Numerical simulation of the radar signal modulation due to the long ocean waves: preparation of the VAGSAT mission

Abstract

1.ABSTRACTVAGSAT has been proposed in 1993 in response to an announcement of opportunity from the French Space Agency for a small-satellite mission. It has then been selected as one of the possible candidates for a future satellite mission. The aim of this project is to measure at the global scale the directional spectra of the ocean surface waves (in the [70-5001 m range of wavelengths).The proposed system is a Ku-band radar (frequency ~1 3 . 4Ghz) using real aperture antenna. The originality of the concept of VAGSAT is to propose an alternative to Synthetic Aperture Radar systems (imaging radar's) which present important limitations over the ocean because of the non-linear character of the imaging process. In this paper, we present the characteristics of VAGSAT and the principle of the measurement. The developments carried out to simulate the radar data and the statistical uncertainties associated with the retrieved wave directional spectra are also presented. 1I.VAGSAT [l]"VAGSAT" is a small satellite mission proposed to measure the directional spectra of the surface ocean waves at a global scale. The proposed system is a Ku-band radar (frequency ~1 3 . 6 Ghz) with a dualbeam pointing towards the surface at nadir and nearnadir (10" incidence from nadir) with a real aperture antenna scanning to cover the horizontal plane over 360" in azimuth. The mission would be based upon a polar orbiting satellite at a 500 km height.With an incidence angle of about lo", a beam aperture of about 2x2", and a platform altitude of about 500 km, the surface footprint will be of about 18 km in diameter. This is quite compatible with the assumptions of a large footprint with respect to the waves to be analyzed.Alarge chirp bandwidth (= 320 MHz) is necessary to minimize the speckle noise. Ths large bandwidth will provide intrinsic range resolution of about 0.50 m, and a corresponding intrinsic horizontal resolution of about 3 m. This resolution is needed to limit the speckle level but is not necessary for the wave analysis. Therefore it is proposed to average the signal over 4 range gates yielding a 12 m horizontal resolution. Each radar sample would be obtained every 26 ms, i.e., every 0.9" in azimuth.