Variability Analysis of High-Frequency Skywave Signals: Insights from Continuous Doppler Measurements between WWV and NJIT

The use of High Frequency (HF; 3-30 MHz) skywave signals is widespread, serving long-range communication needs for aviation, military, government, and amateur radio worldwide. This spectrum is not only crucial for communication, but also intersects with frequencies employed in remote sensing of the near-Earth plasma environment. Fluctuations in ionospheric total electron content and electron density can be understood by measuring the Doppler shift of HF signals due to their interactions with the ionosphere. Through continuous monitoring of Doppler residuals, valuable insights into the impact of geomagnetic activity on the terrestrial ionosphere, stemming from intricate processes within the coupled Sun-Earth plasma environment, can be gained. This study focuses on the analysis of long-term trends in a HF signal sampled at 1 Hz from a broadcast station (callsign WWV) transmitting at 10 MHz, situated in Fort Collins, CO. This radio link is facilitated by a V1 Grape low-IF receiver (callsign K2MFF), at the New Jersey Institute of Technology (NJIT) in Newark, NJ. By examining the difference between the Doppler-shifted received signal and the expected 10 MHz transmitted frequency, we can infer ionospheric variability. This body of work will demonstrate that Doppler residuals acquired during the daytime exhibit greater stability compared to those obtained at night, above the midpoint of signal travel. This night-day contrast persists across almost all 24-hour cycles, barring periods of atypical solar irradiance on the ionosphere, such as during solar flares and eclipses. Furthermore, our findings reveal a significant correlation between daytime measurements and Cauchy statistics, whereas nighttime measurements align more closely with a combination of exponential power and lognormal statistics. This distinction in statistical regimes based on time of day has prompted us to independently characterize long-term trends in the dataset by analyzing the medians of daytime and nighttime Doppler measurements. Lastly, a ray tracing analysis of this radio link was performed in order to evaluate the signal’s various modes of propagation through the Space-Atmosphere Interaction Region (SAIR), and to determine which layers of the ionosphere are responsible for measured Doppler shift across different times of day.