Surface current measurements using airborne visible image time series

Measurements of nearshore water currents are important for a number of reasons, including understanding sediment transport and beach erosion, planning and evaluation of littoral construction projects, and military operations in very shallow water. Because direct measurements are difficult and expensive to make, remote sensing techniques are of great interest. A digital camera system has been used to remotely measure ocean surface currents from an aircraft by estimating the Doppler shift of gravity waves. Navigational data are used to map each of a sequence of images to a geodetic reference frame at the level of the mean ocean surface. The 3-D frequency-wave number spectrum of the luminance modulations in subsets of these mapped data exhibits the dispersion relation for surface gravity waves, and the 2-D velocity vector is retrieved by estimating the Doppler shift of the higher frequency waves in this spectrum, wavelengths in the band of about 5–30 m. The relationship of this technique to that of tracking visible surface features between image pairs is examined with examples. Current retrievals in shallow water outside the surf on a quasi-linear beach and in an exposed tidal inlet channel are favorably compared with simultaneous in situ current measurements, with errors of ∼10% in magnitude and ∼5° in direction.