Title: Doppler shift and spread study for ionospherically propagated signals
Description: Modern, High Frequency (HF) communication techniques, such as spread spectrum and frequency hopping, require precise signal frequency information. The predominant HF propagation path is via the ionosphere, which often produces Doppler frequency shift and spread. This study examined the frequency spectra of selected HF signals traversing short and long mid-latitude paths and one high- latitude auroral zone path. Signal amplitude and Doppler shifts and spreads observed show diurnal, carrier frequency and ionospheric path dependencies. Higher frequency signals experienced more Doppler shift, especially during the daytime. Spectrum spreading was more pronounced at night and was affected by multiple reflections, the auroral oval and field-aligned ionization. Additional signal observations are needed to cover seasonal variations, disturbed ionospheric conditions and solar cycle variations. The impact of Doppler shift and spread on wide-spectrum HF communications also needs to be examined.