Spatial Coherence of Nonlinear, Nonstationary, Non-gaussian Ocean Waves on a One-mile Scale from Scanning Radar Altimeter Data

摘要

This report is complementary to the report of the same title submitted by Borgman and Marrs (Contract No. N00014-98-C-0206) and both relate to the joint proposal of the same title by Borgman, Walsh, and Marrs. The Borgman and Marrs report describes progress on the principal study, using data from the NASA/GSFC Scanning Radar Altimeter (SRA) (Walsh et al., 1989, 1996) to investigate properties of waves in the open ocean related to Mobile Offshore Base (MOB) concerns. That report covers all areas (Long-Term Goal, Objectives, Approach, Impact/Application, Transitions and Related Projects, etc.) of the principal study and they will not be repeated here. This report describes work underway to assess the accuracy of the data being used in the principal study by estimating tilt modulation distortions in the wave topography measured by a scanning radar altimeter. The SRA two-way beamwidth of approximately 1 deg would produce a half-power footprint at nadir of 17.5 m from 1 km height. This is the nominal aircraft height for the high altitude SRA data from the Southern Ocean Waves Experiment (SOWEX) used in the principal study. At the nominal edge of the SRA swath, the cross-track dimension of the footprint increases to 20.3 m because the beam intercepts the sea surface at a 22 deg incidence angle. It would be expected that spatial filtering by the footprint would reduce the apparent amplitude of shorter ocean wavelengths. But we will see that modulation of the radar cross section caused by the tilts of the waves can actually make short waves appear much higher in amplitude at off-nadir locations.