IEEE Transactions on Geoscience and Remote Sensing, 2009
A numerical study of the retrieval of sea surface height profiles from low grazing angle radar ob... more A numerical study of the retrieval of sea surface height profiles from low grazing angle radar observations is described. The study is based on a numerical method for electromagnetic scattering from one-dimensionally rough sea profiles, combined with the "improved linear representation" of Creamer et al for simulating weakly non-linear sea surface hydrodynamics. Numerical computations are performed for frequencies from 2975 to 3025 MHz so that simulated radar pulse returns are achieved. The geometry utilized models a radar with antenna height 14 m observing the sea surface at ranges from 520 m to 1720 m range. The low grazing angles of this configuration produce significant shadowing of the sea surface, and standard analytical theories of sea scattering are not directly applicable. Three approaches for retrieving sea height profile information are compared. The first method uses a statistical relationship between the surface height and the computed radar cross sections versus range (an incoherent measurement). A second method uses the phase difference between scattering measurements in two vertically separated antennas ("vertical interferometry") in the retrieval. The final technique retrieves height profiles from variations in the apparent Doppler frequency (coherent measurements) versus range, and requires that time-stepped simulations be performed. The relative advantages and disadvantages of each of the three approaches are examined and discussed.
IEEE Transactions on Geoscience and Remote Sensing, 2006
A simple method for estimating water depths from multispectral imagery is described and is applie... more A simple method for estimating water depths from multispectral imagery is described and is applied to several IKONOS data sets for which independent measurements of the water depth are available. The methodology is based on a physical model for the shallow-water reflectance, and is capable of correcting for at least some range of water-quality and bottom-reflectance variations. Corrections for sun-glint effects are applied prior to the application of the bathymetry algorithm. The accuracy of the depth algorithm is determined by comparison with ground-truth measurements, and comparisons between the observed and calculated radiances are presented for one case to illustrate how the algorithm corrects for water-attenuation and/or bottom-reflectance variations.
IEEE Transactions on Geoscience and Remote Sensing, 2006
The Naval Research Laboratory WindSat polarimetric radiometer was launched on January 6, 2003 and... more The Naval Research Laboratory WindSat polarimetric radiometer was launched on January 6, 2003 and is the first fully polarimetric radiometer to be flown in space. WindSat has three fully polarimetric channels at 10.7, 18.7, and 37.0 GHz and vertically and horizontally polarized channels at 6.8 and 23.8 GHz. A first-generation wind vector retrieval algorithm for the WindSat polarimetric radiometer is developed in this study. An atmospheric clearing algorithm is used to estimate the surface emissivity from the measured WindSat brightness temperature at each channel. A specular correction factor is introduced in the radiative transfer equation to account for excess reflected atmospheric brightness, compared to the specular assumption, as a function wind speed. An empirical geophysical model function relating the surface emissivity to the wind vector is derived using coincident QuikSCAT scatterometer wind vector measurements. The confidence in the derived harmonics for the polarimetric channels is high and should be considered suitable to validate analytical surface scattering models for polarized ocean surface emission. The performance of the retrieval algorithm is assessed with comparisons to Global Data Assimilation System (GDAS) wind vector outputs. The root mean square (RMS) uncertainty of the closest wind direction ambiguity is less than 20 for wind speeds greater than 6 m/s and less than 15 at 10 m/s and greater. The retrieval skill, the percentage of retrievals in which the first-rank solution is the closest to the GDAS reference, is 75% at 7 m/s and 85% or higher above 10 m/s. The wind speed is retrieved with an RMS uncertainty of 1.5 m/s.
Journal of Atmospheric and Oceanic Technology, 2017
This paper describes a method of processing marine radar signals for the purpose of generating ph... more This paper describes a method of processing marine radar signals for the purpose of generating phase-resolved surface elevation maps as well as statistical measures of ocean surface wave fields. The method is well suited to the processing of data collected by marine radars because it allows for the incorporation of effects dependent on the radar look direction relative to the propagation direction of ocean waves. Applications to Doppler radar and backscattered power measurements are described, and example results are presented using simulated radar data.
