Dragana Perkovic

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.

Surface Velocity Profiles in a Vessel's Turbulent Wake Observed by a Dual-Beam Along-Track Interferometric SAR

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.

Longshore Surface Currents Measured by Doppler Radar and Video PIV Techniques

Mean longshore surface currents within the surf zone were measured using two remote sensing techniques: microwave Doppler radar and optical video. Doppler radar relies on small-scale surface roughness that scatters the incident electromagnetic radiation so that velocities are obtained from the Doppler shift of the backscattered radiation. Video relies on texture and contrast of scattered sunlight from the sea surface, and velocity estimates are determined using particle imaging velocimetry (PIV). This paper compares video PIV and Doppler radar surface velocities over a 1-km alongshore by 0.5-km cross-shore area in the surf zone of a natural beach. The two surface velocity estimates are strongly correlated (R2 ges 0.79) over much of the surf zone. Estimates differ at the outer edge of the surf where strong breaking is prevalent, with radar-estimated velocities as much as 50% below the video estimates. The radar and PIV velocities at particular locations in the surf zone track each other well over a 6-h period, showing strong modulations in the mean alongshore flow occurring on 10-20-min time intervals. In one case, both systems observe a strong eddy-like mean flow pattern over a 200-m section of coastline, with the mean alongshore current changing direction at about the mid surf zone. The good spatial and temporal agreement between the two remote measurement techniques, which rely on very different mechanisms, suggests that both are reasonably approximating the true mean longshore surface velocity.

Ocean Mixed-Layer Depth and Current Variation Estimated From Imagery of Surfactant Streaks

The ability of high-resolution imaging systems to resolve small-scale structure on the ocean surface suggests the possibility of using characteristics of Langmuir circulation to map the surface mixed-layer depth and near-surface current. We illustrate this using synthetic aperture radar and infrared imagery that are collected across the edge of the Gulf Stream (GS), which reveals surfactant streaks, or ldquowindrows,rdquo that are induced by Langmuir circulation. Based on changes in the windrow spacing and orientation, the mixed layer is estimated to deepen from 7 to 12 m across the edge of the GS and the current to increase from about 1 to 2 m/s. These spatial changes compare reasonably well with independent data, suggesting that the approach is plausible. It may also be possible to extract additional environmental information from the windrows.

Initial vector velocity estimates from the UMass Dual Beam Interferometer

The Dual Beam Interferometer consists of two interferometric SARs, one squinted at 20 degrees forward of broadside, and the other 20 degrees aft, to allow estimation of vector surface velocity with only a single aircraft pass. It was developed by the University of Massachusetts and is operated in collaboration with the Naval Research Laboratory. The paper presents initial estimates of surface velocity vectors in the coastal region during high tidal flow. The data were gathered over the barrier islands west of Ft. Myers, Florida, as part of March 2004 deployment. While no attempt has been made at this stage to correct for Doppler contributions from surface waves, the retrieved velocity field clearly follows the expected outflow pattern, and velocity magnitudes agree well with in-situ data.

Microwave radar remote sensing of surface currents in the nearshore region

We present observations of microwave radar backscattered power in the nearshore region, and results of a surface current estimation algorithm using microwave radar data. In low wind and wave conditions, patches of increased backscatter are observed in radar images. Animations of the images show these patches advected by nearshore flows such as wave induced cell circulations and longshore currents

Marine Doppler Radar Surface Current Measurements in the Surf Zone

This paper presents a comparison of microwave radar surface velocity estimates to the estimates derived from video observations in the surf zone. The data presented here were collected during the Nearshore Canyon Experiment (NCEX) in the fall of 2003. The radar estimates are inferred from the Doppler shift of the backscattered radiation while video velocity estimates were produced using particle image velocimetry (PIV) technique. Comparisons of longshore velocity estimates show high spatial correlation within the central surf zone. The comparisons of the near cross-shore velocity shows the importance of bore velocity removal while showing high spatial correlation when the bias is removed. Both alongshore and cross-shore velocity estimates display discrepancies in the breaker and the swash zones.

An airborne pod-mounted dual beam interferometer

Dual beam interferometer (DBI) has been developed by the University of Massachusetts (UMass) to study ocean surface waves and currents in coastal regions. This airborne radar operates at C-band (5.3 GHz) with a bandwidth of 25 MHz and VV polarization. DBI consists of two pairs of microstrip patch array antennas, one squinted 20deg forward of broadside and the other 20deg aft. Each pair of antennas is separated in the along-track direction a distance of 1.23 meters forming an interferometer. Over several years, DBI was flown on a National Oceanic and Atmospheric Administrations WP-3D research aircraft in a number of successful missions collecting the data both over land and ocean. These deployments and subsequent data analysis were carried out in collaboration with Naval Research Laboratory (NRL). This paper describes the hardware integration and use of the system to generate surface current vector maps

Wave Measurements using a Dual-beam Interferometer Near Gulf Stream Boundary

Abstract : A dual-beam interferometric synthetic aperture radar provides two velocity components of the ocean current remotely from a single fight pass. Combining two flight passes, all three orthogonal components of the surface velocity can be retrieved. An experiment was conducted near the Gulf Stream (GS) boundary. A sharp change of the surface velocity of about 1 m/s over a 500 m distance was measured. The wave condition is dominated by a 14-s swell system and low wind velocity. The wave variance inside GS is about twice the wave variance outside the GS in the present data. The large difference in the wave variance is considerably higher than that can be expected from hydrodynamic modulation. An ocean current system with strong current shears such as the Gulf Stream is a wave guide and can trap waves with the right combination of wavelengths and propagation directions. Numerical calculations indicate that the wave properties of the data set may satisfy the conditions for wave trapping by the Gulf Stream.