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AGU Ocean Sciences Meeting (Feb. 2002) COAST abstracts:


Shore-based Mapping of Ocean Surface Currents at Long Range using
5 MHz HF Backscatter

Michael Kosro and J. D. Paduan

Increasing use has been made of HF radio-wave techniques to remotely
sense ocean surface currents, from the Doppler shift they impose upon
backscatter.  Radio frequencies of 11-26 MHz have been most commonly
used in commercial instruments such as SeaSonde, OSCR, and WERA;
these typically allow current mapping to ranges of O(50km). Recently,
we have been operating an array of three SeaSondes designed for lower
frequencies, near 4.8 MHz, between Winchester Bay, Oregon (43.7N) and
Pt. St. George, California (41.8N).  This mode of operation results
in greatly extended range, to O(180km). Preliminary comparisons
with data from upward-looking ADCPs show a strong correlation at
subinertial frequencies; the SeaSonde, measuring the upper 2m, shows
somewhat higher energy in the tidal/inertial band than the ADCP data,
measured at 9m.  Contrary to expectation, these locations have not
shown a strong diurnal modulation in range. Intermittent signal
degradation of a type not seen at 11-28 MHz affects a fraction of
the data.  This degradation appears to be due to scattering from
the lower layers of the ionosphere, and results in distinctive
distortions of the cross-spectra.  Data screening techniques based
on these distortions are being tested.