Showing papers in "Radio Science in 1995"
TL;DR: An MST radar operating at 53 MHz with an average power aperture product of 7 × 108 W m2 has been established at Gadanki (13.5°N, 79.2°E), India as mentioned in this paper.
Abstract: An MST radar operating at 53 MHz with an average power aperture product of 7 × 108 W m2 has been established at Gadanki (13.5°N, 79.2°E), India. The radar development has been accomplished in two phases. In the first phase it was commissioned in ST mode using a partial system comprising one quarter (16 × 16)of the Yagi antenna array and 16 driver units of the transmitters providing an average power aperture product of 4.8 × 106 W m2. In this part we present the radar system description, including off-line data processing, and some sample high-resolution vector wind measurements made in ST mode operation.
293 citations
TL;DR: The parameterized ionospheric model (PIM) as mentioned in this paper is a global model of theoretical ionosphere climatology based on diurnally reproducible runs of four physics-based numerical models of the ionosphere.
Abstract: We describe a parameterized ionospheric model (PIM), a global model of theoretical ionospheric climatology based on diurnally reproducible runs of four physics based numerical models of the ionosphere. The four numerical models, taken together, cover the E and F layers for all latitudes, longitudes, and local times. PIM consists of a semianalytic representation of diurnally reproducible runs of these models for low, moderate, and high levels of both solar and geomagnetic activity and for June and December solstice and March equinox conditions. PIM produces output in several user selectable formats including global or regional latitude/longitude grids (in either geographic or geomagnetic coordinates), a set of user specified points (which could lie along a satellite orbital path), or an altitude/azimuth/elevation grid for a user-specified location. The user selectable output variables include profile parameters (ƒ0F2, hmF2, total electron content, etc.), electron density profiles, and ion composition (O+, NO+, and O2+).
269 citations
TL;DR: An overview of the architecture of the UHF profiler system as it has evolved over the past decade including the development of radio acoustic sounding system (RASS) capabilities is presented in this paper.
Abstract: Developments in UHF profiling at 915 MHz are presented. Recent advances in UHF profiling are traced to early developments beginning about 8 years ago in the Aeronomy Laboratory at 915 MHz using microstrip antennas. This paper presents an overview of the architecture of the UHF profiler system as it has evolved over the past decade including the development of radio acoustic sounding system (RASS) capabilities. Hardware and software components are described and operational performance is summarized from experience gained from many installations, primarily from those in the tropics. Applications to wind profiling, boundary layer height determination, flux measurement, and precipitation profiling are considered.
217 citations
TL;DR: In this article, the authors used the Wideband ionospheric scintillation model (WBMOD) to calculate the S 4 on a two-way channel, where the data from three additional stations (Ascension Island, in the South Atlantic Ocean, Huancayo, Peru, and Manila, Phillipines; data collected under the auspices of the USAF Phillips Laboratory Geophysics Directorate) were used to provide a greater diversity in both latitude and longitude.
Abstract: One of the main limitations of the modeling work that went into the equatorial section of the Wideband ionospheric scintillation model (WBMOD) was that the data set used in the modeling was limited to two stations near the dip equator (Ancon, Peru, and Kwajalein Island, in the North Pacific Ocean) at two fixed local times (nominally 1000 and 2200). Over the past year this section of the WBMOD model has been replaced by a model developed using data from three additional stations (Ascension Island, in the South Atlantic Ocean, Huancayo, Peru, and Manila, Phillipines; data collected under the auspices of the USAF Phillips Laboratory Geophysics Directorate) which provide a greater diversity in both latitude and longitude, as well as cover the entire day. The new model includes variations with latitude, local time, longitude, season, solar epoch, and geomagnetic activity levels. The way in which the irregularity strength parameter C k L is modeled has also been changed. The new model provides the variation of the full probability distribution function (PDF) of log(C k L) rather than simply the average of log(C k L). This permits the user to specify a threshold on scintillation level, and the model will calculate the percent of the time that scintillation will exceed that level in the user-specified scenario. It will also permit calculation of scintillation levels at a user-specified percentile. A final improvement to the WBMOD model is the implementation of a new theory for calculating S 4 on a two-way channel
158 citations
TL;DR: In this article, the authors used a simple ionospheric shell model to derive a map of the ionosphere in the northern hemisphere every 12 hours during the January 1-15, 1993, period.
Abstract: Ionospheric total electron content (TEC) data derived from dual- frequency Global Positioning System (GPS) signals from 30 globally distributed network sites are fit to a simple ionospheric shell model, yielding a map of the ionosphere in the northern hemisphere every 12 hours during the January 1-15, 1993, period.
