Ionospheric radiowave propagation effects observed with a large aperture antenna array.

Abstract

The wavefronts of high frequency (HF) radio waves received after reflection from the ionosphere exhibit both spatial non-linearities and temporal variations which limit the performance of large aperture receiving arrays. The first objective of this investigation was to measure the phase and amplitude of ionospherically propagated signals at several widely spaced antennas in order to relate these parameters to the reflection process. From the amplitudes and phases measured at pairs of spaced antennas, the direction of arrival (DOA) of the signal in both azimuth and elevation was determined. Furthermore. by combining the DOA and reflection height measurements the transmitter location can be estimated from a single receiving site. The second objective of this study was to investigate the ability of the system to determine DOA and transmitter locations correctly. Two seven element antenna arrays were employed with maximum apertures of 1526 m and 294 m respectively. The associated multi-channel receiving and data logging equipment is described together with a pulsed sounding system employed for mode identification. Signals received from several European transmitters exhibited widely differing behaviour and this was interpreted in terms of their modal content. For predominantly single moded signals the observations indicate that the diffracted components normally contribute less than 10% of the received power, moreover the DOA varies in both azimuth and elevation by approximately 1-2° over time periods of several minutes. The use of the smaller array for DF and SSL applications is discussed in detail. In particular, the performance of the system was severly affected by multi-moded propagation. Techniques were developed for recognising periods of single moded propagation, when accurate measurements are to be expected. Good position fixes were obtained when measurements were restricted to these periods provided accurate reflection height information was also available