The GPS Radio Occultation Technique

Radio occultation observations represent a planetary-scale optics experiment 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 acquired using the Global Positioning System (GPS) in order to derive atmospheric quantities of interest including temperature, geopotential and water vapor. Beginning with geometric optics, we derive the Abel integral used to transform the observations into profiles of refractivity. In the process, we characterize why the Abel transform works so well as a first approximation 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 factors 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 observations including the optimal combining of the occultation observations with weather and climate analyses. We describe some key issues in deriving profiles form real data including the correction of clock errors and ionospheric effects, and the estimation of the resolution and atmospheric Doppler in a self-consistent manner. We conclude by summarizing the expected and achieved accuracy and resolution.