bluuebunny/crossref_metadata_2025 huggingface dataset:
Concentrations of radioactive elements in lunar materials
Authors: Korotev Randy L.
Published: February 2004

As an aid to interpreting data obtained remotely on the distribution of radioactive elements on the lunar surface, average concentrations of K, U, and Th as well as Al, Fe, and Ti in different types of lunar rocks and soils are tabulated. The U/Th ratio in representative samples of lunar rocks and regolith is constant at 0.27; K/Th ratios are more variable because K and Th are carried by different mineral phases. In nonmare regoliths at the Apollo sites, the main carriers of radioactive elements are mafic (i.e., 6–8% Fe) impact‐melt breccias created at the time of basin formation and products derived therefrom.
Late Quaternary climate change from δ18O records of multiple species of planktonic foraminifera: High‐resolution records from the Anoxic Cariaco Basin, Venezuela
Authors: Lin Hui‐Ling , Peterson Larry C. , Overpeck Jonathan T. , Trumbore Susan E. , Murray David W.
Published: February 2004

Seasonal trade wind‐induced upwelling along the southern margin of the Caribbean Sea occurs in response to the annual migration of the Intertropical Convergence Zone. Laminated, high deposition rate sediments of the Cariaco Basin, a small anoxic basin on the Venezuelan continental shelf, clearly record large changes in the past intensity of this upwelling. Because sediments of the Cariaco Basin are largely unbioturbated, they offer a natural opportunity to study the stable isotopic records of multiple planktonic foraminiferal taxa and to evaluate their sensitivity to both the modern hydrography and temporal changes in upwelling intensity and climate. Oxygen isotope data (δ18O) from four dominant foraminiferal taxa are presented for the time period covering the last 28 kyr. The δ18O data from Globigerina bulloides, after correction for nonequilibrium precipitation, are used as a monitor of sea surface conditions during the winter‐spring upwelling season. The δ18O data from white Globigerinoides ruber are used as a measure of annual‐average conditions in the near surface, while pink G. ruber data are consistent with use as an index of endmember conditions during the summer‐fall nonupwelling season. Data from the deeper dwelling Neogloboquadrina dutertrei yield information on conditions near the base of the local thermocline. During the last glacial, δ18O data from G. ruber and generally reduced interspecific differences indicate cooling of surface waters over the Cariaco Basin by up to 4°C. This longer‐term cooling does not appear to be related to changes in upwelling intensity along the coast but may instead reflect more regional cooling of the larger Caribbean. Superimposed on this pattern, between 12.6 and ∼10 ka, is a convergence of δ18O data between G. bulloides and N. dutertrei, implying much stronger upwelling during the last deglaciation. This scenario is consistent with other evidence for high productivity at this time. At ∼14 ka, a sharp δ18O depletion event observed in all taxa seems to have been produced by increased freshwater discharge to the southern Caribbean, suggesting either higher regional rainfall or the influence of glacial melting in the Andes. Minimum δ18O values of pink G. ruber around 6–7 ka record warmer summer sea surface temperatures and/or decreased salinity in the mid‐Holocene.
Radiolarian‐based transfer functions for estimating mean surface ocean temperatures and seasonal range
Authors: Pisias Nicklas G. , Roelofs Adrienne , Weber Mysti
Published: February 2004

New radiolarian‐based transfer functions to estimate sea surface temperature (SST) and seasonal range are presented. The transfer fonctions are based on the approach originated by Imbrie and Kipp [1971]. The transfer functions differ from previous studies in the following three important ways: (1) extensions to Q‐mode factor analysis provide an objective method to cull species in the very diverse radiolarian population; (2) a log transform of the relative abundance data is used to normalize the species percent data; and, (3) rather than writing equations for specific seasons, which are not independent data sets, statistically independent equations are developed to predict mean annual sea surface temperatures as well as seasonal temperature range. One hundred and seventy surface sediment samples from the Pacific Ocean are used to generate the SST and season temperature range transfer functions. All samples were counted using a standardized radiolarian taxonomy. Forty one radiolarian species were used in the final regression equation. Q‐mode factor analysis of this data set identified seven assemblages. These assemblages, the tropical, transitional, Antarctic, Bering Sea, western Pacific, central gyre, and eastern boundary current, are named for the oceanographic regions where these assemblages are important. The seven assemblages are used in a regression analysis to predict SST and seasonal temperature range. The standard error of estimates for both mean SST and seasonal temperature range is 1.6°C. Comparison between radiolarian‐based SSTs and SST estimates from alkenone Uk37 in a 20,000 year long record from the northeast Pacific shows excellent agreement. Comparison of mean SST estimates for the last glacial maximum (LGM) based on radiolarian and foraminifera in 10 eastern equatorial Pacific also show excellent concordance. These new LGM estimates suggest that the original Climate: Long‐Range Investigation Mapping, and Prediction (CLIMAP) reconstruction for this region underestimated surface ocean cooling.
The Mid‐Pleistocene climate transition: A deep sea carbon isotopic perspective
Authors: Raymo M. E. , Oppo D. W. , Curry W.
Published: February 2004