128 citations
TL;DR: In this article, the authors describe some of the numerical methods that have been developed for calculations of induced electric fields, current densities, and specific absorption rates for anatomically based heterogeneous models of the human body with increasingly finer resolutions.
Abstract: This paper describes some of the numerical methods that have been developed for calculations of induced electric fields, current densities, and specific absorption rates for anatomically based heterogeneous models of the human body with increasingly finer resolutions. These methods, namely, the impedance method and the finite difference time domain (FDTD) method, have been used for dosimetric calculations for a number of bioelectromagnetic problems for whole-body or partial-body exposures, for far-field or near-field sources, and for CW or transient fields. The paper gives detailed calculations for some recent applications such as currents induced in the user's body by the electromagnetic fields (emfs) of electric blankets using the impedance method, coupling of an ultrawideband pulse using the frequency-dependent FDTD method incorporating dispersive properties of the various tissues, and specific absorption rate distributions in the head for emfs of cellular telephones. Because of accurate modeling of tissue heterogeneities and shapes, these methods are likely to play an increasing role in emerging technologies with bioelectromagnetic concerns.
119 citations
TL;DR: In this paper, a new generation frequency-modulated continuous wave (FMCW) radar using state-of-the-art electronics and computerized data processing is described.
Abstract: This paper describes a new generation frequency-modulated continuous wave (FMCW) radar using state-of-the-art electronics and computerized data processing that greatly enhance the use of the radar as a practical tool for atmospheric research. The system senses at high resolution (∼2-m range and 12 s for obtaining each profile), has ultrasensitivity (< −165 dBm), and is accurately calibrated for the refractive index structure parameter (C2n). The authors present salient features, discuss the calibration procedure, and present and discuss examples of various types of fine detail wave and frontal activity, boundary layer convection, and a light winter storm sensed by the radar over the last 2 years. The authors also show associated data from radiosonde and tower-mounted sensors that are relevant to the episodal events shown from the FMCW radar observations. Since the FMCW radar technique can resolve and sense individual insects, these point targets are shown to act as tracers and enhance flow visualization.
95 citations
TL;DR: In this article, the authors compare measurements of polar cap ionospheric plasma flow over Resolute Bay, Canada, made by a digital ionosonde using the Doppler drift technique with simultaneous measurements at the same location made by the first operational pair of SuperDARN HF radars.
Abstract: We compare measurements of polar cap ionospheric plasma flow over Resolute Bay, Canada, made by a digital ionosonde using the Doppler drift technique with simultaneous measurements at the same location made by the first operational pair of SuperDARN HF radars. During the 3-hour comparison interval the flow varied widely in direction and from 100 to 600 m/s in speed. The two measurement techniques show very good agreement for both the speed and direction of flow for nearly all of the samples in the interval. The difference between the velocities determined by the two techniques has a scatter of about ±35° in direction and ±30% in speed, with no systematic difference above the level of the scatter. The few samples which strongly disagreed were usually associated with strong spatial structure in the flow pattern measured by SuperDARN in the vicinity of the comparison point. The drift speed measured by the ionosonde was independently verified by observing the time taken for polar cap F layer ionization patches to drift between ionosondes sited at Eureka and Resolute Bay. These results confirm that the speed and direction of the polar cap ionospheric convection can be reliably monitored by the ionosonde Doppler drift technique.
92 citations
TL;DR: In this paper, a model was developed for simulating the effects of backscatter from scatterers advected with a mean background wind, which includes features such as aspect sensitivity, gravity wave perturbations, and turbulent motions.
Abstract: A model has been developed for simulating the effects of backscatter from scatterers advected with a mean background wind. This model has been designed to be as simple yet as realistic as possible, allowing the simulation of both spaced antenna and Doppler radars, and includes features such as aspect sensitivity, gravity wave perturbations, and turbulent motions. The model simulates the characteristics of radar backscatter very well at both MF and VHF. Results of the application of the full correlation analysis to model data generated for the spaced antenna configuration are presented, revealing good agreement with the model input velocity. The effects of the sampling time upon the full correlation analysis are investigated, suggesting an upper limit for the successful application of the technique. The triangle size effect of the full correlation analysis is also investigated, confirming that the major cause is the failure to properly compensate for the effects of noise. The application of techniques for the estimation of turbulent velocities and aspect sensitivity from model-generated data have also proven successful.
