Robert W. Jansen

Interpretation of Airphotos and Remotely Sensed Imagery

With the proliferation of easily accessible remotely sensed imagery over the last several years, image analysts from a wide variety of working environments are in high demand. These analysts do not always have advanced technical backgrounds in science. Robert Arnolds useful and timely laboratory manual serves as an adequate introduction to interpreting remotely sensed photographs and imagery. The book poses a graduated set of examples and questions with a generally increasing but low level of sophistication. It is easy to read, and considerable care has been exercised in the layout of the subject index and overall organization of the manual.

Motion Analysis in SAR Images of Unfocused Objects Using Time–Frequency Methods

Synthetic aperture radar (SAR) image formation processing assumes that the scene is stationary, and to focus an object, one coherently sums a large number of independent returns. Any target motion introduces phases that distort and/or translate the targets image. Target motion produces a smear primarily in the azimuth direction of the SAR image. Time-frequency (TF) modeling is used to analyze and correct the residual phase distortions. An interactive focusing algorithm based on TF modeling demonstrates how to correct the phase and to rapidly focus the mover. This is demonstrated on two watercraft observed in a SAR image. Then, two time-frequency representations (TFRs) are applied to estimate the motion parameters of the movers or refocus them or both. The first is the short-time Fourier transform, from which a velocity profile is constructed based on the length of the smear. The second TFR is the time-frequency distribution series, which is a robust derivative of the Wigner-Ville distribution that works well in this SAR environment. The smear is a modulated chirp, from which a velocity profile is plotted and the phase corrections are integrated to focus the movers. The relationship between these two methods is discussed. Both methods show good agreement on the example.

Polarimetric analysis and modeling of multifrequency SAR signatures from Gulf Stream fronts

Using airborne synthetic aperture radar data from the 1990 Gulf Stream Experiment, this paper investigates the polarization and wavelength dependence of radar signatures for narrow fronts with converging flows occurring within the Gulf Stream. The signal-to-background ratios of the cross-polarization backscatter return from a convergent front were found much higher than those of copolarization returns, when the flight path is crossing the front. However, a second convergent front, imaged at 45/spl deg/, showed that the signal-to-background ratios are nearly equal for co- and cross-polarizations. A polarimetric procedure, which has been successfully used to measure terrain slopes and to generate elevation maps, is applied to the convergent front to explain the polarization and imaging geometry dependence of these radar responses. A theoretical modeling of radar modulation using an ocean wave model and a composite-Bragg scattering model, which incorporates the effect of breaking waves, was developed. Calculations with the model agree reasonably well with the radar measurements at various polarizations for three radar frequencies: P-band (68 cm in wavelength), L-band (24 cm), and C-band (5.7 cm).

Gulf Stream surface convergence imaged by synthetic aperture radar

On July 20, 1990, the north edge of the Gulf Stream (36.7°N, 72.0°W) was sampled by the R/V Cape Henlopen and simultaneously imaged by the Jet Propulsion Laboratorys airborne synthetic aperture radar (SAR). Hydrographic measurements show an abrupt surface front separating warm, salty Gulf Stream water in the south from a filament of cool, fresh (<33 practical salinity unit (psu)) water to the north. The filament lies within the stream and is likely water entrained from the continental shelf. The southern boundary of the filament is marked by increased surface wave breaking in a 100- to 200-m-wide zone, accumulations of Sargassum, and an orthogonal velocity change of 20 cm/s. The front is manifested in a sequence of SAR images as a narrow line having returns 1–2 dB higher than background. (A second, transient SAR line occurs near the northern filament boundary.) The observations are compared with model calculations of the surface wave hydrodynamics and radar scattering. The ocean waves are driven by southwesterly 8-m/s winds and interact with the front to produce primarily an enhancement of 2- to 3-m waves over a ≲200-m-wide region centered downwind of the front. Using a composite scattering radar model along with measured breaking-wave statistics, we show that the observed modulations in the radar backscatter can be accounted for through breaking-wave and tilted Bragg wave scattering effects. These results further show that SAR images of the ocean surface can be exploited for detailed study of particular ocean processes.

Subsurface, surface, and radar modeling of a Gulf Stream current convergence

In this paper we investigate the underlying dynamics associated with a strong, line-shaped submesoscale feature that was observed in radar imagery at the boundary between Gulf Stream (GS) and shelf water near Cape Hatteras during the first Naval Research Laboratory High-Resolution Remote Sensing Experiment (HIRES 1). The line-shaped feature, which appears as a pronounced (∼10 dB) increase in radar cross section, extends several kilometers in the east-west direction. In situ current measurements have shown that this feature coincides with the boundary of a sharp current convergence front. These measurements also indicate that the frontal dynamics is associated with the subduction of denser GS water under lighter shelf water. Using the observation that the convergence can be attributed to a hydrodynamic instability at the water interface, we have modeled the resulting subsurface hydrodynamics on the basis of a rigid-lid, two-dimensional solution of the Navier Stokes equation. The calculations of subsurface current flow were used as input to a spectral (wave action) model of wave-current interaction to obtain the surface wave field, which in turn was used to provide input for modeling of radar backscatter. The resulting description also includes the effects of surfactant-induced wave damping on electromagnetic backscatter. Our predictions are compared with real aperture radar imagery and in situ measurements from the HIRES 1 experiment.

