Showing papers on "Polarimetry published in 1996"

A review of target decomposition theorems in radar polarimetry

Abstract: In this paper, we provide a review of the different approaches used for target decomposition theory in radar polarimetry. We classify three main types of theorem; those based on the Mueller matrix and Stokes vector, those using an eigenvector analysis of the covariance or coherency matrix, and those employing coherent decomposition of the scattering matrix. We unify the formulation of these different approaches using transformation theory and an eigenvector analysis. We show how special forms of these decompositions apply for the important case of backscatter from terrain with generic symmetries.

Understanding radio polarimetry. I. Mathematical foundations

Abstract: The measurement of polarized radiation uses entirely different methods at optical and radio wavelengths. As a result, the algebraic analysis of polarimeter performance differs and, in the case of radio interferometry, is unnecessarily complicated. We demonstrate that the mathematical operation of outer matrix multiplication provides the missing link between the two approaches. Within one coherent framework, we then unite the concepts of Stokes parameters and Wolf coherency matrix, the Jones and Mueller calculi from optics, and the techniques of radio interferometry based on multiplying correlators. We relate the polarization performance of a complete radio interferometer to the (matrix) polarization properties of its successive signal processing stages, providing a clear view of how a radio polarimeter works. Our treatment also clarifies the nature of and the relations between the various types of transformations used in optical polarimetry. We develop the analysis from the radio interferometrist's point of view, but include enough background for a wider audience. In a companion paper, we discuss in more detail the application to the calibration of radio interferometer systems; in a third paper we investigate the IAU (1973) radio definition of the Stokes parameters and its precise translation into mathematical form.

Polarimetry and modelling of narrow-line active galaxies

Abstract: iDivision of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts ALI0 9AB 2Department of Astronomy and McDonald Observatory, RLM 15.308, University of Tems at Austin, Austin, TX 78712, USA 3Anglo-Australian Observatory, PO Box 296, Epping, NSW 2121, Australia 'University of Oxford, Nuclear and Astrophysics Laboratory, Keble Road, Oxford OXI 3RH sAffiliated to the Space Sciences Division of ESA, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Wuffield RDdio Astronomy Laboratory, University of Manchester, Jodrell Bank, Macclesfield, Cheshire SKI 1 9DL

Understanding radio polarimetry. II. Instrumental calibration of an interferometer array.

Abstract: In a companion paper, a mathematical formalism to describe the polarimetric response of a radio interferometer was presented. Some of the instrumental parameters, however, are either unknown or poorly known. Here we consider the determination of these parameters both by a traditional radio-interferometry instrumental approach as well as by using optical polarimetry principles. In doing so, we establish links between the two fields. We show that some degrees of freedom cannot be solved for with various calibration or self-calibration schemes. These degrees of freedom are identified with instrumental parameters and physical source properties. The number of unsolvable degrees of freedom is reduced for a long synthesis with alt-az antennas. We also consider the effect of errors in the assumed instrumental parameters on the resultant calibrated data. The polarimetric calibration procedure for some telescopes is reviewed in the context of this analysis.

Cloud observations with a polarimetric 33 GHz and 95 GHz radar

Abstract: The University of Massachusetts' Microwave Remote Sensing Laboratory (MIRSL) has developed a unique high spatial resolution multiparameter radar under sponsorship from the Department of Energy's Atmospheric Radiation Measurement (ARM) program. The Cloud Profiling Radar System (CPRS) uses a single one-meter diameter dielectric lens antenna to make collocated polarimetric and Doppler measurements at both 33 GHz and 95 GHz. The polarization of each transmitted pulse at either frequency can be selected on a pulse-to-pulse basis. The radar and supporting hardware are mounted on a truck that serves as a mobile laboratory. The truck-based platform permits CPRS to operate in remote locations and also serves as an economical means of transporting the system. This paper describes the CPRS hardware and presents preliminary vertically pointing observations of mixed-phase stratus clouds obtained in the summer of 1993 during the first field test of the system. Measurements show Mie scattering in the ice region, melting layer and rain region of the clouds observed. To illustrate CPRS potential for particle sizing, models of differential reflectivity and differential mean Doppler velocity are used to estimate median volume diameter,D 0, from dual-wavelength reflectivity and Doppler measurments of rain.

