Estimation of target altitude in HF surface wave radar

HF surface wave radars (HFSWR) are the only long range ground based radar systems that can detect targets down to sea level, at ranges of tens to several hundreds of kilometers. This unique detection capability is a result of the low attenuation over salt water of vertically polarised HF radio waves, and makes the HFSWR ideally suited to the detection of ships, low flying airborne targets and higher altitude airborne targets. The use of frequencies in the HF band makes the formation of narrow antenna beams difficult, however. Although narrow azimuthal beams can be achieved by deploying linear antenna arrays with horizontal apertures of this order, this approach is simply not viable when trying to achieve narrow beams in elevation. As a result, HF radars (both surface-wave and sky-wave) have traditionally always lacked target height information. This paper presents a novel solution to this problem, in which target height is estimated from the variation of target signal strength with time, using an extended Kalman filter. The process also produces estimates of the target's rate of climb, radar cross section (RCS) and its rate of change of RCS.