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The severity of the threat posed to coastal communities by tsunamis has come into prominence following the event of Boxing Day 2004 when some 230,000 people lost their lives. Many countries have set up tsunami warning systems, or augmented existing systems, but overwhelmingly these derive their information from seismic monitoring stations which detect and locate earthquakes, not measurements of any resulting tsunamis. As a consequence, the false alarm rate is high, which leads to a loss of credibility of issued warnings and an inclination on the part of the public to ignore them. Given our limited inability to predict whether a given seismic event will generate a tsunami, let alone provide quantitative information concerning its amplitude, researchers have sought to develop sensors which can provide reliable and accurate answers to these questions. These sensors fall into two categories. First, there are in situ devices, such as tsunameter stations and tide gauges, which monitor the ocean surface elevation at a single location. The Indian Ocean presently has only a handful of tsunameter stations, the Pacific Ocean more than thirty, as shown in Figure 1. These instruments use bottom-mounted pressure recorders to obtain time series data which can be subjected to spectrum analysis to separate low frequency, long wavelength signals of tsunamis from the much stronger responses due to wind waves and swell. Claimed sensitivity for advanced systems approaches ± 1 cm in 6000 m water depth, however given that the amplitude of a potentially dangerous tsunami may be only a few centimetres in the deep ocean, while waves and swell often exceed several metres in height, the potential for error is high. Moreover, the cost of a single deep sea tsunameter station, together with its deployment may easily exceed $250,000. Accordingly, the overall cost of a reasonably dense network would be enormous. The second category of sensors embraces those which employ remote sensing techniques, either from space, in the form of satellite-borne altimeters, or from land, in the form of HF radars. Each of these remote sensing technologies has demonstrated that it is capable of measuring one or more of the various signatures which characterise a tsunami, with a precision which offers meaningful quantitative estimates of the amplitude of the disturbance. For example, the JASON-1 and Topex/Poseidon satellites which passed over the Boxing Day tsunami while it was propagating across the north-east Indian Ocean were able to detect the associated change in surface elevation and, in the case of Jason-1, a correlated variation in surface roughness inferred from the microwave radar scattering coefficient (Godin et al, 2009).

HF Skywave Radar Performance in the Tsunami Detection and Measurement Role