Simulating GPS Radio Occultation

The GPS radio occultation technique is used frequently to obtain high resolution measurements of the vertical moisture, pressure and temperature profile in the atmosphere. Low Earth orbit satellites are used for detecting the refraction of GPS microwaves as they pass through the atmosphere. The bending angle of these signals is measured, and using the Abel transform the data can be inverted to yield the refractive index of the atmosphere. This data is then processed to produce the atmospheric parameters that are of interest in numerical weather prediction. Complicated signal patterns will arise when strong vertical humidity gradients are present. Such layers may cause ducting of the signal and a lack of occultation data from certain vertical segments. Furthermore, signal multipath may appear, and it is difficult to predict whether the receivers and inversion algorithms can untangle the superposed signals. Although more exact methods are available, the Abel transform is still the standard method for inverting the bending angle data. But in order to evaluate the accuracy of the Abel transform, which is based on geometrical optics, it is necessary to perform simulations of the complete propagation of the microwave field through the atmosphere. This is achieved using a numerical technique called multiple phase screens, wave optics propagator, or the Fourier split step method, which is an algorithm based on the spatial Fourier transform of the scalar Helmholtz equation, and the inclusion of the inhomogeneous refractive index only on certain screens extending vertically through the atmosphere. The aim of the present project funded by the national space research program (NRFP) is to construct a simulation software for performing these type of calculations, in addition to taking into account the detailed characteristics of the new type of high performance receivers constructed by RUAG Space AB, and the in-version algorithms used on the ground for assimilating the data in the numerical weather predictions. There are a few major obstacles that arise when constructing such a software. The approximations used when solving the wave equation will limit the applicability of the results, and care has to be taken so as not to increase the computing time unnecessarily. The surface of the Earth has a finite conductivity, and different signal reflections will result from the particular choice of Earth conductance. Furthermore, since the GPS satellite, the receiving satellite, and the Earth are all moving, complications will arise due to the coordinate systems used. We present the initial results and challenges faced in the construction of such a software