A new scheme for heterodyne polarimetry with high temporal resolution

Abstract: A new scheme for making combined interferometric and polarimetric measurements on tokamak plasmas with high temporal resolution is presented The method, which can be regarded as a generalization of techniques suggested by other workers, can achieve bandwidths in excess of 100 kHz using an optically pumped far-infrared triple laser source A bandwidth of 10 kHz with an accuracy of better than has already been demonstrated Calibration measurements as well as first plasma measurements obtained with the new approach are presented and evaluated

Simultaneous measurements of normal spectral emissivity by spectral radiometry and laser polarimetry at high temperatures in millisecond-resolution pulse-heating experiments : Application to molybdenum and tungsten

Abstract: Spectral radiometry and laser polarimetry are two independent techniques for the measurement of spectral emissivity of materials. In this paper, a high-speed system is described for the rapid measurement of normal spectral emissivity of a specimen based on the simultaneous utilization of the two techniques. One of the goals of this work to ascertain the accuracy of the laser polarimetry technique in measurement of normal spectral emissivity at high temperatures. To accomplish this goal, the normal spectral emissivities, in the vicinity of 0.633μm, of molybdenum and tungsten were measured by the two techniques over the temperature range 2000 to 2600 K. The results obtained by the two techniques are in agreement within 1%. The total uncertainty (two-standard deviation level) in measurement of emissivity by either spectral radiometry or laser polarimetry technique is estimated to be not more than + 2%.

Polarimetric study of solar flares

Abstract: The theory of impact polarization is briefly reviewed. Spectropolaximetry pro-vides a tool to derive the nature, the number flux, and the main characteristics of the angular velocity distribution function of energetic particles accelerated in solar flaxes. As an exemple of application of polarimetry the spatial and temporal characteristics of the linear polarization of the hydrogen Hα line observed in a solar flare is presented.

Polarimetry of young stellar objects — I. Linear polarization of GSS 30

Abstract: We present broad-band, near-infrared (IHKn) imaging linear polarimetry of the young stellar object GSS 30. The overall polarization pattern is centrosymmetric about the illuminating source, IRS 1. The model by Clark & McCall is used to derive the wavelength dependence of the degrees of polarization of the scattered radiation in the nebula. The results suggest that the axis of the nebula is inclined at an angle of - 25°-30° to the plane of the sky, with the northern lobe towards us. The grain size distribution is best fitted by grains covering the range 0.005 to 0.35 ~m. However, the model cannot reproduce the observed polarization pattern across the nebula, which is seen to decrease towards its edge. A polarization disc, i.e. a band of aligned polarization vectors, is seen close to the source. This pattern breaks down to the east of the source where the polarization pattern reverts back to a centro symmetric one. We speculate that a binary system could disrupt the density gradient across the circumstellar disc such that photons can easily escape in this direction and are then scattered (singly) towards us. To the west of the source the pattern of the polarization disc remains intact. The degrees of polarization, both in the line of sight to the source and along the disc, are higher at I and H than they are at Kn. We propose that the wavelength dependence of the polarization here may be produced by a combination of scattering and dichroism, with the grains aligned by a magnetic field parallel to the plane of the disc.

Ultrawideband, polarimetric radar studies of breaking waves at low grazing angles

Abstract: This paper summarizes the results of recent laboratory investigations into the ultrawideband, polarimetric scattering properties of breaking water waves. Breaking waves were produced in a wave tank through chirped wave packets (0.5–1.0 Hz) generated by a programmable wavemaker. An ultrawideband radar system was then used to measure the polarimetric properties of the breakers over the 6–12-GHz band at a grazing angle of 10°. An analysis of the data from these experiments shows interesting dynamics in and differences between the dominant VV and HH polarizations. In particular, the RF frequency responses of the orthogonally polarized signals differ substantially, and both the magnitudes as well as the relative phases can change appreciably over relatively short time intervals. A “bench top” model of a breaker is used to support the hypothesis that this behavior is due to the presence of small, nonlinear structures near the breaker crest and a “double-bounce” scattering mechanism associated with them.

Continuum VLBI Polarimetry of 3C 454.3 at 43 GHz

Abstract: We present 43 GHz VLBA polarization observations of the quasar 3C 454.3 and discuss technical aspects concerning the calibration of millimeter VLBI polarimetry. Significant polarization fine structure is observed near the core of 3C 454.3, with a predominant electric field vector orientation parallel to the projected jet direction. This contrasts with previous centimeter VLBI polarimetry of quasars for which an orthogonal polarization orientation is generally found on larger VLBI scales.

