Ionospheric Profiles for Predicting Nighttime VLF/LF Propagation, Determination of an Effective (Exponential) Model for Use in Design and Deployment of Resources for Communication and Navigation

Abstract

OBJECTIVE Establish a more accurate description of the lower ionosphere than heretofore available, to refine vif and If propagation prediction parameters. BACKGROUND To design and deploy resources to be used in the vif/If Minimum Essential Emergency Communication Network (MEECN) communication links requires a reliable knowledge of V radio signal propagation. Furthermore, assessment of the coverage of the Omega vlf navigation system requires reliable ionospheric profile specification. The Ti-Service Propagation Program was established by the Defense Communication Agency (DCA) to refine the propagation prediction parameters appropriate to vif and If. Many of the experimental data reported here were obtained as part of the Tri-Service Propagation Program. The final propagation analysis and profile assessments were carried out as part of the Omega validation project. Considerable theoretical work in vif and If propagation has been done since the early 1960s. As a result, a number of computer codes were generated and quite successfully applied to experimental data. However, vIf and If propagation strongly depends on the lower ionosphere (D region). Consequently the model of the ionosphere used in propagation predictions must be accurate. An error of 5 km in the effective height of reflection of a nighttime ionospheric model can result in 20-dB errors in signal amplitude calculations. The Tri-Service Propagation Program sought to establish a more accurate description of the lower ionosphere. APPROACH Improvement of the ionospheric model was approached from two separate directions. The first approach was to survey the literature for published profiles of electron density vs height. The lower parts of these profiles were then analyzed to determine the best fit to a profile in which electron density varies exponentially with height. A regression analysis was performed to determine the temporal and geographical variations of the exponential profile. The second approach was to acquire experimentally measured long-path propagation data, then to "fit" these data by calculating the field-strength amplitude along the appropriate propagation path through the use of a variety of ionospheric profiles. The profile was selected that gave the best agreement between calculations and measurements. Propagation Model The computer codes used to calculzte signal levels in this report employ state-of-the. art full-wave solutions to the anisotropic earth-ionosphere waveguide. The lower boundary has an arbitrarily adjustable conductivity. The upper boundary has an arbitrary distribution of electrons and ions with height. Through the use of a mode conversion model, the effects of rapid changes in the waveguide parameters along the direction of propagation are accounted for and the effects of elevated and arbitrarily oriented transmitters and receivers can be calculated. The principal unknown in the earth-ionosphere waveguide model, especially for nighttime propagation, is the ionosphere conductivity, which is in part a function of the electron