Showing papers in "Radio Science in 1998"
TL;DR: In this paper, a technique for retrieving the global distribution of vertical total electron content (TEC) from GPS-based measurements is described, based on interpolating TEC within triangular tiles that tessellate the ionosphere modeled as a thin spherical shell.
Abstract: 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°.
1,148 citations
TL;DR: In this article, a reanalysis of some of these measurements leads to the conclusion that the laboratory data are best represented by a combination of Liebe's [1987] millimeter-wave propagation model (MPM) for the foreignbroadened component of the water continuum and the 1993 version of MPM for the self-broadened components.
Abstract: Measurements, made in different laboratories, of absorption by water vapor in microwave windows are compared with models for the water vapor continuum. A reanalysis of some of these measurements leads to the conclusion that the laboratory data are best represented by a combination of Liebe's [1987] millimeter-wave propagation model (MPM) for the foreign-broadened component of the water continuum and the 1993 version of MPM for the self-broadened component. This combined model is validated by comparison with measurements of atmospheric microwave emission.
555 citations
TL;DR: The Global Positioning System Meteorology (GPS/MET) experiment, which placed a GPS receiver in a low-Earth orbit tracking GPS satellites setting behind the Earth's limb, has collected data from several thousands of occultations since its launch in April 1995.
Abstract: The Global Positioning System Meteorology (GPS/MET) experiment, which placed a GPS receiver in a low-Earth orbit tracking GPS satellites setting behind the Earth's limb, has collected data from several thousands of occultations since its launch in April 1995. This experiment demonstrated for the first time the use of GPS in obtaining profiles of electron density and other geophysical variables such as temperature, pressure, and water vapor in the lower atmosphere. This paper discusses some of the effects of the ionosphere, such as bending and scintillation, on the GPS signal during occultation. It also presents a set of ionospheric profiles obtained from GPS/MET using the Abel inversion technique, and compares these profiles with ones obtained from the parameterized ionospheric model (PIM) and with ionosonde and incoherent scatter radar measurements. Statistical comparison of NmF2 values obtained from GPS/MET profiles and nearby ionosondes indicates that they agree to about ∼20% (1-sigma) in a fractional sense. The high vertical resolution, characteristic of the occultation geometry, is reflected in the GPS/MET profiles which reveal ionospheric structures of very small vertical scales such as the sporadic E.
363 citations
TL;DR: In this article, a tunable temperature profiler based on a highly stable synthe-sizer that can observe at multiple frequencies on the shoulder of the 60 GHz atmospheric oxygen feature was developed.
Abstract: We have developed a tunable temperature profiler based on a highly stable synthe- sizer that can observe at multiple frequencies on the shoulder of the 60 GHz atmospheric oxygen feature. We are developing a similar radiometer to obtain the vertical distribution of water vapor by making observations on the pressure broadened water vapor line from 22 to 29 GHz. Informa- tion on cloud liquid water profiles is also contained in these two wavebands. Various mathemati- cal retrieval methods for temperature, water vapor, and cloud liquid water profiles were tested based on these radiometer designs. These include neural networking, Newtonian iteration of sta- tistically retrieved profiles, and Bayesian "most probable" retrievals. Based on realistic radiome- ter errors and performance, very good retrieval capability is demonstrated. The performance of the various retrieval methods are presented and compared. Examples of profile retrievals are also presented. Data were not binned into seasons to reduce computer time; better retrieval results for all methods would be expected with binning. 1. Motivation for Radiometric Atmospheric Profiling Radiosonde observations (RAOBs) are the fundamental method for atmospheric temperature, wind, and water vapor profiling, in spite of their inaccuracies, cost, sparse temporal sampling and logistical difficulties. A better technology has been sought for decades, but until now, no accurate continuous all weather technology has been demonstrated. Laser radars (LIDARS) and Fourier transform infrared spectrometers can profile temperature and water vapor, but not in the presence of cloud. Our highly stable frequency agile radiometric temperature and water vapor profilers give continuous unattended profile measurements. They also have the capability to profile cloud liquid water, a capability absent in RAOBs and all other systems except for in situ aircraft devices. Applications for this passive radiometric profiling include: weather forecasting and now- casting; detection of aircraft icing and other aviation related meteorological hazards; determina- tion of density profiles for artillery trajectory and sound propagation; refractivity profiles for radio ducting prediction; corrections to radio astronomy; satellite positioning and Global Posi- tioning System (GPS) measurements; atmospheric radiation flux studies; estimation and predic-
244 citations
TL;DR: In this article, the authors measured the permittivity of representative samples of natural seawater, synthetic seawater and aqueous NaCl solutions over the frequency range 3-20 GHz, in 0.1-GHz steps and over the temperature range −2°-30°C in 1° steps.
