Radio Occultation Technique For Ionosphere Detection

The radio occultation technique had been applied successfully to explore the planetary atmosphere and ionosphere since 1960s. It is a new application of Global Navigation Satellite Systems, and one of the most advanced space exploring technique in the 21st century. This technique can observe the profiles of electron density in the ionosphere and the meteorological parameters of neutral atmosphere. It can operate all-weather with long-term stability and global coverage. The global distributed occultation data will greatly promote the space weather researches, numerical weather prediction and climate researches.This thesis focuses on the Ionospheric Radio Occultation (IRO) technique and its application. The main works can be summarized as following:1. The method for the simulation of the radio occultation is introduced, including the orbit simulation and the observation simulation.2. The inversion methods for IRO are studied, which are based on the approximation that the electron distributes symmetrically in local ionosphere. It is important to deal with the upper boundary condition in the inversion of IRO. Here three methods are proposed to do it: calibrated TEC (Total Electron Content) method, inversion method assisted with Chapman model or IRI (International Reference Ionosphere) model. The research shows that the calibrated TEC method is best. If there is no data at the non-occultation side, the inversion method assisted with Chapman model or IRI model is better than the traditional methods– bending angle extrapolation method or ignoring the contribution of the ionosphere above the LEO.3. The error sources of IRO are analyzed. The spherical symmetry approximation of electron density is the main source of the inversion error. The statistical results reveal some characters of the inversion errors. (1) The relative error increases with enhanced solar activity; (2) It is lager in winter than equinox season, and smallest in summer; (3) Generally, it is smaller at middle latitude than low and high latitude; (4) Regardless of season and geomagnetic latitude, it is smaller at daytime than other time.4. The IRO data from COSMIC is compared with measurements of ISR (Incoherent Scanter Radar) and ionosondes. The comparison shows that the electron density profiles of IRO are consistent with the ISR. The peak electron density (NmF2) of IRO is consistent with the measurement of ionosonde, and the correlation coefficient is 0.937, and the standard deviation of their relative differences is 20.7%. The differences between the NmF2 of IRO and ionosondes are studied, and their dependences on season, geomagnetic latitude and local time are consistent with the characters of the inversion errors of the simulation data.5. The effect of the asymmetry of electron density on the inversion of IRO is studied. And an asymmetry factor is defined for IRO. The study shows that the asymmetry factor is consistent with the relative error of inversed NmF2. Then the asymmetry factor is applied to correct the inversion result of IRO, and the inversion error is reduced greatly.6. The three-dimensional (3D) constrained inversion method is studied. With the help of 3D empirical ionosphere model, the method is applied in the inversion of IRO simulation data, and the inversion error is greatly reduced. Then the method is applied in the inversion of real IRO data with IRI2001 as constraint, and the comparison between the inversion result and measurement of ionosonde shows that the inversion result is reasonable and credible. An ionosphere model based on global electron density of IRO is constructed with spherical harmonic function, then the model is used as constraint in the inversion of IRO simulation data, and the inversion error is greatly reduced.7. The IRO data from COSMIC is applied to analyze the climate character of the ionosphere in the low solar activity.8. The exploration of Mars ionosphere with IRO technique is studied with simulation method. The research shows that in the China-Russia joint exploration of Mars, the electron density acquired from satellite-satellite IRO by dual frequencies will reach the unprecedented precision. The satellite-Earth IRO by single frequency can detect the electron density profile of Mars ionosphere in daytime.