Surface currents measured from a sequence of airborne camera images IEEE/OES seventh working conference on current measurement technology

Measurements of nearshore water currents are important for a number of reasons, including emergency response, military operations in very shallow water, understanding sediment transport, and planning/evaluation of littoral construction and dredging projects. Because direct measurements are difficult (and expensive) to make over extensive areas, particularly where the bottom is highly mobile, remote sensing techniques are of great interest. A digital camera system has been used to remotely measure surface currents from a small aircraft by measuring the Doppler shift of short- to medium-length gravity waves. Image sequences are collected by staring at an area about 2 km on a side for 1-2 minutes duration. Aircraft navigation data are used to map and co-register the images to a geodetic reference frame at the level of the mean ocean surface. The 3-D frequency-wavenumber spectrum of the radiance modulations is calculated in a dense grid of sub-regions and the gravity wave dispersion relation is identified in each. The 2-D velocity vector field, as well as water depths, are calculated by fitting the theoretical dispersion surface to the spectrum in each sub-region. Current retrievals in a complex, exposed tidal inlet channel and surrounding shoals are favorably compared with simultaneous in situ measurements. RMS errors are within /spl sim/10% in magnitude and /spl sim/5/spl deg/ in direction for ADCPs on the bottom in the channel center and on a jet ski that was traversing the channel. Bars and shoals are clearly detected, although seemingly erroneous values of the current sometimes occur in locations where the waves clearly are breaking over the shoals. Unfortunately, these locations also are precisely where in situ data are unavailable. These errors are attributed to nonlinear wave and imaging processes that violate the model assumptions. A fix for this problem is hypothesized. The method shows considerable promise for broad area surveys in areas having complex channels and shoals, particularly as maps of bathymetry, directional wave spectra, and ortho-images of the nearby shore can be retrieved with the same image time series used to retrieve the currents.