Abstract: We have measured the permittivity of representative samples of natural seawater, synthetic seawater, and aqueous NaCl solutions over the frequency range 3–20 GHz, in 0.1-GHz steps and over the temperature range −2°–30°C in 1° steps. Additional measurements have been made at spot frequencies (23.8, 36.5, and 89 GHz) and at selected temperatures between −2° and 3O°C. The data from these measurements have allowed us to deduce an interpolation function for e(υ t, S) in the ranges 2 ≤ υ ≤ 20 GHz, −2° ≤ t ≤ 30°C, and 20‰ ≤ S ≤ 40‰ with a precision of 1%. If the frequency range is extended up to 40 GHz, the precision of the interpolation function is about 3%.The data have also allowed us to compare the permittivities of natural seawater, synthetic seawater, and aqueous NaCl solution with the same salinities. Natural and synthetic seawater have the same permittivities within a 1% experimental error estimate. An aqueous NaCl solution has a significantly different permittivity (up to about 6% difference, depending upon the frequency and temperature).
221 citations
TL;DR: In this paper, the authors developed a model of sferic propagation which is based on an existing frequency domain subionospheric VLF propagation code and derived the electron density profile that most closely matched an observed sferric spectrum.
Abstract: Lightning discharges radiate the bulk of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) and extremely low frequency (ELF, 3–3000 Hz) bands. This energy, contained in impulse-like signals called radio atmospherics or sferics, is guided for long distances by multiple reflections from the ground and lower ionosphere. This suggests that observed sferic waveforms radiated from lightning and received at long distances (>1000 km) from the source stroke contain information about the state of the ionosphere along the propagation path. The focus of this work is on the extraction of nighttime D region electron densities (in the altitude range of ∼70–95 km) from observed VLF sferics. In order to accurately interpret observed sferic characteristics, we develop a model of sferic propagation which is based on an existing frequency domain subionospheric VLF propagation code. The model shows that the spectral characteristics of VLF sferics depend primarily on the propagation path averaged ionospheric D region electron density profile, covering the range of electron densities from ∼100 to 103 cm−3. To infer the D region density from observed VLF sferics, we find the electron density profile that produces a modeled sferic spectrum that most closely matches an observed sferic spectrum. In most nighttime cases the quality of the agreement and the uncertainties involved allow the height of an exponentially varying electron density profile to be inferred with a precision of ∼0.2 km.
203 citations
TL;DR: In this paper, an acceleration algorithm for more rapidly computing interactions between widely separated points in the forward-backward method is proposed and results in an O(N) algorithm with increasing surface size.
Abstract: The forward-backward method has been shown to be an effective iterative technique for the computation of scattering from one-dimensional rough surfaces, often converging rapidly even for very large surface heights. However, previous studies with this method have computed interactions between widely separated points on the surface exactly, resulting in anO(N2) computational algorithm that becomes intractable for large rough surface sizes, as are required when low grazing incidence angles are approached. An acceleration algorithm for more rapidly computing interactions between widely separated points in the forward-backward method is proposed in this paper and results in an O(N) algorithm with increasing surface size. The approach is based on a spectral domain representation of source currents and the Green's function and is developed for both perfectly conducting and impedance boundary surfaces. The method is applied in a Monte Carlo study of low grazing incidence backscattering from very rough (up to 10 m/s wind speed) ocean-like surfaces at 14 GHz and is found to require only a small fraction of the CPU time required by other competing methods; such as the banded matrix iterative approach/canonical grid and fast multipole methods.
170 citations
TL;DR: In this article, the effects of mutual impedance between spaced antennas and describe an interferometer which both minimizes these effects and avoids the ambiguities associated with spacings larger than λ/2.
