Michael H. Reilly

Temperature dependence of the short wavelength transmittance limit of vacuum ultraviolet window materials—II theoretical, including interpretations for U.V. spectra of SiO2, GeO2, and Al2O3☆

Abstract The short wavelength transmittance limits of LiF, MgF2, CaF2 and BaF2 crystals have a temperature dependence which is consistent with a model of photoexcitation of localized excitons at momentarily thermally distorted regions of the lattice. The localized exciton is identified as a hole self-trapped in an F2− molecule + excited electron. The anomalous data for LaF3 is hypothesized to be impurity controlled. In order to identify the relevant transitions in SiO2 and Al2O3, a method is developed and employed for an LCAO-MO semiquantitative determination of valence bands, which seems generally advantageous for complex solids with important covalent bonding effects. The preceding model for fluorides is found to be plausible with extension to explain the Al2O3 data, while it is not so for the SiO2 data, which is interpreted to be due to phonon assisted interband transitions. In general, the lowest transitions in the u.v. spectra of SiO2, Al2O3, and, by inference, GeO2, which is structurally isomorphic to SiO2,are found to originate from oxygen lone pair orbital valence bands, and the u.v. spectra of amorphous and crystal specimens of these oxides are tentatively so interpreted, with the assistance of available experimental information.

Upgrades for efficient three-dimensional ionospheric ray tracing: Investigation of HF near vertical incidence sky wave effects

An earlier, three-dimensional ray-tracing program has been upgraded to include magnetic field effects, variable step sizes, improved propagation loss computation, and efficient homing and focusing computation algorithms. A combination of fourth-order Runge-Kutta and Adams methods, with error control and extension to variable step size, has been included. The program is applied to an investigation of high-frequency near vertical incidence sky wave (NVIS) effects. Model simulations of oblique ionograms are found to be consistent with experiment. Implications for NVIS geolocation are discussed.

Ionospheric specification from GPS data and the RIBG ionospheric propagation model

Global Positioning System (GPS) data for phase delay differential between Ll and L2 frequencies are analyzed by a model in which the combined effect of hardware and phase ambiguity terms is represented by a constant for each receiver-satellite path and the ionospheric contribution is represented by the RIBG model (ray trace through the combined ICED (ionospheric conductivity and electron density), Bent, and Gallagher ionospheric models). RIBG contains a detailed global empirical climatological ionospheric model of the electron density versus height profile (EDP) from 80 km up to the plasmapause, which is combined with a full three-dimensional ray trace propagation model. Discrete inverse theory (DIT) is used to fit the driving parameters of RIBG to about 2 hours worth of data from a single GPS receiver at various geographical locations. Validity is demonstrated by the ability of the GPS-updated RIBG model to predict independent measurements of (1) vertical total electron content (TEC) over the ocean out to substantial distances from a GPS receiver, often in excess of 2000 km at midlatitudes, and (2) the electron density versus height profile (EDP). Performance variation with magnetic latitude is noted. Flexibility in RIBG to adjust different driving parameters for different parts of the EDP, in order to obtain a best fit of GPS data, benefits the EDP prediction. Implications of this approach for GPS navigation and other radio system applications are discussed.

GPS data analysis near Puerto Rico for World Day campaigns

Experiments near Puerto Rico around World Ionosphere Days in June 1998 and September 1999 were carried out to develop and test techniques for nowcasting and forecasting ionospheric electron density and space weather during quiet and disturbed geomagnetic conditions. We investigate the use of GPS data for this purpose. We use the Raytrace/ ICED-Bent-Gallagher (RIBG) ionospheric model, line-of-sight propagation, and the assumption of constant differential hardware biases to model two-frequency GPS data. Discrete inverse theory is used to extract model parameters from the data. We avoid the traditional simplifying approximation of a fixed-height spherical shell model for mapping slant to vertical TEC. We show that our method is practical, provides accuracy superior to the traditional methods, and is robust against geomagnetic disturbances.

