Simulations of Radar Imagery of the Ocean Surface

Research Objectives
To develop and improve Massively Parallel Algorithms (MPA's) for processing and simulating radar imagery of the ocean and the associated wave spectra as a means of more fully understanding the effects of environmental forcing from the wind and currents on radar imagery.

Methodology
Comparisons between simulated and processed radar imagery of the ocean are performed to better understand the effects of environmenal forcing from the wind and currents on the imaging process. Radar simulations that incorporate environmental forcing are derived by applying radar backscatter models using simulations of the ocean surface, based on surface wave spectra, S, derived from hydrodynamical calculations involving 4-dimensional numerical grids (consisting of two spatial and two spectral degrees of freedom). These hydrodynamical calculations provide the necessary input for radar simulations. In the radar simulations, radar cross-section is derived either from the Composite Scattering model or the more rigorous Holliday/Thompson backscatter model. Wave-breaking effects are included in an approximate manner, based on the statistical information provided by local wave-breaking criteria (LWBC), derived from S, and on feature modelling of electromagnetic backscatter from breaking waves.

Results
New algorithms for performing electromagnetic backscatter calculations, based on the Holliday/Thompson backscatter model were developed and benchmarked against Composite Scattering model simulations of radar signatures of the HIRES-1 Rip feature. In these new algorithms, the capability for including local changes in incidence angle across the radar image scene were included. Also, simulations of signatures of Gulf Stream (GS) fronts recently identified in SIR-C/XSAR data were performed. These simulations made use of in situ current and wind measurements taken by NRL scientists at the GS boundary that were being made while JPL scientists were constructing images of the identical scene using the spaceborne, Shuttle Radar Laboratory. A comparison between the simulations and the imagery illustrates that wave- current interaction effects are responsible for tilt-induced modulation in radar backscatter which preferentially alters backscatter from particular radar polarizations. Simulations were also performmed incorporating changes in wind direction in the fully 2-dimensional, meandering Gulf Stream front studies of the HIRES Rip feature initiated last year.

Significance
Results have importance for inferring environmental forcing effects in SAR and RAR imagery of the ocean. The research has application in areas associated with ASW/UT, MCM, Littoral Surveillance, and environmental monitoring.

Related Publications

• S. R. Chubb, A. L. Cooper, R. A. Fusina, R. W. Jansen, "Wave-breaking Effects in Radar Signatures from 2-Dimensional Modeling of the HI-RES I Rip Features," IGARSS'97 Digest, 1317 (1997).
• G. M. Nedlin, S. R. Chubb, and A. L. Cooper, "Microwave Scattering from a Slightly Rough Surface of a Medium Possessing a Large Dielectric Constant, and Applications to Air-Water Scattering," IGARSS'97 Digest, 1692 (1997).
• F. Askari, S. R. Chubb, T. Donato, W. Alpers, "Study of Gulf Stream Features with a Multi-Frequency Polarimetric SAR from the Space Shuttle," IGARSS'97 Digest, 1521 (1997).
• S. R. Chubb, A. L. Cooper, R. A. Fusina, R. W. Jansen, "Wave-breaking Effects in Radar Signatures from 2-Dimensional Modeling of the HI-RES I Rip Features," invited talk, presented at the 1997 International Geoscience and Remote Sensing Symposium, 3-9 Aug. 1997, Singapore.
• G. M. Nedlin, S. R. Chubb, and A. L. Cooper, "Microwave Scattering from a Slightly Rough Surface of a Medium Possessing a Large Dielectric Constant, and Applications to Air-Water Scattering," invited talk, presented at the 1997 International Geoscience and Remote Sensing Symposium, 3-9 Aug. 1997, Singapore.
• F. Askari, S. R. Chubb, T. Donato, W. Alpers, "Study of Gulf Stream Features with a Multi-Frequency Polarimetric SAR from the Space Shuttle," invited talk, presented at the 1997 International Geoscience and Remote Sensing Symposium, 3-9 Aug. 1997, Singapore.
• S. R. Chubb, A. L. Cooper, G.M. Nedlin, "Beyond the two-scale composite Backscatter Model for Modelling Radar Return from the Ocean Surface," invited talk, presented at the 1997 International Geoscience and Remote Sensing Symposium, 3-9 Aug.1997, Singapore.

Figure

Absolute Contributions of Component Pieces "Specular Return," and "Tilted Bragg Return," respectively, shown in middle and bottom plots, of the simulated Radar Cross-Sections (RCS's) of the HIRES-1 Rip feature that are obtained from two Radar Backscatter models. The dashed lines correspond to the Holliday/Thompson (HT) Model. The solid lines correspond to the Composite Scattering (CS) Model. The HT model simulations are the first applications of this model to a case involving strong wave-current interaction.