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Background HIV transmission through vertical route can be reduced to a large extent with combination of medical interventions. Apart from maternal HIV status several other epidemiological attributes determine this transmission dynamics. Objective: The objective of this study was to identify various associated factors that determine and modify the risk of HIV transmission from a mother living with HIV to her child. Materials and method: A retrospective cohort-study was conducted with 518 HIV-positive pregnant women with delivering live babies between April 2016 – September 2018. The HIV status of the children was ascertained with polymerase chain reaction. A number of socio-demographic and medical attributes were compared between HIV-positive (41) and HIV-negative babies (477) using bivariate and multivariate methods to identify disease modifying factors. Results Maternal HIV detection during the postnatal period (AOR = 11.2; 5.2 – 23.8), low birth weight (AOR = 2.7; 1.2 – 5.9), and vaginal delivery (AOR = 2.8; 1.01 – 7.7) were significantly associated with vertical transmission of HIV. Lower duration of maternal antiretroviral treatment and higher maternal age (>25 years) were also associated in bivariate analysis. Conclusion The battery of PPTCT (Prevention of Parent to Child Transmission) interventions should be tailored in such a way to address all the epidemiological attributes influencing vertical transmission.

This paper presents numerical analyses of a plasma-column antenna using the finite-difference time-domain (FDTD) technique. We have simulated simple plasma-antenna configurations (monopoles) to demonstrate some of their properties. We have evaluated the field distribution around the plasma antenna, current distribution along the plasma column, radiated power, antenna pattern, and antenna impedance of the plasma column antenna. It has been demonstrated that the plasma antenna can operate with characteristics similar to a copper antenna. Furthermore, the alteration of the plasma properties enables the dynamic reconfiguration of the plasma antenna. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 252–259, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20959

Using some recent Plasma drift data at Arecibo, we present evidences of Electric Field (E) changes induced by sunrise and sunset transitions at both local and conjugate regions. Field perpendicular plasma drifts in the F region are used as unambiguous diagnostic of the Electric Field. We present evidences of a downward velocity started around the conjugate sunrise and reaching minimum around the local sunrise, when it turns around and merges with normal daytime behavior, controlled by the dynamo region. During sunset, the upward drift increase at local sunset and shows a dip at the conjugate sunset, when it turns around. Afterwards, the F region is isolated and behaves in regular night time fashion. The entire behavior can be explained by coupled behavior of the E and F region, where the terminator line switches the coupling and affects charge separation at the boundaries.

Having ionospheric electron density distributions as a function of height, latitude, longitude, and time under different conditions is essential for scientific, technical, and operational purposes. A satellite-based, swept-frequency, HF sounder can obtain electron density profiles on a global scale. We are developing a new generation HF sounder that employs recent developments in technology, electronics, and processing capabilities. It will provide global-scale electron density distributions, contours of fixed densities, maps of ƒoF2, hmax, etc. It will allow us to map irregularities, estimate anomalous propagation and conditions for ducting, determine angles of arrival, etc. It will also be able to perform various plasma diagnostics and, because of new flexibility, will be programmable from the ground to perform a variety of experiments in space. Need for such a system exists through the Department of Defense and several civilian agencies. Some of the novel features of the system include software-based design, direction of arrival estimation and synthetic aperture radar-type operation, onboard processing, and reconfigurable and flexible architecture with multimission capabilities.

