24.1 INTRODUCTION

Beyond-the-horizon ranges up to thousands of nautical miles can be achieved by radar operation in the high-frequency (HF) band (3 to 30 MHz), The longer-range performance is achieved by using sky-wave propagation. Ground-wave propagation over the sea is useful for short but still over-the-horizon distances. HF radar development over the past several decades has led to this capability,1 ^,2 and several operational systems are deployed.3 ^,4 ^,5 Targets of interest are the same as for microwave radar and include aircraft, missiles, and ships. The wavelengths used are of the same order as ocean gravity waves, and this correspondence makes HF radar able to provide information on the waveheight directional spectrum and, by inference, surface winds and ocean currents.6 In addition, this sensor is useful for observing various forms of high-altitude atmospheric ionization such as that due to aurora, meteors, and missile launches.7 ^,8 The wavelengths used and the nature of the transmission path make the spatial resolution coarse when compared with much-higher-frequency radars; however, the doppler resolution can be fine.

For effective radar operation, environmental parameters need to be determined in real time; transmission-path information is generally derived from adjunct vertical and oblique sounders as well as by using the radar itself as a sounder. An ionospheric electron-density model complex enough to enable adequate sounding interpretation is required. Ionospheric or transmission-path statistical forecasts are necessary for radar design and for development of the model, not for real-time operation.