92 citations
TL;DR: In this paper, a series transformation technique for integrals involving Bessel functions and sinusoidal functions is presented. But the main aim is to critically examine the popular series transformation (acceleration) methods which are used in electromagnetic problems and to introduce a new acceleration technique.
Abstract: Most electromagnetic problems can be reduced to either integrating oscillatory integrals or summing up complex series. However, limits of the integrals and the series usually extend to infinity, and, in addition, they may be slowly convergent. Therefore numerically efficient techniques for evaluating the integrals or for calculating the sum of an infinite series have to be used to make the numerical solution feasible and attractive. In the literature there are a wide range of applications of such methods to various EM problems. In this paper our main aim is to critically examine the popular series transformation (acceleration) methods which are used in electromagnetic problems and to introduce a new acceleration technique for integrals involving Bessel functions and sinusoidal functions.
82 citations
TL;DR: In this paper, a comparison of wind velocities at heights from 80 to 98 km have been made using two different techniques, namely meteor drifts and spaced antenna technique, and the agreement between the two techniques is good below 90 km, while above 90 km it is not good.
Abstract: Comparisons of wind velocities at heights from 80 to 98 km have been made using two different techniques. The first method involves the determination of winds using meteor drifts (e.g., Avery et al., 1990; Stubbs, 1973). This was done by observing meteors using the University of Adelaide VHF radar situated approximately 40 km north of Adelaide, Australia, at Buckland Park. The second method used to determine winds was the spaced antenna technique (e.g., Briggs, 1984) using an MF radar at the same site. The two radar systems are independent, the VHF radar operating at 54.1 MHz and the MF radar at 1.98 MHz. The spatial separation of the two radars is approximately 600 m. Simultaneous data obtained from September 10 to 20, 1993, are presented here. The agreement between the two techniques is good below 90 km, while above 90 km we find that the spaced antenna technique yields smaller wind speeds than the meteor drift technique. Several possible reasons for these discrepancies are discussed.
TL;DR: In this paper, a simple model considering flat and very extended sheets was assumed to estimate quantitatively the possible contribution of these temperature gradients within thin layers to the VHF radar vertical power, and the power loss from the flat sheet case at VHF in the vertical direction was shown that partial reflection from atmospheric sheets is an important and generally dominant process at vertical incidence.
Abstract: The interpretation of the radar aspect sensitivity observed at VHF frequencies in the lower atmosphere is still a subject of controversy in the radar community. Indeed, scattering from anisotropic turbulence layers and partial reflection from stable thin horizontally stratified layers are generally proposed without leading to a definitive conclusion. A cause of this persistent discussion has been a lack of in situ high-resolution observations which could identify and describe accurately the atmospheric structures at the origin of the radar aspect sensitivity. The Radars, Scidar and Balloons (RASCIBA 90) campaign (February-March 1990, Aire sur l'Adour, France) was performed using simultaneously colocated VHF ST radars and balloon experiments. The objective was to obtain information about the small-scale structures of the lower atmosphere and then to identify the origin of radar echoes. The main result was produced by the high vertical resolution (20 cm) temperature measurements. For the first time, very strong (positive) temperature gradients within thin layers were detected in the lower atmosphere (at least up to 27 km). Such an observation allowed then investigation of the partial reflection interpretation using available bifrequency radar measurements: a 45-MHz radar for which typical 15-dB vertical enhancement is observed just above the tropopause and a 72.5-MHz radar for which an aspect sensitivity of the same order is presented for the first time at this frequency. A simple model considering flat and very extended sheets was assumed to estimate quantitatively the possible contribution of these temperature gradients to the VHF radar vertical power. Radar and reconstructed power reflection coefficient profiles are in good agreement in shape and in level showing that partial reflection from atmospheric sheets is an important and generally dominant process at vertical incidence. Furthermore, the effect of sheets of limited extent is investigated using simple theoretical considerations, and this model shows that the correction to the infinite sheet approximation is probably weak. The sheet distortion effect is also approximately evaluated applying Gaussian rough models. The power loss from the flat sheet case at VHF in the vertical direction is weak for a mean roughness height lower than a few tens of centimeters and for larger heights if the correlation length of the irregularities is larger than a few hundreds of meters. Sheet generation mechanisms are also briefly discussed using two previously published models: the viscosity wave model (Hocking et al., 1991) and “sheet and layer” model (Gossard et al., 1985). Neither of the models in their present forms seems to be able to account for the observations.
TL;DR: In this article, the authors analyze the early TIPP observations and compute several parameters that describe the events: the location of the satellite at the time of reception, the energy in each pulse, the separation between pulses, the duration of each pulse and the dispersion of each pulses.