Complete descriptions of spatial and temporal distributions of the ionospheric plasma are necessary for scientific understanding and for several practical applications involving communication, radar, navigation, etc. Ionospheric diagnostic techniques, on the other hand, provide spatially and temporally incomplete information. A framework for data fusion is needed which would be self-consistent in terms of various input data and would also provide spatial-temporal extrapolations. Incorporation of a physical ionospheric model as a framework for assimilation of ionospheric (ionosonde) data was demonstrated by Ganguly [1990, 1991]. The free parameters of the physical model were adjusted using the observational data, and then the adjusted model was used to predict the state of the ionosphere at a different place and time. Most recently, the approach has been extended to incorporate total electron content (TEC) observation from ground and/or satellite-borne receivers and beacons, as well as data from a variety of other sources such as oblique and vertical sounders, radar, radio propagation, incoherent scatter, optical observations, in situ measurements, etc. This TEC-based reconstruction utilizes novel tomographic techniques and allows integration of any satellite constellation including GPS. Data from other sensors are integrated into the complete system, and the fusion is performed in conjunction with some physical/empirical ionospheric model. This provides a self-consistent four-dimensional ionosphere. The system can accept any reasonable ionospheric model and any amount of diverse input data covering a wide range of space, time, and observational methods.

Measurements of total electron content of the ionosphere by recording group delay or differential phase of transionospheric satellite signals have been used for reconstruction of ionospheric profiles. Integrated electron content measurements contain little information of ionospheric height distribution and require precise observations. These measurements are difficult for VHF and higher frequencies and require reasonably sophisticated instruments. Faraday rotation measurements, on the other hand, require relatively simple instruments and were widely used throughout the world. Faraday rotation of HF, VHF, and lower UHF band signals contains an additional term, e.g., a known function of the magnetic field. The functional form depends on the ray path and orientation of the magnetic field and in general is a complex quantity. Since the magnetic field distribution in the Earth's ionosphere is known with very high precision, this information can be used to obtain improved reconstruction of the ionospheric profiles. The magnetic field components have a strong dependence with the height, and this provides additional information regarding the ionospheric height retrieval. We have performed simulations using these techniques, and we present results using both simulated data as well as Faraday rotation data measured from several stations. We compare the sensitivities and reconstruction of ionospheric profiles using group delay and Faraday rotation techniques and demonstrate the improvements with the use of Faraday rotation data. The most difficult (in terms of radio propagation) regions, i.e., those near the equatorial regions, are simulated, and data from several stations, i.e., those near Calcutta, India, as well as those from several stations in Africa, are used. For the first time, we present the results of the reconstruction of the ionosphere using both simulated and observed Faraday rotation data. Large amounts of Faraday rotation data have been acquired during the 1960s and 1970s. These data contain a history of the ionosphere. The present technique would allow retrieval of ionospheric profiles over extended regions and could allow inferences to be drawn regarding the long-term changes in the ionosphere. The technique can also be incorporated in future satellite beacons, and the combination of group delay and Faraday rotation techniques will allow us to obtain superior reconstructions.

The current collection of magnetospheric observations consists almost entirely of in situ measurements at isolated spacecraft positions. A novel remote sensing technique for simultaneously imaging the plasma density N and the magnetic field B is proposed. The imaged region of the magnetosphere can be several Earth radii in extent. Simultaneous measurements of both group delay and Faraday rotation of radio waves propagating through the magnetosphere yield both total electron content (TEC) and Faraday rotation angle, which is the integral of electron density weighted by some known function of the magnetic field and ray path. These data can then be inverted to obtain estimates of the three-dimensional spatial distribution of N and B. The geometry of the ray paths determines the number of components of B that can be resolved. Simple multisatellite simulations and common data processing techniques introduce this new application of tomography: Magnetospheric TEC measurements yield a map of N, which is then combined with Faraday rotation angle measurements to produce a two-component map of B in the plane of observation. The successful tomographic reconstructions provide a straightforward illustration of the practicality of using this technique to map the magnetospheric plasma density and magnetic field in two or three dimensions. Furthermore, simultaneous images of N and B are of great topical interest to geospace science because of their direct and immediate relevance to current questions regarding magnetospheric structure and dynamics.

An extensive series of incoherent scatter studies of the ionospheric D-region was carried out at the Arecibo radar facility during 1978 and 1979. They included several full-day sequences of electron density measurements over a range of altitudes, and also included a sequence during the serendipitous occurrence of a large solar flare. For the solar flare event simultaneous data on solar X-ray fluxes in several wavelength bands were available from the GOES-2 and ISEE-3 satellites.

