Simulation of synthetic aperture radar (SAR) raw signal pertaining to an ocean scene is presented. It embodies the distributed surface model, the hydrodynamic modulation, and the Bragg dominant scattering mechanism. Simulation experiments are provided.

On ocean SAR raw signal simulation

An analytic expression is derived for the spectrum of a synthetic aperture radar image in terms of the surface reflectivity covariance function. It is shown that within the severe constraints imposed by the short integration time, the third (temporal) dimension of the spectrum can be extracted by varying the processor focus parameter. The results also illustrate the dependence of the image spectrum on the scene coherence time, as well as various nonlinear effects associated with the velocity bunching mechanism. Example calculations are presented for two cases corresponding to data sets collected during the Tower Ocean Wave and Radar Dependence Experiment.

An analytic representation of the synthetic aperture radar image spectrum for ocean waves

Theoretical comparisons are made of different models for synthetic aperture radar (SAR) imaging of ocean waves. The velocity bunching model, distributed surface model, Lyzenga model, Canadian Center for Remote Sensing/Radarsat (CCRS/RADARSAT) model, and the Ocean Research and Engineering (ORE) model are compared analytically with respect to their description of the SAR imaging process and the radar backscatter process. The study concentrates on the temporal description of the backscatter process and its impact on SAR imaging of the ocean. Sample simulations are conducted so as to quantify some of the similarities and differences of the aforementioned models. Other SAR imaging models are discussed, as they pertain to the comparisons noted above. The analytical and numerical simulations show that the Lyzenga model, the CCRS/RADARSAT, model and the ORE model are similar for most commonly occurring sea states. The velocity bunching model is similar to these models when the SAR integration time is small, but it tends to deviate for long integration times. The distributed surface model is shown to describe the imaging process adequately when the radar backscatter is highly correlated in space and time.

Models for synthetic aperture radar imaging of the ocean: A comparison

The SAR ocean images obtained in the Tower Ocean Wave and Radar Dependence Experiment (TOWARD) are carefully analyzed at different focus settings and compared with simulated results based on various theories for imaging surface waves. The agreement between the experimental data and all of the SAR simulations except one is found to be favorable. There is also surprisingly close agreement among the different theoretical models themselves, suggesting a closer fundamental similarity among the contending theories than was previously thought. It is shown that the width of the focusing curve has an inverse dependence on the SAR integration time. For the TOWARD conditions it is found that the image modulation due to azimuth-traveling surface waves is greater than that due to range-traveling waves.

Focusing simulations of synthetic aperture radar ocean images

The Tower Ocean Wave and Radar Dependence experiment was conducted between October 1984 and January 1986 to study the mechanisms involved in the SAR imaging of the ocean surface. Measurements obtained included in situ capillary and short gravity waves, long surface waves and internal waves, ambient current and detailed meteorological measurements, and stereophotography. None of the hypotheses on SAR imaging of long surface waves considered are able to explain all of the present SAR observations.

Tower Ocean Wave and Radar Dependence Experiment: An overview

As part of the Tower Ocean Wave and Radar Dependence Experiment (TOWARD) objectives, the mechanisms of SAR imaging of ocean waves are investigated using L band SAR data over the Naval Ocean Systems Center tower. This paper provides experimental evidence needed to validate the differing hypotheses. Various processing methods are investigated to generate spectra with large degrees of freedom. The results show that waves traveling in the aircraft direction are most detectable at focus settings in the range 10.0–15.0 m/s, which is consistent with the Marine Remote Sensing Experiment (MARSEN) observations reported by Jain and Shemdin (1983). Waves traveling in the direction opposite to the aircraft are most detectable at settings equal to −5.0 to −15.0 m/s. The SAR imaging system acts as a low-pass filter with the peak of the ocean wave height spectrum occurring at higher wave numbers compared with the peak in the SAR image spectrum.

E. K. Tajirian

Papers

Multifocus processing of L band synthetic aperture radar images of ocean waves obtained during the Tower Ocean Wave and Radar Dependence Experiment