Research On GNSS Atmopsheric Radio Occultation Technique

Recently, radio occultation technique using the Global Position System for observing the Earth's atmosphere, one of the most advanced space exploring technique in the 21st century, has developed rapidly and become the focus of a lot of scientists both here and abroad. The radio occultation technique gives a new method of exploring the Earth's atmosphere. This technique can observe global neutral atmosphere and ionosphere. The principle of the technique is to install the GPS receiver at the satellite of LEO and do limb sounding for GPS satellite. When the radio signal spread through the atmosphere and ionosphere, the signal path will be bended because of the variety of refractivity, from which we can retrieve the atmospheric refractivity, pressure, density, temperature, and water vapor, also we can obtain the electron density in the ionosphere. It can operate all weather with long-term stability and global coverage. And its product has high precision. Occultation sounding is the supplement of traditional measurements. The occultation data of global distributed will greatly promote the precision of numerical weather prediction and research on the space near the Earth, and in addition, it can be applied to study on climate and global change. The occultation technique is of great importance to research on atmosphere model and data assimilation. The GPS radio occultation technique has shown a great future in the field of climate, meteorology, ionosphere, and geodesy.This thesis focuses on the retrieval technique of atmospheric occultation. The basic theory and system construction of radio occultation technique are introduced all around. The main arithmetics of geometric optic method and sliding-spectral method in the occultation inversion are studied in-depth. The arithmetics of high precision and the software for processing atmospheric occulation data have been completed. The occultation data of GPS/MET, CHAMP, and COSMIC are processed using the method we developed. Some factors that influence the precision of retrieval and the characteristics in atmospheric inversion are discussed. At the same time, we analyze the data of the experiment of mountain-based radio occultation.The main works can be summarized as following:1. The method for GPS/LEO radio occultation simulation is introduced in this thesis. Using the simulation method, it can be calculated of atmospheric excess phase received by GPS receiver using the satellite orbit data when the atmospheric background is known. Study on simulation is the basic of validate of occultation and error anlysis. And it is important to study and develop occultation technique.2. The geometric optic method in atmospheric occultation inversion is introduced. We discuss some important process in detail, such as check up and control for Doppler shift data, data pretreatment, the local curvature correction, ionospheric calibration, optimal estimation of the bending angle and calculating the parameters of occultation quality. And the software for processing the occultation data is developed. The occultation data of GPS/MET, CHAMP, and COSMIC are processed, and we compare our results and results of GFZ and UCAR. A lot of statistic and validation of data are done in this thesis. Results show that our arithmetic has high precision.3. The factors that affect the precision of atmospheric inversion are discussed in-depth. There are three types of error source in atmospheric occultation included of satellite measurements errors, GPS receiver measurements errors and the errors from the method of inversion. We analyze the error sources using simulative method. And the characteristics in atmospheric inversion are discussed. The effects of The Earth's oblateness and ionosphere on the retrieval are analyzed. Results show that: local curvature calibration and ionosphere calibration are very important for GPS occultation inversion of high precision. Ionospheric residual influence is the main reason of restricting the retrieval precision from 30 km to 60 km also, and it is necessary to develop new ionosphere calibration method for atmosphere inversion of higher precision. In addition, we analyze effect of the orbit error, velocity error, clock error and shift and observation error of phase of carrier wave on inversion.4. The multipath propagation that affects the inversion in the lower atmosphere is studied. To solve this problem, the sliding-spectral method has been studied. Sliding-spectral method is applied to COSMIC radio occultation event data. Through the comparative analysis of our results and that of UCAR, it can be concluded that sliding-spectral method can reduce the effect of multipath propagation of lower atmosphere, and can retrieve the atmospheric parameters more accurately.5. The COSMIC atmospheric radio occultation data is compared with global radiosondes. The comparison shows that the atmospheric temperature profiles are consistent with radiosondes. The mean difference of temperature between radio occultation and radiosonde is less than 1K from 10km to 30km.6. The mountain-based GPS radio occultation is studied. Using the data from the mountain-based observation experiment at Mt Wuling in Hebei Province during August 1-29, 2005, the profiles below the height of receiver are retrieved successfully. We compared the refractivity of the altitude in which the receiver is located with the result of automatic weather station. The result illustrates a good agreement in mountain-based GPS occultation and automatic weather station. Mountain-based occultation has potential applications for lower atmosphere sounding.