[1] Accurate characterization of ionospheric parameters such as total electron content (TEC) and scintillation (signal fluctuation due to ionospheric irregularities) is critical to all users of GPS, whether the ultimate goal is measurement in navigation, geodesy, ionospheric, or atmospheric studies. Improved absolute TEC measurement accuracy is demanded by many global ionospheric characterization schemes, where small errors can be magnified in 3-D tomographic profile reconstructions. We present research showing that there are three errors, or biases that typically result from characterizing TEC with GPS receiver data. These biases are (1) estimation, instead of measurement of receiver differential code bias (DCB); (2) ionospheric divergence of pseudorange-code-derived TEC resulting from code smoothing; and (3) delay of pseudorange TEC as a result of code smoothing. We present results of ionospheric data collected with a receiver that mitigates these biases to demonstrate the utility of improved accuracy, particularly for ingestion into tomographic reconstructions, but also for conversion from slant to vertical TEC.

Plasma's conductive and dielectric properties are discussed in this paper. Plasma has properties very different from those of metallic conductors. In physical experiments, the flexibility of the apparatus is often limited. For example, if one wishes to experiment with different dimensions and shapes of plasma columns, those must be built beforehand. Numerical simulation can ease these difficulties, serving as a tool to evaluate the potentials of plasma antennas at low cost and with flexibilities in selecting various complex geometries. It is also useful for analyzing the antenna characteristics by varying plasma properties such as the dimensions of the column, the plasma density, the collision frequency, etc. There are many different ways to simulate the electromagnetic interactions in plasma. Two major schemes that are used in FDTD simulations are the direct integration (DI) and the recursive convolution (RC) methods

We present a novel multiband antenna design for GPS applications at three different frequencies (L/sub 1/-1575.42MHz, L/sub 2/-1227.60MHz, and L/sub 5/-1176.45MHz). The future GPS system will introduces an L/sub 5/ capability, allowing the possibility of extended services using the new GPS signal. The designed antenna employs the aperture-coupled stacked patch configuration and achieves improved performance in terms of bandwidths, axial ratios, cross-polarization rejection levels and multipath rejection characteristics. To mitigate the multipath interference, a novel structure (vertical choke ring) using multiple metallic disks with center cylinders of different diameters was introduced. The use of the vertical choke ring helps achieve very low backlobes, without the need to increase the diameter of the ground plane, by suppressing the surface waves and the edge diffractions. The designed antenna has been fabricated and tested. The measured performance of the designed antenna is presented in the paper.

In this paper we present two novel multiband antenna designs for multiple frequency applications. The first of these, which is based on fractal concepts, is a Sierpinski gasket type of fractal configuration, printed on a dielectric substrate backed by a ground plane. We propose a novel approach to enhancing its multiband performance, in terms of impedance matching characteristics as well as radiation patterns, at three operating frequencies. The second design combines a rectangular microstrip patch with a cylindrical dielectric resonator, and operates at multiple frequencies. Specifically, we describe a stacked configuration of a dielectric resonator antenna on a microstrip patch for three frequency bands. Circular polarization is obtained by utilizing four coaxial feeds in phase quadrature. This configuration is found to exhibit good cross-polarisation rejection characteristics.

