A New Technique to Improve the Electron Density Retrieval Accuracy: Application to FORMOSAT-3/COSMIC Constellation

摘要

Deriving ionospheric density profiles from radio occultation measurements has become a useful tool to help determining and study vertical ionospheric 3D structures at global scale, overcoming the limitations of ionosonde measurements, which can only provide profiles up to the maximum frequency and at localized places. Moreover, the recent deployment of the FORMOSAT-3/COSMIC constellation, 6 LEO (Low Earth Orbit) satellites with GPS receivers on board which could theoretically provide around 2,500 occultations per day, would allow for the first time ever global electron density monitoring (indeed, structures plus time evolution) with high resolution. Therefore, and in order to take advantage of such amount of occultations, the aim of this work is to test and refine possible retrieval techniques. In this context, the use of GPS phase and bending angle observables are going to be analyzed with a more realistic modeling: the improved Abel transform. The details of the adaptation of the separability algorithms to bending angle observables along with a proposal for clock calibration are discussed. The comparatives provided in the section devoted to results, taking ionosonde data as reference, will show that the improved Abel transform provides a better determination of the electron density for both observables versus the classical Abel retrieval (27% vs. 49% of RMS regarding ionosondes). Concerning the use of one observable and another, their performance is quite equivalent (discrepancies of about 2%). The better knowledge of the detailed structure of the Ionosphere will have a positive impact on the prediction capacity and accuracy of GNSS systems, in particular, for precise positioning.