Abstract: The measurement of the directions of radio meteors with an interferometric system is beset by two problems: (1) The ambiguity in the measured directions for antennas spaced by more than λ/2 and (2) the effects of mutual impedance when the antennas are spaced at λ/2 and less to avoid these ambiguities. In this paper we discuss the effects of mutual impedance between spaced antennas and describe an interferometer which both minimizes these effects and avoids the ambiguities associated with spacings larger than λ/2. We have modeled a version of this design numerically and show that under ideal conditions an interferometer of total span 4.5λ can yield directions accurate to about 0.3° with a signal-to-noise ratio of 20 dB. Finally, we have tested the design with observations from the 1996 Geminid and 1997 Quadrant meteor showers and find that even without a ground plane, the interferometer provides unambiguous directions to an accuracy of the order of 1.5°.
168 citations
TL;DR: In this article, simulations of a tomography system based on data from the Global Positioning System and a low Earth-orbiting satellite were made using a four-dimensional stochastic model of ionosphere perturbations.
Abstract: Using a four-dimensional stochastic model of ionosphere perturbations, simulations are made of a tomography system based on data from the Global Positioning System and a low Earth-orbiting satellite. The perturbations are departures from a simple time-independent reference state. The spatial structure is parameterized in terms of empirical orthogonal functions (EOFs) in the vertical and spherical harmonics in the horizontal. The horizontal covariance structure is specified by variance and correlation length scales as functions of latitude and longitude. Time dependence is modeled as a first-order Markov process with a 6-hour timescale and white-noise forcing. A Sun-fixed coordinate system is used so that ionospheric features are more nearly steady in time. A Kaiman filter is used to objectively assimilate the simulated data into the simple time-dependent model. In addition to solving for the three-dimensional electron density field at each time step, the procedure solves for instrumental biases. The simulations show that the fractions of resolved variance for vertical EOF modes 1, 2, and 3 are 0.99, 0.93, and 0.73, respectively. The resolution of the vertically integrated total electron content is 0.99.
142 citations
TL;DR: In this article, a linearly constrained mathematical formulation is provided for the problem of coherent radar imaging, and an imaging technique based on constrained optimization is introduced and shown to exhibit higher resolution and resistance to interfering signals.
Abstract: A linearly constrained mathematical formulation is provided for the problem of coherent radar imaging. In contrast to studies of field-aligned irregularities in the ionosphere, where the technique has previously been applied, lower atmospheric imaging is complicated by the fact that the scattering structures are not aligned along any single baseline. As a result, a two-dimensional generalization of the brightness distribution was required. It is shown that Fourier-based imaging is a special case of this general formulation. Furthermore, an imaging technique based on constrained optimization is introduced and shown to exhibit higher resolution and resistance to interfering signals. These techniques were applied to data from the middle and upper atmosphere radar in Shigaraki, Japan. The experiment was conducted during the Baiu season, which is characterized by significant precipitation events.
130 citations
TL;DR: In this article, a study of the distribution and dynamics of mesoscale (tens of kilometers to tens of meters) electron density irregularities in the dayside auroral region was performed at Ny Alesund, Svalbard, by measuring the effects of these irregularities on the amplitude scintillation of 250-MHz transmissions from a quasi-stationary polar satellite as well as the amplitude and phase scintillations of 1.6-GHz signals from Global Positioning System (GPS) satellites.
Abstract: Satellite scintillation, all-sky optical imager, and digisonde observations were coordinated during a cusp campaign conducted at Ny Alesund, Svalbard (78.9°N, 11.8°E 75.7°N corrected geomagnetic latitude, over the period January 4–15, 1997. This paper is focused on a study of the distribution and dynamics of mesoscale (tens of kilometers to tens of meters) electron density irregularities in the dayside auroral region. This study has been performed at Ny Alesund, Svalbard, by measuring the effects of these irregularities on the amplitude scintillation of 250-MHz transmissions from a quasi-stationary polar satellite as well as the amplitude and phase scintillation of 1.6-GHz signals from Global Positioning System (GPS) satellites. These GPS scintillation measurements were augmented by the use of dual-frequency (1.2 and 1.6 GHz) GPS phase data acquired at the same station by the Jet Propulsion Laboratory for the International GPS Geodynamic Service (IGS). The continuous 250-MHz scintillation observations explored the daytime auroral ionosphere 2° poleward of Ny Alesund and showed that the scintillation spectra are often broad, as may be expected for irregularities in a turbulent flow region. Such irregularity dynamics were detected poleward of the nominal cusp region over the interval of 0600–1500 magnetic local time. The period of observations included the magnetic storm of January 10–11, 1997, when GPS observations of the IGS detected polar cap patches with total electron contents of 3×1016 m−2 and large-scale (tens of kilometers) phase variations at the GPS frequency of 1.6 GHz that corresponded to temporal gradients of 2×1016 m−2 min−1. However, amplitude scintillations at the GPS frequency of 1.6 GHz could not be detected in association with these large-scale phase variations, indicating that the irregularities with wavelengths less than the Fresnel dimension of 400 m were below the detectable limit. This is shown to be consistent in the context of enhanced ionospheric convection determined by digisonde and scintillation spectra.