[1] Scintillations are caused by ionospheric irregularities and affect the amplitude, phase, dispersion, and related parameters of GPS signals. Both L1 and L2 are affected in a somewhat uncorrelated fashion. Adequate understanding of the effects that ionospheric scintillation has on GPS signals is essential in order to produce a GPS receiver that is immune to the degradation of signal caused by these scintillation-related effects. Access to the internal workings of commercial GPS receivers is not provided to users. However, in order to understand, control, and mitigate scintillation effects, it is necessary to have access to tracking loops and associated components such as local oscillator. The software GPS receiver developed by the Center for Remote Sensing, Inc. allows the user to access its internal workings; hence facilitating advanced development of GPS receivers under different conditions. In this paper we present the results of analysis of GPS signals under scintillating conditions. Raw signals under conditions of ionospheric scintillation at Ascension Island have been processed using our software GPS receiver in order to derive the scintillation parameters. The receiver has been configured to provide stable operation during scintillation. We describe the receiver architecture, particularly the portion involving the tracking filters and related components, and demonstrate their performances under scintillating conditions. The software-based approach used by us allows receiver implementations which are suitable for ionospheric monitoring as well as for reliable operation during scintillating conditions. The improved architecture may be used for several other applications.

We present a novel conformal multiband antenna design, based on fractal concepts, for application at three different frequencies in this paper. Although a whole host of conventional fractal type monopole and dipole configurations have been designed in the past, they are not well suited for conformal applications. In this paper we describe a Sierpinski gasket type of fractal configuration, printed on a dielectric substrate backed by a ground plane, and propose a novel approach to enhancing its multiband performance in terms of impedance matching characteristics as well as radiation patterns at three operating frequencies.

A two-layer configuration of the Sierpinski gasket patch antenna is investigated, and a new technique for improving the matching characteristics and the radiation pattern is proposed. We demonstrate that the original two-layer design does not exhibit the desired performance from the point of view of the matching and radiation pattern, and show how this can be modified to improve its performance.

The practical feasibility of detection and characterization of underground structures using electromagnetic sources in the ELF/VLF range is being investigated. For this, we investigate issues associated with sensors. One important objective is to examine the feasibility of using magnetic field measurements alone. We have developed unique 3-D e.m. models which can adequately cover arbitrary geometries, inhomogeneous ground, various excitation schemes and most importantly can handle a wide range of frequencies. We are currently performing numerical simulations using the 3-D e.m. code. We have established the need of 3-D modeling and there are no other codes which allow complete solutions for e.m. problems without any approximations. The results of these simulation will provide the experimental requirements. It is impractical to measure E fields from a stand-off distance. We are pursuing simulations to see if only the magnetic field measurement could provide the desired information. We have developed the most sensitive magnetic sensors which can be used from a remote location (such as from UAV). We plan to use these sensors for practical demonstration of magnetic-field-only measurements.

The effects of high-power RF fields generated in the context of a strategic defense system on the atmosphere and ionosphere are discussed. The significance of density perturbations, electron accelerations, IR emissions, optical emissions, UV emissions, generation of RF noise field, and air breakdown due to the fields are discussed. The impact of these physical changes on communication, jamming, surveillance, and tracking are noted.

Measurements of electron density at seven D region altitudes were made with the Arecibo radar during a class X solar flare on August 18, 1979. Measurements of solar X ray fluxes during the same period were available from the GOES 2 satellite (0.5–4 A and 1–8 A) and from ISEE 3 (in four bands between 26 and 400 keV). From the X ray flux data we computed ionization rates in the D region and the associated chemical changes, using a coupled atmospheric chemistry and diffusion model (with 997 chemical reactions and 19 vertical levels). The computed electron densities matched the data quite well after we had adjusted the rate coefficients of three reactions, namely, O−2 + O2(1Δ) → e + O2 + O2, O+2 + O2 + M → O+4 + M, and O+4 + O → O+2 + O3. We also discuss the hierarchies among the many flare-induced chemical reactions in two altitude ranges within the D region and the possible effects of two separate isomeric forms of O+4.

The width of the spectra of signals incoherently scattered from 85 to 93 km at Arecibo shows significant variations during sunrise and sunset periods. After sunset, the width decreases substantially for all these heights, indicating a 20–30% decrease in the quotient of neutral temperature to neutral number density T N ). Some increase in effective positive ion mass could also be possible. Somewhat opposite situations are evidenced during sunrise for heights above 87.5 km. At 85 km, on the other hand, the width decreases during sunrise. This is due to the disappearance of negative ions, which were formed during the night hours.

