Stereo observation of lakes and coastal zones using ASTER imagery

One of the unique features of the visible and near-infrared (VNIR) subsystem of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is the near-infrared back-looking view, which is obtained 55 s later than the corresponding nadir view. Although the stereoscopic capability derived from a combination of these two views was originally designed for use in land applications, serendipitous observation has shown that it also represents a promising source of data over water bodies. In fact, when coupled with surface temperature measurements derived from the sensor's thermal-infrared (TIR) component, stereo viewing forms a powerful observational capability that can be exploited in the monitoring of lake and marine environments. One important aspect introduced by the stereo system is the capability to measure the short-term displacements associated with surface currents and waves by comparing the nadir and back-looking views. In initial results presented here, surface motion was deduced from the displacement of a ship wake appearing in the Sun glitter regime of an ASTER image of the Izu Shoto islands, which lie to the southwest of Tokyo Bay, Japan. Within the natural limitations imposed by spatial and temporal resolutions, this measurement was found to be consistent with simultaneous surface current values derived from a nearby shore-based High-frequency (HF) radar system. The success of this basic comparison introduces the possibility that stereo imaging from space, if well tuned to follow the motion of surface slicks and other surface features within the Sun glitter regime, could provide a future means of observing surface current on a global scale. Data from the VNIR stereo system also highlights the influence of observational geometry. In the case of a surface with Sun glitter, this often manifests as a brightness reversal between the nadir and back-looking views in which relatively bright (dark) features at nadir appear dark (bright) in the back-looking view. Such a region of brightness reversal (ROBR) can be indicative of a locally rough or smooth surface texture as may be found, for example, in the vicinity of internal waves (rough) or ship wakes (smooth). Simple analysis based on a Back to Nadir ratio of relative radiances shows that the signal strengths measured by ASTER within a ROBR can yield a crude estimate of the mean square surface slope, which is an important parameter in theories of sea surface roughness imaging by devices such as Synthetic Aperture Radar (SAR). Several other potential applications of VNIR stereo data in coastal zone and lake monitoring are considered here, including the study of internal waves (in the Strait of Gibraltar), derivation of wind velocity at cloud height, swell-wave propagation, suspended sediment transport, marine traffic monitoring and ship wake patterns. As the reflectivity of water in the near infrared reverses from low to high in the presence of Sun glitter, most of the applications discussed here involve glitter and hence some discussion of the influence of sensor view geometry is presented. The general conclusion of the paper is that future missions such as the Advanced Land Observing Satellite (ALOS), which aim to deploy high spatial resolution stereo viewing systems primarily for land applications, should also return data of considerable value in oceanography and limnology, particularly if their observational geometry can be adjusted to permit the imaging of Sun glitter from water surfaces. (C) 2005 Elsevier Inc. All rights reserved.