State-of-the-art in Sea Surface Topography Measurements with GPS

Upon impinging on the ocean surface, the signal from the GPS transmitters (L-band) is reflected primarily in the specular (forward) direction, in an amount dependent on surface roughness and angle of incidence. An airborne (or spaceborne) receiver, could collect such scattered signal forming a multistatic radar system, in principle capable of intercepting bounces from several areas of the ocean simultaneously. Analogously to the case of a conventional radar altimeter, the reflected GPS signal acquired by the receiver is the average power versus time. This waveform is derived as a function of viewing geometry, system parameters, and surface roughness. The waveform is crucial for the derivation of the sea surface topography (from its leading edge) or wind speed and direction (from its trailing edge). This work summarizes the accomplishments of several years of research into the phenomenology of GPS reflections and the development of tools for understanding the signal use as an altimetry measurement. We examine properties of the modeled received power as a function of surface state and scattering geometry. In particular we investigate the spatial-temporal coherence properties and statistics of the measured reflected GPS signal that describes variability from one sample to another. This information is needed to choose an optimal strategy for a successful signal processing. Its impact on the accuracy of sea surface topography measurements both from airborne and orbital platforms is addressed. A characterization of error and related spatial resolution in relation to existing instruments is discussed. In the case of an Earth orbiting receiver, high-gain, multiple narrow-beam antenna arrays are needed to ensure both a strong enough signal and a multistatic wide-swath surface overview. Additionally, a constellation of GPS receivers would provide increased sampling density of the ocean at reduced temporal repetion. This would allow for the global monitoring of mesoscale eddies, fast barotropic waves and other features currently beyond the capabilities of existing instruments.