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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

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.

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.

[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.

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.

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.

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.