This paper explores the possibility of estimating smallscale vector wind fields from SAR data by ... more This paper explores the possibility of estimating smallscale vector wind fields from SAR data by applying certain dynamical constraints to the wind field. The method is illustrated for the case of a diverging surface flow field that is intended to represent a precipitationinduced downdraft. A simulated radar cross section map generated from this flow field is used to reconstruct the wind field by choosing the wind direction at each point so as to minimize the vorticity. The method works well for this case because the input wind field was chosen to be irrotational. For cases in which the vorticity is not zero, it may be possible to generalize the method by applying other dynamical constraints.
: The long-term goal is to provide a more detailed understanding of the surface roughness and rad... more : The long-term goal is to provide a more detailed understanding of the surface roughness and radar backscatter variations associated with nonlinear internal waves under a variety of environmental conditions. The objective is to determine the effects of wave-current interactions and nonlinear surface wave interactions on the development of the short wave spectrum in the presence of oceanic internal waves.
Public reporting burden for the collection of information is estimated to average 1 hour per resp... more Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. To measure vector wind fields over the global oceans on a continuous basis using satellite-borne microwave radiometers 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT Same as Report (SAR) 18. NUMBER OF PAGES 2 19a. NAME OF RESPONSIBLE PERSON
This project is part of a larger effort to use the high resolution capabilities of synthetic aper... more This project is part of a larger effort to use the high resolution capabilities of synthetic aperture radar to map variations in ocean surface wind and wave fields on spatial scales that are not currently resolved by other spaceborne systems such as scatterometers, radiometers, and altimeters.
This report describes the SAR data collection and processing activities performed to date for the... more This report describes the SAR data collection and processing activities performed to date for the SAR Internal Wave Signature Experiment (SARSEX). During SARSEX, a total of 88 passes of X- and L-band SAR data were collected over two test areas by the ERIM/CCRS CV-580 SAR System. These data were collected over an instrumented ocean site located 100 km south of Long Island, NY and a calibrated reflector array site. A description of the CV-580 SAR System is presented. The location of each SAR pass, along with a listing of the SAR and aircraft parameters, are given. All SAR data have been precision optically processed and reviewed. A selected subset of data have been digitally processed and a preliminary engineering assessment of the quality of the SAR imagery performed. The results of this assessment are presented, as are representative examples of optically-and digitally-processed SAR imagery collected during SARSEX. Finally, the analysis plans for the SAR data are discussed, includin...
The effects of nonlinear energy transfer on the development of the short wave spectrum are evalua... more The effects of nonlinear energy transfer on the development of the short wave spectrum are evaluated using a diffusion approximation and a modification of this approximation to include nonlocal effects. Both formulations were used to compute the evolution of a JONSWAP-type spectrum, and the results are compared with direct numerical simulations. Terms corresponding to each of these formulations were then incorporated into the wave action equation, and the resulting equation was numerically integrated using a second-order Runge-Kutta method. The results show an increase in the angular width of the spectrum and in the spectral density at high wave numbers as compared with solutions of the action equation without the nonlinear energy transfer term. Example results are presented for the case of a moderately strong internal wave in a light wind, and implications for the remote sensing of these waves using microwave radar are discussed.
