A proposed approach for increasing the azimuthal resolution in HF radar

A method is proposed for improving the azimuthal resolution of HF radar several times beyond the limitation due to ionospheric scattering. Azimuthal resolution improves directly with array size, in accordance with standard array theory, until the beamwidth shrinks to a few degrees. Further increase in array size does not produce a dependable resolution improvement. In radio parlance, the signal is said to be decorrelated across the aperture. This "decorrelation" is due to scattering by the nonhomogeneous ionosphere. The scattered energy accounts for only a portion of the received wave. The remainder, an almost specular component, not only has a very much narrower angular distribution of its energy, but appears to have a much longer (perhaps 100 times) time constant of fluctuation. This difference in fluctuation rate permits separation of the two components by filtering the received radar echo. The decorrelating component is thus reduced, and the available azimuthal resolution, i.e., the resolution limitation due to scatter, correspondingly increases. When literature data are applied to the theory, a several-fold increase in available azimuthal resolution is predicted.