The effect of the high latitude ionosphere on superresolution direction finding

The direction-of-arrival of HF signals (9.292 MHz) propagated on a 2100 km South-to-North path in the high latitude ionosphere were investigated on three geomagnetically quiet days in January 1996. A simple Beam forming and some superresolution direction finding (SRDF) algorithms, Loaded Capon, Iterative Null Steering (INS) and MUSIC, were utilised to measure the azimuth, elevation and amplitude of this signal. The validity of the SRDF Spread Maximum Likelihood (SML) algorithm was tested. The array aperture and shape was changed by using 12 and 8 antenna arrays. The performance of the five algorithms was compared under a wide variety of propagation conditions.;The effect of the integration time and the time over which the estimates are averaged on the measurements has been evaluated. For a fixed integration time value, an increase in the time/average resulted in a decrease in the standard deviation of the bearing estimates for all algorithms and array geometries.;Simulations of enhanced ionisation structures (patches) drifting across the great circle path agreed well with the observed azimuth and elevation swings. The simulated trajectories were often consistent with the direction of the convection flow.;The behaviour of experimental observations was reproduced by two simulations. In the first, the effect of one signal spread in azimuth was studied. In the second, two spread signals were modelled with a variety of azimuth spreads, angular separation and relative amplitude. While Loaded Capon and INS were able to resolve the two spread signals at angular separations close to and above the signal spread, MUSIC succeeded for unspread signals only.;For Loaded Capon and INS secondary energy arose which closely tracked the stronger signal - a feature commonly observed in the data. The theory of the effect of non-zero bandwidth signals on the two largest eigenvalues of the covariance matrix (Zatman, 1998) has been applied to the case of an angular spread source and numerical results from this compared to simulations.