, coincident shuttle imaging radar-B synthetic aperture radar (SAR) imagery and wave measurements... more , coincident shuttle imaging radar-B synthetic aperture radar (SAR) imagery and wave measurements from airborne instrumentation were acquired. The two-dimensional wave spectrum was measured by both a radar ocean wave spectrometer and a surface contour radar aboard the aircraft. In this paper we compare two-dimensional SAR image intensity variance spectra with these independent measures of ocean wave spectra to verify previously proposed models of the relationship between such SAR image spectra and ocea•n wave spectra. The results illustrate both the functional relationship between SAR image spectra and ocean wave spectra and the limitations imposed on the imaging of short-wavelength, azimuth-traveling waves. 1. INTRODUCTION The first spaceborne synthetic aperture radar (SAR) used to estimate ocean wave spectra from space was aboard the Seasat satellite in 1978. In spite of the short (3 months) life of the satellite, substantial quantities of SAR wave imagery were acquired. The relatively modest portion of th9se data, which were fully digitally processed to produce geometrically and radi•ometrically corrected SAR imagery and associated image intensity variance spectra, revealed a general correlation between the location of spectral peaks in the image spectra and independent surface measurements and wave model predictions [Beal et al., 1983, 1986a, b]. However, the lack of independently measured, twodimensional wave spectra with wave number and angular resolution comparable to those of SAR image spectra precluded verification of any thorough description of the relationship between SAR image intensity variance spectra and ocean wave slope or height variance spectra. Additionally, the imaging of azimuth-traveling (traveling parallel to the satellite ground track) waves in Seasat SAR imagery was observed to be limited. Two-dimensional image spectra clearly exhibited a loss of spectral response at large azimuth wave numbers. The effect was observed to become more pronounced in higher sea state conditions, making the measurement of short azimuth-traveling waves the most significant limitation on the use of spaceborne SAR wave imagery to estimate ocean wave spectra [Beal et al., 1986a; Monaldo and Beal, 1986' Alpers et al., 1986]. A SAR achieves fine-scale range (perpendicular to the satellite ground track) resolution by fine-scale timing of the return trip travel time of the radar pulse (Figure t). Azimuth resolution, many times finer than the antenna footprint on the surface, is achieved by examining the Doppler shift of the return signals. When the Doppler shift of a scatterer is zero, the azimuth position of the scatterer is assumed •to he the same as the along-track position of the SAg platform. Azimuth resotu-Copyright 1988 by the American Geophysical Union. Paper number 8C0089. 0148-0227/88/008C-•89505.• tion is then limited by the accuracy with which Doppler shifts can be determined. Although there may be considerable disagreement on the details, all investigators attribute the observed loss in azimuth image resolution to ocean surface motion distorting the Doppler return signal from the surface [
An integrated hydrodynamic-electromagnetic model which combines a full-spectrum numerical solutio... more An integrated hydrodynamic-electromagnetic model which combines a full-spectrum numerical solution of the wave action equation with a two-scale composite radar backscatter model is described, and results are presented for several internal wave cases observed during recent synthetic aperture radar experiments. The combined model predicts radar backscatter modulations significantly larger than those predicted by a simple Bragg scattering model under relatively high wind speeds (> 5 m/s), particularly at short wavelengths (X band) and for horizontal polarization. Furthermore, these predictions are in general agreement with observations in some cases where the simple Bragg model fails. Under other conditions, particularly at lower wind speeds and for vertical polarization, the predictions of the composite model are not significantly different from those of the Bragg model. In some of these cases, both models appear to underpredict the observed radar modulations, indicating that additional hydrodynamic and/or electromagnetic effects are present which are not included in either model. 1. INTRODUCTION Oceanic internal waves are visible on synthetic aperture radar (SAR) images because of variations in surface roughness which are related to the surface currents induced by the internal waves. These surface current variations are similar to those caused by flow over shallow bottom topographic features. Hence the problem of modeling the surface roughness and radar backscatter variations associated with internal waves is essentially the same as that for shallow bottom features. Previous treatments of this problem [e.g., Shuchman, 1982; Lyzen•la et al., 1983; Alpers and Hennin•ls, 1984; Alpers, 1985; Thompson and Gasparovic, 1986] have used an action balance equation to compute variations in wave spectral density, coupled with a simple Bragg scattering model to relate these spectral perturbations to variations in the radar cross section and SAR image intensity. This simplified