ANALYSIS OF DIRECTION-OF-ARRIVAL ALIASING FOR MF HF DOPPLER-SORTED INTERFEROMETRY MEASUREMENTS OF IONOSPHERIC DRIFT/

The determination of accurate direction of arrival (DOA) of echoes is paramount when performing Doppler-sorted interferometry (DSI) at MF/HF to measure the drift of ionospheric plasma. Important factors affecti ng the accuracy include inadequate signal-to-noise ratio (SNR) and coh erency of ionospheric echoes. As the SNR deteriorates, there is an inc rease in the errors in the phases measured at the individual antennas of an array, and this leads to ambiguities in DOA equivalent to direct ional aliasing. In this paper we model this effect, the effects of Dop pler-frequency aliasing, and the finite resolution of Doppler spectra. We thereby show the importance for vertical incident sounders of reje cting echoes with large off-vertical angles, and the choice of appropr iate signal-processing parameters for the accurate estimation of ionos pheric motions. As an example, we consider the situation for vertical incident ionosondes and show that a low SNR can produce spurious echoe s at relatively large zenith angles, which can, however, be avoided or minimized by the suitable choice of operating parameters.