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.

ARTS, the Atmospheric Radiative Transfer Simulator – version 2.2, the planetary toolbox edition

“…The implementation focuses on the attenuation of the power between a transmitter of a coherent signal and the receiver position, but some other aspects are also covered by the auxiliary variables provided. The latter includes a basic treatment of radio occultation, i.e., when a coherent microwave signal, such as from GNSS, is recorded by either a satelliteor a ground-based receiver in order to determine certain atmospheric properties (e.g., Kursinski et al, 2000;Nilsson and Elgered, 2008). Only an overview of these additions is given here; for details, see Eriksson et al (2011c) and the built-in documentation.…”

Preconditions to ground based GPS water vapour tomography

“…the total water vapour between the GPS satellite and the GPS ground-station (Kursinski et al, 2000). The 3-D water vapour distribution can be reconstructed from a large number of such integrated values by means of tomographic techniques (Flores et al, 2000a;Subbarao et al, 1997;Kunitsyn and Tereshchenko, 2003).…”

Ray Tracing Simulation in Nonspherically Symmetric Atmosphere for GPS Radio Occultation

“…Because the earth is spinning the shape of the earth is oblate which causes the local curvature center of refractivity contours around the globe to vary with respect to the center of the earth. An ellipsoid can be used to represent the geoid shape due to gravity and hydrostatic balance (Kursinski et al 2000). The earth's shape in this research is assumed to be an ellipsoid.…”