Ocean Environment Sensing using Polarimetric and Interferometric SAR

New methods have been investigated which use fully polarimetric synthetic aperture radar (POLSAR) image data to measure ocean wave slopes. Independent techniques have been developed to measure wave slope spectra in both the radar azimuth and range directions. Wave spectra measured using the new methods are compared with spectra developed using conventional SAR intensity-based methods, and with spectra from in situ buoys. Wave-current interactions may also be measured using the same measurement techniques [1]. NASA/JPL/AIRSAR L-band image data from California coastal waters and from the New York Bight are used in the studies. NRL has also recently lead two collaborative field experiments that feature Along-Track Interferometric SAR (AT-INSAR) systems. In April, 2003, NRL, NASA JPL and UCLA collaborated in a study of sub-mesoscale coastal eddies that featured the NASA/JPL/AIRSAR. NRL has also recently collaborated with the University of Massachusetts in a deployment of their Dual Beam Interferometer on the west coast of Florida. This paper presents preliminary data from both of these experiments. I. POLSAR MEASUREMENT OF OCEAN WAVE SLOPES Synthetic aperture radar images of ocean surface waves have been used with intensity-based algorithms to measure physical parameters such as wave slope spectra [2]. SAR instruments operating at a single polarization base their measurements on wave-induced backscatter cross-section modulations. These measurements require a parametrically complex modulation transfer function (MTF) to relate wave properties to the SAR measurements. The studies reported here investigate the feasibility of using polarimetric SAR (POLSAR) data to measure ocean wave slopes in both the radar azimuth and range directions. In the Fourier-transform domain, this orthogonal slope information may be used to estimate a complete directional ocean wave slope (or height) spectrum. Motion-induced “velocity-bunching” effects still present difficulties for wave measurements in the azimuth direction. The advantage of using these new POLSAR algorithms is, however, that a nearly direct physical measurement of the slope is made which does not require the use of a nonlinear, complex MTF. Modulations of the polarization orientation angle are largely caused by waves traveling in the azimuth direction. A method [3] that senses modulations of is used to measure wave slopes in the azimuth direction. Slopes smaller than 1 are measurable by this method. An eigenvector/eigenvalue decomposition parameter alpha described in [4] is used to measure wave slopes in the orthogonal range direction. Waves in the range direction cause modulation of the local incidence angle that, in turn, also modulate the value of alpha. From these azimuth and range slope pairs, a complete directional wave spectrum may be estimated. Measurements of can also be used to study ocean wave-current interactions such as those produced by internal waves [1]. NASA/JPL/AIRSAR L band ocean scatter data has been used in the studies. Comparisons will be made of ocean wave spectra measured using this new POLSAR method, spectra produced from intensity images, and conventional National Data Buoy Center (NDBC) buoy spectra. A. Orthogonal Slope Measurement Pairs It has been shown by Schuler et al [3] that by measuring the orientation angle shift in the polarization signature one may determine a combined effect of the surface tilts. In particular, the shift in the orientation angle is related to the azimuth/range surface tilts and the local incidence angle. This relationship, derived by Lee [4] and Pottier [5] is: cos tan sin tan tan (1) where, , tan , tan , and are the shift in the orientation angle, the azimuth slope, the ground range slope, and the radar incidence angle, respectively. According to (1), the azimuth tilts may be estimated from the shift in the orientation angle if the range tilt is known. The orthogonal range slope can be estimated using the value of local incidence angle associated with alpha for each pixel. Since for the ocean surface, the tilt angles are small, the denominator in (1) may be approximated by sin . Thus, for the ocean surface, the azimuth slope may be written as tan (sin ) tan . Knowledge of the azimuth slope, tan and the range slope tan provides complete slope information for each image pixel. 1) Orientation Angle Measurements: Azimuth Direction Wave Spectra POLSAR data is represented, for single-look complex data, by a scattering matrix. Single-look complex, or multi-look complex data, is represented by a covariance (or coherency) matrix. An orientation angle shift causes rotation of all these matrices about the line of sight. Since the orientation angle information is embedded in the POLSAR data, several methods have been developed to estimate azimuth slope induced orientation angles for the land and sea. The “polarization signature” method and the “circular polarization” methods have proven to be the two most effective. A complete discussion of these methods and the relation of the orientation angle to orthogonal slopes and radar parameters are given in Lee et al. [4]. AIRSAR data (1994) at L-band imaging a coastal area near the Gualala River in northern California was Report Documentation Page Form Approved