  The Alderney Race has been identified as a future site for the development of
tidal energy, due to its bidirectional strong current reaching 5 m/s during
spring tides. This hydrodynamics is very difficult to measure by in situ or
remote sensing means. High-frequency coastal radars can provide a synoptic and
near-real-time view of such a complex circulation, but the classical processing
algorithms are not adapted to the extreme situation of strongly sheared
currents. We propose an improved high-resolution direction-finding technique
for the azimuthal processing of such radar data. It uses phased-array systems
and combines the advantages of the usual beam-forming technique to eliminate
many problems related to the distortion of Doppler spectra by extreme currents.
The method is evaluated with a unique data set of radar measurements at two
radar frequencies (13 and 24.5 MHz) and three spatial resolutions (200, 750,
and 1500 m). The radar-based surface currents are analyzed in the light of a
high-resolution numerical model and also compared with in situ measurements.
While high azimuthal resolution can be achieved in this way, it is shown that
the typical range resolutions of 750 and 1500 m are insufficient to account for
the strong spatial variations of the surface current at some specific times and
locations.
