation and phase shift

System-induced depolarisation of single-polarised satellite beacon signals can be cancelled with vector cancellation. It is demonstrated that the residual error after application of this bias removal technique depends only on the polarisation tilt angle and the XPD of the satellite antenna. It appears that for the calculation of the atmospheric-induced differential attenuation A and phase shift β, correction of the system-induced differential phase shift for the copolar and crosspolar signals is necessary. An experiment is described to measure this system-induced differential phase shift. It is demonstrated that it is possible to clearly distinguish between rain and ice depolarisation by looking at the scatterplots of the calculated A and β against measured copolar attenuation.

Attenuation of co- and cross-polarized electric fields of waves through a layer of dielectric spheroids

A recent model (see D.A. de Wolf et al., ibid., vol.38, p.1317-25, Sept. 1990) for analyzing radar reflections from the layer of melting ice hydrometeors is adjusted and expanded for analyzing line-of-sight attenuation of copolar and crosspolar signals at wavelengths that are comparable to the particle sizes. Expressions for copolar and cross-polar attenuation discrimination are derived. It is shown with a parametric study that these quantities have the expected behavior. >

Electromagnetic wave propagation and scattering in rain and other hydrometeors

This paper reviews the theories and the results of numerical calculations of interactions between hydrometeors and microwaves or millimeter waves propagating in media containing these hydrometeors. At frequencies above about 5 GHz, rain, snow, or ice crystals in high altitudes cause various degrading effects on the performance of communication links. The attenuation of the transmitted signals is a well-known example of such effects. Because of the demand for more communication channels, operating frequencies of both terrestrial and earth--scace communication systems become increasingly higher, and the communication technology becomes more and more sophisticated with the use of digital techniques, or the use of both orthogonaly polarized channels. These have given rise to the need of acquiring more detailed knowledge of interactions between hydrometeors and waves than is necessary for the calculation of attenuation. To obtain such knowledge, we have to know the detailed microphysical properties of hydrometeors, such as shape, size distributions, or the motion of hydrometeors while they are falling. These properties, some obtained previously by meteorolgists and some obtaianed recently by radio engineers to meet the requirements of present radio problems, are reviewed first. Various methods of calculations, and the detailed knowledge obtained from calculations of the single scattering properties of hydrometeors, whose sizes are generally comparable to wavelength, are then reviewed. With the information of these scattering properties, various propagation phenomena are discussed from a theoretical point of view. Specific subjects treated include attenuation, cross polarization due to rain or ice crystals in high altitudes, the methods of depolarization cancellation, rain scattering other then forward and backward which is necessary in the estimation of rain scatter interference between two communication links operating at the same frequency, various multiple or incoherent scattering effects on copolar and cross-polar signals, pulse shape, channel transfer characteristics, etc. The aim of this review is to provide fundamental background informations on the interactions between hydrometeors and radio waves, and hence the more practical investigations, such as long-term statistics of attenuation, are beyond the scope of this review.

Phase fluctuations of cross-polarized signals at 4 GHz in fine weather on earth-space paths

On an earthspace propagation path with the low elevation angle of 10 deg, the phase between co- and crosspolar signals occasionally showed rapid and irregular fluctuations during fine weather. These fluctuations were generally significant during the daytime, and were strongly in phase with the occurrence of the copolar amplitude scintillations. This could be attributed to the combined effects of the crosspolar phase pattern of the receiving antenna and small fluctuations of the angle of arrival of the radiowaves.

Relative phase measurements at 30 GHz between copolar and induced crosspolar signals produced by ice particles on a satellite-to-ground link

Observations of relative phase variations between the copolar and crosspolar signals, produced by ice particles, on a 30 GHz satellite-to-ground link are presented. A mechanism, involving differential phase shift between orthogonal components of a linearly polarised wave, is discussed.

Copolar and cross-polar signals received at 30 GHz from ATS-6

From the end of July 1975 until August 1976 the reception of signals from the geostationary satellite ATS-6was possible since at that time the satellite was brought into a position above Victoria Lake at 35° East. The copolar signal showed attenuation which varies strongly with the amount of rain. The rain was measured by a rain gauge located close to the 3 meter precision Cassegrain antenna. The attenuation measured at the satellite link did not always correspond with the attenuation measured at the nearby line of sight link at 34 GHz. For example during heavy thunderstorms the attenuation might have reach values as high as 20 dB or more. For smaller attenuations comparison is possible with a radiometer experiment at 30.1 GHz. In general this comparison shows good results for attenuations less than 10 dB. The crosspolar signal measured showed values of — 55 dB to about — 20 dB relative to the copolar depending upon several reasons. First of all the rain showed mostly a strong increase in the crosspolar signal but satellite movements and errors in the antenna pointing will also increased this level. Various examples are shown. It appears that the copolar — cross-polar isolation reached the lowest value at attenuations of 3–6 dB. Although the time that the satellite was available for measurements was limited, some statistic results can be given despite the fact the signal transmitted from the satellite was not always constant.

Phase of crosspolarised signals on microwave satellite links

For a linearly polarised satellite link in the 4–30 GHz range, ‘anomalously high’ crosspolar signals caused by differential phase shift should always exhibit a phase of nearly ±90 relative to the wanted polarisation. This principle might permit an adaptive cancellation system using only one variable parameter (amplitude); it does not necessarily apply for circular polarisation, a possible advantage of linear over circular polarisation on practical links.