TL;DR: In this article, a new method for determining the scattering and absorption behavior of test samples was applied and further developed by application of a multiple scattering model, which was applied with homogeneous slabs of dry, natural snow samples measured over a frequency range from 11 to 94 GHz.
Abstract: The interaction of microwaves with the natural snow cover strongly depends on the complex structure of the snowpack. In order to quantify this dependency, dedicated experiments were performed with homogeneous slabs of dry, natural snow samples measured over a frequency range from 11 to 94 GHz. A new method introduced by Matzler and Wegmuller [1995] and Weise [1996a] for determining the scattering and absorption behavior of test samples was applied and further developed by application of a multiple scattering model. Homogeneous samples of dry snow were (1) investigated using a set of portable, linearly polarized Dicke radiometers at frequencies of 11, 21, 35, 48 and 94 GHz, (2) characterized by temperature, grain size and shape, density and permittivity, and (3) structurally analyzed by digitized snow sections in order to obtain statistical information of the snow structure i.e. the autocorrelation function. During the winters 1994/1995 and 1995/1996 additional measurements of snow samples were made to extend the variability of the investigated snow types. Up to now, 20 samples, representing alpine snow in winter (that is, without melt metamorphism) have been collected during three winter campaigns. Here, we present the method and the radiative transfer model and show how it can be inverted to obtain scattering and absorption coefficients. A first assessment of the snow sample data is also presented. The results show good agreement between the measured and the theoretical absorption coefficient. The scattering coefficient turns out to be a strong function of frequency and correlation length as expected from Rayleigh scattering. However, distinct differences can be noted.
TL;DR: In this article, the authors describe the use of the ionosphere as a virtual mirror to extend radar range by an order of magnitude with use of a matched filter Doppler processing.
Abstract: Efforts to extend radar range by an order of magnitude with use of the ionosphere as a virtual mirror started after the end of World War II. A number of HF radar programs were pursued, with long-range nuclear burst and missile launch detection demonstrated by 1956. Successful east coast radar aircraft detect and track tests extending across the Atlantic were conducted by 1961. The major obstacles to success, the large target-to-clutter ratio and low signal-to-noise ratio, were overcome with matched filter Doppler processing. To search the areas that a 2000 nautical mile (3700 km) radar can reach, very complex and high dynamic range processing is required. The spectacular advances in digital processing technology have made truly wide-area surveillance possible. Use of the surface attached wave over the oceans can enable HF radar to obtain modest extension of range beyond the horizon. The decameter wavelengths used by both skywave and surface wave radars require large physical antenna apertures, but they have unique capabilities for air and surface targets, many of which are of resonant scattering dimensions. Resonant scattering from the ocean permits sea state and direction estimation. Military and commercial applications of HF radar are in their infancy.
TL;DR: In this paper, a simple two-dimensional ray tracing scheme is developed and is applied to several propagation problems of importance to over-the-horizon radar, in particular, it is used as the basis of schemes for the simulation of oblique and backscatter ionograms.
Abstract: A simple, two-dimensional ray tracing scheme is developed and is applied to several propagation problems of importance to over-the-horizon radar. In particular, it is used as the basis of schemes for the simulation of oblique and backscatter ionograms. The efficiency of the scheme makes it ideal for applications where fast calculations are required, and an algorithm for inverting backscatter ionogram leading edges is developed as a particular example.
TL;DR: An algorithmic structure for a global optimum distortion correction scheme which can function under arbitrary combinations of signal corruption phenomena is presented, based on multiply recursive estimation of distortion parameters.
Abstract: HF sky wave radar signals are subjected to a wide variety of distortion and contamination effects as they propagate through the ionosphere. Signal processing techniques designed to remove or at least mitigate these effects have been in use for some years, but they do not address the problem of simultaneous occurrence of multiple distortion mechanisms, which may reduce drastically the efficacy of the ameliorative processing. In this paper we present an algorithmic structure for a global optimum distortion correction scheme which can function under arbitrary combinations of signal corruption phenomena. The scheme is based on multiply recursive estimation of distortion parameters. Results from a prototype implementation of this scheme show substantial gains over traditional processing sequences. The new approach is expected to enhance the remote sensing capabilities of HF sky wave radars significantly.
