Analysis and validation of GPS/MET radio occultation data in the ionosphere

Global Positioning System (GPS) radio occultation signals received by a low Earth orbit (LEO) satellite provide information about the global distribut ion of electron density in the ionosphere. We examine two radio occultation inversion algorithms. The first algorithm utilizes the Abel integral trans form, which assumes spherical symmetry of the electron density field. We te st this algorithm with two approaches: through the computation of bending a ngles and through the computation of total electron content (TEC) assuming straight line propagation. We demonstrate that for GPS frequencies and for observations in LEG, the assumption of straight-line propagation (neglectin g bending) introduces small errors when monitoring the F-2 layer. The secon d algorithm, which also assumes straight-line propagation, is a three-dimen sional (3-D) inversion constrained with the horizontal structure of a prior i electron density fields. As a priori fields we use tomographic solutions and the parameterized real-time ionospheric specification model (PRISM) whe n adjusted with ionosonde data or ground-based GPS vertical TEC maps. For b oth algorithms we calibrate the occultation data by utilizing observations from the part of the LEO that is closer to the GPS satellite. For inversion s we use dual-frequency observational data (the difference of L1 and L2 pha se observables) which cancel orbit errors (without applying precise orbit d etermination) and clock errors (without requiring synchronous ground data) and thus may allow inversions to be computed close to real time in the futu re. The Abel and 3-D constrained algorithms are validated by statistically comparing 4 days of inversions with critical frequency (f(o)F(2)) data from a network of 45 ionosonde stations and with vertical TEC data from the glo bal network of GPS,ground receivers. Globally, the Abel inversion approach agrees with the f(o)F(2) correlative data at the 13% rms level, with a negl igible mean difference. All tested 3-D constrained inversion approaches pos sess a statistically significant mean difference when compared with the ion osonde data. The vertical TEC correlative comparisons for both the Abel and 3-D constrained inversions are significantly biased (similar to 30%) by th e electrons above the 735-km LEO altitude.