On Airborne Doppler Navigation System Application for Measurement of the Water Surface Backscattering Signature and Estimation of the Near-Surface Wind

Abstract

Abstract: A method for measuring the water surface backscattering signature and estimating the near-surface wind vector over water using the airborne Doppler navigation system in addition to its standard navigational application is discussed. A case of an airplane circle flight measurement of the azimuth normalized radar cross section curve of the water surface in the range of middle incidence angles is considered. The system op-erates in the scatterometer mode and uses a fore-beam directed to the right side at a typical mounting angle in the vertical plane that is not so far from nadir at a straight flight. Wind vector is recovered from the azimuth normalized radar cross section curve obtained. Algorithms for measuring the water surface backscattering signature and extracting the wind speed and direction are proposed. Keywords: Doppler navigation system, Scatterometer, Water surface backscattering signature, Sea wind, A lgorithm. INTRODUCTION On the global scale, the information about sea waves and wind, in general, could be obtained from a satellite using active microwave instruments: Scatterometer, Synthetic Aperture Radar and Radar Altimeter [1]. However, for the local numerical weather and wave models as well as for a pilot on an amphibious airplane to make a landing decision, the local data about wave height, wind speed and direction are required. Many researchers have been investigating the microwave backscattering signatures of the water surface and solving the problem of remote measuring of the wind speed and direction over water [2-13]. However, the mechanics of interactions between water surfaces and microwaves have not been well studied in detail. The typical method for describing sea clutter is in the form of the normalized radar cross section (NRCS), the statistical distribution of the NRCS, the amplitude correlation and the spectral shape of the Doppler returns [14]. To describe the radar backscatter from the water surfaces, three major scattering models are used: the Kirchhoff or physical optics model, the composite-surface or two-scale model, and the Bragg model. The Kirchhoff model assumes a perfectly conducting surface (unless it is modified to include the Fresnel reflection coefficient) and applies from small to intermediate incidence angles without shadowing effects. Apart from the implicit dependence on the Fresnel