Along-track interferometric synthetic aperture radar measurements of a small fiberglass-hull boat were made in various wind and wave conditions and for different measurement geometries and boat speeds. The data collected show three different cases: 1) the boat signature is visible both in the backscattered power and interferometric phase images; 2) the boat signature is visible only in the interferometric phase image, and 3) the boat signature is not visible in either image. From a preliminary analysis of the data, we conclude that the angle between radar look direction and the nominal velocity vector of the boat significantly affects boat detection. The worst cases for detection are when those vectors are collinear or oppositely directed. The best detection cases appear to be for the case, when boat velocity vector and radar look direction are orthogonal or when the boat is stationary.

Tides and river slope are fundamental characteristics of estuaries, but they are usually undersampled due to deficiencies in the spatial coverage of water level measurements. This study aims to address this issue by investigating the use of airborne lidar measurements to study tidal statistics and river slope in the Columbia River estuary. Eight plane transects over a 12-h period yield at least eight independent measurements of water level at 2.5-km increments over a 65-km stretch of the estuary. These data are fit to a sinusoidal curve and the results are compared to seven in situ gauges. In situ– and lidar-based tide curves agree to within a root-mean-square error of 0.21 m, and the lidar-based river slope estimate of 1.8 × 10^−5 agrees well with the in situ–based estimate of 1.4 × 10^−5 (4 mm km^−1 difference). Lidar-based amplitude and phase estimates are within 10% and 8°, respectively, of their in situ counterparts throughout most of the estuary. Error analysis suggests that increased measurement accuracy and more transects are required to reduce the errors in estimates of tidal amplitude and phase. However, the results validate the use of airborne remote sensing to measure tides and suggest this approach can be used to systematically study water levels at a spatial density not possible with in situ gauges.

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.

The deep ocean, continental shelf, and surf zone are defined by their unique physical processes and dynamics. The nearshore region from about 50 m water depth to the outer edge of the surf zone (SZ) is known as the inner shelf. This region is characterized by overlapping and interacting surface and bottom boundary layers. At the offshore side of the inner shelf, instabilities from wind-driven currents and fronts create cross-shelf meanders and eddies. In addition, energetic nonlinear internal waves (NLIWs) are ubiquitous on the inner shelf.

To understand and predict the exchange of water properties (heat, gases, sediment, pollutants, biota) across the inner shelf over a range of temporal and spatial scales, the Office of Naval Research Inner Shelf Dynamics Departmental Research Initiative (Inner Shelf DRI) is coordinating field observations (in situ and remote sensing) coupled to numerical modeling efforts on a 50-km section of coast off Vandenberg Air Force Base, California, located in the vicinity of Point Sal. The overall goal is to develop and improve the predictive capability of a range of numerical models to simulate the 3D circulation, density, and surface wave field across the inner shelf associated with a broad array of physical processes and complex bathymetry.

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.

A radiometric calibration scheme that exploits time-domain backprojection is evaluated using a squinted frequency-modulated continuous wave radar and corner reflectors. We find that the calibration scheme compensates for most of the variation in image intensity due to changes in aircraft attitude, and that for good signal to clutter ratios, the return from the corner reflectors varies by for most cases by around 1 dB, although a 3 dB difference was observed in one case.

The relationship between synthetic aperture radar (SAR) signatures of depth-limited breaking waves and wave height is studied. Wave height is estimated from SAR images using an empirically derived relationship that exploits the azimuthal shift in SAR images associated with moving scatterers. This relationship is derived from in situ measurements rather than from an idealized model of breaking waves as was done in a previous study. We find that the lengths of the SAR signatures are correlated with the observed significant wave height (the correlation coefficient is 0.78) for a range of wave conditions. The relationship between the wave heights and velocity bandwidths from the field data is similar to that between simulated (with a Boussinesq surface wave model) wave heights and velocity ranges (correlation coefficient = 0.82).

The concept of using orthogonal flight tracks to measure two components of surface velocity with along-track interferometric (ATI) synthetic aperture radar (SAR) has existed since the first ATI SAR measurements by Goldstein and Zebker in 1987. While this approach is geometrically optimal for measuring surface currents, it is limited in that the measurement can only be made over the intersecting area of the two SAR images. Furthermore, it is not feasible for a space-based implementation. To overcome these limitations, a single-platform dual-beam ATI SAR approach was evaluated, and first implemented at the University of Massachusetts, and subsequently during the Wavemill proof of concept project and as a miniaturized ATI SAR by the Applied Physics Laboratory at the University of Washington. In this paper, we present dual-beam ATI SAR measurements in a tidally-driven estuary, and compare the estimated surface velocity field with in situ drifter measurements.