TL;DR: In this paper, a new dielectric permittivity model based on the standard Debye theory has been developed for remote sensing applications over the ocean below 40 GHz, together with polynomial interpolations at the millimeter frequencies 85.5 and 89 GHz.
Abstract: As part of a measuring program dedicated to the analysis of the dielectric properties of seawater in the frequency range 3–89 GHz, a new dielectric permittivity model based on the standard Debye theory has been developed for remote sensing applications over the ocean below 40 GHz, together with polynomial interpolations at the millimeter frequencies 85.5 and 89 GHz. The aim of this paper is to test the relevance of these new dielectric measurements through statistical comparisons of radiative transfer predictions with satellite and airborne radiometric data between 18 and 89 GHz. A radiometric sensitivity analysis to the permittivity measurement errors is proposed, which yields a sea surface brightness temperature accuracy of at least 0.5 K below 20 GHz, 1 K at 24 GHz, and 1.5 K at 37 and 89 GHz. At frequencies less than 40 GHz, superiority of the revised Debye model is pointed out over the most commonly used model of Klein and Swift [1977]. At millimeter frequencies the new permittivity expressions deviate significantly from the standard Debye predictions, especially at low temperature, suggesting the influence of a second “high-frequency” Debye relaxation. Our comparisons with radiometric data at 89 GHz and in the channel 85.5V of the special sensor microwave imager tend to support this hypothesis. The results emphasize the importance of an adequate modeling of the complex permittivity of seawater as input to the surface emissivity models, at any frequency of the microwave spectrum, and augur interesting outputs in both in-flight calibration and interpretation of satellite data.
TL;DR: In this paper, the authors derived microwave land surface emissivities from special sensor microwave imager (SSM/I) observations using collocated visible/infrared satellite observations (ISCCP) data.
Abstract: Microwave land surface emissivities are derived from special sensor microwave imager (SSM/I) observations. Cloud-free SSM/I observations are first isolated with the help of collocated visible/infrared satellite observations (International Satellite Cloud Climatology Project (ISCCP) data). Then, the cloud-free atmospheric contribution is calculated from an estimate of the local atmospheric temperature-humidity profile (National Centers for Environmental Prediction (NCEP) analyses). Finally, with the surface skin temperature derived from IR observations (ISCCP estimate), the surface emissivity is calculated for all the SSM/I channels. The method is applied to the SSM/I data for the globe for October 1991. Correspondences between geographical patterns of the emissivities and surface characteristics are briefly analyzed to investigate the potential of microwave emissivities to monitor vegetation phenology and surface properties at regional and continental scales.
TL;DR: In this paper, a simple experimental system is described and experimental results of high spatial resolution for the near and far field are shown, and a special reconstruction algorithm was developed and is described theoretically and validated by the experiments.
Abstract: Aperture synthesis is an attractive alternative to conventional passive microwave imaging systems for remote sensing applications Whenever a high spatial resolution is desired or the imaging process should work like an optical camera or when microwave remote sensing in the near field has to be distance adaptive, conventional systems cause a lot of problems However, aperture synthesis for practical applications requires a lot of single receivers and correlators This can cause other problems because of the large number of possible imperfections To get an estimation of such effects, the imaging expressions for the aperture synthesis principle are developed for the near- and far-field conditions of a real system To support the theory and to get an estimation of the feasibility of this imaging method, a simple experimental system is described and experimental results of high spatial resolution for the near and far field are shown For the near-field case a special reconstruction algorithm was developed and is described theoretically and validated by the experiments A discussion of the temperature resolution of aperture synthesis concludes the paper
TL;DR: In this paper, measurements of Doppler spread, multipath spread, and signal-to-noise ratio have been made on four high-latitude high-frequency (HF) communications paths.
Abstract: Measurements of Doppler spread, multipath spread, and signal-to-noise ratio have been made on four high-latitude high-frequency (HF) communications paths. The measurement system and analysis techniques are outlined, and an analysis of the data pertinent to the design of robust HF data modems is presented. A summary of the spreads that are exceeded for 5% of time is presented for each path. Doppler spreads range from 2 to 55 Hz, while multipath spreads range from 1 to 11 ms. Physical interpretations of the data are made, and the data are related to the measured performance characteristics of an HF data modem to estimate the modem availability on the paths considered. When there is mode support, availabilities range from 64% to 100% for a signal-to-noise ratio of 0 dB, although the data indicate that the availabilities can generally be increased by optimizing frequency selection.
