Simultaneous Measurements of Temporal and Spatial Phase Structure Functions of an HF Skywave Signal at Mid-Latitudes

This paper presents the results of data collected with an HF/VHF interferometer in southern Maryland of skywave propagating signals from WWV near Ft. Collins, Colorado and CHU near Ottawa, Ontario.  Using a four-antenna array with spacings of ~100-350 m near Pomonkey, Maryland, a four-hour test data collection was made of the 10 MHz signal from WWV in Feb. 2020.  Measurements of the temporal and spatial phase structure functions (SFs) from this collection were used to constrain the coherence time and length as well as the relationship between the two.  The temporal SFs indicated coherence times of ~1-4 s.  By scaling the temporal SFs to match the spatial ones, it was found that the ratio of coherence length (on the ground) to coherence time was about $\sim$200 m s^-1 until near sunset when this ratio became highly variable and was ~400-800 m s^-1.  A 30-day campaign in Oct. 2022 that used two of the four antennas to collect data from the 7.85 MHz skywave of CHU yielded similar results.  These showed the coherence ratio systematically increasing from ~150 m s^-1 at 17 UT (noon local time) to ~250 m s^-1 at 22 UT (17 local time).