A test of ocean surface-current mapping with over-the-horizon radar

A two-day test with a decametric over-the-horizon (OTH or ionospheric) radar in Virginia attempted to map the radial component of ocean surface currents over a 210 000 km/sup 2/ area that includes the Florida Straits and parts of the Gulf of Mexico as distant as 1500 km. Ionospheric motions distort and bias individual measurements, but their effects are mitigated by a combination of strategies that take advantage of the different space and time scales of oceanic and ionospheric motions. In addition, nearby land echoes are used as zero Doppler references to correct for ionospheric shifts. The result is a composite picture of the Florida Current and ancillary surface flows with 10-15-km resolution. The picture agrees quantitatively with known currents in the region, but reveals dynamical features with new detail. Concurrent sea-surface topography in the Gulf of Mexico, derived using tandem altimetric observations from the TOPEX/Poseidon and ERS-1 satellites, confirms that a region where the OTH radar measures a southwestward current greater than 1 m/s/sup -1/ coincides with the confluence of the Tortugas Gyre and the Gulf of Mexico Loop Current. These results suggest that consistent surface current maps can be constructed by using OTH radar to repeatedly interrogate a region of interest, perhaps over several days.