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

As a priori fields we use tomographic solutions and the parameterized real-time ionospheric specification model (PRISM) when adjusted with ionosonde data or ground-based GPS vertical TEC maps. For both algorithms we calibrate the occultation data by utilizing observations from the part of the LEO that is closer to the GPS satellite. For inversions we use dual-frequency observational data (the difference of Ll and L2 phase observables) which cancel orbit errors (without applying precise orbit determination) and clock errors (without requiring synchronous ground data) and thus may allow inversions to be computed close to real time in the future.

The Abel and 3-D constrained algorithms are validated by statistically comparing 4 days of inversions with critical frequency (f 0 F 2) data from a network of 45 ionosonde stations and with vertical TEC data from the global network of GPS ground receivers. Globally, the Abel inversion approach agrees with the f 0 F 2 correlative data at the 13% rms level, with a negligible mean difference. All tested 3-D constrained inversion approaches possess a statistically significant mean difference when compared with the ionosonde data.

The vertical TEC correlative comparisons for both the Abel and 3-D constrained inversions are significantly biased (~30%) by the electrons above the 735-km LEO altitude.