Abstract: Recent observations of transient radio impulses by an Earth-orbiting satellite appear to be quite unlike any previously reported. They appear as pairs of brief (a few microseconds), noiselike bursts, separated by a few tens of microseconds, and are dispersed in a way that implies subionospheric origin. Over 300 of these events have now been observed. These “transionospheric pulse pairs” (TIPPs) have not yet been associated with any known source, although thunderstorms are suspected. The observations, made by the Blackbeard instrument on the ALEXIS satellite, are digitized records of the electric field in a passband from about 25 to 100 MHz. Ground-based observations of lightning in this band appear quite different, even accounting for ionospheric dispersion: bursts of short pulses last hundreds of microseconds and have much lower power (when propagated to the satellite) than TIPP events. Signals that resemble the ground-based data have been observed by Blackbeard but, being much weaker, are much less likely to trigger the instrument than are the strong pulse pair events. In this paper we analyze 97 of the early TIPP observations. We compute several parameters that describe the events: the location of the satellite at the time of reception, the energy in each pulse, the separation between pulses, the duration of each pulse, and the dispersion of each pulse. The statistical distributions of these parameters provide clues to and constraints on possible source mechanisms. The possibility that the pulses might be the direct and reflected signals from a high-altitude source is considered and cannot be rejected by the data.
TL;DR: In this paper, the performance of the L band boundary layer radar (BLR) with respect to the observation height range and the wind measurement reliability has been examined on the basis of simultaneous meteorological observations.
Abstract: An L band (1357.5 MHz) boundary layer radar (BLR) has been in continuous successful operation in Serpong, Indonesia (6.4°S, 106.7°E), since November 1992. The performance of the BLR with respect to the observation height range and the wind measurement reliability has been examined on the basis of simultaneous meteorological observations. We have found that there are two types of strong echo structures appearing systematically in the equatorial planetary boundary layer with diurnal variations on clear days. The first type is the striking appearance of a strong echo layer ascending from below 300 m (in the morning) to above 3–5 km (in the afternoon), which is identified with a diurnal variation of the top of the mixing planetary boundary layer. As expected, it is higher in the Indonesian equatorial region than in midlatitudes. Another type is layered echoes appearing at 2–3 km heights from nighttime to morning, which seem to be coincident with humidity gaps.
TL;DR: In this article, a simple image domain ray tube integration formula is presented to efficiently compute the inverse synthetic aperture radar (ISAR) image of a complex target by the shooting and bouncing ray (SBR) technique.
Abstract: A simple image domain ray tube integration formula is presented to efficiently compute the inverse synthetic aperture radar (ISAR) image of a complex target by the shooting and bouncing ray (SBR) technique. Contrary to the conventional approach where the ISAR image is obtained by inverse Fourier transforming the computed scattered field data over frequency and aspect, this image domain formula gives the contribution of each ray to the overall ISAR image directly. Under the small angle approximation and utilizing the bistatic-monostatic equivalence, the image domain ray tube integration formula is determined in closed form. Simulation results using the SBR-based code “Xpatch” show that the direct image domain method results in good image quality and superior time performance when compared to the conventional frequency aspect approach.
TL;DR: The weight of experimental evidence indicates that an induced E field according to Faraday's law of induction during magnetic field exposures elicits cellular effects, and several studies have suggested that an ELF B field by itself or in combination with a static B field may elicit cellular effects.