Updated climatological model predictions of ionospheric and HF propagation parameters

The prediction performance of ionospheric conductivity and electron density (ICED), RADAR C, and Ionospheric Communications Analysis and Predictions Program (IONCAP) climatological ionospheric models is evaluated for different regions and sunspot number inputs. With conventional sunspot numbers, based on the observed solar 10.7-cm flux, ƒ0F2 prediction errors at Boulder for a particular month are no less than 18%. Use of the U.S. Air Force Global Weather Center (AFGWC) global sunspot number SSNE, extracted from ionosonde data for use with the ICED model, gives 15% errors. A dramatic improvement involves the use of single-station ionospheric data to update the driving parameters of an ionospheric model for near-real-time (NRT) predictions of ƒ0F2 and other ionospheric and HF circuit parameters. For middle latitudes the improvement extends out thousands of kilometers from the update point to points of comparable corrected geomagnetic latitude. At very large distances an update scheme which exploits a strong global dependence on geomagnetic activity shows promise. For the purpose of NRT predictions of the entire electron density profile the use of two distinct effective sunspot numbers for the bottomside, coupled with one or more model parameters for the topside, is suggested for model updates from a single station.

Virtual heights for oblique path calculations

The propagation model of the Ionospheric Communications Analysis and Predictions Program (IONCAP) program, which is designed for fast throughput calculations in the prediction of high-frequency sky wave system performance, makes use of virtual heights, as calculated from known height profiles of the electron density. Use of these for oblique path calculations relies on the well known flat Earth theorems of Breit-Tuve and Martyn, with additional curved Earth correction terms. These approximate corrections are examined in the light of exact results from other models, and alternative strategies are discussed.

Thin-phase screen estimates of TID effects on midlatitude transionospheric radio paths

The thin-phase screen model for ionospheric irregularity perturbations to transionospheric radio propagation is redefined. It is argued that the phase screen normal should be along the line of sight (LOS) between a receiver on the ground and a space transmitter, rather than in the zenith direction at the point of intersection with the LOS, which is traditional. The model is applied to a calculation of TID strength thresholds for the occurrence of multipath and scintillation. The results are in sharp disagreement with the traditional model, which predicts thresholds lower by an order of magnitude in typical cases. Midlatitude observations of TID strengths are reviewed, and it is found that multipath thresholds can be exceeded under one or more favorable circumstances, which include frequencies below about 100 MHz, low elevation angles, winter, night, atmospheric gravity wave velocity near the magnetic field direction and away from parallel with the LOS, and low solar activity.

Sounder Updates for Statistical Model Predictions of Maximum Usable Frequencies on HF Sky Wave Paths

A method is presented for the short-term prediction of maximum usable frequencies (MUFs) in a large communications region. It is shown how ionospheric measurements from a network of ionospherit sounders can be used to update sunspot number or solar 10.7 cm flux inputs to a climatological MUF prediction model, MINIMUF in this case, which is then used to predict MUFs on paths throughout the region. Analysis of mid-latitude oblique-incidence sounder data sets indicates the advantage gained from single-path sounder updates of flux for MUF predictions on adjacent paths. Under specified conditions a further dramatic improvement in MUF prediction accuracy is found from spatial interpolation of sounder-updated flux values. MUF prediction accuracies within 0.5 MHz are obtained for fairly modest sounder network deployments, in which the sounder midpath point distributions and updating frequency satisfy particular requirements.

Global Considerations for Utilization of Real-Time Channel Evaluation Systems in HF Spectrum Management

Real-Time Channel Evaluation (RTCE) devices are coming into increased use in modern management of HF communications systems. The earliest techniques used in channel evaluation exploited vertical incidence pulse sounders (VIS) and these devices are still being used for some applications. An oblique incidence sounding technology has also been developed employing both pulse and chirp waveform approaches. There are a variety of RTCE devices which are identified in the paper but the major emphasis is placed on the OIS chirp sounder. Using this device as a canonical channel evaluator, the implications for global, theater, and local HF resource management may be addressed. As one might suspect, there are both advantages and disadvantages which may accrue from construction of a sounder network. Issues include: data applicability, data collection and dissemination, network size and cost, network ECM vulnerabilities, optimum network architecture, and system component reliabilities to name a few. These issues are outlined in the paper.