Five δ13C records from the deep ocean, extending back to 1.3 Ma, were examined in order to constrain changes in mean ocean carbon isotope composition and thermohaline circulation over the 41‐ to 100‐ka climate transition. These data show that significant perturbations in mean ocean carbon chemistry were associated with the mid‐Pleistocene climate transition. Notable features of the last 1.3 Myr are (1) a pronounced ∼0.3‰ decrease in mean ocean δ13C between 0.9 and 1.0 Myr, followed by a return to pre‐1.0 Ma values by 400 ka B.P., which we propose was due to the onetime addition of isotopically depleted terrestrial carbon to the ocean, possibly associated with an increase in global aridity (and decrease in the size of the biosphere) across the 41‐ to 100‐ka transition; (2) no change in the Atlantic‐Pacific (A‐P) δ13C gradient over the last 1.3 Myr, suggesting no change in mean ocean nutrient content accompanied the addition of light carbon; and (3) stronger vertical nutrient fractionation in the North Atlantic in the middle Pleistocene between sites 607 and 552, suggesting weaker North Atlantic Deep Water formation at this time relative to the early and late Pleistocene. We also find evidence for a more pronounced deep recirculation gyre in the western North Atlantic basin in the early Brunhes, as evidenced by “aging” of deep northern basin water (site 607) relative to deep water in the equatorial Atlantic (site 664).
Evidence for a volcanic cooling signal in a 335‐year coral record from New Caledonia
Authors: Crowley Thomas J. , Quinn Terrence M. , Taylor Frederick W. , Henin Christian , Joannot Pascale
Published: February 2004

Although volcanic cooling events have been detected in tree ring records, their occurrence in marine records has received much less attention. Herein we report results from a 335‐year oxygen isotope record (1657–1992) from a New Caledonia coral indicating that as many as 16 interannual‐scale cooling events occur within 1 year of a volcanic eruption as determined by ice core records. There are also pentadal/decadal‐scale cooling events beginning in 1675, 1813, and 1903 that immediately postdate volcanic eruptions. However, the interannual correspondences are complicated by the fact that some of the cooling events also coincide with El Niños, which cause cooling in this part of the western South Pacific. If our conclusions are substantiated by further work, occurrence of distinct volcanic cooling signals may enable refinement of coral chronologies by use of the “event stratigraphic” approach, with the most promising correlation horizons being associated with the following eruptions: 1808 (Unknown), 1813–1821 (several eruptions), 1835 (Coseguina), 1883 (Krakatau), and possibly 1963 (Agung).
Pliocene carbonate accumulation along the California Margin
Authors: Ravelo A. C. , Lyle M. , Koizumi I. , Caulet J. P. , Fornaciari E. , Hayashida A. , Heider F. , Hood J. , Hovan S. , Janecek T. , Janik A. , Stax R.
Published: February 2004

Recent modeling studies call on increased ocean heat transport to explain high‐latitude warming observed for intervals throughout the middle Pliocene. Possible vehicles for ocean heat transport are the poleward arms of the subtropical gyres. Sites from the California margin (Ocean Drilling Program Leg 167) provide monitors of wind field within the eastern arm of the gyre which may be an indication of basin‐wide subtropical gyral strength. At most sites (water depths from 1106 to 4212 m) CaCO3 mass accumulation rate (MAR) was highest in the middle Pliocene (3.5–2.0 Ma). This high CaCO3 MAR “event” is attributed primarily to higher CaCO3 production due to higher offshore upwelling associated with the zone of the greatest wind stress curl. Thus, in the middle Pliocene, there was enhanced wind stress curl along the California margin, and possibly enhanced North Pacific sub‐tropical gyral circulation and meridional ocean heat advection.
Evidence for large earthquakes at the Cascadia Subduction Zone
Authors: Clague John J.
Published: February 2004

Large, historically unprecedented earthquakes at the Cascadia subduction zone in western North America have left signs of sudden land level change, tsunamis, and strong shaking in coastal sediments. The coastal geological evidence suggests that many of the earthquakes occurred at the boundary between the overriding North American plate and the subducting Juan de Fuca plate. This hypothesis is consistent with geodetic measurements and the results of geophysical modeling, which indicate that part of the plate boundary is locked and accumulating elastic strain that will be released during a future large earthquake. Arguments based on potential amounts of seismic slip and likely rupture areas suggest that most or all of the plate boundary earthquakes were magnitude 8 or larger events. The last earthquake or series of earthquakes, about 300 years ago, ruptured the entire 1000‐km length of the subduction zone; if it was a single quake, it probably exceeded magnitude 9. Other earthquakes may have ruptured one or more segments of the subduction zone or may have occurred on faults in the North American plate. Recurrence intervals are uncertain because of difficulties in identifying and dating earthquakes. In southwestern Washington state, intervals for the seven most recent earthquakes average about 500 years but range from less than 200 years to 700–1300 years. Future research on Cascadian plate boundary earthquakes will probably focus on (1) the relation between plate boundary and crustal earthquakes, (2) earthquake magnitude, (3) the areal extent and severity of seismic ground motions, (4) ages and number of past plate boundary earthquakes, and (5) land level changes preceding earthquakes.
Anisotropy of the Earth's inner core
Authors: Song Xiaodong
Published: February 2004