Results from measurements of nearshore ocean processes with an frequency-modulated continuous wave along-track interferometric synthetic aperture radar are presented. Surface flow measured with the radar in a tidally-driven inlet compares well with in situ measurements by drifters. Data from a larger inlet system, in which stratification is important, shows regions of strong mixing and dramatic changes in the velocity field that are associated with bathymetric features.

We present a technique to extract geophysical parameters of nearshore ocean breaking waves (e.g., wave height, wave velocity) from ATI SAR data. The technique exploits the azimuthal shift induced by moving scatterers in SAR imagery. We find that length of the azimuthal streaks are correlated with breaking wave height (correlation coefficient is 0.78) for a range of wave conditions. We also outline a technique for highlighting breaking waves in SAR imagery using the ATI SAR images.

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.

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.

Shadowing and modulation of microwave backscatter by ocean waves are studied using coherent X-band radars. Two types of shadowing are investigated: geometric shadowing (complete blockage of incident rays) and partial shadowing (polarization-dependent diffraction combined with weak scatterers). We point out that the frequency of occurrence of zero signal-to-noise ratio samples cannot depend on the incident power level or the polarization if geometric shadowing occurs but can if partial shadowing exists. We then compare this behavior with observations, and show that the data do not support the hypothesis that geometric shadowing plays a significant role in low-grazing-angle microwave scattering from the ocean surface. Furthermore, our data indicate that partial shadowing only depends significantly on polarization for the steep waves found near shorelines. We also study the modulation of microwave backscatter by ocean waves using these data by looking at the phase differences between received power and scatterer velocity. These phase differences appear to be rather well explained by standard composite surface theory at VV polarization, having values that are positive looking up wave and negative looking down wave. For HH polarization, however, breaking effects come into play and overshadow composite surface effects of free waves. They cause the phase difference to be near zero for up wave looks and near 180° for down-wave looks. A simple model that involves both breaking and freely propagating waves but does not include any shadowing effects is shown to account for observed phase differences at both polarizations to within about 10°.

Wave number-frequency spectra measured with remote sensing systems consist of energy along the ocean wave dispersion relation and additional features that lie above and below this relation. At low frequencies a feature passing through the origin as a straight line is observed while one or more high-frequency features exhibit substantial curvature. Here we utilize images obtained on the open ocean from microwave Doppler shifts, which are directly related to scatterer velocity and allow us to calculate expected wave-wave interaction effects. We show that the strongest features lying off the first-order dispersion relation are not primarily due to second-order interactions, breaking caused by wind turbulence, advection by turbulence, or shadowing. The low-frequency feature can be seen traveling in the opposite direction to swell when looking nearly crosswind. We show that the most probable cause of these features is the interference of long ocean waves, which causes breaking near local maxima of surface slope. Doppler spectra observed by the radars indicate that the maximum speed reached by water particles on the open ocean is less than 6 m/s and usually close to the speed of the low-frequency feature in the wave number-frequency spectrum. Since this is much less than the phase speeds of dominant wind waves and swell, neither of these waves can be the breaking wave. Rather, we hypothesize that the superposition of these waves steepens short gravity waves on the surface, which then break to produce water parcels traveling near their phase speed, the speed observed by the radar.

The Dual-Beam Interferometer is an airborne instrument that combines two along-track interferometric synthetic aperture radars observing the surface below at different squints. This configuration allows retrieving vector velocities of surface flows in a single aircraft pass. The system was designed by the University of Massachusetts and saw several deployments in the early 2000s.

An imagery of a boat with a rather pronounced wake system captured during one of these flights is the subject of this paper. The velocity of the vessel is estimated based on the "train off the tracks" displacement in one of the looks. This estimate, which will be affected by uncertainty in target position due to smearing, is then used to remove unknown phase biases in the interferometric channels. Retrieved velocity variations are examined along cuts traversing the wake, with the focus on its narrow "turbulent" part. We find that the reconstructed velocities 150 m behind the boat are intuitively satisfying, but we are unable to fully account for the cross-wake component of velocity at 400 m behind the vessel.

Vertically polarized backscattered power, Doppler velocity, and interferometric height were measured in the nearshore ocean region using a high-resolution imaging microwave radar. The interferometric surface displacement measurements are compared with normalized radar cross sections (NRCS) and Doppler velocities, and with in situ pressure sensor estimates of wave height. The rms uncertainty in the interferometric surface displacement is computed from the data. The interferometric surface displacement measurements are intuitively satisfying in that positive displacements are generally associated with pixels that have larger NRCS values. Interferometric surface displacements compare well with heights derived from in situ pressure sensors, and we provide some possible explanations for the differences.