A High Frequency (HF) radar system comprising a skywave transmit channel and surface wave receive channel is studied. Simple analytic expressions for the resolution of this radar system are determined by considering the spreading of radar signals in Doppler and angle during the ionospheric propagation. The detection of ocean surface targets within the patch of ocean surface illuminated by the radar beam is then determined by comparing the radar cross section of the ocean patch with typical vessel radar cross sections. It is shown that a hybrid sky-surface wave radar can detect low-Doppler ships down to 30 dBm 2 cross section during the day and down to 40 dBm 2 at night, during calm ionospheric conditions. During periods of spread F ionospheric turbulence, where the temporal correlation time decreases by an order of magnitude, the corresponding vessel detection capability decreases by two orders of magnitude.Index Terms-over-the-horizon radar, high frequency radar, ocean clutter.

[1] The detection performance of high frequency surface wave radar (HFSWR) and high frequency over-the-horizon radar (OTHR) systems is heavily influenced by the presence of radar clutter. In HFSWR systems, the clutter has its origins in vertical-incidence ionospheric reflections, whereas in OTHR systems, the origin is Bragg backscatter from plasma structures in the auroral zone. This paper models the spreading of the radar clutter signal in the Doppler and angle-of-arrival domains that arises from forward-scattering effects as the radar pulse propagates through regions of ionospheric plasma irregularities. The models use a geometric optics approach to determine the power spectrum of the radar signal phase. This power spectrum is then used to simulate three-dimensional space-time-range radar data cubes. The accuracy of the models is tested by comparing the simulated data to measured data cubes. As an application, the data are then used to evaluate the performance of the newly developed fast fully adaptive (FFA) space-time adaptive processing (STAP) scheme to improve the extraction of target echoes from a clutter background.Citation: Ravan, M., R. J. Riddolls, and R. S. Adve (2012), Ionospheric and auroral clutter models for HF surface wave and over-the-horizon radar systems, Radio Sci., 47, RS3010,

Modulation of electrojet currents by amplitude-modulated HF heating waves creates ionospheric antenna currents to generate ELF/VLF waves. The dependence of the generation efficiency on the HF heating wave modulation scheme and frequency is studied experimentally and numerically. Three heating wave modulation schemes: (1) rectangular wave, (2) sine wave, and (3) half-wave rectified wave are examined. The experimental results show that the half-wave rectified wave modulation scheme is the most efficient one to generate signals at the modulation frequency and its second harmonic, corroborating theoretical predictions. The equations modeling the radiation process and experimental measurements include the contribution from the near field, in addition to the far field, of the antenna current and the resonant effect on the wave intensity generated in the resonator formed by the conducting ground and the plasma layer in the lower ionosphere. The numerically calculated dependency of the radiation intensity on the modulation frequency is shown in good agreement with that measured experimentally for all three modulation-schemes.