Abstract: Worldwide interest in the biological effects of ELF (extremely low frequency, <1 kHz) electromagnetic fields has grown significantly. Health professionals and government administrators and regulators, scientists and engineers, and, importantly, an increasing number of individuals in the general public are interested in this health issue. The goal of research at the cellular level is to identify cellular responses to ELF fields, to develop a dose threshold for such interactions, and with such information to formulate and test appropriate interaction mechanisms. This review is selective and will discuss the most recent cellular studies directed at these goals which relate to power line, sinusoidal ELF fields. In these studies an interaction site at the cell membrane is by consensus a likely candidate, since changes in ion transport, ligand-receptor events such as antibody binding, and G protein activation have been reported. These changes strongly indicate that signal transduction (ST) can be influenced. Also, ELF fields are reported to influence enzyme activation, gene expression, protein synthesis, and cell proliferation, which are triggered by earlier ST events at the cell membrane. The concept of ELF fields altering early cell membrane events and thereby influencing intracellular cell function via the ST cascade is perhaps the most plausible biological framework currently being investigated for understanding ELF effects on cells. For example, the consequence of an increase due to ELF fields in mitogenesis, the final endpoint of the ST cascade, is an overall increase in the probability of mutagenesis and consequently cancer, according to the Ames epigenetic model of carcinogenesis. Consistent with this epigenetic mechanism and the ST pathway to carcinogenesis is recent evidence that ELF fields can alter breast cancer cell proliferation and can act as a copromoter in vitro. The most important dosimetric question being addressed currently is whether the electric (E) or the magnetic (B) field, or if combinations of static B and time-varying B fields represent an exposure metric for the cell. This question relates directly to understanding fundamental interaction mechanisms and to the development of a rationale for ELF dose threshold guidelines. The weight of experimental evidence indicates that an induced E field according to Faraday's law of induction during magnetic field exposures elicits cellular effects. An E-field-mediated interaction has interesting consequences for microdosimetry at the cellular level and is mechanistically consistent with an interaction at the cell surface, since the E field does not penetrate beyond the cell membrane. Recently, several studies have suggested that an ELF B field by itself or in combination with a static B field may elicit cellular effects. Thus in addition to E-field-mediated effects, other interaction mechanisms as yet not fully understood may operate at the cellular level; this complexity is in contrast to the case for ionizing radiation. In addition to the question of an exposure field metric, the biological state of the target cell is important in ELF interactions. Biological factors such as cell type, cell cycle, cell activation, age of donor animal, passage number of cell line, presence of specific growth/mitogenic factors, temperature, shape, and cell density/packing during exposures have been shown to play a role in mediating ELF interactions with cells. Most recently, reports of single-cell studies usher in a new direction for research that can be termed microbioelectromagnetics. Single-cell digital microscopy introduces a new approach to answer the above questions with potential for real-time microdosimetry and bioeffects limited only by the spatial resolution of state-of-the-art microscopy, which is approximately 0.1 /μm. Digital imaging microscopy should therefore permit the quantitative assessment of spatial and temporal features of ELF field interactions within living single cells.
TL;DR: In this paper, a comparison of drift measurements made at Sondre Stromfjord show that the apparent velocities measured by the digisonde (DGS) are in good agreement with the drift veloities observed by the collocated incoherent scatter radar (ISR).
Abstract: Comparison of drift measurements made at Sondre Stromfjord show that the apparent velocities measured by the digisonde (DGS) are in good agreement with the drift velocities observed by the collocated incoherent scatter radar (ISR). Data from December 5 to 9, 1991, show the mean DGS velocities to be within 50 m s−1 of the ISR velocities, that is, within the uncertainty levels of each instrument. The analysis highlights the dominance of the electric field in controlling the plasma motion. The measured velocities are generally height independent, as would be expected for an E field mapped along the magnetic field lines from high altitudes to ionospheric heights. In addition, the comparative analysis is used to study an ionospheric event where a large section of ionization was removed from the daytime cusp region by a strong anti-Sunward drift when the interplanetary magnetic field (IMF) Bz component changed orientation.
TL;DR: In this paper, the authors used the finite difference time domain (FDTD) method to predict the interaction between the human body and the electromagnetic field generated by a cordless telephone and found that about 15% of the antenna input power at 900 MHz is absorbed in the head.
Abstract: This paper shows the capability of the finite difference time domain (FDTD) method to predict the interaction between the human body and the electromagnetic field generated by a cordless telephone. Both the influence of the human head on the performance of the cordless telephone and the energy deposited in the human head have been determined. The interaction has been evaluated for a simple dipole model and for an accurate “box” model of the telephone. The FDTD method is a versatile method for refining the antenna model. The results show that the use of the accurate box model in combination with a realistic model of the head derived from a nuclear magnetic resonance image is a prerequisite for accurate determination of the near fields induced in the head. The total amount of power absorbed in the head has been compared to the radiated power. From our calculations we found that about 15% of the antenna input power at 900 MHz is absorbed in the head.
TL;DR: In this paper, the authors investigate the response of polarimetric backscattering at 24-cm wavelength to the thickness of Arctic sea ice in leads and first-year ice features.
