Response of high frequency radar to meteor backscatter

The response of a High Frequency (HF) Radar System to echoes backscattered from underdense meteor trails is calculated. Three propagation modes are identified according to whether the echo is received along the direct (line-of-slight) path, or along two possible paths from beyond the horizon involving ionospheric reflection. The system response contours in terms of meteor radiant position on the sky are presented in the altitude-azimuth and celestial ecliptic coordinate systems. Diurnal echo rate curves are deduced for point radiants, which correspond to meteor showers and for a density distribution of radiants which is appropriate to sporadic meteors.The calculations are compared with observations of integrated meteor echo power from sporadic meteors made with an experimental radar system at frequencies throughout the HF band. Satisfactory agreement is reached between predictions and observations as functions of time of day, radar frequency and range. The extension of observations to include ionospherically propagated echoes permits meteor echo rates to be simultaneously monitored over an area of the Earth's surface of the order of 10⁶ km² with a single radar system.A greater than normally accepted echo rate is required to explain our observations. However, we believe that this enhanced rate is consistent with the true echo height distribution and the attenuating effects of trail initial radius and diffusion, which are particularly severe at the radar frequencies normally used for meteor detection near and beyond the top of the HF band. Our echo rate is consistent with the meteoroid cumulative mass distribution which may be inferred from a simple interpolation between satellite and visual measurements.