Fading statistics of bistatically scattered GPS signals detected from ocean and land in low earth orbit

Abstract

Signals from the Global Positioning System (GPS) are constantly being scattered off the entire Earth's surface. These signals can be detected using a low Earth orbiting receiver and have the potential to be used to remotely sense the Earth's ocean, land and ice surface conditions. It is known that the received signals are linked to the ocean surface roughness and height, land surface water conditions and sea ice concentration. However, in order to better understand the potential and limitations of this new technique, the scattered signals fading (or speckle) noise must be properly understood. As consecutive observations are captured from the scattering surface, the fading noise on the signal is gradually reduced to reveal the true signals power profile across the surface. The ultimate accuracy of measurements obtainable using bistatically reflected GPS signals will depend on how accurately this true signal can be estimated in the presence of fading noise. For the case of bi-statically reflected GPS signals the fading noise is of interest over different ranges of delays on the surface. From a space-based platform the signal power is normally detected over a very large surface area, covering a wide range of delays and frequencies, each with unique fading noise statistics. Using selected signals from the UK-DMC satellites bistatic GPS experiment it has been possible to examine several reflected signals from a range of surfaces. This paper will present the observed fading statistics of four signals collected under different conditions; calm ocean, rough ocean, ocean with swell present and a reflection from land. The measurement fluctuations will be presented as a function of the number of averaged looks and time delay across the surface.