Research On Observing The Earth's Atmosphere With The GNSS Radio Occultation Technique

The rapid development of Global Navigation Satellite System (GPS, GLONASS and the planned Galileo) has not only enhanced its applications in positioning, navigation and timing, it has also influenced many other research areas greatly. The application of GNSS in the research of climate and atmosphere has brought to the birth of GNSS Meteorology, which includes Ground-based GNSS Meteorology and Space-based GNSS Meteorology. As animportant direction of GNSS Meteorology, Space-based GNSS Meteorology-----research ofEarth's atmosphere with GNSS radio occultation techniques has drawn great attention all over the world. Because of its merits such as wide covering, even distributing and good vertical resolution, the research achievements will be useful for the improvement of the precision of Numerical Weather Prediction and the global climate change.In this thesis, with GPS occultation as an example, the theory and methods of occultation are summarized systematically. The data processing and the design of the orbit of LEO satellite are studied thoroughly. All the methods and conclusions are applicable for other GNSS occultations. The main research work and related conclusions are as following:1. The development history of GNSS and some conventional atmosphere sounding techniques are introduced. The international and internal research trends of this subject are summarized. It is pointed out that at present, the internal research level is still inferior to the international level. In data processing, some problems still exist such as that only GPS/MET data has been used and that there is a lack of statistical validation for the inversed results. In the orbit design of LEO satellite, some computational and analytical results that are more persuasive are needed. There still do not exist software systems which can process large amount of observed data automatically and to carry through the orbit design of the LEO satellite. To solve these problems is the aim of this thesis.2. As the research basis for this thesis, some characteristics of the Earth's atmosphere are introduced. All the atmosphere models and NWP models used in the research, including the simple Bi-exponential model for neutral atmosphere, the MSISE-90 dry air model, the ECMWF and the NCEP NWP models, the NeUoG ionosphere model are introduced respectively.3. Occultation events, including rising events and setting events are the basic observations for GNSS occultation. For GPS occultation, an occultation event happens when the signals of a GPS satellite pass through the Earth's atmosphere and are received by the receiver at a LEO satellite. In a setting event which lasts for about 1min, the excess phase delay of the GPS signal caused by the Earth's atmosphere increases from about 1mm when the signal path is tangential to mesopause to about 1km when it becomes tangential to the surface of the Earth. In this thesis, the concept of radio occultation is explained. The principle of retrieving bending angle and refractivity and the procedure to get atmospheric parameters through Abel inversion method are presented in detail. Two ionospheric correction methods, traditional linear correction of phases and the linear correction of bending angles are explained. The vertical and horizontal resolutions of the inversed profiles are introduced.4. The statistical optimization method for the upper boundary of bending angles is discussed. With the observed orbit data of CHAMP mission, two kinds of retrieved temperature profiles, one with the statistical optimization method and the other not, are compared. It is found that different values of boundary height ZU lead to different temperature results without using the statistical optimization method. On the contrary, the temperature profiles are in accordance with each other below 40km with that method. The computation results verify the effect of the statistical optimization method. In this thesis, the approach to correct the impact of the Earth's oblateness is also discussed. With the 3D ray tracing code developed by DMI, using real CHAMP orbit ephemerides and MSISE-90 dry air model, the ionosphere free excess phase delays are simulated under different schemes in the forward modeling. The simulated data are then retrieved respectively. It is found that when using an ellipsoid to simulate the Earth in the forward modeling but not taking into account the difference between the center of refraction and the center of the ellipsoid in inversion, we get temperature results with large errors. The errors decreased obviously when the center of refraction and the radius of curvature are corrected.5. Aiming at the inversion of real Level2 data, two data pre-processing tasks, coordinate transformation of satellites and low-pass filtering of the excess phase delays, are taken. The flow chart of the retrieving procedure is drawn based on the theory and methods mentioned above. Taking the results of GFZ as reference, the retrieved profiles of a CHAMP occultation