Abstract: We investigate observationally and theoretically the response of polarimetric backscattering at 24-cm wavelength to the thickness of Arctic sea ice in leads and first-year ice features. We employ backscattering data acquired by the Jet Propulsion Laboratory airborne synthetic aperture radar (SAR) during March 1988 in the Beaufort Sea, together with nearly simultaneous passive microwave imagery acquired by the U.S. Navy Ka band radiometric mapping system. We find that 24-cm copolar ratios and copolar phases vary strongly with apparent ice thickness. We observe copolar phase shifts between −10° and −50° (relative to multiyear ice phases) for new ice features in the imagery, as well as positive copolar phases in a first-year ice feature. Copolar ratios also vary with apparent thickness, from values larger than those expected theoretically for seawater to values slightly lower than those expected for thick ice. We derive a signature model based on scattering from a rough air/sea ice interface with realistic vertical profiles of brine volume and relative permittivity beneath. Model predictions for copolar ratios and phases show ice thickness-dependent variations consistent with those observed. We present simulation results showing that plausible ice thickness variations between pixels in a multilook average diminish, but do not eliminate, the signature response to thickness. This suggests that direct thickness estimation of sea ice in leads may be possible using polarimetric SAR at wavelengths of 24 cm or longer.
TL;DR: In this article, a linear combination of orthonormalized simple Chapman profiles is used to extract the required vertical profile for three different test cases, as follows: an arbitrary vertical profile is assumed; for convenience, the coefficients of these functions are found using a nonlinear optimization procedure in conjunction with repeated MaxEnt calculations.
Abstract: Ionospheric tomography is viewed as an inverse problem, the direct problem of which is linear. The total electron content (TEC) is the integral of the electron density along rays from a satellite to a set of receivers on the ground. If the continuous ionosphere is replaced by a discrete set of finite-sized cells, in each of which the density is constant, then the integrals become simple sums of density times ray lengths in each cell. The equation of the direct problem is then a simple matrix equation. Since the electron density distribution is positive and normalizable, it is isomorphic to a probability distribution for which the maximum entropy method (MaxEnt) yields a unique solution which uses all available information and is maximally noncommittal with respect to all unavailable information. The ionospheric problem is distinct from other forms of tomography in that the rays from the satellite to the receivers are all nearly vertical; horizontal rays which would carry information on the average vertical profile are not available. Without such information the problem cannot be solved; but the nearly vertical rays do carry some such information, although very weakly. The present method is able to extract the required vertical profile for three different test cases, as follows: An arbitrary vertical profile is assumed; for convenience a linear combination of orthonormalized simple Chapman profiles is used; the coefficients of these functions are found using a nonlinear optimization procedure in conjunction with repeated MaxEnt calculations. These test results are unique: There is enough information in the simulated TECs to determine a unique reconstruction in all three test cases. No artifacts appear in the reconstructions, and the resolution is excellent.
TL;DR: In this article, an event-based approach is developed and applied to 3 years of data of a four link transhorizon path over the English Channel (La Manche) and the signal enhancements observed on the chart recordings during anomalous propagation are selected and, according to their dynamical signatures, categorized into three main patterns of ducting, layer reflection, and high k factor diffraction.
Abstract: An event-based approach is developed and applied to 3 years of data of a four link transhorizon path over the English Channel (La Manche). The signal enhancements (events) observed on the chart recordings during anomalous propagation are selected and, according to their dynamical signatures, categorized into three main patterns of ducting, layer reflection, and high k factor diffraction. The statistical analysis carried out on this event basis gives the correlation between the signal levels of different links and the cross correlation between signal levels and local N index derived from surface meteorological monitoring. The results reveal the fundamental properties of the anomalous propagation regimes. Using case studies of several interesting events, the meteorological processes and resultant impact on transhorizon transmission are also explained. This extensive work contributes to a better understanding of anomalous propagation mechanisms present in transhorizon links.
TL;DR: In this paper, a simple and computationally efficient methodology is demonstrated for treating the interaction of radio waves near grazing above rough surfaces, which is applied to the sea from HF through the microwave.
Abstract: A simple and computationally efficient methodology is demonstrated for treating the interaction of radio waves near grazing above rough surfaces. We apply it to the sea from HF through the microwave. Results using actual finite seawater conductivity depart significantly from those of a perfectly conducting profile for vertical polarization; for horizontal polarization there is essentially no difference. Shadowing behind wave crests is surprisingly shallow for vertical polarization even when the occluded depth is tens of wavelengths. This methodology gives expressions for the effective surface impedance at grazing as modified by the roughness when height scales greatly exceed the radio wavelength, augmenting prior expressions valid at HF for the sea obtained from perturbation theory. These effective impedances can then be used in formulations for radiation and surface wave propagation/diffraction at and below the horizon.
TL;DR: The fundamental relationship between self-similar arrays and their ability to generate radiation patterns which possess fractal features is examined in this article, where the theoretical foundation and design procedures are developed for using fractal arrays to synthesize fractal radiation patterns having certain desired characteristics.
