Lawrence E. Young

Oceanography with GPS

This paper discusses a new technique for ocean remote sensing based on detecting GNSS signals, i.e. those transmitted from the Global Positioning System (GPS) or GALILEO (in the future), after they are reflected off the ocean surface, to measure sea surface topography and roughness (from which wind speeds are retrieved) at high spatial resolution and rapid temporal coverage. The topography measurement is suitable for the global monitoring of ocean eddies, which is currently precluded by the existing altimeters, due to their track separation and repeat cycle. The basic description of the technique is provided and the connection between the engineering measurement and the ocean descriptors (altimetry and winds) is outlined. A discussion of measurement accuracy and resolution is provided, and the concept of combining many independent measurements to provide an “average” over a given cell size and time interval, as opposed to repeat measurements, is outlined. To address the issue of systems needs for global coverage, an estimate of the scaling of accuracy with the number of available receivers of a hypothetical remote sensing system is given, showing that a constellation of 6 receivers with reflection antenna gain of 30 dB would be adequate for eddy resolution and monitoring. Furthermore, the latest experimental results demonstrating a measurement precision of 5 cm in a few minutes from airplane flying over rough ocean and 2 cm/sec from fixed-site over a lake are summarized, together with the basic features of a data acquisition/processing system developed for investigating the measurement phenomenology. Lastly, the requirements on the GPS receiver technology necessary to track and process reflections on board, and those of the antenna system necessary to capture all available reflections, are discussed.