V. Hesany

Slope-induced nonlinearities on imaging of ocean waves

The MTF (modulation transfer function) is the primary descriptor used to relate the variations of the electromagnetic backscatter from the ocean to the ocean wave parameters. The major assumption involved in the definition of the MTF is a linear relation between the backscattered power and the long-wave slope. An investigation of this assumption is presented showing that the backscattered power and wave-height are nonlinearly related at least for tilt modulation. The nonlinear relation between the backscattered power and the long-wave slope is analyzed for both vertically and horizontally polarized backscatterers. For HH polarization, at maximum slopes of 0.1 rad, the second harmonic contribution is 37% for a pointing angle of 20 degrees and reaches its minimum of roughly 19% at a pointing angle of 50 degrees . For VV polarization, the first-order amplitude and harmonic nonlinearities are always smaller than the corresponding values for HH polarization and are significant only at angles near vertical. >

Sea Spikes at Moderate Incidence and Their Relation to Position on the Waves

Most models of radar backscatter from the sea ignore sea spikes, the nonlinear effects that result in large excursions above the local mean signal. They are strong enough to change the mean scattering level significantly, and they may cause streaks in synthetic aperture radar (SAR) ocean images. A threshold based on short signal excursions above the local mean is used to identify spikes. With this method, spikes can be found in regions of low signal level. An autoregressive spectral method is used to identify the locations of the spikes on the dominant waves. Sample results from Ka-band measurements in the North Sea made during the SAXON-FPN experiment are given. It is shown that spikes can occur anywhere on the dominant wave, although they are most prevalent on the front face.

Microwave backscatter spatial variations in response to low winds and ocean fronts

Comparisons among measurements of sea surface Ku-band backscatter (dual polarized), wind speed, air and sea temperatures and friction velocity, heat and moisture fluxes were made from a manned airship platform off the Oregon coast in October 1993 during a month long field experiment. The broad objective of the experiment was to observe air/sea interactions with high spatial resolution and temporal coincidence of numerous sensors, in a coastal environment. The blimp platform enabled the measurements of surface conditions that displayed rapid spatial variations along its path. Among the most interesting observations were the frequent large amplitude drops and rises of the azimuthally averaged radar cross section (between 20/spl deg/ and 50/spl deg/ incidence angles) associated with wind variations in the range from 1 to 4 m/s. These observations demonstrate a minimum wind speed threshold for wave generation, which is being measured on the ocean for the first time after having been studied previously only in indoor wave tanks.

Comparison of Slope, Wave Height, and Radar Spectra, Slope and Hydrodynamic Modulation of Radar Scatter from the Sea

Description of Ocean waves usually depends on point measurements of wave height. The important vector slope of the ocean must usually be derived either from point measurements and linear assumptions or from pitch-and-roll buoys that cannot be in the radar footprint. We developed a 35-GHz vector slope gaugehat- terometer using a single switched-beam antenna. It can measure three adjacent height profiles of the ocean, from which we can derive two orthogonal components of the slope. We used the vector slope gauge during the SAXON-FFN experiment in November, 1990. Simultaneous measurements of the orthogonal components of the long-wave slopes and the backscattered power permit determination of the relative contribution to the overall modulation of the radar signal by slope modulation and hydrody- namic modulation of Bragg-resonant ripple amplitude.

Spatial variations of sea surface microwave backscatter in response to wind, wind stress and surface current variations

Comparisons among submesoscale sea surface K/sub u/-band backscatter, wind speed, air and sea temperatures and friction velocity were made to observe the dynamic responses in time and space of the centimeter waves within kilometer sized areas. A unique experimental configuration using a Blimp as a radar platform has made it possible to conduct K/sub u/-band radar cross section observation with a full 360/spl deg/ azimuth sweep, for simultaneous vertical and horizontal polarizations. Measurements were made in October 1993 off the Oregon coast from an airship that also carried a suspended package of meteorological and physical instruments that is positioned close to the sea surface.

Spatial variations of microwave backscatter magnitude and angular distribution: response of the short waves to wind and surface changes

Comparisons among submesoscale sea surface Ku-band backscatter, wind speed, air and sea temperatures, friction velocity and laser altimeter wave height spectra were made to observe the dynamic responses in time and space of the centimeter waves within kilometer sized areas. Measurements were made in October 1993 off the Oregon coast from an airship carrying a dual polarized Ku-band radar with a suspended package of meteorological and physical instruments, and a laser altimeter.

The response of the short wave directional spectrum to rapid wind and surface variations using microwave backscatter

For the first time, it has been possible to conduct airborne K/sub u/-band (14 GHz) radar cross section measurements with a full 360/spl deg/ azimuthal sweep of the sea surface combined with simultaneous measurements of coincident wind speed, friction velocity and numerous other physical variables at the air-sea interface within a kilometer-sized area. Measurements were conducted from a blimp during September and October of 1993 off the coast of Oregon. The broad objective of the experiment was to understand air/sea interactions in a coastal environment using a variety of remote (active and passive), and in-situ sensors.

Slope and hydrodynamic modulation of radar scatter from the sea

Microwave backscatter from the ocean surface is largely due to Bragg scattering from short surface ripples. Modulation of the signal results from changes in the local angle of incidence as the local slope changes, and from variations in Bragg ripple amplitude. The slope (tilt) modulation can be modeled as a memoryless nonlinear system. The hydrodynamic modulation results from a nonuniform distribution of the amplitude of the small-scale ripples over the large-scale waves. For azimuthally traveling waves the hydrodynamic modulation dominates, while for waves propagating in other directions, both tilt and hydrodynamic modulation are significant. The authors developed a 35-GHz radar vector slope gauge (VSG) to measure the orthogonal components of the surface slopes within the radar footprint. Simultaneous measurements of the surface slope and radar cross section permit determination of the relative contribution of slope and hydrodynamic modulations to the overall fluctuation of the radar signal. The authors present a method for separating effects due to the surface tilting from hydrodynamic effects. They include a sample result based on this approach with data from the SAXON-FPN experiment in November, 1990.<<ETX>>

Autoregressive modeling for ocean wave — radar modulation transfer function

The standard technique for computing ocean wave — radar modulation transfer functions (MTFs) depends on nonparametric fast Fourier transform (FFT) methods. In this paper we suggest a new approach applying an autoregressive (AR) parametric spectral estimator. The analysis shows that the two-channel autoregressive model produces smooth estimates of the MTF and permits better frequency resolution than FFT techniques. The feed-across effect inherent in the methods developed to evaluate the multichannel AR coefficients causes a negligible effect on MTF. The AR-based MTF depends weakly on the record length; for steady meteorological conditions the AR model of the MTF for a 2-min data run is comparable with ones calculated using 10 min or more of data. In the same situation the FFT technique is unreliable for records of less than 10 min. The appropriate order of the multichannel AR model can vary without causing significant change in the MTF. In most cases any order between 8 and 16, depending on radar system parameters and meteorological conditions, will give acceptable results.