The inner core has the intriguing property that seismic waves traveling parallel to the Earth's spin axis arrive earlier than those traveling parallel to the equatorial plane. The travel time is some 6 s faster from pole to pole than across the equator. This difference is not the result of different polar and equatorial radii of the inner core, for it would require the inner core to be elongated at the poles by more than one third of its radius. Rather, this 6‐s travel time difference is the result of the inner core's being anisotropic; wave speeds differ for different directions of wave propagation. This anisotropy, discovered only in the last decade, is characterized by cylindrical symmetry about an axis approximately aligned with the Earth's spin axis. The anisotropy is believed to be due to a preferred orientation of anisotropic iron crystals composing the inner core, but the mechanisms responsible for creating such a preferred orientation remain uncertain. With the anisotropy now well established as a basic property of the inner core, its quantification and that temporal variations of its orientation have become a vital tool for probing the structure, composition, and dynamics of the Earth's deep interior.
Broadband seismology and noise under the ocean
Authors: Webb Spahr C.
Published: February 2004

Most of our understanding of the Earth's interior has been derived from measurements from global seismic networks, although no network has ever been truly “global” because some 71% of the Earth's surface is underwater. The resulting gaps in coverage produce a biased and incomplete image of the Earth. Work has begun toward establishing permanent observatories on the deep seafloor, although the technical difficulties remain severe. Will these stations be useful, and where and how shall they be established? Data from seafloor observatories will be of poorer quality than continental site data because the sea surface is an important and local source of broadband noise. This noise is derived from wind and waves through direct forcing at long periods and by nonlinear coupling to elastic waves at short periods. Our understanding of the generation and propagation of seismic noise and of wind and wave climatology can be used to predict the temporal and geographical variability of the noise spectrum and to assess likely sites for permanent seafloor observatories. High noise levels near 1 Hz may raise detection limits for short‐period, teleseismic arrivals above mb = 7.5, limiting the usefulness of many seafloor sites. Noise levels in deep boreholes will be 10 dB quieter than those at the seafloor, but sensors buried short distances below the seafloor may also provide comparable noise levels and fidelity. The retrieval of data from permanent seafloor observatories remains an unsolved problem, but long‐term temporary arrays of ocean bottom seismometers are now being used in regional scale experiments using earthquakes as sources. Such experiments are likely to be less successful in the Pacific basin than in either the Indian Ocean or North Atlantic Ocean because of higher noise levels.
A meteor scatter prediction model and its application to adaptive beam steering
Authors: Akram Ammad , Cannon Paul S.
Published: February 2004

A computer model to predict the underdense meteor arrival rate over a forward meteor scatter communications link is presented. The model incorporates important effects such as major shower streams, a nonuniform radiant distribution, and antenna polarization coupling. A particularly useful aspect of the model is its capacity to predict the passage of sporadic and shower hotspot regions across the sky and thereby provide directional information to drive an adaptive beam steering system. Directional data from a phased array reception system in the United Kingdom has been used to determine the diurnal arrival distribution of sporadic meteors. This diurnal variation is broadly reproduced by the model and suggests that just two daily changes in the direction of a high‐gain beam would offer considerable advantage over a fixed beam system. This paper shows that further improvements in system performance can be achieved with accurate predictions of the time of appearance and location of shower streams. A comparative study between monthly predictions and experimental data from a high‐latitude link in Greenland over a year shows reasonable agreement but highlights the need for higher‐resolution radiant data.
Main ionospheric trough as a channel for MF propagation in the magnetosphere
Authors: Blagoveshchensky D. V. , Dobroselsky K. A. , Maltseva O. A.
Published: February 2004

By interpretation of the long‐delayed echoes (L=3.2) observed on the frequency 1.8 MHz, the most probable mechanism of propagation was found to be wave guiding in the odd magnetospheric channel such as the main ionospheric trough. The method of numerical simulation of the middle wave propagation allowed us to formulate (1) the conditions of trapping and keeping waves in a channel and (2) the relations between the positions of the transmitter, the trough, and the plasmapause corresponding to both the echo appearance and the wave passing to the magneto‐conjugate region as far as the Earth's surface. Similar conditions and relations are satisfied in a wide range of frequencies and L shells of the transmitter and may be used for finding the position of the trough and the plasmapause.
Superposition of signals in ionospheric multipath channels
Authors: Västberg Anders , Lundborg Bengt
Published: February 2004

