Experimental and Computational Methods to Analyze Complex Doppler Behavior of Ionospherically Induced Doppler Shifts on HF Signals

The HamSCI community has been studying apparent frequency shifts in the reception of HF skywave signals from radio station WWV in Ft. Collins, CO. Causes for frequency shifts in the received signal are recognized as complex and varied. Leading candidates are Doppler shifts resulting from dynamic changes in refraction layer height and the behavior of modes at incidence angles at the cusp between escape into space and refraction back to earth. Observations have shown the most radical frequency disturbances occur during the diurnal transitions between night and day, with the morning transitions exhibiting more radical behavior than evening. Other changes in solar radiation such as passage of an eclipse shadow or solar flares produce similar results. In all cases the frequency swings were found to follow the rate of change of propagation path length. Specific behaviors studied include mode splitting, where the Doppler shift diverges into multiple overtone-related tracks,  modes that abruptly manifest and disappear during the transition, and asymptotic behavior where Doppler tracks exhibit a rapid frequency change followed by extinction. This paper describes experiments and analytical procedures devised to better understand these phenomena that include 1. Development of a virtual height geometric model to relate path length to layer height, 2. The complimentary processes of calculating Doppler shifts from layer height changes and layer height changes from Doppler shifts, and 3. Analysis of intermittent low-Doppler shift modes and correlation with with ionosonde data to help identify E layer modes.. Plausible explanations for several aspects of observed frequency swings are postulated.