Unconstrained inversion of waveheight spectra from SAR images

A procedure for inverting the nonlinear relationship between the waveheight spectrum and the SAR image spectrum is presented, and this procedure is evaluated using simulated data as well as actual ERS SAR data collected near Duck, NC. Results of this nonlinear inversion are compared with those obtained from a quasi-linear estimation procedure using simulated data, in order to illustrate the effects of nonlinearities in the imaging process. These effects include the well-known azimuth falloff effect as well as the generation of harmonics which appear in the background region of the spectrum. The nonlinear inversion technique is able to reproduce the input image spectrum to high accuracy, although the wave spectrum obtained by this procedure is not necessarily the same as the input wave spectrum. In general, the estimated wave spectrum is quite similar to the portion of the input wave spectrum within the SAR passband region, but none of the energy outside the passband is recovered. The background signals due to nonlinear effects can cause large errors in the quasi-linear estimation procedure because these signals appear in regions of the spectrum where the SAR modulation transfer function (mtf) is small. Results using actual SAR data also indicate that energy within the passband is recovered fairly accurately, although energy outside the passband is clearly lost     

Multispectral bathymetry using a simple physically based algorithm

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.     

Measurement of the surface tension of water using microwavebackscatter from gravity-capillary waves

A technique developed for measuring the surface tension of fluid based on the resonant scattering of microwave radiation from gravity-capillary waves on the surface of the fluid is discussed. The method was implemented by generating monochromatic water waves in a tank, illuminating them with microwave radiation, and then tuning the water-wave frequency until Bragg resonance was observed, that is, until the backscattered power was observed to reach its maximum value. At resonance, the wavelength and wave frequency can be calculated from the physical parameters of the system and from the Doppler shift of the scattered radiation. Laboratory experimental results indicate an accuracy on the order of ±1.6 dyn/cm for the surface-tension measurement. This accuracy is in agreement with an error model based on the width of the Bragg resonance line. The technique is, in principle, nonintrusive and thus can be used for observing the properties of surfactants on water surfaces     

The Contribution of Wedge Scattering to the Radar Cross Section of the Ocean Surface

This letter considers the contribution to the radar cross section of the ocean surface due to scattering from edges for which the local radius of curvature is small compared with the radar wavelength. An analytic expression based on the method of equivalent currents is given for such scattering and is evaluated for several assumed sets of parameters. This contribution is shown to augment the Bragg scattering cross section in regions where the latter underestimates the measured radar cross section, while remaining smaller than the Bragg component elsewhere.     

Azimuth falloff effects in two-antenna SAR measurements of oceanwave spectra

Estimates of ocean wave spectra obtained from conventional synthetic aperture radar (SAR) images suffer from the azimuth falloff effect which limits the range of azimuth wavenumbers that can be observed by such systems. This effect was earlier predicted to be modified by the use of a second receive antenna which is displaced from the first in the along-track direction. The authors present experimental evidence which is consistent with these theoretical predictions and suggests that the azimuth falloff limitations inherent in conventional SAR systems can be overcome by the use of two or more appropriately spaced antennas     

Numerical calculations of radar scattering from sharply peakedocean waves

A numerical solution of the surface-current integral equation is used to calculate the radar backscatter for several wavelike surfaces. The surfaces used in this study represent solutions of the water wave equations for finite amplitude, irrotational gravity waves, with steepnesses selected so as to produce minimum radii of curvature ranging from zero to a value equal to the electromagnetic wavelength and with wave amplitudes ranging from about one-half to five times the electromagnetic wavelength. The results are used to evaluate the importance of edge diffraction effects on backscatter from sharply peaked ocean waves     

Comparison of WindSat brightness temperatures with two-scale model predictions

Predictions of the polarized microwave brightness temperatures over the ocean are made using a two-scale surface bidirectional reflectance model combined with an atmospheric radiative transfer model. The reflected atmospheric radiation is found to contribute significantly to the magnitude and directional dependence of the brightness temperatures. The predicted brightness temperatures are also sensitive to the form of the shortwave spectrum. Calculations are made using a new physically based model for the wave spectrum, and preliminary comparisons are made with WindSat observations at 10.7, 18.7, and 37 GHz, for wind speeds ranging from 0-20 m/s and for vertically integrated atmospheric water vapor concentrations from 0-70 mm. Predictions of the mean (azimuthally averaged) brightness temperatures for vertical and horizontal polarization agree quite well with WindSat observations over this range of wind speeds and water vapor concentrations. The predicted azimuthal variations of the third and fourth Stokes parameters also agree fairly well with the observations, except for the fourth Stokes parameter at 37 GHz. Further adjustments of the wave spectrum are expected to improve the agreement.     

Multiwavelength optical pyrometer for shock compression experiments

A system for measurement of the spectral radiance of materials shocked to high pressures ( approximately 100 GPa) by impact using a light gas gun is described. Thermal radiation from the sample is sampled at six wavelength bands in the visible spectrum, and each signal is separately detected by solid-state photodiodes, and recorded with a time resolution of approximately 10 ns. Interpretation of the records in terms of temperature of transparent sample materials is discussed. Results of a series of exploratory experiments with metals are also given. Shock temperatures in the range 4000-8000 K have been reliably measured. Spectral radiance and temperatures have been determined with uncertainties of 2%.