This paper discusses signal intensity calculations for ionospheric HF radio waves using the geometrical optics approximation. To calculate the total signal intensity for an ionospheric channel, all the multipath and multimode amplitudes must be superimposed with their proper polarizations and phases. The basic method for calculating multipath interference with geometrical optics is presented. Using this method, the phenomenon of polarization fading is investigated and, in particular, the frequency‐selective fading of a wideband HF channel due to the beating of the ordinary and extraordinary modes is calculated and compared with actual measured data.
Surface currents induced on a dielectric half‐space by a Gaussian beam: An extended validation for point matching method of moment codes
Authors: Johnson Joel T.
Published: February 2004

An approximate analytical solution for electric surface currents induced on a planar dielectric half‐space by a normally incident Gaussian beam is presented. This solution provides a useful tool for testing discretization methods in point matching moment method codes for penetrable rough surface scattering which use a Gaussian beam “tapered” incident field to avoid edge effects. For one example case illustrated, the surface current solution obtained can be shown to satisfy standard impedance boundary condition integral equations at the beam center to within 0.01% accuracy, and a separation of individual term contributions to the entire integral equation enables a detailed check of moment method discretization procedures to be performed.
Ionospheric total electron content and slab thickness determined in Australia
Authors: Breed A. M. , Goodwin G. L. , Vandenberg A‐M. , Essex E. A. , Lynn K. J. W. , Silby J. H.
Published: February 2004

Ionospheric total electron content (TEC) and slab thickness (τ) have been determined for southern Australia from July 1991 to June 1995 using Global Positioning System (GPS) satellite reception and ionograms (at 5‐min intervals) recorded at Salisbury, South Australia, and other Australian ionograms of the Ionospheric Prediction Service Radio and Space Services, Australian Department of Administrative Services. Seasonal, diurnal, and latitudinal variations in TEC and slab thickness are investigated. The removal of possible error sources in GPS measurements such as satellite and receiver biases is considered. Preliminary procedures are outlined in which protonospheric electron content is separated from GPS TEC measurements (up to 20,000 km height) by subtracting Navy Navigation Satellite System (NNSS) measurements (up to 1000 km height). Although slab thickness is substantially constant, there is a trend for increased values at times of reduced solar influence, such as when approaching sunspot minimum. A preliminary report is given of an extension of the current work at Salisbury by using Australian Surveying and Land Information Group GPS receivers at other Australian locations. A comparison is made between the experimental data and values derived from ionospheric models. The two models considered are the international reference ionosphere model (IRI90) and the parameterized ionospheric model ((PIM) version 1.4, February 1996). The GPS TEC measurements near solar minimum were consistently of the order of 5–10 TEC units greater than the model TEC predictions. This difference is attributed to the inclusion of protonospheric TEC in the GPS measurements but not in the model predictions.
High‐frequency noise and spectrum occupancy measurements for Virginia and Texas with comparisons to International Radio Consultative Committee predictions
Authors: Rodriguez Serafin P.
Published: February 2004

There is ongoing interest in the high‐frequency (HF) band of the spectrum, both in terms of the noise levels and the available spectrum bandwidth. In particular, current HF radar systems such as the U.S. Navy's Relocatable Over‐the‐Horizon Radar (ROTHR) or the Australian Jindalee radar must operate against the external noise and share the spectrum with other HF band users. Current estimates of external noise levels are based on International Radio Consultative Committee (CCIR) [1988]. These predictions are based primarily on measurements made several decades ago at a few sites around the world. There are no current generally accepted estimates for available spectrum bandwidth. The Radar Division at the Naval Research Laboratory, in support of the U.S. Navy's ROTHR, developed a semiportable HF spectrum monitoring system to make HF noise and available bandwidth measurements. HF measurements were made over the 5‐ to 28‐MHz frequency range at three locations. Measurements were made in Virginia at the end of August 1994, in Texas at the end of November 1994, and aboard the U.S.S. Ashland off the coast of Virginia in May 1995. These measurements are presented, as well as comparisons with CCIR predictions for the same period.
Equatorial scintillation and systems support
Authors: Groves K. M. , Basu S. , Weber E. J. , Smitham M. , Kuenzler H. , Valladares C. E. , Sheehan R. , MacKenzie E. , Secan J. A. , Ning P. , McNeill W. J. , Moonan D. W. , Kendra M. J.
Published: February 2004

The need to nowcast and forecast scintillation for the support of operational systems has been recently identified by the interagency National Space Weather Program. This issue is addressed in the present paper in the context of nighttime irregularities in the equatorial ionosphere that cause intense amplitude and phase scintillations of satellite signals in the VHF/UHF range of frequencies and impact satellite communication, Global Positioning System navigation, and radar systems. Multistation and multifrequency satellite scintillation observations have been used to show that even though equatorial scintillations vary in accordance with the solar cycle, the extreme day‐to‐day variability of unknown origin modulates the scintillation occurrence during all phases of the solar cycle. It is shown that although equatorial scintillation events often show correlation with magnetic activity, the major component of scintillation is observed during magnetically quiet periods. In view of the day‐to‐day variability of the occurrence and intensity of scintillating regions, their latitude extent, and their zonal motion, a regional specification and short‐term forecast system based on real‐time measurements has been developed. This system, named the Scintillation Network Decision Aid, consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250‐MHz transmissions from two longitudinally separated geostationary satellites. The scintillation index and zonal irregularity drift are processed on‐line and are retrieved by a remote operator on the Internet. At the operator terminal the data are combined with an empirical plasma bubble model to generate three‐dimensional maps of irregularity structures and two‐dimensional outage maps for the region.
Possibilities of the near‐space environment radio tomography
Authors: Kunitsyn V. E. , Andreeva E. S. , Razinkov O. G.
Published: February 2004

