摘要:

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 anLband 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 rep

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13829

年代:1988

数据来源: WILEY

摘要:

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 atLband is reason

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13837

年代:1988

数据来源: WILEY

摘要:

As part of the Tower Ocean Wave and Radar Dependence Experiment (TOWARD) objectives, the mechanisms of SAR imaging of ocean waves are investigated usingLband 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 s

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13849

年代:1988

数据来源: WILEY

摘要:

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.

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13859

年代:1988

数据来源: WILEY

摘要:

The synthetic aperture radar (SAR) images obtained with the Jet Propulsion Laboratory'sLband system in the Tower Ocean Wave and Radar Dependence (TOWARD) oceanographic experiment are compared with images generated by a simulation program implementing a SAR ocean imaging model based on two‐scale hydrodynamic and electromagnetic scattering approximate models. A critically regarded test of a theory of SAR ocean imaging is its prediction of the best image focus dependence on surface motion. The primary means of comparison here is an estimate of the best focus parameter that uses a subimage cross‐correlation technique. The focus parameter estimates for both the actual and simulated images show (1) a reversal in sign with reversal in dominant longwave direction relative to the SAR direction, (2) a magnitude increase with an increase in magnitude of the angle between the dominent long wave and SAR axes, and (3) an independence of the altitude and also the range‐to‐velocity ratio. All equivalent velocity estimates are of the order of the dominant longwave phase velocity, normalized by the SAR vehicle velocity, and agree well qualitatively though there are some quantitative differences. These behaviors are in agreement with a related analytical prediction. The “visibility” of the long waves' SAR image artifacts, for this TOWARD data set, increased with increased altitude and also with increased range‐to‐velocity ratio for both the actual and simulated images, as was predicted by a related analysis. Agreement of spectral density estimates of the actual and simulated SAR images generally required an order of magnitude increase in the strength of the hydrodynamic interaction of the long and short waves. This may indicate, as have surface measurements in TOWARD and other experiments, that the two‐scale model does not fully describe this nonlinear interaction. With this interaction adjustment, a comparison of actual and simulated images using an alternative focus criterion showed very close agreem

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13867

年代:1988

数据来源: WILEY

摘要:

Reliable sea surface slope time series, using a laseroptical receiver system deployed on a wave follower, are analyzed to yield slope frequency spectra of the ocean surface up to 300 Hz. The results show significant differences when compared with the Pierson and Stacy (1973) model. An empirical model is proposed in this paper that is consistent with the observed slope spectra. The newly proposed model is compared with other more recently advanced shortwave spectral models.

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13883

年代:1988

数据来源: WILEY

摘要:

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 thek−4distribution, wherekis 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

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13891

年代:1988

数据来源: WILEY

摘要:

A tower‐mounted optical device is used to measure the two orthogonal components of the sea surface slope. The results indicate that an unstable stratification at the air‐sea interface tends to enhance the surface roughness. The presence of a long ocean swell system steers the primary direction of shortwave propagation away from wind direction, and may increase or reduce the mean square slope of the sea surf

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13903

年代:1988

数据来源: WILEY

摘要:

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 ag

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13913

年代:1988

数据来源: WILEY

摘要:

A knowledge of the structure of current systems over their total depth, especially of western boundary currents such as the East Australian Current, is important in assessing global transports. Many measurements have been performed on these currents and the generation of climatological statistics for them should assist in assessing the representativeness of conditions obtained in any particular study. To examine the structure, in a statistical sense, of the East Australian Current system over most of its depth, archived hydrological station data are analyzed to produce profiles of various statistics of temperature and dynamic height perturbations (e.g., means, standard deviations, and correlation coefficients as a function of vertical separation). These are compared with the same statistics derived for two other locations with less vigorous mesoscale perturbations. Nondimensionalized geostrophic velocity profiles are also obtained. The method uses all the individual profiles from a given area and does not need series of oceanographic sections or areal surveys. It is found that the magnitude of perturbations and their vertical length scales are larger in the East Australian Current region than in either of the other two locations.

ISSN:0148-0227    

DOI:10.1029/JC093iC11p13925

年代:1988

数据来源: WILEY