Probabilistic identification of high-frequency radar backscatter from the ground and ionosphere based on spectral characteristics

We present an analysis of the spectral characteristics of 1-hop HF radar ground scatter and 1/2- and 11/2-hop ionospheric scatter as measured by the Super Dual Auroral Radar Network. Our objective is to determine criteria that separate signals scattered from the ground and the ionosphere. We find that for both ground scatter and ionospheric scatter that the probability density function of backscatter Doppler velocity decreases exponentially with velocity, but with significantly different e-folding velocities for the two types of backscatter. We use this observation to separate the total probability density of Doppler velocity and spectral width into two component distributions. This process yields the posterior probability that a signal of given Doppler velocity and spectral width is ground scatter. The resulting criterion for classification of a particular signal as ground scatter, v < 33.1 m/s + 0.139w - (0.00133 s/m)w(2), significantly reduces the probability that a signal will be erroneously classified as ionospheric scatter, while only moderately increasing the probability that an ionospheric scatter signal will be erroneously classified as ground scatter. Finally, we validate the ground scatter probability function by demonstrating that the backscatter virtual height increases as expected with increasing probability of ground scatter.