Showing papers on "Polarimetry published in 1988"

Radar polarimetry: analysis tools and applications

Abstract: The authors have developed several techniques to analyze polarimetric radar data from the NASA/JPL airborne SAR for Earth science applications. The techniques determine the heterogeneity of scatterers with subregions, optimize the return power from these areas, and identify probable scattering mechanisms for each pixel in a radar image. These techniques are applied to the discrimination and characterization of geologic surfaces and vegetation cover, and it is found that their utility varies depending on the terrain type. It is concluded that there are several classes of problems amenable to single-frequency polarimetric data analysis, including characterization of surface roughness and vegetation structure, and estimation of vegetation density. Polarimetric radar remote sensing can thus be a useful tool for monitoring a set of Earth science parameters. >

A C-Band Coherent Polarimetric Radar for Propagation and Cloud Physics Research

Abstract: At DFVLR in Oberpfaffenhofen, West Germany, a recently developed coherent polarimetric C-band radar is now in operation for research in atmospheric physics and radio wave propagation physics. Its capabilities provide opportunities to investigate the microphysics and dynamics of clouds and precipitation and the kinematics of mesoscale systems with high sampling rates and good resolution in space and time. They include the measurement of scattering (S-) and Stokes matrices by virtue of polarization agility and the ability to receive echoes on the corresponding co- and cross-channels. Investigations of hydrometeor volumes are discussed and first measurement results are presented.

Optimal polarizations for achieving maximum contrast in radar images

Abstract: There is considerable interest in determining the optimal polarizations that maximize contrast between two scattering classes in polarimetric radar images. A systematic approach is presented for obtaining the optimal polarimetric matched filter, i.e., that filter which produces maximum contrast between two scattering classes. The maximization procedure involves solving an eigenvalue problem where the eigenvector corresponding to the maximum contrast ratio is an optimal polarimetric matched filter. To exhibit the physical significance of this filter, it is transformed into its associated transmitting and receiving polarization states, written in terms of horizontal and vertical vector components. For the special case where the transmitting polarization is fixed, the receiving polarization which maximizes the contrast ratio is also obtained. Polarimetric filtering is then applies to synthetic aperture radar images obtained from the Jet Propulsion Laboratory. It is shown, both numerically and through the use of radar imagery, that maximum image contrast can be realized when data is processed with the optimal polarimeter matched filter.

Linear birefringence effects on fiber-optic current sensors.

Abstract: The occurrence of linear birefringence is inevitable when dealing with fiber optics. Intrinsic birefringence can be minimized, but deploying the fiber on an experiment will introduce stress birefringence due to bending and pressure. We have studied the effects of this extraneous linear birefringence on the measurement of current-induced circular birefringence in a fiber which also has a strong twist circular birefringence bias. Orienting the analyzing polarizer by a proscribed procedure gives minimum error. Quantitative error limits on the current for given fiber coil radii and winding tensile stress are calculated. Additional restrictions on the fiber lead-in and lead-out sections are discussed.

Millimeter-wave polarimetric measurements of artificial and natural targets

Abstract: The millimeter-wave polarimeter (MMP), a scatterometer system uses the HP 8510A vector network analyzer for coherent processing of the received signal, provides the polarization and phase measurement capability needed to measure the complete scattering matrix of a given target. A calibration and measurement technique that was used with the MMP at 35 GHz to measure the scattering matrix for both distributed and point targets is described. The measurement accuracy was analyzed by comparing theoretical and measured values for a set of conducting spheres and finite-length conducting cylinders. As an extension of the analysis to natural targets, the scattering matrix was measured for a series of twigs and various smooth and rough surfaces. >

Polarimetry at 1. 3 mm using MILLIPOL - methods and preliminary results for Orion

Abstract: This paper describes a polarimeter for use at wavelengths near 1 mm, designed to be self-contained and portable. Only minor modifications should be required to adapt this instrument for use on any of several millimeter and submillimeter telescopes. The polarimeter system and data-taking techniques are described, and a preliminary measurement is reported of the polarized dust emission from the Orion KL region at 1.3 mm using the NRAO 12 m telescope. The results are similar to previous polarization measurements of Orion at far-infrared and submillimeter wavelengths. The magnetic field direction implied by the polarization position angle is parallel to that found in the surrounding Orion region using optical and near- to midinfrared polarimetric techniques. 17 references.

