Simulation results are used to assist algorithm development for extracting the ocean surface significant wave height (SWH) from microwave scattering of satellite broadcast signals from the ocean surface. The analysis shows that the Doppler frequency spectrum width is a reliable parameter for extracting the SWH. The numerical experiments include both deep and shallow water conditions. The correlation between Doppler frequency spectrum width and SWH is robust and applicable to all wind speeds, inverse wave ages, and water depths tested in this study (5–21 m/s, 0.8–2.0, and 3 m– $\infty $ , respectively).

The dual-beam interferometer is an airborne instrument that combines two vertically polarized C-band along-track interferometric synthetic aperture radars (AT-InSARs) observing the surface below at different squints. The system was designed by the University of Massachusetts and saw several deployments in the early 2000s. An imagery of a small vessel with a rather pronounced wake pattern captured during one of such flights is the subject of this letter. Specifically, the interferometric phase in the turbulent wake exhibits a conspicuous banding structure that is still visible at distances more than 1 km behind the craft. The phase signatures from the fore and aft looks are combined to retrieve both longitudinal and lateral velocity components along cuts traversing the wake 400 and 750 m behind the boat. The results identify appreciable variations in the longitudinal velocity across the turbulent wake which are apparently consistent with the combined effect of the hull drag and the propeller backwash. A persistent pattern for the lateral component is also observed but is harder to interpret without the detailed knowledge of the vessel. The examples demonstrate the utility of AT-InSAR and, particularly, of a dual-beam AT-InSAR, for studies of centerline ship wakes. Readily available velocity signatures of a turbulent wake obtained with such systems can help with vessel classification tasks.

Methods for improving efficiency in computing bistatic scattering from ocean-like surfaces are presented. The methods focus on approximations for the Kirchhoff integral required in evaluating the predictions of multiple theories of bistatic scattering from the ocean surface including the physical optics (PO) and small slope approximation (SSA) approaches. A representation of the integral in terms of the probability density function of a symmetric alpha stable random variable is examined for near-specular scattering as has been proposed previously in the literature. Methods for evaluating this form are presented for azimuthally varying surfaces having a general spectrum model as well as a fully analytical expression for surfaces described by an isotropic Pierson-Moskowitz spectrum. The resulting forms are also compared and contrasted with the standard geometrical optics (GO) theory of surface scattering to provide insight into the cutoff wavenumber used in the standard GO method. Additional two-scale and small roughness approximations to the integral are also investigated for the prediction of scattered fields outside the near-specular region.

This work presents data from a recent field campaign in which an S-band passive radar using noncooperative satellite-based illumination was used to measure bistatic radar scattering from the sea surface. Significant wave height (SWH) information from a nearby Waverider buoy was reported concurrent with passive radar measurements. These simultaneous data collections fostered a study of the sensitivity of passive radar amplitude and Doppler measurements to SWH. This work demonstrates that Doppler and amplitude measurements with passive radar using satellite-based illumination can provide a promising approach to coastal remote sensing of SWH and mean wave direction, particularly when the scene is illuminated by multiple transmitters with a large physical separation between them.

Numerical experiments are carried out to explore the retrieval of ocean surface significant wave height (SWH) with the Doppler spectrum width (DSW) of microwave bistatic scattering from the ocean surface. Variations of DSW with incidence and scattering angles, wave height, wind speed, wave age, local water depth, and swell condition are described. Physical interpretation is offered on the relationship between SWH and DSW through the surface wave orbital velocity. Among the various environmental factors, the swell exerts the largest influence on the resulting relationship between DSW and SWH. The analysis considers a stationary transmitter and receiver only in order to provide insight into the impact of wave motions on the Doppler signatures.

A recent ship-based field campaign demonstrated the use of a maritime multi-channel reflectometer using Sirius-XM broadcast radio downlinks as Signals-of-Opportunity. This campaign involved the collection of a large data set of bistatic ocean surface scattering measurements, which coincided with ground truth data detailing local oceanographic parameters. The experiment was unique in that most of its measurements were taken well outside the plane of incidence, with illumination from an oblique angle. This study offers a novel opportunity to explore the fidelity of electromagnetic models (coupled with oceanographic models) to predict radar scattering in these unique geometries. Match-ups are shown between measured and modeled bistatic normalized radar cross sections and range profiles, with the intention of both 1) supporting shore- and pier-based reflectometry through bistatic surface scattering model validation and 2) demonstrating the utility of using reflectometry for oceanic remote sensing from a sea-faring vessel.