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High Frequency radar is based on the process of backscattering of electromagnetic waves (frequencies used for
this project are between 24 and 27 MHz) from the rough sea surface and can be used to measure surface currents
and ocean wave parameters. In contrast to microwave radars, the transmitted electromagnetic wave is guided by
the sea surface following the curvature of the earth beyond the horizon. Ranges of about 50 km can be achieved.
The technique has been rapidly developed over the past 25 years.
The Doppler spectrum of HF radar signals backscattered from the rough moving sea surface contains prominent peaks
(due to first-order Bragg scatter) which are shifted and broadened by the radial velocity of the underlying ocean
current. The radial current field and directional properties of the short, ~2-s Bragg waves are derived from the
first-order peaks using modern DF (direction-finding) methods to obtain the echo bearing from the directional-antenna
signals. These first-order peaks are surrounded by a higher-order spectral continuum containing information about the
long ocean waves, from which the wave height directional spectral data are extracted.
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