Abstract: The fundamental relationship between self-similar, that is, fractal, arrays and their ability to generate radiation patterns which possess fractal features is examined in this paper. The theoretical foundation and design procedures are developed for using fractal arrays to synthesize fractal radiation patterns having certain desired characteristics. A family of functions, known as generalized Weierstrass functions, are shown to play a pivotal role in the theory of fractal radiation pattern synthesis. These functions are everywhere continuous but nowhere differentiable and exhibit fractal behavior at all scales. It will be demonstrated that the array factor for a nonuniformly but symmetrically spaced linear array can be expressed in terms of a Weierstrass partial sum (band-limited Weierstrass function) for an appropriate choice of array element spacings and excitations. The resulting fractal radiation patterns from these arrays possess structure over a finite range of scales. This range of scales can be controlled by the number of elements in the array. For a fixed array geometry, the fractal dimension of the radiation pattern may be varied by changing the array current distribution. A general and highly flexible synthesis technique is introduced which is based on the theory of Fourier-Weierstrass expansions. One of the appealing attributes of this synthesis technique is that it provides the freedom to select an appropriate generating function, in addition to the dimension, for a desired fractal radiation pattern. It is shown that this synthesis procedure results in fractal arrays which are composed of a sequence of self-similar uniformly spaced linear subarrays. Finally, a synthesis technique for application to continuous line sources is presented which also makes use of Fourier-Weierstrass expansions.
TL;DR: In this article, the authors examined the sensitivity of the 915-MHz profiler to precipitation and showed that deep precipitating clouds were present more than 25% of the time over a 10-month period.
Abstract: Precipitation echoes obtained with 915-MHz wind profilers operated in the tropics provide useful information on the structure of precipitating cloud systems. This paper examines the sensitivity of the 915-MHz profiler to precipitation. Observations of precipitation echoes at Manus Island, Papua New Guinea, illustrate profiler performance capabilities for observing several types of tropical convective systems. Special attention is given to the occurrence of deep precipitating clouds which are often present without rain at the surface. Analyses of profiler data from Manus Island show that over a 10-month period, deep precipitating clouds were present more than 25% of the time. The importance of the deep precipitating clouds to the heat budget of the tropical atmosphere is briefly discussed.
TL;DR: In this article, a radio sounder outside the plasmasphere could provide nearly continuous remote density measurements of the magnetopause and plasmosphere, as well as other important density features elsewhere in this region.
Abstract: A radio sounder outside the plasmasphere could provide nearly continuous remote density measurements of the magnetopause and plasmasphere, as well as other important density features elsewhere in this region. Using digital integration and tuned reception at frequencies from a few kilohertz to a few megahertz with 400-m to 500-m tip-to-tip dipole antennas and 10 W transmitter power, such a sounder would be capable of 10% density resolution and 500 to 1300 km spatial resolution in only a few minutes at distances of up to 4 RE. By providing such detailed observations of its principal density structures, such a sounder would then clearly revolutionize magnetospheric research.
TL;DR: Bessel series expansions for the incomplete Lipschitz-Hankel integral are derived by using contour integration techniques to evaluate the inverse Laplace transform representation for Je0(a, z) as mentioned in this paper.
Abstract: Bessel series expansions are derived for the incomplete Lipschitz-Hankel integralJe0(a, z). These expansions are obtained by using contour integration techniques to evaluate the inverse Laplace transform representation for Je0(a, z). It is shown that one of the expansions can be used as a convergent series expansion for one definition of the branch cut and as an asymptotic expansion if the branch cut is chosen differently. The effects of the branch cuts are demonstrated by plotting the terms in the series for interesting special cases. The Laplace transform technique used in this paper simplifies the derivation of the series expansions, provides information about the resulting branch cuts, yields integral representations for Je0(a, z), and allows the series expansions to be extended to complex values of z. These series expansions can be used together with the expansions for Ye0(a, z), which are obtained in a separate paper, to compute numerous other special functions, encountered in electromagnetic applications. These include: incomplete Lipschitz-Hankel integrals of the Hankel and modified Bessel form, incomplete cylindrical functions of Poisson form (incomplete Bessel, Struve, Hankel, and Macdonald functions), and incomplete Weber integrals (Lommel functions of two variables).
TL;DR: In this paper, the inverse problem of restoring dynamic and statistical parameters of traveling ionospheric disturbances (TIDs) from variations of angles of arrival and Doppler frequency shift of HF radio signals on one-hop oblique radio paths is solved.