Recent progress in imaging polarimetry

Abstract: Recent instrumental developments in imaging polarimetry allow array detec-tors to reach a polarimetric sensitivity of 1 × 10−4 of the intensity. New instrumental effects appear at these levels of sensitivity and generate spurious polarization signals with amplitudes of up to 5 × 10−4. Here I discuss these effects and present methods to avoid them. Polarized spectra with an rms noise of 6 × 10−6 may then be obtained. Furthermore a method is brought to the readers attention that allows polarization measurements at the 1 × 10−4 level with regular array detectors, e.g. in the near-infrared.

Specular null polarization theory: applications to radar meteorology

Abstract: Specular null polarization theory (SNPT) has been recently introduced for the case of coherent scattering where a 2/spl times/2 scattering matrix is sufficient to describe the scattering process. In this paper, SNPT is extended to the case of incoherent scattering. Optimum polarization states are derived and the results are discussed in relation to the classic radar optimum polarizations. In traditional radar polarimetry, modeling of the radar receive/transmit network is included in the radar voltage equation and consequently this affects the optimum polarizations and polarization responses of scatterers. SNPT eliminates this effect and therefore allows for a more direct analysis of scatterers. Modeling of ensembles of precipitation particles is used to illustrate the results of the analysis.

Mueller matrix imaging polarimetry: An overview

Abstract: The design and operation of a Mueller matrix imaging polarimeter (MMIP) are presented. The instrument is configurable to operate in transmission, reflection, retroreflection, and variable-angle scattering to make a wide variety of polarimetric measurements. The sample may be a single element such as a lens, polarizer, retarder, spatial light modulator, or beamsplitter; the tested sample may also be an entire polarization-critical optical system containing many elements. The MMIP instrument combines a dual-rotating retarder polarimeter with high-resolution imaging capacity. Well-calibrated known polarized light states are incident upon the sample and the exiting state is precisely analyzed. By measuring a series of different generated and analyzed state, the Mueller matrix can be determined. `Decomposing' the measured Mueller matrix into retardance, diattenuation, and depolarization components can give a complete description of the sample's effect on an arbitrary light state. In one system configuration, the MMIP measures the polarization of a set of ray paths through a sample. Another configuration measures the sample's point spread matrix, a Mueller matrix relating the polarization state of a point object to the distribution of intensity and polarization across the image. The MMIP instrument and measurement capabilities are described along with an assortment of previous results.

Weather Radar Polarimetry-Trends Toward Operational Applications

Abstract: This is a version of a speech presented at the 27th Conference on Radar Meteorology. Relative advantages of polarimetry are contrasted with the advantages accrued by the introduction of radar into meteorology and by the addition of Doppler measurements. A description of present interests as perceived by the author follows, and possible future trends are suggested.

Polarimetric detection of objects buried in snowpack by a synthetic aperture FM-CW radar

Abstract: Presents experimental results of polarimetric detection of objects buried in a natural snowpack by a synthetic aperture FM-CW radar. First, the principle of polarimetric imaging in the Co- and Cross(X)-pol radar channels is outlined based on the scattering matrix and the characteristic polarization states for a specific target. Then, polarimetric measurements were carried out to detect objects buried in a natural snowpack 230 cm deep. The targets included two orthogonally placed metallic plates, an ice layer within the snowpack, and a human body. It is shown that the polarimetric FM-CW radar clearly enhances target signatures and that it serves to significantly improve detection in snowpack. Several polarimetric detection results are displayed, demonstrating the potential capability of characteristic polarization imaging and the usefulness of FM-CW radar.

Distances to cataclysmic variables using linear polarimetry

Abstract: A model independent method is proposed for estimating the distances to Cataclysmic Variables (CVs) using linear polarimetry. A polarization versus distance relation is constructed by measuring the linear polarization of field stars along the line of sight to the CV. The distance of the CV can be estimated by measuring its polarization and by assuming its intrinsic polarization is negligible in relation to the interstellar polarization, an assumption that appears to be true in most cases. We show that an accuracy of 6% is achievable by estimating the distance to VW Hyi to be 82 ± 5 pc. Rough distance estimates of other CVs with measured polarization are also given.