Seasonally averaged quiet-day F region ionospheric E × B drift observations from the Millstone Hill, St. Santin, Arecibo, and Jicamarca incoherent scatter radars are used to produce a model of the middle and low-latitude electric field for solar minimum conditions. A function similar to an electrostatic potential is fitted to the data to provide model values continuous in latitude, longitude, time of day, and day of the year. This model is intended to serve as a reference standard for applications requiring global knowledge of the mean electric field or requiring information at some location removed from the observing radars. This article contains supplementary material.

The Arecibo 430 MHz incoherent scatter radar was used to observe the diurnal variation of electron concentration in the 6–100 km altitude region on 14 August 1977. This report is an evaluation of the technique and includes a fairly complete discussion of errors involved. Although interference remains a serious problem, the results are useful down to about 60 km altitude and a minimum density of about 50 electrons cm−3. Characteristic statistical plus systematic errors indicate that an observed 100 electrons cm−3 value actually lies between 50 and 180 electrons cm−3 assuming no interference. Observed variations of electron concentration include not only those due to basic solar control but also at least one wavelike feature characterized by phase shift with altitude. These results should prove particularly useful as constraints to time dependent models of the D-region chemistry.

For the first time a ground based technique, i.e. incoherent scatter observation of the mesospheric spectra has been utilized in identifying the ‘transition height’ of simple molecular ions to complex cluster ions around 80–90 km in the D-region. This transition height also matches with the electron density ledge. A substantial diurnal variation of this height is observed. The transition height varies by about 10km during the course of a day, the lowest being 80 km, near noon. There is also a strong likelihood that both the neutral temperature and effective positive ion mass vary during the course of a day.

The results obtained with ionospheric modification experiments over the three years preceding the XXI General Assembly of URSI in 1984 at Florence are reviewed. The topics discussed include weak electromagnetic sidebands observed using a single pump frequency, the HF-induced plasma line at 3.175 MHz and its similarity to the plasma lines observed using higher HF frequencies near Tromso, the HF-enhanced plasma line observed with the 50 MHz radar, the HF-induced plasma line with a doublehumped spectrum below threshold. HF-induced plasma line spectra with height discrimination using a new technique, the HF-induced plasma line and ion line spectra obtained with two pumps differing in frequency by a few kHz, narrow features such as the OTSI in the HF-enhanced plasma line and ion line spectra observed by a new technique, the use of such narrow features for measuring the line-of-sight electron drift velocity, the discovery of a radical qualitative change in the spectrum of the HF-induced plasma line as the HF power (CW) is increased or as the duty cycle is changed while pulsing, observations of the temporal development of the enhancement of the thermal plasma line at the peak of the F 2-layer by electrons accelerated during ionospheric heating studies of artificial density stratification resulting from the standing wave nature of the heating wave and strong electromagnetic sidebands generated by two Powerful HF radio waves differing from each other by some tens of Hz. Theoretical work on soliton formation and on VLF generation by HF heating is briefly mentioned, as well as experimental studies of self-focusing by observing the scintillation of extraterrestrial radio sources, direct conversion and studies of short scale field-aligned irregularities by VHF radar backscatter during ionospheric modification.

Incoherent scatter spectral observations at Arecibo have been extended down to 59 km. For the first time vertical velocities in the mesosphere have been observed using this technique. The velocities unambiguously reveal a wavelike structure growing with height and varying in time. Above around 75 km, the vertical wavelength is around 16 km and the time period about 4 hours ± 20 minutes. Vertical velocities near 90 km were as large as 1 ms−1 and the apparent vertical energy flux was reasonably constant above 75 km. Below 75 km the energy flux was substantially lower, suggesting a mechanism where the gravity waves acquire significant energy from the diurnal tidal component trapped below the mesopause. The estimated adiabatic density and temperature changes near 90 km, estimated using simple idealized theories, were around 11%. The observed changes in spectral width were also found to be in harmony with estimated density and temperature changes to within 25%.