A number of radio tomography schemes of the ionosphere and near‐space environment (Earth's magnetosphere, plasmasphere, and protonosphere) are analyzed by utilizing multisatellite systems. A new approach to space‐time tomography is proposed. It is shown that application of high orbital systems of Global Positioning System and Global Navigation Satellite System type with ground‐based receivers is promising in the case of space‐time tomography. Computer simulation results showed the possibility of realizing radio tomography of the near‐space environment with the help of some multisatellite systems.
On spectra of ionospheric conductivity variations during a heating experiment
Authors: Pashin Anatoly B. , Lyatsky Wladislav B.
Published: February 2004

Appearance of the harmonics in the spectrum of artificial emissions generated during heating experiments has been reported earlier. Usually, it relates with square modulation of HF powerful wave and distortion of this time dependence due to characteristic time of electron heating/cooling. In the EISCAT‐Heating experiment in November 1993, a quasi‐sinusoidal modulation regime was used. Observations show the clear harmonic structure in artificial emissions in this case too. In the present paper we discuss a generation of harmonics of a modulation frequency due to nonlinear dependence of value of the ionosphere conductivity disturbance from effective radiated power (ERP). A numerical model is used for calculations of the spectrum of the integral conductivity disturbance variation as a function of electron density profile and ERP. For the modulation frequency in the range of 1–100 Hz, the characteristic time of electron heating/cooling was neglected, and intensity ratios of the second and third harmonics to the fundamental frequency reach 0.4 and 0.15, respectively. For the modulation frequency 1375 Hz, time integration of the electron energy balance equation was used. The different characteristic time constants of the electron heating/cooling at different altitudes make the behavior of the intensity ratio and phase shift of the harmonics of the integral conductivity disturbance variations more complicated. The values of the harmonic amplitude ratio in this case decrease to 0.25 and 0.1.
Ionosphere tomography with data from satellite reception of Global Navigation Satellite System signals and ground reception of Navy Navigation Satellite System signals
Authors: Leitinger R. , Ladreiter H.‐P. , Kirchengast G.
Published: February 2004

GPS/MET, a multichannel Global Positioning System (GPS) receiver onboard the small research satellite MicroLab 1, is the first example of a research tool of great importance for ionospheric research. In the near future, other satellites with GPS/GLONASS (Global Navigation Satellite System (also GNSS)) receivers will be launched. Their main purpose is lower atmosphere research, but because of the necessity to correct for plasma influences, “ionospheric” data will be available as a side product. The occultation of GNSS signals offers the possibility to gain very good quality height profiles of electron density by means of classical inversion techniques. The profiles are averaged horizontally. This paper concentrates on the possibility to complement inversion results with electron content data gained on the ground using beacon signals of low orbiting satellites (e.g., the U.S. Navy Navigation Satellite System (NNSS)). The data combination offers several possibilities for ionospheric tomography. Several GNSS scanning satellite scenarios are assessed, and their ionospheric imaging/tomography merits are discussed. An example result for the inversion of GPS/MET data is shown. The results of simulations with model ionosphere data are used to demonstrate tomographic reconstruction techniques based on the combination of “space” and “ground” electron content. The simulation results have direct applicability to observed data.
The effect of large‐scale ionospheric gradients on backscatter ionograms
Authors: Russell C. J. , Dyson P. L. , Houminer Z. , Bennett J. A. , Li L.
Published: February 2004

This paper presents the results of the synthesis of a range of backscatter ionograms using ray tracing through model ionospheres. The backscatter ionograms were obtained by the Jindalee over‐the‐horizon radar facility at Alice Springs in northern Australia. Sample ionograms obtained during 1990 were used, and the study concentrated on reproducing effects due to sunrise‐sunset gradients and the equatorial anomaly. Backscatter ionograms were synthesized using both analytical and numerical ray tracing through ionospheric models based on FAIM (fully analytic ionospheric model). To make the synthesis realistic, signal strength was calculated taking account of ray divergence, ionospheric absorption, and antenna patterns. Analytical ray tracing produced quite realistic results when horizontal gradients were small but did not reproduce prominent features observed during sunrise‐sunset or when propagation occurred through the equatorial anomaly region. Since the analytical ray tracing was restricted to a single vertical profile which could be tilted, this result shows that gradients in ionospheric electron density, rather than simple tilts, are most significant in determining propagation characteristics. Numerical ray tracing through ionospheric models based on the FAIM model reproduced dominant features of backscatter ionograms for those situations when analytical ray tracing proved inadequate. Major seasonal variations were also reproduced. Overall, the results of this initial study show that many premier features on backscatter ionograms, including the power variation of the backscattered signals, can be realistically modeled using ray tracing and ionospheric models. Further work is required before all the detailed structure of backscatter ionogram traces can be synthesized and accurately interpreted in terms of ionospheric structure.
Experiments on Bragg and non‐Bragg scattering using single‐frequency and chirped radars
Authors: Lee P. H. Y. , Barter J. D. , Beach K. L. , Hindman C. L. , Lake B. M. , Rungaldier H. , Thompson H. R. , Yee R.
Published: February 2004

