X band microwave backscattering from ocean waves

Abstract

Backscattering experiments at microwave frequencies were conducted off the west coast of Scotland in the summer of 1991. Using a dual-polarization, eightfrequency, X band, coherent scatterometer mounted on the bow of a boat, we measured time-resolved backscattering from ocean waves at a range of grazing angles from 10 ø to 70 ø. From the grazing-angle-dependent signals and their Doppler spectra, we are able to differentiate Bragg scattering from non-Bragg scattering and resolve "peak separation" between the vertical and horizontal polarizations. We observe instances of "super" events, i.e., instances when the horizontal polarization return power equals or exceeds the vertical polarization power at particular frequencies. We find that "super" events occur not only at low grazing angles but at any grazing angle for upwind viewing directions and obtain statistics for such occurrences as a function of grazing angle. We study the coherence properties of scatterers and find strong evidence that at low grazing angles, lifetime-dominated, non-Bragg scattering contributes noticeably to returns of both polarizations, but is dominant in providing returns for the horizontal polarization. We examine "spiking" events and find that they can be related to, but need not be limited to, breaking wave events. By comparing the data of upwind runs with cross-wind and circle runs, we obtain wind direction dependence of Doppler spectra, which further assists in the identification of scattering mechanisms. 2591 2592 LEE ET AL.' X BAND MICROWAVE BACKSCATTERING FROM OCEAN WAVESTable 1. Frequency Pairs for Microwave Scatterometer Polarization, GHz Pair Vertical Horizontal 1 9.020 9.021 2 9.170 9.171 3 9.320 9.321 4 9.470 9.471 four quadrature mixers, one for each transmitted frequency. Each mixer generates an in-phase (I) and quadrature (Q) signal with a frequency response of 0 to 1000 Hz. With complex amplitude thus generated for each frequency, there are 16 channels of output. During an experiment the 16 signals are recorded digitally on a multichannel cassette recorder.An absolute calibration of the scatterometer system [Barter et al., 1993] was conducted in a large (10 m x 10 m x 30 m) anechoic chamber using spheres and cylinders of various sizes as well as corner reflector targets. Measurements consisted of establishing the output power and receiver gain of each of the eight frequency channels, the system radiation patterns in two perpendicular planes in the forward half sphere, the range dependence of the signal power, and the cross-polarization isolation of the scatterometer system. The essential results of the calibration are as follows: total power (eight channels), -1 W; nominal receiver gain each channel, -60 dB; system radiation pattern, approximately Gaussian main lobe; azimuthal plane -3 dB beam width, 8.7 ø for VV and 10.3 ø for HH; vertical plane -3 dB beam width, 11 ø for VV and 9.6 ø for HH; average -3 dB beam width, 9.5ø; antenna gain, 26.6 dB; cross-polarization isolation, >35 dB; image rejection ratio, --45 dB. As an ex...

Investigation of Low-Grazing-Angle Microwave Backscattering From Threedimensional Breaking Sea Waves

“…The wind speed varies with the incidence angle. Although Figure 8 shows the HH polarized scattering is weaker than the V V counterpart in each incidence direction, the strong increase of polarization ratio (HH/V V ) in LGA proves the existence of nonBragg scattering, which is referred as "fast signals" in [16]. Different from the classical TSM, the modified composite model agree with the measured data closely.…”

“…It is consequently found that the Kudryavtsev model is higher than our model in HH polarization and weaker than our model in V V polarization. The influence of breakers scattering on polarization ratio is shown in Figure 8, along with previous published measurements data of the ocean [16]. The experiment is conducted at X band, and the wave direction is upwind.…”

“…The simulated Doppler spectrums including breaking waves are compared with the measured time-integrated Doppler spectra of wind waves [16] in Figure 10. The operating frequency of the microwave scatterometer is in X band, and the incidence angle varies from 35 • to 80 • .…”

“…The wave height is denoted as H. The wind drift is directly related to the breaking waves. Research has shown a good approximation to the wind drift velocity v w [16] v w = αU, α = 2.6 − 5.5%.…”

“…Although the sea surface scattering is successfully modeled based on the Bragg theory in small and moderate incidence angles [6][7][8][9][10][11][12][13], some discrepancies between the predictions and experiment observations still cannot be explained. The terminology "super events" refers to the special phenomenon observed from the measured sea clutters, which includes sea spikes [14], polarization independence [15], faster scatters [16] and expanded width of Doppler spectrum [17].…”