TL;DR: In this paper, a theoretical analysis of the angular resolution of two-dimensional interferometric radiometers for Earth observation from low-orbit satellites and its degradation due to spatial decorrelation effects is presented.
Abstract: A theoretical analysis of the angular resolution of two-dimensional interferometric radiometers for Earth observation from low-orbit satellites and its degradation due to spatial decorrelation effects is presented. The analysis extends basic effects known in the context of radio astronomy (application with narrow field of view, very few baselines) and in one dimension (ESTAR L band, few baselines interferometric radiometer) to the wide-field-of-view, many-baseline, high-resolution two-dimensional system required by Earth observation applications and computes beam width, encircled energy (or main beam efficiency), and side lobe level as a function of windowing (apodization) to allow for an optimum angular versus radiometric resolution trade-off. It is found that the extension of the Barlett window (which has a poor performance in one dimensional signal processing) to two dimensions produces high-quality results, comparable or better than those of Gaussian and Blackmann windows. Theory is extended to hexagonally sampled systems based on a ? or Y-shaped instrument, with hexagonal- and star-shaped support regions in the visibility space, respectively. The superior performance of the latter over the former for the same number of antennas and correlators is quantified and details of the angular resolution of one instrument of this kind, MIRAS, under development by the European Space Agency, are presented. For this radiometer Gaussian or Barlett windows should be used for good radiometric sensitivity or spatial resolution, respectively. In both cases the effects of decorrelation within the small alias-free field of view are negligible. It is also found that the impact of hardware imperfections which exist within the strict requirements of the specifications have a negligible effect on the angular resolution. Finally, experimental angular resolution results with a laboratory breadboard in a focused near-field setup are presented and compared to the theoretical predictions.
TL;DR: In this article, seasonal profiles from 5 to 20 km above mean sea level of the inner scale have been estimated based on the kinematic viscosity and eddy dissipation rates which were determined from 5 years of nearly continuous 49.25MHz radar observations at White Sands Missile Range, New Mexico.
Abstract: There are very few reliable results of the inner and outer scales of turbulence in the remote atmosphere. Knowledge of these parameters is of high interest to the propagation and remote sensing communities. Seasonal profiles from 5 to 20 km above mean sea level of the inner scale have been estimated based on the kinematic viscosity and eddy dissipation rates which were determined from 5 years of nearly continuous 49.25-MHz radar observations at White Sands Missile Range, New Mexico. Inner scale values were found to increase from about 1 cm at 5 km to near 7 cm at 19 km altitude. Outer scale profile determinations were made using a method proposed by Tatarskii [1971] that involves vertical wind shear and the eddy dissipation rate, both derived from the longterm VHF radar measurements. The outer scale decreased from about 60 m at 5 km altitude for all seasons to 12–20 m at 15 km (depending on season) and then increased to 22 m at 19 km. Seasonal differences in the inner and outer scales and background meteorological conditions are also presented and discussed.
Abstract: An improved model for the absorption of the atmosphere near the 22-GHz water vapor line is presented. The Van Vleck-Weisskopf line shape is used with a simple parameterized version of the model from Liebe et al. [1993] for the water vapor absorption spectra and a scaling of the model from Rosenkranz [1993] for the 20 to 32-GHz oxygen absorption. Radiometrie brightness temperature measurements from two sites of contrasting climatological properties, San Diego, California, and West Palm Beach, Florida, were used as ground truth for comparison with in situ radiosonde-derived brightness temperatures under clear-sky conditions. Estimation of the new model's four parameters, related to water vapor line strength, line width and continuum absorption, and far-wing oxygen absorption, was performed using the Newton-Raphson inversion method. Improvements to the water vapor line strength and line width parameters are found to be statistically significant. The accuracy of the new absorption model is estimated to be 3% between 20 and 24 GHz, degrading to 8% near 32 GHz. In addition, the Hill line shape asymmetry ratio was evaluated on several currently used models to show the agreement of the data with Van Vleck-Weisskopf based models and to rule out water vapor absorption models near 22 GHz given by Waters [1976] and Ulaby et al. [1981], which are based on the Gross line shape.