A collection of laboratory experiments on Bragg and non‐Bragg scattering, mainly from water surfaces, are conducted using a radar system which can be operated in a frequency‐chirped, range‐resolved mode, or a single‐frequency mode without range resolution. A Bragg wave generator is used to generate monochromatic, plane gravity‐capillary water waves with which Bragg resonance and Rice's theory are examined in some detail at small grazing angles. The Bragg resonance, which is sharp for single‐frequency operation, is broadened for a chirped system where the broadening is proportional to the chirp bandwidth. For single‐frequency operation, Bragg resonances are found to be Lorentzian; the resonance width has been used to infer the spatial decay rate (the imaginary wave number) of the Bragg waves, and the results are in agreement with that obtained from radar RCS and wave‐height probe measurements. For scattering from water surfaces, Bragg and non‐Bragg scattering are distinguished by the fact that the former process yields polarization by diffraction where HH is always less than VV, while the latter process usually yields polarization by reflection where HH is usually greater than VV. Fresnel reflection, a prime example of non‐Bragg scattering, is also studied using metal dihedral and labyrinth targets. We point out that although a fine range resolution is desirable, in some cases, it may lead to “phantom binning.” Other physics issues related to non‐Bragg scattering using chirped systems, such as the effects of multiple scattering and the dependence of frequency chirp on the dielectric constant, are also examined and discussed.
An integral equation approach to the prediction of indoor wave propagation
Authors: De Backer Bernard , Olyslager Frank , De Zutter Daniël
Published: February 2004

A two‐dimensional method of moments solution technique for the simulation of electrically large structures is presented. This technique is based on a boundary integral equation description, and was designed to keep CPU time and memory requirements within acceptable limits even for problems of several thousands of unknowns. This is achieved through an optimized calculation of the interaction integrals leading to the moment method interaction matrix and through a so‐called impedance matrix transformation of this matrix to a sparse form. The resulting sparse system of linear equations is efficiently solved using a conjugate gradient algorithm. The entire solution technique is applied to the simulation of indoor wave propagation problems and is illustrated by two examples.
Estimation of total electron content using GPS data: How stable are the differential satellite and receiver instrumental biases?
Authors: Sardón Esther , Zarraoa Néstor
Published: February 2004

The main source of error in the estimation of TEC (total electron content) from dual Global Positioning System (GPS) data is the effect of the differential satellite and receiver instrumental delay biases. These biases are normally estimated simultaneously with the TEC. However, the additional estimation of the instrumental biases may constitute an insurmountable burden in some practical applications like real‐time estimation of TEC, or the estimation may be difficult or correlated to the ionospheric parameters, particularly in situations where the TEC behavior may be harder to model (equatorial or auroral zone, ionospheric storms, etc.). A priori values of the instrumental biases, estimated under good conditions or with global networks, could solve those problems if we could determine how stable those instrumental biases are in time and how often we need to check or reestimate their values. In this paper we will present our estimation of the GPS satellite and receiver instrumental biases from 19 months of data and the study of their variation during that time. We will also show some situations of changes in the instrumental biases and the possible influence of antispoofmg (AS). The main conclusion of this work is that the variation of the estimated differential GPS satellite biases during the 19 months is smaller than 1 ns (1 ns = 2.86 × 1016 e/m2) in most of the cases, with a mean RMS of 0.15 ns. For the GPS receivers used, that variation is greater than for the satellites, with the larger variations corresponding to physical changes in the receivers. The difference of the estimated differential instrumental biases between two consecutive days is in practically all cases smaller than 0.5 ns for the GPS satellites and smaller than 1 ns for the GPS receivers. Regarding the influence of AS, we have detected some significant changes in the instrumental biases of some satellites and some stations whether AS is activated or not. Our main conclusion is that due to the stability of the GPS instrumental biases, only an estimation or calibration of them (under optimal conditions) from time to time is required.
Scintillation and simultaneous rain attenuation at 12.5 GHz to satellite Olympus
Authors: Matricciani Emilio , Mauri Mario , Riva Carlo
Published: February 2004

