Radar Dependence Experiment Research Articles

Overview of SAR Chesapeake tower experiment

Synthetic aperture radar (SAR) imaging of the ocean surface promises to be an important tool in ocean remote sensing. The long‐range goal of the work reported here is to understand SAR imaging in sufficient detail that airborne and/or spaceborne SARs become reliable tools for oceanic research and operational use.TOWARD (Tower Ocean Wave and Radar Dependence Experiment) [see Shemdin, 1988, 1990], a predecessor experiment to the SAR and X‐Band Ocean Nonlinearities Experiment at Chesapeake Light Tower (SAXON:CLT), provided a comprehensive data set to test the theoretical concepts advanced on SAR imaging of the ocean surface. A significant achievement of TOWARD is determining that none of the then available theories on SAR imaging of long surface waves could explain all the SAR observations satisfactorily. Improved models immediately followed as a consequence of the TOWARD finding. In spite of the significant insights gained in TOWARD, gaps remain in understanding radar backscatter from the sea surface.

Cross sections and modulation transfer functions at L and Ku bands measured during the Tower Ocean Wave and Radar Dependence Experiment

Normalized radar cross sections and modulation transfer functions (MTFs) for microwave backscattering from the sea surface have been measured at both L‐ and Ku‐ Bands during the Tower Ocean Wave and Radar Dependence Experiment. Long waves during the experiment were usually not generated by the local wind, so a unique opportunity was afforded to investigate the effects of arbitrary wind, wave, and antenna angles on the backscatter. Data were collected using horizontal polarization at L‐ Band and vertical polarization at Ku‐ Band. Cross sections at L‐ Band are shown to be isotropic with respect to wind‐antenna angle and nearly independent of wind speed except at the lowest wind speeds. Ku‐ Band cross sections, on the other hand, show the expected wind‐antenna angle anisotropy and wind speed dependence. The Ku‐ Band cross sections agree well in magnitude with previous wave‐tank and satellite measurements, casting doubt on the dependence of cross section on antenna height which has been suggested in the literature. At both frequencies the data suggest that cross sections may be lowered slightly when long waves propagate at large angles to the wind. Consistent with the cross section results, MTFs at L‐ Band are shown to depend little on wind speed, while those at Ku‐ Band decrease with wind speed. These results agree with previous studies. At both frequencies, MTFs depend little on wind‐antenna angle for a constant wave‐antenna angle. On the other hand, for a constant wind‐antenna angle, MTFs decrease with increasing wave‐antenna angle. Implications of these results for radar imagery of the sea surface and for the theoretical interpretation of the origin of MTFs are discussed.

Tower Ocean Wave and Radar Dependence Experiment: A synthesis

The Tower Ocean Wave and Radar Dependence Experiment (TOWARD) was conceived to provide an experimental data base to resolve the disparity among the different synthetic aperture radar (SAR) imaging theories of the ocean surface. This goal has been achieved through publication of two collections of scientific papers of which the second is part of this issue. There is now a framework and consensus on how SAR images the ocean surface.

Focusing simulations of synthetic aperture radar ocean images

The SAR ocean images obtained in the Tower Ocean Wave and Radar Dependence Experiment (TOWARD) are carefully analyzed at different focus settings and compared with simulated results based on various theories for imaging surface waves. The agreement between the experimental data and all of the SAR simulations except one is found to be favorable. There is also surprisingly close agreement among the different theoretical models themselves, suggesting a closer fundamental similarity among the contending theories than was previously thought. It is shown that the width of the focusing curve has an inverse dependence on the SAR integration time. For the TOWARD conditions it is found that the image modulation due to azimuth‐traveling surface waves is greater than that due to range‐traveling waves.

Theory for synthetic aperture radar imaging of the ocean surface: With application to the Tower Ocean Wave and Radar Dependence Experiment on focus, resolution, and wave height spectra

A one‐dimensional model is developed to simulate azimuthal synthetic aperture radar (SAR) imaging of the ocean surface. The model developed here is general and can admit both the “distributed surface” and the “velocity bunching” approaches. Computer simulations are performed to test the validity of this model. The simulations show that the time‐dependent modulation patterns due to the radar cross section variation and the velocity bunching effects give optimum focusing around half the phase velocity of the long wave. It is shown that the focus dependence is due to an incoherent integration process and not due to motion induced phase errors. It is argued that the radar reflectivity is correlated over distances small in comparison with the SAR resolution. In the simulations the radar reflectivity is assumed to be spatially decorrelated. Also given are the relative importance of the various effects that degrade the azimuthal resolution. It is shown that the limited radar temporal coherence due to the velocity spread of short waves degrades the azimuthal resolution of the SAR. This coherence time depends on sea state and radar wavelength. It is shown that the orbital acceleration effects are small in comparison with the smearing due to the velocity spread. It is found that the degraded azimuthal resolution of the radar minimizes the nonlinear effects of velocity bunching. The simulations show that in the Tower Ocean Wave and Radar Dependence Experiment (TOWARD), SAR imaging at L band is reasonably linear.

