A Method for Estimating the Spatial Coherence of Mid-Latitude Skywave Propagation Based on Transionospheric Scintillations at 35 MHz

The results of a study aimed at assessing the utility of transionospheric 35 MHz scintillation measurements toward cosmic radio sources for estimating the level of spatial coherence in high frequency (HF) skywave systems are presented. An array of four antennas in southern Maryland called the Deployable Low-band Ionosphere and Transient Experiment was used. Two of the antennas within a ∼350-m north/south baseline were used to monitor 35-MHz intensity variations of two bright cosmic sources, Cygnus A and Cassiopeia A. The other two antennas, which were within a ∼420-m east/west baseline, recorded the 7.85 MHz skywave from the CHU radio station near Ottawa, Ontario. These HF measurements were used to quantify the level of spatial coherence by measuring the amplitudes of the cross correlation of the two antennas' recorded voltages relative to the received power, which were typically ∼0.5 to 0.9, but occasionally near zero. An approximate scaling method was developed to estimate the expected cross-correlation amplitude based on the 35-MHz scintillations. This method assumes a single layer of irregularities at the reflection height that affects relatively small changes in electron density, the latter of which is generally appropriate for mid-latitudes. It also assumes that the irregularity distribution follows that of the background electron density. These calculations typically captured the day-to-day variations in spatial coherence well (correlation coefficient r ≃ 0.6) while only marginally reproducing hour-to-hour variations (r ≃ 0.2). Thus, this method holds promise as an economical and passive means to assess the spatial coherence expected for skywave propagation within a given mid-latitude region.