This paper reports the statistical relationship between 1‐s averaged rain attenuation (A, in decibels) and standard deviation (σ, in decibels) of simultaneous tropospheric scintillation in 1‐s intervals, derived from high‐resolution (50 samples/s) experimental 12.5‐GHz attenuation time series recorded at Spino d'Adda (45.4°N) in a 30.6° slant path to satellite Olympus during an observation time of approximately 1 year. The statistical relationship can be fitted by the power law σ=0.068A5/12 derivable from a thin turbulent layer model. A similar relationship was also previously obtained in the same slant path at 19.77 GHz. As a consequence, we have concluded that at least up to 20 GHz, the modeling is independent of frequency.
Scattering by a conducting elliptic cylinder with a multilayer dielectric coating
Authors: Caorsi Salvatore , Pastorino Matteo , Raffetto Mirco
Published: February 2004

A solution to the electromagnetic scattering of a transverse magnetic plane wave due to a perfectly conducting elliptic cylinder coated by a lossless, nonmagnetic, and elliptic multilayer dielectric is proposed. Despite the lack of orthogonality of the eigenfunctions of the field inside different layers, an efficient recursive procedure for the computation of the solution is devised. It is based on series expansions of the fields in terms of Mathieu functions and on a Galerkin approach. An outline of the procedure is given, and some numerical results, concerning both the field quantities and the radar cross section per unit length, are provided.
Quasi‐linear series in three‐dimensional electromagnetic modeling
Authors: Zhdanov Michael S. , Fang Sheng
Published: February 2004

We have recently introduced a quasi‐linear (QL) approximation for the solution of the three‐dimensional (3‐D) electromagnetic modeling problem. In this paper we discuss an approach to improving its accuracy by considering the QL approximations of the higher‐order. This approach can be considered the natural generalization of the Born series. We use the modified Green's operator with the norm less than 1 to ensure the convergence of the higher orders QL approximations to the true solution. This new approach produces the converged QL series, which makes it possible to estimate the accuracy of the original QL approximation without direct comparison with the rigorous full integral equation solution. It also opens principally new possibilities for fast and accurate 3‐D EM modeling and inversion.
A global high‐resolution microwave emission model for the Earth
Authors: Schiavon G. , Ferrazzoli P. , Solimini D. , de Maagt P. , Baptista J. P. V. Poiares
Published: February 2004

This paper reports on a numerical model developed to simulate vertically and horizontally polarized microwave emission from the Earth in the frequency range between 5 and 50 GHz and at various angles of observation, with a 1°×1° (latitude by longitude) spatial resolution, taking into account seasonal variations. The principal motivation of the model is the evaluation of the noise antenna temperature of telecommunications satellites, which is required to calculate the uplinkG/Tfor satellite‐borne receivers. The results of the study, however, prove useful in a number of remote sensing applications. To implement the model, significant types of surface, such as bare soil, nonarboreous vegetation, forests, snow, glacier and sea ice, and ocean, have been identified, and their emissivity properties have been determined by the available theoretical and/or empirical models.The millimeter‐wave propagation model ofLiebe[1993] has been used to compute the atmospheric contribution. Profiles from actual radio soundings collected during a 10‐year period over the globe have been used to take into account major climatic variations. The various contributions from the surface and the atmosphere have been finally combined to obtain the theoretical global brightness temperature of each 1°×1° pixel. The numerical model has been validated by comparing on a pixel‐by‐pixel basis the theoretical brightness temperature with those measured by the special sensor microwave imager (SSM/I) radiometer in the year 1992 at 19.35, 22.235, and 37.0 GHz at the available polarizations. The discrepancies between model and experimental brightness temperatures have been noted, and actions have been taken to reduce the differences. In its present configuration, the global emission model yields brightness temperature estimates which differ all over the Earth by less than 12 K rms from those measured by the SSM/I.
A fast multipole approach to analyze scattering from an inhomogeneous bianisotropic cylindrical object embedded in a chiral host
Authors: Shanker Balasubramaniam , Han Sang‐Kyun , Michielssen Eric
Published: February 2004

This paper presents a computationally efficient algorithm to compute oblique scattering from a bianisotropic object immersed in a chiral host. The analysis is performed in terms of Beltrami fields, as these form a natural basis for representing waves in a chiral medium. Volume integral equations are derived solely in terms of the axial components of the Beltrami fields and are discretized using pyramidal basis functions. The integral equations are solved iteratively using a fast multipole accelerated method of moments technique with a computational complexity of O(N1.4) per iteration. A windowed translation operator is introduced, which further reduces the computational complexity of the algorithm to O(N1.33). Our numerical results are validated against Mie‐type solutions for anisotropic cylinders and shells. Results for scattering from large bianisotropic corrugated surfaces are presented.
Interpolation techniques to improve the accuracy of the plane wave excitations in the finite difference time domain method
Authors: Oğuz Uğur , Gürel Levent
Published: February 2004

