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High-frequency (HF) surface wave radars provide the unique capability to continuously monitor the coastal environment far beyond the range of conventional microwave radars. Bragg-resonant backscattering by ocean waves with half the electromagnetic radar wavelength allows ocean surface currents to be measured at distances up to 200 km. When a tsunami propagates from the deep ocean to shallow water, a specific ocean current signature is generated throughout the water column. Due to the long range of an HF radar, it is possible to detect this current signature at the shelf edge. When the shelf edge is about 100 km in Responsible Editor: Aida Alvera-Azcárate This article is part of the Topical Collection on Multiparametric observation and analysis of the Sea.front of the coastline, the radar can detect the tsunami about 45 min before it hits the coast, leaving enough time to issue an early warning. As up to now no HF radar measurements of an approaching tsunami exist, a simulation study has been done to fix parameters like the required spatial resolution or the maximum coherent integration time allowed. The simulation involves several steps, starting with the Hamburg Shelf Ocean Model (HAMSOM) which is used to estimate the tsunami-induced current velocity at 1 km spatial resolution and 1 s time step. This ocean current signal is then superimposed to modelled and measured HF radar backscatter signals using a new modulation technique. After applying conventional HF radar signal processing techniques, the surface current maps contain the rapidly changing tsunami-induced current features, which can be compared to the HAMSOM data. The specific radial tsunami current signatures can clearly be observed in these maps, if appropriate spatial and temporal resolution is used. Based on the entropy of the ocean current maps, a tsunami detection algorithm is described which can be used to issue an automated tsunami warning message.

A high-frequency (HF) ocean radar system is a shore-based remote sensing system to simultaneously monitor ocean surface currents, waves and wind far beyond the horizon. The system operation is based on electromagnetic wave propagation coupling to salty water. Depending on operational frequencies, which are usually chosen between 5 and 30 MHz, a radar coverage of ocean surface may be extended up to 300 km offshore. The primary output of these radar systems is well used for various applications such as ocean current and wave mapping, vessel traffic service, search and rescue, monitoring of pollutants drift, and ocean sciences. Observations of the 2011 Japan tsunami event and recent meteotsunami events by HF radar technology confirmed that ocean radar systems are capable to measure tsunami-induced surface current velocity in real time. If the shelf edge width extension occupies tens of kilometers then the first appearance of specific tsunami currents can be monitored by an HF radar system in advance, already starting at the shelf edge. Hence, the radar measurements may be utilized to raise a tsunami alert. Moreover, the ocean radar can be a valuable tool to support Tsunami Early Warning Systems. The National Multi-Hazard Early Warning System in Oman launched in 2015 already includes a network of phased-array WERA ® ocean radar systems to provide real-time tsunami monitoring. The radar measurements are considered to confirm a tsunami pre-warning from seismic, tide gauge and buoy components of the system.

High-Frequency (HF) radars are operated in the 3-30 MHz frequency band. For oceanographic applications low transmit power HF radar systems have been developed, which use surface electromagnetic wave propagation along the salty ocean surface. The WERA HF radar system transmits a power of 30 watts but achieves detection ranges up to 200 km, which are far beyond the conventional microwave radar coverage. Hence the radar system can be used for coastal monitoring. Due to external noise, radio frequency interference, and sea clutter the radar detection capability is limited. Real measured radar data from the WERA system were recorded for a 12-hour period. The measured radar data were correlated with simultaneously available Automatic Identification System (AIS) information, which shows current ship position, speed, course and type. This paper presents statistical analysis of maximum detectable range and target reflectivity to estimate the radar performance in case of different cargo ship sizes. Based on the order-statistic constant false alarm rate (OS-CFAR) detection rule the potentially detectable target range and aspect angle of different ship categories were analyzed. I.978-1-4244-5812-7/10/$26.00 ©2010 IEEE

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