MEASUREMENTS OF DIRECTION-OF-ARRIVAL OF HF SKY WAVE SIGNALS BY MEANS OF A WIDE-APERTURE ANTENNA-ARRAY AND 2 SUPER RESOLUTION DIRECTION FINDING ALGORITHMS

From measurements of the amplitude and phase of ionospherically propag ated HF radio signals received at several spaced antennas, the directi on of arrival of the signals in both azimuth and elevation can be dete rmined. A series of measurements has been undertaken using a wide aper ture antenna array on signals received from several European broadcast stations at distances of up to about 1400 km, and the directions of a rrival estimated by means of two superresolution algorithms; MUSIC and an iterative null steering method (an approximate maximum likelihood estimator) based on the IMP and DOSE algorithms. A detailed comparison of the performance of the two algorithms has been undertaken, and it is concluded that the null steering algorithm is significantly better than MUSIC at resolving the various propagation modes, particularly fo r short signal analysis periods.