INSAR imagery of surface currents, wave fields, and fronts

The authors demonstrate the ability of interferometric radar imagery to determine both relative and absolute surface velocities in the open ocean. Absolute phase calibration is accomplished by noting the azimuthal displacement of range-travelling targets-demonstrating for the first time that under favourable circumstances phase calibration can be achieved in open-ocean in the absence of ground truth. The high resolution of radar imagery permits observation of sharp velocity discontinuities, e.g. the Gulf Stream boundary and the wave field. The recent SIR-C/X-SAR shuttle missions dramatically emphasize the experimental and observational aspects of space-based radar. The combination of absolute velocities, high spatial resolution, and wide-area coverage suggest that interferometric radar imagery can provide a unique and powerful aid both for studies of global circulation patterns and detailed analysis of slope/shelf water interactions with ocean currents. In particular, the authors employ this measurement of the surface currents and wave field near a velocity front to help refine and bound results of their modeling of calculated radar images of the front. The results of this paper are compared with available ground truth. >

Sub-aperture analysis of high-resolution polarimetric SAR data

High-resolution polarimetric SAR imagery is used to illustrate aspects of sub-aperture SAR image analysis. Sub-aperture analysis is employed to highlight the temporal variation of radar backscatter within the full aperture. These changes arise from motion within the scene and from angular variation of the backscatter. Two EMISAR full polarimetric, single-look images of the Storebaelt area in Denmark were considered. These L- and C-band images have a resolution of approximately 2 meters. The EMISAR resolution compares favorably to the resolution of full polarimetric SIR-C imagery. The diversity of this coastal environment provides a rich and varied scene for sub-aperture analysis.

Radar backscatter from breaking waves in Gulf Stream current convergence fronts

Bright linear features have been observed in radar imagery taken near the Gulf Stream (GS) boundary on two separate occasions. In each case, these have been observed directly over strong current convergences. Progress has been made in understanding the origin of these signatures through simulations that incorporate environmental forcing from the winds and currents. These simulations significantly underestimate the backscatter unless wave-breaking (WB) effects are included at least approximately. Using a new, quasistatistical procedure that generalizes and quantifies earlier procedures for including WB effects, the authors have been able to successfully simulate the magnitude and behavior of these signatures. The approach combines the statistically based, composite model of radar backscatter with a deterministic feature model that relates backscatter from breaking waves to a particular geometrical model of a spilling breaker. This is accomplished using localized criteria, defined by local wave crest acceleration, to determine the probability of breaking, and by extending the feature model so that its unknown parameters may be evaluated directly from wave-current interaction calculations. The new approach provides an estimate of the critical crest acceleration of a potentially breaking wave, as a function of wind speed, that agrees with independent measurements.

Adventures in SAR processing

The image formation process associated with coherent imaging sensor is particularly sensitive to and is often corrupted by non-stationary processes. In the case of SAR, non- stationary processes result from motion within the scene, variable radar cross section, multi-path, topographic variations, sensor anomalies, and deficiencies in the image formation processing chain. Conversely, stationary processes result in image signatures that appear literal to the eye, e.g., urban infrastructure, vegetation, and natural terrain. In analyzing SAR signal history two objectives unfold. One is to obtain a well-focused image devoid of distortions and non-literal artifacts. The second objective is the detection and value-added exploitation of the non-stationary signatures. Note that the roles of signal and clutter are reversed for these two objectives. The notion that joint time-frequency (JTF) techniques may prove useful in accomplishing these objectives has spurred limited investigations into the field of coherent radar imaging systems. This paper addresses SAR image formation processing, the complex response function for a point source, and SAR JTF image formation implementations. Each of these topics is described within the context of applying JTF processing to all aspects of SAR image formation and analysis.

Hydrodynamic and radar modeling of surface features observed in Gulf Stream boundary regions

The perturbed ocean wave field and the radar return associated with Gulf Stream boundary current convergence fronts (Rips) are modeled. The surface wave field is simulated using the full-wave action density equation including the effects of wind input, wave/current interaction, and dissipation. Radar return from the ocean surface is computed using a tilted-Bragg scattering model and wave breaking statistics obtained using modelled wave dissipation results. The effect of wave breaking is examined for a range of commonly observed Rip conditions and compared with observations made during the NRL Gulf Stream 90 (Valenzuela, 1991), and Hi-Res Ocean experiments (Mied, 1992).<<ETX>>