The GPS radio occultation technique

Radio occultation observations represent a planetary-scale optics experimen t in which the atmosphere acts as a lens and alters the propagation velocit y and paths of microwave signals passing through it. In this paper we revie w the process of inverting the radio occultation observations acquired usin g the Global Positioning System (GPS) in order to derive atmospheric quanti ties of interest including temperature, geopotential and water vapor. Begin ning with geometric optics, we derive the Abel integral used to transform t he observations into profiles of refractivity, In the process, we character ize why the Abel transform works so well as a first approximation for deriv ing atmospheric profiles. We discuss the resolution of the observations and the improvements that can be achieved via the backpropagation concept wher e the receiver's position is effectively moved closer to the limb of the Ea rth in post-processing. We discuss several factors that complicate the obse rvations in the Earth's troposphere including critical refraction and atmos pheric multipath. Critical refraction refers to the situation where the ben ding becomes so great that the occulted signal disappears whereas atmospher ic multipath refers to the situation where multiple signal paths connect th e transmitter and receiver. We describe the derivation of temperature, pres sure and water vapor from the observations including the optimal combining of the occultation observations with weather and climate analyses. We descr ibe some key issues in deriving profiles from real data including the corre ction of dock errors and ionospheric effects, and the estimation of the res olution and atmospheric Doppler in a self-consistent manner. We conclude by summarizing the expected and achieved accuracy and resolution.