Inversion of ionospheric GPS occultation data

Based on the assumption of the local spherical symmetric ionosphere, we propose two inversion methods with which ionospheric density profiles are extracted from the dual frequencies and single frequency radio occultation data respectively. The estimation errors of the dual-frequency method come from errors of the carrier phase measurements, while the errors of the single frequency method mainly are determined by the precision of the pseudo-range measurements. Since the pseudo-range measurements tend to be about 100 times noisier than those of the carrier phase, the retrieval precision with the dual-frequency method may be generally higher than that with the single frequency. However, slightly different paths of carrier L1 and L2 will bring errors to estimations of the dual-frequency method. The radio occultation simulation data with a 3 dimensional ray-tracing program are used to evaluate these inversion methods. Results show that the inversion profiles agree very well with the given model ionosphere, which indicates reliabilities and accuracies of both methods. Then, two inversion methods are applied to the processing of GPS/MET ionospheric occultation data. The inversion results show reasonable ionospheric profiles. Inversion profiles with the single-frequency method are consistent with those of the dual-frequency method, which provide the theoretical inversion base for ionosphere radio occultation observations with a single frequency GPS receiver onboard LEO.