HR: 18:05h
AN: OS34A-08    [Abstracts]
TI: A Multi-frequency Beam-forming HF Radar for Tsunami Detection
AU: * Trizna, D B
EM: dennis@isr-sensing.com
AF: Imaging Science Research, Inc, 6103B Virgo Court, Bukre, VA 22015-3249, United States
AB: We discuss a new multi-frequency beam-forming HF radar design for robust detection and tracking of tsunami waves from 200 km distances, providing continuous coverage of the tsunami wave pattern after it impinges on the continental shelf. The method works by mapping ocean currents at long range using traditional HF radar method of radial Bragg line Doppler shift measurements. The tsunami is detected by anomalous spatial patterns of higher than normal Bragg-line shifts due to the large orbital wave of the series of tsunami wave crests as they impinge on the continental shelf. An approach using beam forming of 16 or 32 antenna elements provides an update every five minutes or less, while Direction-of-Arrival method systems using just a few antenna elements inherently require of the order of 30 to 60 minutes for a reliable current map. The multi-frequency radar provides a more robust capability than the single frequency HF radar for at least two reasons. First, because the HF channel user spectrum suffers diurnal variability in channel occupancy due to the ionosphere changing with time of day, low frequencies can become contaminated with user noise, so that maximum range for reliable detection not achieved. Under this condition, one would rely on quiet higher HF frequencies that lie above the Maximum Usable Frequency (MUF) for ionospheric reflection propagation. Alternatively, for daylight operation when low frequency utilization can be used to minimize surface wave propagation loss, the sea state might not be sufficiently active to allow long range coverage needed for reliable detection, due to the lack of ocean wave spectral energy at the Bragg-resonant wave frequency. Thus, single- frequency radars, operating in the 4-6 MHz range to minimize propagation losses to achieve long-range coverage, would suffer due to low wind conditions. The multi-frequency HF radar discussed here allows one to dynamically choose the optimum frequency from a set of 8 to 16, as allowed by local radio frequency allocation authorities, assuming just a single frequency is used at a time. Alternatively, one may choose four to eight frequencies operating simultaneously, to allow the clearest channel and best signal strength channel for enhanced processing. We will discuss the tradeoffs on multiple frequency use versus single optimum frequency use, both using a multi-frequency radar capability. Results of preliminary testing of a prototype system at Duck, NC will also be presented.
UR: http:www.isr-sensing.com
DE: 4217 Coastal processes
DE: 4512 Currents
DE: 4528 Fronts and jets
DE: 4564 Tsunamis and storm surges
DE: 4594 Instruments and techniques
SC: Ocean Sciences [OS]
MN: 2007 Joint Assembly