Ionospheric Variability's Impact on HF Propagation: Insights from Grape V1 Doppler Residuals and PHaRLAP Ray Tracing

This study investigates the impact of ionospheric variability on HF (3-30 MHz) skywave propagation, a critical aspect of long-range radio communication. Using a dedicated HamSCI Grape V1 low-IF receiver (K2MFF) at NJIT, we analyze long-term Doppler residual measurements of the 10 MHz WWV signal from Fort Collins, CO. Our observations reveal a strong correlation between stable daytime propagation and a Cauchy distribution of Doppler residuals, while sporadic nighttime conditions align more with a combination of exponential power and log-normal statistics. This pattern is consistent across our dataset, barring dates of atypical solar irradiance on the ionosphere, such as during high-class solar flares and solar eclipses. We compare these findings with PHaRLAP ray-tracing simulations using an IRI ionosphere model, allowing us to visualize the signal's ray path and pinpoint the ionospheric altitudes (100-400 km) responsible for observed features. Raytracing results attribute daytime signal stability to a superposition of multi-hop modes, while nighttime oscillations linked to TIDs are measurable due to 1-hop dominance. By examining diurnal variations in Doppler residuals, this work provides amateur radio operators with a better understanding of ionospheric dynamics and their impact on HF communication, ultimately contributing to improved prediction of propagation conditions and more effective use of the HF spectrum. Additionally, this work provides a foundation for interpreting real-time propagation data and understanding the effects of solar activity and ionospheric variations on signal quality and reliability for amateur radio communication.