Polarization determination utilizing two arbitrarily polarized antennas

Abstract: Microwave polarimeter systems typically consist of two orthogonally polarized receiving antennas. Formulas for extracting the unknown polarization antennas with known polarization states are presented. Numerical results are generated showing the size of the errors made by assuming perfect orthogonal polarizations (either circular or linear). >

Application Of Radar Polarimetry To Forestry

Abstract: In order to understand L-band multipolarization radar measurements of forested areas, a model for the forest polarization signature was developed. The model is based on backscatter from dielectric cylinders which represent branches and trunks. In the model the Stokes matrices corresponding to several different scattering mechanisms is calculated, combining the results to get the total Stokes matrix. Comparison of model predictions with radar measurements shows that the model can accurately predict the forest polarization signature.

Polarization change of high-frequency electromagnetic waves in magnetically sheared plasmas

Abstract: The polarization change of an electromagnetic wave passing through a magnetized plasma turns out to be of renewed interest because of the possibility, offered by polarimetry measurements in the infrared range of frequencies, of determining the current density profile for a tokamak plasma. Because of magnetic shear, acting as a coupling parameter between the characteristic modes, the fraction of energy in each mode is not a constant, and a non-WKB treatment is necessary in order to describe the evolution of the wave polarization. The present work shows that this evolution, together with the one of the mode energy, is described by a set of four first-order differential equations. The solutions of this system are discussed in the general case, and in significant limiting cases.

Control: Application to Faraday Rotation

Abstract: An all-fiber-optic system for the control and modulation of the azimuth of a linearly polarized beam is described. The experi- mental configuration is basically that of a Mach-Zehnder interfer- ometer but with the birefringence of the two fiber arms controlled to produce orthogonal circular states of polarization which recombine in the final directional coupler. Azimuth control is then achieved by rel- ative phase modulation using a piezo-electric element. The system has required the use of monomode fiber directional couplers of accurately characterized polarization properties, and the development of fiber lin- ear retarders. The application of the system in the measurement of electric currents via Faraday rotation is described, although extensions to other types of optical instrument are equally feasible. I. INTRODUCTION ECENTLY we have reported fiber-optic techniques R for producing a linearly polarized optical beam of controllable azimuth ( 13. These systems ofler consider- able advantages over conventional ones in terms of size, cost, and compatability with fiber-optic probes for remote measurements. However, the system described previ- ously were hybrid in that only limited use was made of fiber optics, and many conventional components, includ- ing wave plates and beamsplitters, were also required. We have therefore developed new techniques which allow the replacement of all the previously used conventional com- ponents to realize an all-fiber system, with consequent substantial advantages of mechanical stability and mini- aturization. As we have described previously (l), (2) polarization azimuth control and measurement forms the basis of a range of optical instrumentation and experimental tech- niques. Examples include ellipsometry, polarimetry and a wide range of light scattering experiments. To illustrate the potential of the present system, we have applied it to the measurement of electric current via Faraday rotation produced in the fiber guided beam. However. the signal processing schemes are equally applicable to the mea- surement of other circular birefringence phenomena, for example, optical activity. 11. TtwoRY

Optical to mid-infrared polarimetry of OH 0739-14

Abstract: Optical and mid-IR polarimetry and optical photometry are presented for OH 0739 - 14 (= OH 2318 + 42), and previous NIR polarimetry is confirmed The wavelength dependence of the polarization is modeled A model using 50-nm silicate grains fits the data at optical and NIR wavelengths but does not do well at the 10-micron silicate feature The model with 100-nm ice grains fits the data well except at the L band, which is near the 308-micron ice absorption feature The source is most likely to contain a mixture of these two grain species

Detecting polarized light at levels below 1 ppm.