Incoherent scatter observations of mesospheric spectra at Arecibo have shown the presence of zones of large negative ions in the altitude range 85–90 km. These large negative ion regions are somewhat sporadic in nature and are observed on roughly about one third of the evenings. On at least one occasion, somewhat similar observations have been made during the noon period.

(1970). A High-powered Equipment for Studying Absorption and Partial Reflections in the Lower Ionosphere. IETE Journal of Research: Vol. 16, No. 3, pp. 223-230.

When the ionosphere is illuminated with two strong HF signals, nonlinear mixing processes can take place near the region of reflection. The first direct experimental evidence of such processes occurring in ionospheric plasma are presented in this paper. The experiments were performed at Arecibo, where the HF ionospheric modifier was split to operate as two pure isolated and independent pumps over the frequency range of 3-8 MHz and the frequency difference between the two pumps was varied between a fraction of a Hz and several kHz. The purity of the trans- mitted signals was always ensured. Strong nonlinear effects were observed when the frequency difference was several Hz. Specifically, the HF signal from the ionosphere contained sidebands which were separated by multiples of the difference (Δf) between the two pumping frequencies. The first sidebands were typically 20-40 db below the pump signal. The strengths of the first sidebands increased, when the difference (Δf) between the two pumping frequencies decreased. Below about 5-10 Hz, the power in the first sidebands varied almost in Δf−n fashion, with n between 1 and 2. The power in the sidebands was also a strong nonlinear function of the incident pump field strength, showing the presence of a threshold. It is suggested that studies of such nonlinear mixing processes would be valuable both for diagnostic and practical usage of ionospheric plasma.

Controlled ionospheric modification can be used for disrupting as well as facilitating C3I systems. After briefly describing the results achieved with the present day ionospheric modification facilities we present a scenario for the generation of strong ionospheric modification and the impact of such a situation to the communication community.

Plasma line observations obtained from incoherent radar backscatter have been used as a ground-based method for deriving information about the size and anisotropy of the ionospheric photoelectron fluxes. In the past, data interpretation has been confined to altitudes above the F2 peak. Measurements below the F2 peak consistently show an anisotropy in the ratio of the downshifted to upshifted amplitudes of 20–50% when it is generally assumed that diffusion processes dominate. We describe calculations of the plasma line intensity which use a multiangle multienergy calculation of the photoelectron distribution function. The calculated electron flux exhibits a small low-altitude low-energy anisotropy which is reflected in the plasma line measurements. Given anisotropic elastic electron-neutral cross sections, the flux anisotropy arises when the local mean free path is of the order of the local scale height. The net effect is conversion of a spatial density inhomogeneity into a velocity distribution anisotropy.

For the first time, vertical electron drift velocities in the F region of the ionosphere have been directly determined using the Doppler shift of the electron component of the incoherent-scatter spectra. Simultaneous measurements of ion drift velocities were also made. A large difference between the electron and ion velocities of the order of 100 ms−1 with corresponding currents of 10−5 Am−2 was observed at sunrise. The most likely source of these currents appears to be the action of the local thermospheric wind on the large conductivity gradients associated with the sunrise line.

Plasma drift measurements taken at Arecibo during the solar minimum period of 1974-1977 are examined to determine their average behavior in the E, F2, and F regions. The drifts are generally diurnal in the E region and semidiurnal in the F1 region. These lower thermospheric drifts are set up by polarization fields generated by propagating and in situ atmospheric tides. In the F region the diurnal component is more pronounced, especially in the zonal direction. The magnitude of the drifts is of the order of 25-30 m/s (or 1 mV/m). Enhanced geomagnetic activity appears to increase the westward component of the drift in agreement with the theory of the ionospheric disturbance dynamo (Blanc and Richmond, 1980). Nighttime drifts appear to be at least partly explained in terms of polarization fields.