scattering model also allowed a simplification in the hydrodynamic calculation, since it required the spectral perturbations at only one wavelength (the Bragg wavelength). Comparison of the results of this simplified hydrodynamicelectromagnetic model with actual SAR images showed fairly good agreement at L band (23 cm) wavelengths but very poor agreement at X band (3 cm) wavelengths. Since it is known that surface waves with wavelengths much longer than the Bragg wavelength can influence the radar return via surface tilt effects, the next logical extension of the earlier modeling work was to include these effects using a two-scale or composite surface scattering model [e.g., Valenzuela, 1978]. This extension is described in the present paper, and results are presented for cases corresponding to internal wave data collected during the Georgia Strait Experiment and the SAR Internal Wave Signature Experiment (SARSEX). In order to facilitate calculation of surface wave spectral perturbations over the entire spectrum of waves, a numerical
Airborne C band synthetic aperture radar (SAR) data were collected during the Norwegian Continent... more Airborne C band synthetic aperture radar (SAR) data were collected during the Norwegian Continental Shelf Experiment (NORCSEX) carried out in March 1988. Unique signature variations in the SAR backscatter due to upper ocean circulation features and wind fronts were found. It is documented that a current jet of 0.3 m s -• with a shear of about 0.3 x 10 -3 s -• is correlated with a narrow, bright curved line in the backscatter. In comparison a rapid wind shift of 9 m s-• resulted in a sharp transition from dark to brighter backscatter regions. The corresponding profiles of backscatter modulation across these features are expressed by a peak of about 2 dB in contrast to a steplike increase of 5-8 dB. This suggests that SAR image expressions of upper ocean circulation features and wind fronts can be distinguished and classified. On the basis of this classification, we attempt to quantify the dominating marine geophysical variables. This method for systematic interpretation of SAR images should be further validated with the use of airborne or satellite data such as from the first European Space Agency remote sensing satellite, ERS 1. 1. INTRODUCTION The first European Space Agency (ESA) remote sensing satellite (ERS 1) will be launched into polar orbit in spring 1991. ERS 1 will be equipped with an active microwave sensor package including a combined synthetic aperture radar (SAR) and scatterometer and a radar altimeter. Seasat demonstrated that the combination of such spaceborne active microwave sensors can provide measurements of surface wind, waves, and ocean surface currents (Beal et al., [1981]; see also the Seasat II collection of papers in the Journal of Geophysical Research, volume 88, number C3, 1983). In order to simulate and evaluate the ERS 1 detection capability, the pre-launch ERS 1 Norwegian Continental Shelf Experiment (NORCSEX '88) was carried out off the west coast of Norway, centered at 64øN, in March 1988. The NORCSEX program was selected by ESA under the Announcement of Opportunity and will continue with two post-launch validation experiments in summer and fall 1991. The sampling strategy of NORCSEX '88 was to measure simultaneously near-surface wind, waves, and current and their interaction by coordinated use of remote sensing and in situ data [Johannessen, this issue]. In particular, C band SAR measurements were acquired using the Canadian Convair-580 aircraft with the aim of investigating the feasibility of classifying and quantifying variations in SAR backscatter due to upper ocean circulation features and wind fronts. Synthetic aperture radar imaging of the ocean surface at incidence angles from 20 ø to 70 ø is principally caused by resonant Bragg scattering, whereby the transmitted radar waves are scattered by short gravity waves of approximately the radar wavelength [Wright, 1978; Hasselmann et al.,
Capillary-gravity wave spectra are measured using a scanning laser slope gauge (SLSG), and simult... more Capillary-gravity wave spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary-gravity waves is in reasonable agreement with measurements by J•ihne and Riemer (1990) obtained from measurements in a large wind-wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope-frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long waves are responsible for a large Doppler shift of capillary-gravity waves. Close examination of combined wavenumber-frequency slope spectra also reveals significant smearing of the spectra in the frequency domain due to long waves. These observations confirm that spatial measurements (wavenumber spectra measurements) are essential for characterizing short capillary-gravity waves, since this strong Doppler shift will dramatically change apparent frequency spectra. in idealized conditions [Hara and Mei, 1994]. The parameterization of the surface roughness in practical conditions still relies heavily on empirical observations [e.g., Charnock, 1955]. Many theoretical, as well as experimental, studies are available on the generation, the nonlinear wave-wave interaction, and the dissipation of the capillary-gravity waves
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Papers by David Lyzenga