event are verified. For CHAMP and GPS/MET, with the results of GFZ and UCAR as reference respectively, the retrieved results of all the occultation events in a whole day are verified statistically. Because the approach to get ECMWF data is limited, the inversion method of water vapor content is only studied through simulation. The inversion system Occlnversion and the forward modeling system DelaySimu are introduced.6. The error sources in the occultation observation and inversion are analysed systematically. All the factors are attributed to 3 categories: those relate to the instrument, those relate to the occultation geometry and those relate to the retrieval methods. The impacts of two aspects of satellite orbit determination errors, the radial direction position error of GPS and LEO satellites and the along-ray direction velocity error of LEO satellite, are considered. With CHAMP real orbit ephemeredes, according to different schemes, radial direction position errors of two satellites and along-ray direction velocity error of LEO satellite are added and corresponding excess phase delays are simulated. Temperature and pressure profiles under different schemes are retrieved. The comparision between the errors of these profiles shows that the impact of radial direction position errors of two satellites can be ignored under precise orbit determination condition and that the error of the retrieved temperature profile is less than 0.5K below 30km when the along-ray direction velocity error of the LEO satellite is less than 0.1 mm/s.7. In this work, the impact of random errors of excess phase delay on retrieved bending angle is deduced theoretically. With CHAMP real orbit, the ionosphere free excess phase delay is simulated. Random errors of 1cm and 1mm are added on the excess phase delay and then the bending angles are retrieved. It is found that the corresponding bending angle errors are less than ± 10 μ rad and ± 1 μ. rad respectively. Under the assumption that the errors ofbending angles are uncorrelated, some simulant computions are done on the propagation of Abel inversion and the hydrostatic equilibrium formulae. The correlation coefficient curves of refractivity, pressure of different heights and Std curves of retrieved atmospheric parameters are drawn.8. To develop the satellites for GNSS occultation mission, the orbit design of the LEO satellite is of great importance. Because the orbits of LEO satellites are comparatively low, precise orbit generating techniques need to be adopted. In traditional LEO orbit simulations, simple orbit generating techniques such as circular orbit, Kepler ellipse orbit and the Kepler orbit only taking into account the low orders of Earth's gravitation field model are used. In this work, higher orders of Earth's gravitation field model are taken into account, which is helpful to improve the veracity of the simulation. The force model for generating LEO satellite orbit and the model of occultation antenna models are stated. OccultSimu, a software system to design the LEO satellite orbits, is developed.9. The impact of main orbit parameters of a single LEO satellite on the distribuation and the number of occultation events is analysed and some conclusions are drawn. It is found that the adjustment of Argument Of Latitude (AOL), Right Ascension of Ascending Node (RAAN), orbit height and inclination does not affect the longitudinal distribuation of occultation events too much. The event number decreases when the orbit height is increased. The change of orbit inclination has obvious impact on the latitudinal distribuation of events. The occultation events mainly happen over low-latitude sites when the inclinations are low. On the contrary, there will be an even distribuation with high inclination.10. Considering the great potential of LEO constellation to improve the time and space resolution of occultation observation compared with a single LEO satellite, the impact of different satellite number and different k parameter of rose type LEO constellation on the time and space distribution of occultation events are studied. It is found that a constellation composed at least of 4 satellites can improve the time and space resolution of occultation observation over certain region obviously. If the constellation parameter k is zero or the greatest value, the time distribution of occultation events is not even. With other permitted k value, the time distribution is better and when k equals to the number of LEO satellites minus 3, the number of occultation events is preferable.11. A software system to optimize the LEO satellite orbit parameters is developed and computation is done to optimize the orbit of LEO satellite aiming to take GPS occultation observation over certain region. At the same time, with OccOptRose, a system developed to optimize the parameters of rose type LEO constellation, the optimal k parameters of rose type LEO constellations with different number of satellites for the occultation observation over certain region are computed and corresponding conclusions are drawn.