The importance of matching the phase velocity of the incident plane wave to the numerical phase velocity imposed by the numerical dispersion of the three‐dimensional (3‐D) finite difference time domain (FDTD) grid is demonstrated. In separate‐field formulation of the FDTD method, a plane wave may be introduced to the 3‐D computational domain either by evaluating closed‐form incident‐field expressions or by interpolating from a 1‐D incident‐field array (IFA), which is a 1‐D FDTD grid simulating the propagation of the plane wave. The relative accuracies and efficiencies of these two excitation schemes are compared, and it has been shown that higher‐order interpolation techniques can be used to improve the accuracy of the IFA scheme, which is already quite efficient.
A global mapping technique for GPS‐derived ionospheric total electron content measurements
Authors: Mannucci A. J. , Wilson B. D. , Yuan D. N. , Ho C. H. , Lindqwister U. J. , Runge T. F.
Published: February 2004

A worldwide network of receivers tracking the transmissions of Global Positioning System (GPS) satellites represents a new source of ionospheric data that is globally distributed and continuously available. We describe a technique for retrieving the global distribution of vertical total electron content (TEC) from GPS‐based measurements. The approach is based on interpolating TEC within triangular tiles that tessellate the ionosphere modeled as a thin spherical shell. The high spatial resolution of pixel‐based methods, where widely separated regions can be retrieved independently of each other, is combined with the efficient retrieval of gradients characteristic of polynomial fitting. TEC predictions from climatological models are incorporated as simulated data to bridge significant gaps between measurements. Time sequences of global TEC maps are formed by incrementally updating the most recent retrieval with the newest data as it becomes available. This Kalman filtering approach smooths the maps in time, and provides time‐resolved covariance information, useful for mapping the formal error of each global TEC retrieval. Preliminary comparisons with independent vertical TEC data, available from the TOPEX dual‐frequency altimeter, suggest that the maps can accurately reproduce spatial and temporal ionospheric variations over latitudes ranging from equatorial to about ±65°.
Propagation factor errors due to the assumption of lateral homogeneity
Authors: Goldhirsh Julius , Dockery Daniel
Published: February 2004

Propagation conditions are frequently assessed by making a single measurement of the vertical refractivity and injecting this profile into a propagation model from which the two‐dimensional propagation factor field is calculated. Such an assessment tacitly assumes lateral homogeneity of the refractivity environment. That is, the refractive index is assumed to vary only as a function of height above the Earth's surface. Hitherto, the authors, using only case studies, have determined errors in propagation assessment employing the above assumptions. The objective of this paper is to characterize the errors in the propagation factor field using a statistical database of multiple arrays of refractivity profiles covering many days. In this effort, errors in the propagation factor field at 1, 3, and 10 GHz are derived for the case in which single refractive index profile measurements are used to calculate the two‐dimensional propagation factor field. This “error” field is subsequently compared with the “benchmark” or “truth” propagation factor field calculated using an array of refractivity profiles. The errors are statistically deduced by combining measurements from 30 days over defined two‐dimensional areas. The methodology for which the above type of propagation assessment is addressed may also be used to establish temporal and lateral spacing requirements employing similar statistical databases.
Estimation of midlatitude rainfall parameters from satellite microwave radiometers using the area‐time integral concept
Authors: Wang Yiping , Alishouse John C. , Ferraro Ralph R. , Song Ran
Published: February 2004

Weather Surveillance Radar‐1988 Doppler (WSR‐88D) level II data and special sensor microwave imager (SSM/I) brightness temperature data were collocated to further investigate the use of the area‐time integral (ATI) technique in the estimation of satellite microwave rainfall amount based on the storm area coverage information. The concept of a strong relationship between the areal extent and time duration of precipitation and the total amount of precipitation seems to have been extant in radar meteorology for some time, primarily in the tropics. This strong relationship is again demonstrated using WSR‐88D data. However, it was found that this correlation strongly depends on the Z‐R relationship used in computing rain rate from radar reflectivity data. This study conducted two experiments: (1) using one Z‐R relationship for the whole storm rain area and (2) separating rain into two parts, convective and stratiform, and applying two Z‐R relationships. The area‐wide average rain rate from the storm as viewed from the satellite was calculated from the ATI formalism based on the relationships developed from the WSR‐88D data. The estimates were compared with the SSM/I 85‐GHz scattering algorithm estimates and verified using Next Generation Weather Radar (NEXRAD) observations and hourly rain gauge measurements. The results reveal that the ATI technique can be used as an alternative approach for determining and validating satellite precipitation parameters. The separation of convective and stratiform precipitation improved the relationship between the area‐wide average rain rate and fractional rainfall area and consequently improved the accuracy of satellite rainfall estimation.
Performance of a numerical iterative solution of the surface current integral equation for surfaces containing small radii of curvature
Authors: Ericson Eric A. , Lyzenga David R.
Published: February 2004

An iterative method for numerically solving the surface current integral equation is applied to wedge‐like surfaces in order evaluate its applicability to surfaces that give rise to substantial edge diffraction. The convergence properties and accuracy of this approach are examined by comparing its results with the analytic Sommerfeld solution for incidence angles of 45° and 85° and scattering angles from −90° to 90°. The accuracy and computational efficiency of these results suggest that this method may be suitable for studying scattering from a variety of one‐ and two‐dimensional surfaces that contain regions of large surface curvature.