Investigating The Sensitivity of Delay Doppler Maps to Wind Direction Using Ambiguous Stare Processing

Benjamin J. Southwell

Abstract: The application of Global Navigation Satellite System - Reflectometry (GNSS-R) to remote sensing of the ocean surface has received much attention in recent years. The launch of TechDemoSat-1 and later, the CYGNSS constellation, saw the first dedicated GNSS-R instruments configured as scatterometers flown in space. Processing Delay Doppler Maps (DDMs) to estimate wind speed has been done by averaging the received signal power surrounding the specular point and sometimes over multiple contiguous DDMs. Such averaging results in the smearing of the spatial footprint of the window due to the receivers high velocity in low Earth orbit. Recently, stare processing has been applied to DDMs collected by TechDemoSat-1. This method tracks a fixed point on the surface as it moves through the delay-Doppler domain providing multiple looks at the point at various incidence angles. The stare point is selected so that it lies on the ambiguity free line to improve the achievable resolution of the method. However, as shown in this paper, this results in a sampling of the surface slopes PDF that is unable to resolve both wind speed and direction. A weak dependence of the normalized peak SNR to wind direction has been observed in the data collected by TDS-1 indicating that DDMs collected by spaceborne receivers are sensitive to wind direction. Whilst sensitivity to wind direction has been demonstrated for airborne platforms, the global coverage provided by spaceborne platforms make them preferable for oceanography applications. Thus, it is of interest to develop processing methods that will facilitate the estimation of wind direction using GNSS-R receivers on spaceborne platforms. In this paper, the ambiguous stare processing concept is introduced which tracks a number of fixed points simultaneously. All but one of these points are chosen so that they do not fall on the ambiguity free line. Whilst degrading the resolution, the ambiguity provides sufficient diversity in the sampling of the surface slopes so that sensitivity to wind direction is achieved. Ambiguities in the wind direction are introduced due the nature of the bivariate uniform distribution used to model the surface slopes. Additional ambiguities are observed when analyzing simulated stare profiles which are found to be heavily dependent on the direction of the receiver velocity, and, for some scattering geometries, are not present. Finally, the impact on the achievable resolution is investigated when combining observations from multiple stare points. It is shown that the achievable resolution is worse when observing multiple stare points but a larger portion of the resolution cell contributes to the measured signals.
Published in: Proceedings of the 31st International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2018)
September 24 - 28, 2018
Hyatt Regency Miami
Miami, Florida
Pages: 2819 - 2833
Cite this article: Southwell, Benjamin J., "Investigating The Sensitivity of Delay Doppler Maps to Wind Direction Using Ambiguous Stare Processing," Proceedings of the 31st International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2018), Miami, Florida, September 2018, pp. 2819-2833.
https://doi.org/10.33012/2018.15859
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