Abstract: In the model of a perfectly reflecting surface the inverse problem of restoring dynamic and statistical parameters of traveling ionospheric disturbances (TIDs) from variations of angles of arrival and Doppler frequency shift of HF radio signals on one-hop oblique radio paths is solved. The technique for determining dispersion law, fluctuation spectrum, geometry, direction, and velocity of TID motion is described.
TL;DR: In this paper, the authors studied phase scintillation and ionospheric irregularities at northern polar latitudes using data collected from the United States Navy Navigation Satellite System (NNSS) satellites.
Abstract: Radio transmissions from Navy Navigation Satellite System (NNSS) satellites have been used to study phase scintillation and ionospheric irregularities at northern polar latitudes. The observations were made at Ny Alesund, a station in the vicinity of the daytime cusp and under the polar cap on the nightside. The analysis concentrates on data obtained over an extended period when more than 7000 satellite passes were recorded to provide estimates of the σф phase scintillation parameter for some 300,000 data intervals. The results show a marked difference in scintillation and irregularity occurrence with season. During the summer months the main feature is a maximum around magnetic noon, apparently associated with irregularities in the cusp region. In winter, irregularities are found at all times of day though there is a noon maximum that extends in a latitudinal belt into the afternoon and evening. The premidnight sector shows the highest occurrence levels in the polar cap. Investigation of signal fading rates indicates that doppler shifting resulting from irregularities convecting rapidly towards the station in the premidnight polar cap may contribute to the occurrence patterns. Under quiet geomagnetic conditions during summer the irregularities form a belt essentially in the morning sector, following the auroral oval. There is expansion to lower latitudes with increased geomagnetic activity and increased occurrence in the afternoon. In winter, irregularities can be seen at all hours even under quiet conditions, with the afternoon and evening showing greatest occurrence at times of geomagnetic disturbance. The pattern of scintillation occurrence with respect to the station indicates that the irregularities may be in the form of field-aligned sheets of low axial ratio. In contrast to observations of L shell confined sheets at auroral latitudes there are indications from the current work that the irregularities are stretched in the antisunwards convective flow across the polar cap.
TL;DR: In this paper, mean winds and tides have been measured with the new MF radar at London, Ontario using a 1-year continuous set of data, the mean winds are compared with a recent empirical model.
Abstract: Mean winds and tides have been measured with the new MF radar at London, Ontario. Using a 1-year continuous set of data, the mean winds are compared with a recent empirical model. The diurnal and semidiurnal tides are discussed, and these are compared with results from other similar midlatitude ground-based radars. There are interesting similarities and discrepancies, although we note that some of the initial comparisons use data from different years. The diurnal and semidiurnal tides are also compared with recent tidal models. Tidal characteristics are generally consistent with the models, but again some differences are also noted.
TL;DR: In this article, a novel approach to AGW retrieval is introduced, which is based on realistic modeling of AGW/TID physics, constrained by TIDs measured by the incoherent-scatter technique.
Abstract: With the advent of the era of thermosphere/ionosphere weather modeling there is an increasing need to routinely acquire information on important regional-scale phenomena in the upper atmosphere like atmospheric gravity waves (AGWs) in the thermosphere and their ionospheric signatures, the traveling ionospheric disturbances (TIDs). AGW information is especially desirable in view of its role in the momentum and energy balance of the upper atmosphere. Measuring AGWs directly is difficult, however, so they are usually traced by the TIDs, which can be routinely observed by the powerful incoherent-scatter technique even in all fundamental ionospheric parameters. Thus an important problem, and the central concern of this study, is how to infer, as comprehensively as possible, AGW information from measured TIDs. This problem was tackled up to now by direct inversion of TID parameters into AGW parameters, but severe limitations, which we shall briefly discuss, are inherent in this approach. Contrasting with this, we introduce a novel approach to AGW retrieval: realistic modeling of AGW/TID physics, constrained by TIDs measured by the incoherent-scatter technique. This method is found to be capable of yielding comprehensive AGW information, that is, the full polarization and dispersion information. We applied the method to four TID events, which cover different seasons and solar activities during quiet magnetic conditions, from European EISCAT radar data. One-to-one comparisons of modeling results with experimental data are presented and discussed. These show the feasibility and scope of the method and its potential to discriminate between TIDs from AGWs and non-AGW sources. In view of the satisfactory results, we finally address the potential of the method for routine processing of incoherent-scatter data with respect to AGWs, which could provide valuable AGW information for other research.