A novel bistatic scattering matrix measurement technique using a monostatic radar

Abstract: Presents a new technique for measuring the bistatic scattering matrix of point targets using a monostatic radar. In this technique, the complexity of the traditional bistatic measurement setup and difficulties in retaining the phase coherence between the transmitter and the receiver are circumvented completely. The bistatic measurement is performed using a wideband, polarimetric, monostatic radar in conjunction with a rotatable ground plane positioned behind the target. Assuming that the distance between the target and the ground plane is larger than the radar resolution, the desired bistatic response (image contribution) can be isolated from the unwanted backscatter. Noting that the radar operates in the backscatter mode and using the reciprocity theorem, it is shown that the measured cross-polarized responses (/spl sigma//sub vh/and /spl sigma//sub hv/) cannot be determined uniquely. To rectify this problem, additional independent measurements are required. Additional equations for characterizing the cross-polarized components are obtained by placing an anisotropic lossless slab over the perfectly conducting flat surface. The validity and accuracy of the new bistatic measurement technique is demonstrated by measuring a number of point targets with known theoretical bistatic responses. Also, a new approach for determining the effective dielectric constant of dense random media based on the new bistatic measurement technique is developed.

Development of a ground-based, polarimetric synthetic aperture radar

Abstract: A description of an indoor SAR measurement facility is given and a series of results is presented to demonstrate the imaging properties of the system. The aim of the work is to understand better the microwave backscatter characteristics of vegetation and soils. Based around a vector network analyser, the system uses synthetic pulse techniques to obtain high resolution images. The principles of the technique are presented, together with a description of the imaging algorithm used. Calibration procedures are implemented successfully and the images provide useful quantitative RCS information. The polarimetric capability of the system is demonstrated for both metallic and vegetation targets. The system is a precursor to an outdoor polarimetric ground-based synthetic aperture radar (GB-SAR), designed to be easily and rapidly deployable at widely separated measurement sites.

Gamma-Ray Instrument for Polarimetry, Spectroscopy and Imaging (GIPSI)

Abstract: We present an instrument concept called GIPSI that uses germanium strip detectors in an imaging system to provide narrow line sensitivity less than 8.0 multiplied by 10-6 gamma cm-2s-1 at 100 keV in a 2 week exposure (3 sigma), and which has a point spread function (spatial resolution of approximately 20 arc minutes rms. The germanium strip detectors also make an excellent polarimeter by capitalizing on the angular dependence of the Compton scattering cross section. Gamma-ray polarimetry in the energy band around 60 - 300 keV is an interesting area of high energy astrophysics where observations have not been possible with the technologies employed in current and past space missions. We have tested a prototype detector with polarized beams and have measured a modulation factor of approximately 0.8 at 100 keV. A sensitive instrument can be realized on a modest space mission or a long duration balloon flight. Linear polarization can be detected in sources such as the Crab Pulsar, Cen A, Cyg X-1, and solar flares down to less than 5% of the source flux. The proposed instrument would have a collecting area of 400 cm2.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Determination of both the electron density and the poloidal magnetic field in a tokamak plasma from polarimetric measurements of phase only

Abstract: It is shown that it is possible to measure both the plasma density and the poloidal magnetic field simultaneously by polarimetry, purely from phase measurements. The latter are advantageous compared with previously proposed amplitude measurements. The phase measurements can be performed either by modulating the input polarization (two alternative configurations for this modulation are considered), or by modulating the output polarization. The merits of the three schemes are discussed. The polarimeter required for these measurements is simpler and more robust than the conventional interferometer - polarimeter presently used for the combined density and magnetic field measurements.

Wideband Interferometric Sensing and Imaging Polarimetry

Abstract: Wideband Interferometric Sensing and Imaging Polarimetry (WISIP) has become an important, indispensible tool in wide area military surveillance and global environmental monitoring of the terrestrial and planetary covers. It enables dynamic, real time optimal feature extraction of significant characteristics of desirable targets and/or target sections with simultaneous suppression of undesirable background clutter and propagation path speckle at hitherto unknown clarity and never before achieved quality. WISIP may be adopted to the detection, recognition, and identification (DRI) of any stationary, moving or vibrating targets or distributed scatterer segments versus arbitrary stationary, dynamical changing and/or moving geo-physical/ecological environments, provided the instantaneous 2x2 phasor and 4x4 power density matrices for forward propagation/backward scattering, respectively, can be measured with sufficient accuracy. For example, the DRI of stealthy, dynamically moving inhomogeneous volumetric scatter environments such as precipitation scatter, the ocean/sea/lake surface boundary layers, the littoral coastal surf zones, pack ice and snow or vegetative canopies, dry sands and soils, etc. can now be successfully realized. A comprehensive overview is presented on how these modern high resolution/precision, complete polarimetric co-registered signature sensing and imaging techniques, complemented by full integration of novel navigational electronic tools, such as DGPS, will advance electromagnetic vector wave sensing and imaging towards the limits of physical realization. Various examples utilizing the most recent image data take sets of airborne, space shuttle, and satellite imaging systems demonstrate the utility of WISIP.