The spectral reflectance of ship wakes between 400 and 900 nanometers

This technical note describes the use of an airborne hyperspectral imaging sensor (HICO - Hyperspectral Imager of the Coastal Ocean) to record the spectral reflectance characteristics of centerline ship wakes in the 400 to 900 nm wavelength region. Data were collected for a target of known provenance (the United States Coast Guard Cutter Kittiwake) off the Wai´anae coast of O´ahu, Hawai´i, on 8 April 2010. HICO acquired data in 60 spectral bands by flying along the long axis of the wake while the vessel travelled at three speeds (~3.6 m s^-1 or 7 knots; ~7.2 m s^-1 or 14 knots; and ~11.2 m s^-1 or 21 knots). A flying altitude of ~1500 m yielded a spatial resolution of ~1.5 m. Spectral profiles along and across the wake axes are presented which show how the spectral reflectance of the centerline wake varies spatially and temporally as a function of vessel speed. Length (and to a lesser extent, width) vary in proportion to speed. In common with previous studies and model predictions, the wakes show a pronounced greening of the wake (i.e. enhanced reflectance at ~550 nm), with evidence for elevated reflectance at 750-800 nm. Resampling the data from its raw 1.5 m spatial resolution yields insights into how the turbulent wake becomes spectrally inseparable from the background water as spatial resolution decreases (i.e. becomes increasingly coarse). Using a simple statistical test, the wake becomes spectrally similar to the background ocean as the resolution approaches 60 m.