Ocean surface currents obtained from microwave sea‐echo Doppler spectra

Abstract

Ocean currents may be determined from the positions of Bragg peaks in Doppler spectra of radar echoes from the sea surface. In this paper the applicability of this technique at microwave frequencies is investigated. Advantages of high range resolution and sensitivity to currents within centimeters of the ocean surface ensue from the short Bragg wavelength at microwave frequencies. A disadvantage is that the technique is restricted to low wind speed, fetch‐limited sea conditions for which individual Bragg peaks are resolvable in the Doppler spectra. Experiments at low grazing angles with an S band (3 GHz) Frequency‐modulated interrupted continuous wave radar show that in these conditions surface drift currents can be measured to better than ±2 cm s−1 accuracy over a large range of azimuth angles with respect to the wind direction. In conditions under which the Bragg peaks are not resolvable, currents may be measured by using local wind direction measurements to correct the mean Doppler velocity for the relative contributions of the advancing and receding Bragg peaks. Experiments revealed that the accuracy of current measurements obtained in this manner was better than ±5 cm s−1. The technique was applied to the mapping of tidal currents in an inlet, demonstrating the significant potential of the radar measurements for nearshore and coastal dynamics studies.