Mark A. Sletten

Ultrawide-band radar observations of multipath propagation over the sea surface

An ultrawide-band radar system centered at 10 GHz has been developed for sea-scatter research and was recently deployed on a research pier in the North Atlantic. The radar is based on a time-domain reflectometer module for a sampling oscilloscope. Using transient excitation of a traveling wave-tube amplifier, the system generates 200-ps wide pulses with a 10-GHz center frequency and a peak power of approximately 1 KW. The resulting range resolution is approximately 3 cm. The system was used to investigate low-grazing angle, multipath effects in the ocean environment using a trihedral corner reflector, mounted 45 cm above the water surface. The ultrahigh-range resolution of the system allows spatial separation of the direct and indirect echoes from the trihedral. In addition, a comparison of the indirect vertically- (VV) and horizontally- (HH) polarized echoes illustrates the effects of Brewster angle damping. The implications of these effects for sea-clutter statistics are briefly discussed.

Sea surface velocity vector retrieval using dual-beam interferometry: first demonstration

The dual-beam interferometer consists of two interferometric synthetic aperture radars (InSARs), one squinted at 20/spl deg/ forward of broadside, and the other 20/spl deg/ aft, to allow measurement of vector surface velocity with only a single aircraft pass. Estimates of surface velocity vectors in the coastal region during high tidal flow are presented. The data were gathered over the barrier islands west of Fort Myers, Florida, as part of a March 2004 deployment. Whereas no detailed bathymetry data were available, high-quality aerial photography appears to be a useful tool in inferring bottom topography and possible current obstructions. The retrieved velocity field clearly follows the expected outflow pattern. While comparisons with tidal current magnitudes predicted by the U.S. National Ocean Service do reveal discrepancies of up to 0.5 m/s, these differences are most likely due to the contribution of ocean surface waves to the overall InSAR velocity measurement. Velocity retrievals for the same area based on the data from different tracks show good consistency. The results constitute the first demonstration of vector retrieval of the surface velocity field with a single-pass InSAR system and confirm the robustness of the dual-beam interferometry principle.

Statistical Properties of Low-Grazing Range-Resolved Sea Surface Backscatter Generated Through Two-Dimensional Direct Numerical Simulations

Statistical properties of the X-band sea clutter are studied using 2-D direct numerical simulations. Surfaces are modeled as realizations of a Gaussian random process with the Pierson-Moskowitz or Elfouhaily spectrum. The Creamer transform is further applied to account for the lowest-order surface nonlinearities. Backscattered field at a given frequency is found using the first-principles boundary integral equation (BIE) technique. Calculations are repeated at a number of frequencies, which allows synthesizing the surface response to a pulse as short as 2.2 ns (the corresponding spatial resolution is 0.33 m). Large-scale Monte Carlo trials are used to evaluate the correlation properties and to obtain the probability distributions for the vertically- and horizontally-polarized clutter. This paper concentrates on the incident angle of 85deg (5deg grazing), with a few results for moderate 60deg incidence also reported for comparison. The effects of variations in wind speed (sea state) and radar resolution on the clutter statistics are investigated. An L-band example (with proportionally longer pulse) helps explore the role of a different electromagnetic (e/m) wavelength. The simulation technique also allows for the isolation and examination of the impacts of certain e/m and hydrodynamic approximations, including the replacement of rigorous solution to the BIE by a simpler analytical scattering model. The amplitude statistics of the simulated backscatter are compared to the Weibull and K distributions that are often used to describe surface clutter

An airborne, real aperture radar study of the Chesapeake Bay outflow plume

An airborne, real aperture radar (RAR) has been used to study the fronts associated with the Chesapeake Bay outflow plume during spring outflow conditions. The RAR produced images of the ocean surface with a range resolution of 10 m, an azimuthal resolution of approximately 30 m, and an image size of 2.5 km × 24 km. Two sampling strategies were utilized: one to synoptically map the entire mouth of the Chesapeake Bay at roughly hourly intervals; and a second to capture the rapid evolution of particular features. In addition, flight times were chosen such that over the course of the entire experiment, data were collected over all phases of the semidiurnal tidal cycle. Three distinct frontal signatures were observed in the imagery. A primary front extended from inside the estuary along the Chesapeake Channel to an anticyclonic turning region east of Cape Henry, and then extended southward along the coast toward Cape Hatteras. This is the classic expression of the plume front, inertial turning region, and coastal jet. A second front with a north-south orientation was observed approximately 20 km east of the bay mouth. This secondary front appears to mark the residual offshore density gradient. A third front was identified east and south of Cape Henry, within 2 km of the coast. This front appears to mark the inshore edge of the plume and has not been documented previously. Time sequences of the imagery indicate that when moving in a clockwise sense around the primary front, the frontal translation speed varies systematically from 20 cm/s in the northern section to 50 cm/s in the south. The position of the primary front and the locations and trajectories of small-scale frontal cusps suggest that bathymetry may be both a significant determinant of the front location as well as a source of along-front variability. These observations are possible due to the airborne RARs ability to collect high-frame rate image sequences, a capability that is not shared by present space-based radar systems.