TL;DR: In this article, a simple model for the radiometric determination of tropospheric transmittance is based on an isothermal troposphere, which characterizes the radiation and temperature properties of the troposphere using ground temperature, ground relative humidity, and radiometer data.
Abstract: A simple model for the radiometric determination of tropospheric transmittance is based on an isothermal troposphere In this model the key parameter is the weighted mean tropospheric temperature Tm, which characterizes the radiation and temperature properties of the troposphere Statistical approaches in modeling this parameter are presented here by using ground temperature, ground relative humidity, and radiometer data In order to determine the statistical coefficients for Tm modeling and the parameters used in the transmittance retrieval algorithm, radiosonde data were used in a millimeter-wave propagation model for a site in the Swiss central plane and an Alpine site Various observing geometries at different millimeter-wave frequencies were considered A determination of Tm from ground temperature was achieved with a rms error between 4–5 K for the low-altitude site and 3–4 K for the high-altitude site By incorporating relative humidity or radiometer data, an improvement of up to 25% relative to these values results, depending on frequency and site The zenith transmittance estimations for the low-altitude site with our best model have a rms error of 05% at 38 GHz, 1% at 94, 110, and 142 GHz, 15% at 115 GHz, 2% at 204 GHz, and 35% at 279 GHz, whereas for the high-altitude site all rms errors are below 1% The inclusion of radiometric information at 20 and 31 GHz did not provide any additional improvement, which was confirmed by actual measurements at 142 GHz
TL;DR: In this article, the authors show that the disagreement is caused by the different sampling of the data for the two instruments when the two data sets are reduced to include only data that are sampled simultaneously, and find an agreement between all estimated model parameters, given their statistical uncertainties.
Abstract: Microwave radiometer data and radiosonde data from the time period 1981-1995 have been used to study long-term trends in the integrated precipitable water vapor (IPWV) The two instruments have operated 37 km apart on the Swedish west coast Model parameters are estimated for the entire data sets as well as for subsets of the data The IPWV model parameters are a mean value, a linear drift with time, and the amplitude and phase of an annual component The radiosonde data, which are uniformly sampled in time, show an increase in the IPWV of 003 mm/yr with a statistical standard deviation of 001 mm The microwave radiometer data, which are not at all uniformly sampled in time, show -002+/-001 mm/yr We show that the disagreement is caused by the different sampling of the data for the two instruments When the two data sets are reduced to include only data that are sampled simultaneously, we find an agreement between all estimated model parameters, given their statistical uncertainties This suggests that if the microwave radiometer had also been operating continuously over the 15-year period, its data would have implied a linear trend similar to the result obtained from the radiosonde data The general quality of the data, in terms of the short time scatter, has been improved over the time period The root mean square (RMS) difference between the IPWV measured by the radiometer and by the radiosondes was 21 mm during the first 5 years and was reduced to 16 mm during the last 4 years These values include the real difference in the IPWV between the two sites The bias, radiometer-radiosonde, was 01 mm for the whole data set and varied between -02 and 09 mm for smaller data sets of a few years
TL;DR: An inversion technique is presented for retrieving vertical profiles of atmospheric temperature and vapor from the brightness temperatures measured by a ground-based multichannel microwave radiometer and the surface measurements of temperature and relative humidity and has proven to be flexible, showing a good capability of exploiting information provided by other instruments.
Abstract: An inversion technique is presented for retrieving vertical profiles of atmospheric temperature and vapor from the brightness temperatures measured by a ground-based multichannel microwave radiometer and the surface measurements of temperature and relative humidity. It combines a profile expansion over a complete set of natural orthogonal functions with a neural network which performs the estimate of the coefficients of the expansion itself. A simulation study has been carried out, and the algorithm has been tested by comparing its retrievals with those obtained by means of linear statistical inversion applied on the same data sets. The analysis has been limited to the case of profiles with clouds in order to test the ability of the neural network to face nonlinear problems. The technique has proven to be flexible, showing a good capability of exploiting information provided by other instruments, such as a laser ceilometer. A fault tolerance evaluation has also been considered, which showed interesting properties of robustness of the algorithm.
TL;DR: In this article, a sinusoidal model has been successfully used to approximate and subtract the first-order Bragg peaks, which spread and mask slow targets when using Fourier Doppler processing on short dwells (fewer than 10 s).
