A method for estimating the spatial coherence of mid-latitude skywave
propagation based on transionospheric scintillations at 35 MHz
Abstract
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. This was done using an array of four
antennas in southern Maryland called the Deployable Low-band Ionosphere
and Transient Experiment (DLITE). Two of the antennas within a
~350-m north/south baseline were used to monitor 35-MHz
intensity variations of two bright cosmic sources, Cynus 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
~0.5-0.9, but occasionally near zero. A method was
developed to estimate the expected cross-correlation amplitude based on
the 35-MHz scintillations. This method assumes the case of weak
scattering, which is generally appropriate for mid-latitudes, and that
the irregularity distribution follows that of the background electron
density. These calculations typically captured the day-to-day variations
in spatial coherence quite 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.