Analysis of radar sea return for breaking wave investigation

[1] Low-grazing angle backscattering data collected by a coherent dual-polarized radar installed on a fixed tower in the ocean are analyzed to investigate the properties of sea spikes attributable to wave breaking. The distribution of breaking wave speed is narrow-banded with an average speed between 2.0 and 2.6 m/s in mixed seas with wind speeds between 7 and 14.5 m/s. The corresponding breaking wavelength is between 2.5 and 4.3 m. The length or velocity scale of wave breaking is not proportional to the length or velocity scale of the dominant wave. This observation reflects the localized nature of the breaking process and may have significant implications on quantifying various breaking properties such as the energy dissipation or area of turnover by breaking waves. The fraction of sea spike coverage generally increases with wind speed but the trend of increase is modified by the intensity and relative direction of background swell. Parameterizations of sea spike coverage needs to take into consideration both wind and wave factors. Similarities and differences between sea spikes and whitecaps are discussed. In particular, while both quantities show a similar power law dependence on wind speed, the fraction of sea spike coverage is considerably higher than that of whitecap coverage. This result reflects the prevalence of steep features that produce quasi-specular facets and short-scale waves bounded to intermediate waves during breaking. These quasi-specular facets and bound waves contribute significantly to enhancing the radar sea return but may not entrain air to produce whitecap signature.

Observations of an inshore front associated with the Chesapeake Bay outflow plume

Abstract Preliminary observations are reported of a recurring front located near Cape Henry, Virginia, USA. The front occurs on the right-hand side, looking seaward, of the buoyant plume discharging from the Chesapeake Bay and separates the plume from a band of relatively dense seawater confined against the Virginia coast. The front thus appears to be of a type similar to the inshore plume front reported by Sanders and Garvine for the Delaware Bay. Similar to an estuarine tidal intrusion front, the Cape Henry front evolves to a prominent V-shaped planform during flood tide and subduction of fluid along the front may provide a means for recirculation of near-surface material.

Multipath scattering in ultrawide-band radar sea spikes

This paper presents sea scatter data collected with an ultrawide-band (UWB) polarimetric radar system that indicates that multipath scattering plays an important role in the generation of sea spikes. The radar system used in this study produces short pulses with a bandwidth of approximately 3 GHz centered at 9 GHz for a range resolution of approximately 4 cm. Pulse-to-pulse switching allows collection of the microwave echoes produced by all four combinations of linear transmit and receive polarizations [vertical-transmit vertical-receive (VV), horizontal-transmit horizontal-receive (HH), horizontal-transmit vertical receive (HV), and vertical-transmit horizontal-receive (VH)] each of which is collected by a sampling oscilloscope utilizing equivalent time sampling. In June 1996, upwind sea scatter data at grazing angles of 10/spl deg/, 20/spl deg/, and 30/spl deg/ were collected while the system was deployed on a research pier on the Outer Banks of North Carolina. An analysis of the strongest echoes (sea spikes) from this data set is presented and discussed. First, the cumulative distribution functions are presented. Second, an increase of approximately 5 dB is shown to occur in the polarization ratio (HH/VV) of the strongest echoes as the grazing angle decreases from 30/spl deg/ to 10/spl deg/. Third, differences in the spatial and spectral characteristics of the VV and HH spikes are described. Through comparisons with laboratory results and a simple scattering model, these observations are explained by the presence of a multibounce scattering mechanism. The use of the model to extract wave height from the sea-spike frequency response is also explored.

Ultrawideband, polarimetric radar studies of breaking waves at low grazing angles

This paper summarizes the results of recent laboratory investigations into the ultrawideband, polarimetric scattering properties of breaking water waves. Breaking waves were produced in a wave tank through chirped wave packets (0.5–1.0 Hz) generated by a programmable wavemaker. An ultrawideband radar system was then used to measure the polarimetric properties of the breakers over the 6–12-GHz band at a grazing angle of 10°. An analysis of the data from these experiments shows interesting dynamics in and differences between the dominant VV and HH polarizations. In particular, the RF frequency responses of the orthogonally polarized signals differ substantially, and both the magnitudes as well as the relative phases can change appreciably over relatively short time intervals. A “bench top” model of a breaker is used to support the hypothesis that this behavior is due to the presence of small, nonlinear structures near the breaker crest and a “double-bounce” scattering mechanism associated with them.

A pod-based dual-beam SAR

A dual-beam along-track interferometric synthetic aperture radar that is entirely self-contained within an aircraft pod has been developed by the University of Massachusetts to study sea surface processes in coastal regions. The radar operates at 5.3 GHz with a bandwidth of up to 25 MHz. System hardware is described. Initial test flights aboard the National Oceanic and Atmospheric Administrations WP-3D research aircraft were performed to evaluate system performance over land and water surfaces. Imagery were collected for fore and aft squinted beams, though no interferometric data were collected. Notable look-angle dependences are observed in the sea surface normalized radar cross section under very low wind conditions.

An ultrawideband, polarimetric radar for the study of sea scatter

An ultrawideband radar system is described which has the capability of making pulse-to-pulse polarimetric measurements of the dynamic water features responsible for radar backscatter from the sea. The fast risetime voltage step produced by a Tektronix time-domain reflectometer (TDR) is used to excite a 6-12-GHz amplifier, producing a short (15 cm) radar pulse, A pair of 2- to 18-GHz antennas and appropriate pulse-to-pulse transmit and receive switching capability allows the collection of four consecutive equivalent-time-sampled pulses, one for each combination of the linear transmit and receive antenna polarizations. A polarimetric scattering matrix is then obtained at a sequence of frequencies across the 6-12-GHz band through the Fourier transform of each of the four waveforms and the application of an ultrawideband, polarimetric calibration procedure. The effect of motion on the computed scattering matrix is discussed and quantified, as this is an important consideration for polarimetric investigations of the water features responsible for radar sea scatter, A technique is then presented which compensates for the effects of target translation during the sampling interval. Scattering measurements of several rigid targets and of small breaking waves in a wave tank are used to illustrate the unique capabilities of this system and its applicability to sea scatter studies. >