The GPS Radio Occultation Technique

Abstract

Radio occultation observations represent a planetary-scale optics ex periment in which the atmosphere acts as a lens and alters the propagation velocity and paths of microwave signals passing through it. In this paper we review the process of inverting the radio occultation observations ac quired using the Global Positioning System (GPS) in order to derive atmo spheric quantities of interest including temperature, geopotential and wa ter vapor. Beginning with geometric optics, we derive the Abel integral used to transform the observations into profiles of refractivity. In the pro cess, we characterize why the Abel transform works so well as a first ap proximation for deriving atmospheric profiles. We discuss the resolution of the observations and the improvements that can be achieved via the backpropagation concept where the receiver's position is effectively moved closer to the limb of the Earth in post-processing. We discuss several fac tors that complicate the observations in the Earth's troposphere including critical refraction and atmospheric multipath. Critical refraction refers to the situation where the bending becomes so great that the occulted signal disappears whereas atmospheric multipath refers to the situation where multiple signal paths connect the transmitter and receiver. We describe the derivation of temperature, pressure and water vapor from the observa tions including the optimal combining of the occultation observations with weather and climate analyses. We describe some key issues in deriving profiles fr om real data including the correction of clock errors and iono spheric effects, and the estimation of the resolution and atmospheric Dop pler in a self-consistent manner. We conclude by summarizing the expected and achieved accuracy and resolution.