Preliminary Results of the NASA/Goddard Space Flight Center Short Pulse Radar Ocean Waves Sensing Program

The Goddard Space Flight Center 13.9 GHz 12 ns pulse compression radar was flown on extended NASA CV-990 aircraft missions in 1977 and 1978 for the primary purpose of advancing a simple microwave radar technique for measuring ocean wave directional spectra from aircraft and satellite platforms, and secondarily for advancing in general the state-of-knowledge of microwave interaction with sea and ice surfaces. The radar is a versatile, rather complex, instrument having basically three functional modes that are associated with three different antenna systems. These modes are: a. Altimeter (ALT. A 12\deg \times 12\deg nadir-pointing horn antenna is used for significant wave height measurements to within the 1.5 m resolution capability of the radar. This well-known technique makes use of the time-stretch of the leading edge of the average pulse return in the presence of waves. b. Real Aperture Imaging Radar (SLAR). A 1/2\deg \times 40\deg slotted waveguide antenna mounted on the aircraft instrument sled is used for wave imagery at low altitude ( 16\deg nadir angle and having a 10\deg elevation and 4\deg azimuth beamwidth, was used in the DWS mode. (In the 1977 mission an adjustable elevation angle PC antenna fixed at broadside azimuth was used to study incidence angle dependence.) The DWS concept, briefly, makes use of the azimuthal averaging of ocean wave contrast features to isolate Fourier contrast wave components traveling in the radar look direction. Spectral analysis of the return for the contrast wave modulation over 360\deg azimuth angle yields a directional contrast spectrum that is in a first approximation proportional to the directional spectrum of total wave slope variance. Modulation signal-to-Rayleigh clutter noise ratios are improved by Doppler time domain filtering prior to the spectral analysis. A preliminary analysis of the digital data from flights in the Northeastern Pacific over several environmental buoys and ocean weather ship PAPA, and from a flight in the Norwegian Sea over an instrumented weather ship during storm conditions is presented. The directional modulation spectrum is related to the directional wave height spectrum through a physical scattering model: essential similarity with the directional slope spectrum is shown. The short pulse DWS and its sister dual-frequency DWS concepts are discussed with reference to future satellite DWS instrument design.