Abstract: Some instrument considerations for detecting very low levels of light polarization -- below 1 ppm --are discussed. Recent linear and circular polarimetry of the whole disk of the Sun, at levels down to 3 x 10 -7 , is reviewed as an outstanding example. Further developments and applications are indicated.

Adaptation of a Ku-band microwave scatterometer for polarimetry measurements of crop canopy

Abstract: The adaptation to a polarimeter of a dual polarized, Ku-band, continuous wave, frequency modulated, microwave scatterometer is described. Polarimetry measurements for a field of hard red spring wheat show that the polarized component of the backscattered radiation differs only slightly from the polarization of the illuminating radiation but that the unpolarized backscattered radiation depends on incidence angle and is sensitive to the polarization of the trasmitted signal.

Polarimetric Radar Measurements of Rain and Radar Derived Attenuation and Depolarization in Rain at 20/30 GHz

Abstract: Propagation properties of rain are derived from radar measurements. The radar measurements of rain have been performed using the polarimetric DFVLR weather radar. Measurements of the horizontal reflectivity, differential reflectivity and linear depolarization ratio in rain are presented. The mean canting angle of raindrops is directly derived from measurements at several linear polarizations. The radar data is corrected for propagation effects which occur on the way between radar and scatter volume. Attenuation and depolarization are derived from the corrected radar data for the frequencies of planned satellite-earth links at 20 and 30 GHz.

Current density profile determination via polarimetry

Abstract: Extensive experimental and numerical studies have been performed to assess the feasibility of polarimeter systems to be installed on a number of devices. The systems employ an amplitude ratio technique similar to that of Soltwisch, with a 45° polarizer angle to divide probe beam equally between two mixer arrays. At present, a nine‐channel system has been constructed to perform measurements of central q value and also the current density profile by scanning the entire system. Laboratory test results will be discussed together with installation plans. The system has the advantage that the amplitude ratio is very insensitive to ellipticity, which can be determined from the phase difference between equivalent detector signals. These properties make the polarimeter attractive, especially for high‐field, high‐density plasmas where the elliptization effects are large. In this case Ψp ≠const λ2∫neBpz.

Some considerations in remote sensing of rain by polarization diversity radars

Abstract: The author investigates the coherency matrix measurement for polarization scanning, with emphasis on propagation effects in precipitation media. An analytical description of propagation effects for partially polarized backscatter, using linear polarization as a basis is developed. A comparison is made with a formulation based on circular polarization. For this comparison, and for verifying the analytical models, consecutive radar polarimetric measurements at scanning linear polarization, on the rainstorm described here and on other forms of precipitation are examined. >

Formulation of the proper equations for developing standards in radar polarimetry

Abstract: Several crucial inconsistencies have been found in the basic equations of radar polarimetry. In particular, the pertinent formulations of the polarization state definitions given in the IEEE/ANSI Standard 149-1979 are in error. These and other inconsistencies and conceptual errors are discussed. The authors provide the correct formulae for the proposed revision of the polarimetric standards together with a well-defined and consistent procedure for measuring target scattering matrices in both monostatic and bistatic arrangements. Further, the proposed procedure can be applied to an arbitrary measurement process in any general elliptical polarization basis. >

Polarimetry of Symbiotic Stars

Abstract: In an effort to bring out the variations in the polarization parameters of symbiotic stars that result from their binary character, polarimetry in R, I IR bands was initiated at the Byurakan station of the Leningrad University Observatory in 1974. In addition, U, B, V measurements for some stars have been made. Three telescopes have been used, with mirror apertures of 48, 48, 62 cm.