Wind stress measurements during the Tower Ocean Wave and Radar Dependence Experiment

During the Tower Ocean Wave and Radar Dependence Experiment (TOWARD), near‐continuous measurements of the wind drag were conducted using the dissipation technique. An intercomparison between these measurements and direct stress magnitudes using a sonic anemometer was performed over 3 days of the experiment. The results indicated that the dissipation technique compared well to the directly determined values when conditions were steady and neutral; otherwise, the dissipation method performed poorly. When neutrally stratified data were used, the drag coefficient exhibited a systematic dependence on both surface tension and wave age.

Multifocus processing of L band synthetic aperture radar images of ocean waves obtained during the Tower Ocean Wave and Radar Dependence Experiment

As part of the Tower Ocean Wave and Radar Dependence Experiment (TOWARD) objectives, the mechanisms of SAR imaging of ocean waves are investigated using L band SAR data over the Naval Ocean Systems Center tower. This paper provides experimental evidence needed to validate the differing hypotheses. Various processing methods are investigated to generate spectra with large degrees of freedom. The results show that waves traveling in the aircraft direction are most detectable at focus settings in the range 10.0–15.0 m/s, which is consistent with the Marine Remote Sensing Experiment (MARSEN) observations reported by Jain and Shemdin (1983). Waves traveling in the direction opposite to the aircraft are most detectable at settings equal to −5.0 to −15.0 m/s. The SAR imaging system acts as a low‐pass filter with the peak of the ocean wave height spectrum occurring at higher wave numbers compared with the peak in the SAR image spectrum.

Directional measurement of short ocean waves with stereophotography

Stereophotographs of the sea surface, acquired during the Tower Ocean Wave and Radar Dependence experiment (TOWARD), are analyzed to yield directional wave height spectra of short surface waves in the 6‐ to 80‐cm range. The omnidirectional wave height spectra are found to deviate from the k−4 distribution, where k is the wave number. The stereo data processing errors are found to be within ±5%. The omnidirectional spectra yield 514 degrees of freedom for 30‐cm‐long waves. The directional distribution of short waves is processed with a directional resolution of 30°, so as to yield 72 degrees of freedom for 30‐cm‐long waves. The directional distributions show peaks that are aligned with the wind and swell directions. It is found that dynamically relevant measurements can be obtained with Stereophotography, after removal of the mean surface associated with long waves.

Tower Ocean Wave and Radar Dependence Experiment: An overview

The Tower Ocean Wave and Radar Dependence Experiment (TOWARD) is a coordinated effort, involving both field experimentation and theoretical investigations, to address the divergent hypotheses on the mechanisms involved in synthetic aperture radar (SAR) imaging of the ocean surface. In the formulation of the experiment it was recognized that three distinct disciplines were to be addressed: (1) hydrodynamics, (2) radar backscatter, and (3) SAR image processing. The field operations were executed in three 6‐week segments during the period October 1984 to January 1986. The Naval Ocean Systems Center tower, located offshore of Mission Beach, San Diego, California, was used as the focal point for the field operations. The primary measurements included in situ capillary and short gravity waves, long surface waves and internal waves, ambient current and detailed meteorological measurements, stereophotography, tower‐based multifrequency radars and an L band SAR deployed on board the NASA CV‐990. The single most significant result is the determination that none of the available hypotheses on SAR imaging of long surface waves could be demonstrated to explain all the SAR observations obtained in TOWARD. Specific results are reported.

An analytic representation of the synthetic aperture radar image spectrum for ocean waves

An analytic expression is derived for the spectrum of a synthetic aperture radar image in terms of the surface reflectivity covariance function. It is shown that within the severe constraints imposed by the short integration time, the third (temporal) dimension of the spectrum can be extracted by varying the processor focus parameter. The results also illustrate the dependence of the image spectrum on the scene coherence time, as well as various nonlinear effects associated with the velocity bunching mechanism. Example calculations are presented for two cases corresponding to data sets collected during the Tower Ocean Wave and Radar Dependence Experiment.

Incorporation of Stratification Effects on the Oceanic Roughness Length in the Derivation of the Neutral Drag Coefficient

Abstract Based on the assumption that, over the sea, the roughness length of the wind profile scales with the wind stress, a new formulation that describes the drag coefficient as a function of the given neutral drag coefficient and stability is derived. The new formulation is compared to an earlier formulation where roughness changes with stability were ignored. The two are then illustrated with data collected from both the Marine Remote Sensing Project (1979) and the Tower Ocean Wave and Radar Dependence Experiment (1984). It was found that when the surface roughness was allowed to depend on wind stress (and therefore stability), the stratification correction to the neutral drag coefficient was larger than for the case when the roughness length was not allowed to vary.