Gordon Farquharson

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.

Contrast-Based Phase Calibration for Remote Sensing Systems With Digital Beamforming Antennas

A contrast-based phase calibration algorithm for digital beamforming remote sensing radars using three contrast metrics is presented. The algorithm corrects time-varying antenna array phase errors that defocus digital beamforming remote sensing radar imagery. Amplitude errors are treated by equalizing the received powers in all elements. As such, the algorithm does not produce an absolute (or radiometric) calibration vector for the array. The performance of the algorithm is studied using a combination of simulated and real radar data under various conditions and is compared with a clutter-based calibration algorithm. An analytical proof showing that maximizing the expected value of the 4-norm metric is equivalent to phase-calibrating the image, except for a linear phase offset, is provided. We find that the clutter calibration algorithm performs best for statistically homogeneous scenes but that the contrast-calibration algorithms perform better with scenes with larger contrast ratios.

Dual-beam ATI SAR measurements of surface currents in the nearshore ocean

Surface flow is estimated from a dual-beam along-track interferometric synthetic aperture radar at the mouth of a tidally-driven estuary. The measurements show strong variation in the surface flow around jetties and seem to be associated with bathymetry. The surface flow estimates compare well with in situ drifter measurements made around the same time.

Development of an Environmental and Ship Motion Forecasting System

Military sea basing operations include mooring ships together offshore and transferring cargo and equipment between them. A newly developed Environmental and Ship Motion Forecasting (ESMF) System will facilitate these operations by providing predictions of ship motions in waves. Coherent forecasts of the ship motions are provided through remote sensing of the ambient waves and using these waves as input to a predictive ship motion simulation. Key technologies developed in support of the ESMF system include: a custom-built wave sensing radar; a least squares inverse wave retrieval algorithm; a ship motion model for performing rapid seakeeping simulations; and a robust peer-to-peer system architecture. The ESMF system was tested extensively in a demonstration aboard the R/V Melville with very good results, often achieving correlations of forecast-to-realized signals of better than 80% over 30 minute intervals.Copyright

ATI SAR simulation shows signatures of complex objects

This paper shows results of simulations of single point and complex target signatures of along track interferometric SAR radar. The SAR images of the moving target with motion in along track direction show the typical smearing and displacement of the target signature in case of squint-looking radar. The interferometric image shows the rainbow-like signature for moving target. The behaviors of targets signatures in a good agreement with the obtained experimental results.

Phase calibration of an along-track interferometric FMCW SAR

Techniques to estimate and correct range-dependent phase differences between receivers in an FMCW ATI SAR are studied. Both techniques reproduce the range-dependent phase ripple seem in the ATI interferograms.

High-Resolution Non-Hydrostatic Modeling of Frontal Features in the Mouth of the Columbia River

Airborne data measured during the recent RIVET II field experiment has revealed that horizontally distributed thermal fingers regularly occur at the Mouth of Columbia River (MCR) during strong ebb tidal flows. The high-resolution, non-hydrostatic coastal model, NHWAVE, predicts salinity anomalies on the water surface which are believed to be associated with the thermal fingers. Model results indicate that large amplitude recirculation are generated in the water column between an oblique internal hydraulic jump and the North Jetty. Simulation results indicate that the billows of higher density fluid have sufficiently large amplitudes to interrupt the water surface, causing the prominent features of stripes on the surface. The current field is modulated by the frontal structures, as indicated by the vorticity field calculated from both the numerical model and data measured by an interferometric synthetic aperture radar.

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.

Gulf Stream observations obtained with the UMass Dual Beam Interferometer and an infrared camera

The Dual Beam Interferometer (DBI) developed by University of Massachusetts (UMass) and an infrared camera operated by the Naval Research Laboratory (NRL) were jointly deployed during a March 2004 flight campaign off the east coast of Florida. Data collected produced simultaneous observations of features over the Gulf Streams western boundary by both instruments. Use of instruments with different imaging capabilities enabled extraction of wind direction as well as current velocity measurement.

Oblique Internal Hydraulic Jumps at a Stratified Estuary Mouth

AbstractObservations and analyses of two tidally recurring, oblique, internal hydraulic jumps at a stratified estuary mouth (Columbia River, Oregon/Washington) are presented. These hydraulic features have not previously been studied due to the challenges of both horizontally resolving the sharp gradients and temporally resolving their evolution in numerical models and traditional observation platforms. The jumps, both of which recurred during ebb, formed adjacent to two engineered lateral channel constrictions and were identified in marine radar image time series. Jump occurrence was corroborated by (i) a collocated sharp gradient in the surface currents measured via airborne along-track interferometric synthetic aperture radar and (ii) the transition from supercritical to subcritical flow in the cross-jump direction via shipborne velocity and density measurements. Using a two-layer approximation, observed jump angles at both lateral constrictions are shown to lie within the theoretical bounds given by th...