HF surface wave radar for tsunami alerting: from system concept and simulations to integration into early warning systems

An important property of the high-frequency (HF) band (3-30 MHz) is the unique ability of HF signals to propagate over long distances and to illuminate the Earth's surface beyond the horizon. Historically, over-the-horizon (OTH) HF radar systems were divided into two main categories: (1) those using sky wave propagation in different ionospheric layers and reaching thousands of kilometers and (2) others using the surface (ground) wave mode when the signals couple well to the conducting ocean surface and hence potentially enabling propagation and observations hundreds of kilometers from shore (depending on the radar parameters and operating conditions). Excellent descriptions of both types of OTH systems, including their characteristics and functionality, appear in [1]. Originally, both types of OTH radar were intended to fulfill military tasks of air and maritime surveillance far beyond the horizon. However, other systems, generally categorized as HF ocean radars, have been developed for civil applications, with remote sensing of the sea surface as a primary goal. These applications include spatial mapping of current velocity, significant wave height, wave spectra, and surface winds. A large number of experimental and commercial HF ocean radar systems have been deployed w.orldwide for remote-sensing purposes [2], [3].