Cross sections and modulation transfer functions at L and K u bands measured during the Tower Ocean Wave and Radar Dependence Experiment

Normalized radar cross sections and modulation transfer functions (MTFs) for microwave backscattering from the sea surface have been measured at both L- and Ku- Bands during the Tower Ocean Wave and Radar Dependence Experiment. Long waves during the experiment were usually not generated by the local wind, so a unique opportunity was afforded to investigate the effects of arbitrary wind, wave, and antenna angles on the backscatter. Data were collected using horizontal polarization at L- Band and vertical polarization at Ku- Band. Cross sections at L- Band are shown to be isotropic with respect to wind-antenna angle and nearly independent of wind speed except at the lowest wind speeds. Ku- Band cross sections, on the other hand, show the expected wind-antenna angle anisotropy and wind speed dependence. The Ku- Band cross sections agree well in magnitude with previous wave-tank and satellite measurements, casting doubt on the dependence of cross section on antenna height which has been suggested in the literature. At both frequencies the data suggest that cross sections may be lowered slightly when long waves propagate at large angles to the wind. Consistent with the cross section results, MTFs at L- Band are shown to depend little on wind speed, while those at Ku- Band decrease with wind speed. These results agree with previous studies. At both frequencies, MTFs depend little on wind-antenna angle for a constant wave-antenna angle. On the other hand, for a constant wind-antenna angle, MTFs decrease with increasing wave-antenna angle. Implications of these results for radar imagery of the sea surface and for the theoretical interpretation of the origin of MTFs are discussed.

[1]  John C. Daley,et al.  Variation of the NRCS of the sea with increasing roughness , 1971 .

[2]  W. Plant,et al.  Modulation of coherent microwave backscatter by shoaling waves , 1978 .

[3]  Werner Alpers,et al.  Airborne measurements of the ocean radar cross section at 5.3 GHz as a function of wind speed , 1986 .

[4]  R. Legeckis,et al.  Modulation of sea surface radar cross section by surface stress: Wind speed and temperature effects across the Gulf Stream , 1980 .

[5]  Peter Hoogeboom,et al.  A Discrepancy between Ground-Based and Airborne Radar Backscatter Measurements , 1982, IEEE Transactions on Geoscience and Remote Sensing.

[6]  T. W. Thompson,et al.  Seasat Sar cross‐section modulation by surface winds: Goasex observations , 1981 .

[7]  S. Larsen,et al.  Wind stress measurements during the Tower Ocean Wave and Radar Dependence Experiment , 1988 .

[8]  K. Hasselmann,et al.  The two-frequency microwave technique for measuring ocean-wave spectra from an airplane or satellite , 1978 .

[9]  T. Gerling.  Structure of the surface wind field from the Seasat SAR , 1986 .

[10]  W. Plant.  A two-scale model of short wind-generated waves and scatterometry , 1986 .

[11]  A.K. Fung,et al.  Radar determination of winds at sea , 1979, Proceedings of the IEEE.

[12]  William J. Plant,et al.  Parametric dependence of ocean wave-radar modulation transfer functions , 1983 .

[13]  D. Ross,et al.  On the detectability of ocean surface waves by real and synthetic aperture radar , 1981 .

[14]  William J. Plant,et al.  The dependence of X band microwave sea return on atmospheric stability and sea state , 1985 .

[15]  H. Masuko,et al.  Measurement of microwave backscattering signatures of the ocean surface using X band and Ka band airborne scatterometers , 1986 .

[16]  William J. Plant,et al.  Ocean wave‐radar modulation transfer functions from the West Coast Experiment , 1980 .

[17]  Wolfgang Keller,et al.  Measurement of the ocean wave-radar modulation transfer function at 4.3 GHz , 1986 .

[18]  David B. King,et al.  Relationship between Hurricane Surface Winds and L-Band Radar Backscatter from the Sea Surface , 1979 .

[19]  F. J. Wentz,et al.  A model function for ocean radar cross sections at 14.6 GHz , 1984 .

[20]  R. Moore,et al.  Short gravity and capillary wave spectra from tower-based radar , 1984 .

[21]  Werner Alpers,et al.  Measurement of the ocean wave-radar modulation transfer function at 35 GHz from a sea-based platform in the North Sea , 1986 .

[22]  J. Wright,et al.  Microwave scattering and the straining of wind‐generated waves , 1975 .

[23]  William J. Plant,et al.  The Modulation Transfer Function: Concept and Applications , 1989 .