Abstract: Coherent sea-clutter cancelation is shown to be an effective method for the exposure of ships with HF OTH (high-frequency over-the-horizon) sky wave radar. A sinusoidal model has been successfully used to approximate and subtract the first-order Bragg peaks, which spread and mask slow targets when using Fourier Doppler processing on short dwells (fewer than 10 s). (Short dwells are required for aircraft surveillance over the vast HF radar coverage area.) The new clutter canceling algorithm has been successfully applied to both synthetic and real OTH data from the U.S. Navy's Relocatable Over-the-Horizon radar (ROTHR) (AN/TPS-71), and results are presented.
TL;DR: In this article, the development of a novel VHF radar designed to measure winds and temperatures in the planetary boundary layer is described, which operates at 54.1 MHz and is compact and easily transportable.
Abstract: The development of a novel VHF radar designed to measure winds and temperatures in the planetary boundary layer is described. The radar operates at 54.1 MHz and is compact and easily transportable. The antenna system consists of 12 Yagis grouped into three subarrays arranged in the form of an equilateral triangle. Transmission takes place on the whole array, and reception takes place on the three subarrays, with winds measured by the spaced antenna technique over a height range between 300 and 3000 m. Results from field trials conducted in southern Australia in a variety of meteorological conditions are presented. Comparisons with high-resolution radiosondes launched from the radar site show excellent agreement, with rms differences between radiosonde and radar wind components being about 1.5 m s−1. Observations carried out in rain show that echoes from precipitation are clearly distinguishable from clear-air echoes. Unlike UHF radars, this means that vertical air velocities can be measured during precipitation, and the evolution of drop-size distributions can be studied down to low altitudes. It is shown that temperatures derived from a radio acoustic sounding system are measured up to heights near 2 km, depending on background wind conditions.
TL;DR: In this paper, the authors analyzed the dispersion properties of leaky-wave modes in the vicinity of cutoff for several representative waveguiding structures, and showed that the presence of branch points in the complex frequency plane can be used to identify important regions in both the frequency and wavenumber planes.
Abstract: In this paper we analyze characteristics of the dispersion function for leaky-wave modes in the vicinity of cutoff for several representative waveguiding structures. Our principal purpose is to demonstrate that in the vicinity of leaky-wave cutoff in open-boundary waveguides (in the spectral-gap region), dispersion behavior is controlled by the presence of branch points in the complex frequency plane. A similar situation occurs for the ordinary modes of homogeneously filled, perfefctly conducting cylindrical waveguides. These closed waveguides admit to simple analysis, leading to an explicit dispersion function which indicates frequency domain branch points. For open-boundary waveguides, the presence of frequency domain branch points is obscured by the necessity of numerically solving an implicit dispersion equation. A set of sufficient conditions is provided here which defines these branch points in a unified manner for both open and closed waveguides. Identification of these points allows for rapid determination of important and interesting regions in both the frequency and wavenumber planes and leads to increased understanding of dispersion behavior, especially in the case of dielectric loss. Examples are shown for several waveguiding geometries to demonstrate the general nature of the presented formulation.
TL;DR: In this paper, an experiment for compensating interferometer phase fluctuations due to the turbulent troposphere was conducted with the paired antennas method (PAM) using different sky frequencies for a target and reference source.
Abstract: An experiment for compensating interferometer phase fluctuations due to the turbulent troposphere was conducted with the paired antennas method (PAM) using different sky frequencies for a target and reference source. A celestial source 3C279 was observed at 146.81 GHz with the Nobeyama millimeter array (NMA), while a geostationary satellite was observed as a reference source at 19.45 GHz with commercially available antennas. Each of the antennas was installed near one of the NMA antennas, thus giving us 10 pairs of nearly parallel baselines up to 316 m. Large fluctuations in the interferometer phase of 3C279 were mostly eliminated on almost all baselines by subtracting the reference phase multiplied by a ratio of the observing frequencies. The standard deviations of the compensated interferometer phase were under the level of 20° where the angular separation between the two sources was within 2°, while those of the original phase were typically at the level of 40°–50°. The phase compensation was much improved to the level of the differential excess path length of 0.06-mm rms (11° at 146.81GHz) in inserting a time lag proportional to the separation angle to the reference phase time series. These results have practical significance for the millimeter or submillimeter wave interferometry because there is rarely a suitable reference source in the vicinity of scientifically interesting sources at such high frequencies. The present experiment shows that it is quite effective in the PAM to use the reference phase at centimeter wave for compensating the millimeter-wave phase for future large millimeter-wave arrays.