2-FREQUENCY CORRELATION-FUNCTION OF THE SINGLE-PATH HF CHANNEL - THEORY AND COMPARISON WITH THE EXPERIMENT

The two-frequency mutual coherence of HF electromagnetic field fluctua tions caused by ionospheric irregularities in an oblique radio path is studied. The single scattering approximation is first developed and t hen extended to stronger field fluctuations by applying Rytov's approx imation. Of considerable interest for wideband HF communication applic ations is the two-frequency, two-time correlation function of the chan nel [u(omega,t(1))u(omega+Omega,t(1)+t)], where u(omega,t) is the com plex amplitude of the radio wave transmitted at a frequency omega, mea sured by a receiver at time t. Our results show that this particular q uantity behaves as though there were no diffraction effects (no Fresne l-filtering effects). Thus the correlation time to is close to Lambda / nu regardless of the ratio between the irregularity size and the Fre snel length. Here Lambda is the irregularity size and nu is the compon ent of the drift velocity perpendicular to the ray path. Typical scale s for the two-frequency mutual correlation function are studied, and s imple physical interpretations are developed. For example, the correla tion bandwidth Omega(c), is determined by the condition that the rays connecting the transmitter and receiver at omega and omega + Omega(c), are separated from each other (in the vicinity of the reflection poin t) by a distance of the order of Lambda. A procedure is described whic h allows the size of irregularities and components of plasma drift to be estimated from one of the mutual correlation functions and from the autocorrelation function. This procedure was applied to measurements from the University of Illinois sounder (transmitter is located in Pla tteville, Colorado, receiver in Urbana, Illinois). In the examples con sidered, irregularities with a size of the order of a few hundred mete rs drifting with a velocity 20-100 m s(-1) can explain the fading. The observed magnitude of fading is consistent with an rms irregularity f luctuation of the order of a few tenths of one percent.