Effects of Strong Scattering on Transionospheric Propagation

Abstract

A JAN12 1990Appro~d for pUbic releas; Cstrbtln is wianited. Approved for public release; distribution is unlimited. 01 12 072 NAVAL OCEAN SYSTEMS CENTER ABSTRACT PWl&mn 200 w ds). .This report presents preliminary results of an effort to model the effects of ionospheric electron density irregularities on transionospheric propagation. The focus of the report is on propagation at HF and low VHF, where strong multiple scattering is often encountered. A strong scattering model is therefore used to predict the effects on such parameters as the phase autocorrelation function, the spectrum of intensity fluctuations, and the fluctuation in angle of arrival. Reqrements for improvements in the model and directions for later work are discussed. PREFACEThis report presents the results of modeling the effects of electron density irregularities on transionospheric propagation. The objective of this effort has been to provide a tool for determining, in real time, the expected effects of such irregularities on the performance of a given transionospheric monitoring system. While these effects can take several forms, we concentrate here on the -ariation in the phase of the signal due to scattering caused by such irregularities.In this report, we shall be mainly interested in remote locating systems. Such systems rely on accurate measurements of the phase difference between separated receive antennas to provide estimates of apparent angular location (azimuth/ elevation). Fluctuations in this phase difference due to scattering caused by electron density irregularities result in a consequent fluctuation in the apparent location of the transmitter. System performance can be seriously degraded if the correlation in the phase fluctuation between antennas is low or the mean square fluctuation in phase is large. In any case, the accurate determination of the phase fluctuations provides an estimate of the error for a given location estimate and thus provides important information for the system user.Another feature that must be addressed under strong multiple-scattering conditions is the reduction of correlation in the intensity of the signal. Under such scattering conditions, defocusing by large scale* irregularities can cause deep small-period fades in signal strength. For sufficiently strong scattering, these signal fades may have a spatial extent on the order of the size of the locating system (tens of meters). This loss of signal makes the system unreliable at best and, in some cases, inoperable.This area of investigation is not completely new; there exists a large body of related work. What is different about this effort is the extension to low VHF and HF operational frequencies. Previous work has, for the most part, dealt with higher frequencies where, in general, the relatively simple single-scatter models are applicable. Also, at these frequencies, one can generally ignore the refractive effects of the background ionization. This simplifies the implementation of the models considerably.At lower frequencies, strong multiple refra...