Abstract
A three-dimensional ray tracing model with aiming algorithms for global positioning system (GPS) signal is proposed to make simulations conform to the realistic radio occultation (RO) signal propagation. The two aiming algorithms used in this study ensure the initial and end point ray trajectories are located in the prescribed region. In past studies, the ray tracing techniques applied to the RO signal simulation usually assumed a spherically symmetrical atmosphere for simplicity. The exact GPS and low earth orbit (LEO) satellite locations are not considered in the simulation. These two assumptions make the simulation unrealistic for GPS signal propagation in the RO technique. In the proposed model, the shape of the earth is assumed as an ellipse. The information from European Centre for Medium-Range Weather Forecasts (ECMWF) analysis is used to setup the atmosphere in the simulation. Two aiming algorithms are developed to determine the initial signal propagating direction to make the simulated signal start from the prescribed GPS satellite position and end in the close vicinity of the LEO satellite position. An ideal spherical symmetric atmospheric structure is used to verify the ray tracing model. The fractional difference between real and simulated refractivity results is less than 0.1%. Otherwise, the GPS and LEO satellite position in the Formosat-3/COSMIC observation and the ECMWF analysis, considering the earth’s flattening, is also used to verify the aiming algorithms. All of the simulated signals end in close vicinity to the LEO satellite position in the simulation results.
Highlights
The radio occultation (RO) technique has been used to explore the atmosphere of planets in the solar system (Fjeldbo et al 1971; Woo and Ishimaru 1974; Berman and Ramos 1980)
The main procedure input parameters are the times for the RO events, the initial and end points of the signal trajectories which correspond to the global positioning system (GPS) and low earth orbit (LEO) satellite positions
(2) The atmosphere described by the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis was recorded on the 172th day of 2008
Summary
The radio occultation (RO) technique has been used to explore the atmosphere of planets in the solar system (Fjeldbo et al 1971; Woo and Ishimaru 1974; Berman and Ramos 1980). When the ray tracing technique is used to support the RO observation and retrieval methods, the refractive index profiles of the earth’s atmosphere is often assumed to be spherically symmetric (Beyerle and Hocke 2001; Jensen et al 2003). Based on the spherical symmetry assumption, the Abel transform, which is an integral transform pair relating bending angle profiles and refractivity profiles (Fjeldbo et al 1971), can be used under this assumption in the retrieval process In such cases, the orbits of GPS and LEO satellites and the signal trajectories are assumed to be in the same plane as the earth’s center
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