Simulating GPS radio occultations using 3-d numerical ray tracing

Ray tracing is an important tool for the operation of GPS L-band frequency signal propagation; in particular GPS RO techniques where accurate and near real-time results are often required. Ray tracing techniques are commonly used for calculating the path of an electromagnetic signal in a medium specified by a position dependent refractive index, such as the Earth's atmosphere. In this study three dimensional numerical ray tracing techniques are used to simulate GPS L-band signals received by the LEO satellites and primarily focusing on regions of increased electron density gradients such as the equatorial anomaly. The down range refractive gradients on the GPS to LEO signal paths will be investigated. The ionosphere is a birefringent medium due to the presence of the Earth's magnetic field. The effects of the Earth's magnetic field on the transmitted signals and the paths of the ordinary and extraordinary signals are investigated

Real-time retrieval of precipitable water vapour from GNSS precise point positioning

Global Positioning System (GPS) meteorology (GPS-MET) as a novel approach for precipitable water vapour (PWV) sounding using ground-based GPS receivers has been conducted since earlier 1990s. Further research to date is based on post-processing or near-real-time processing using differenced GPS observations. It still remains a challenging task at high temporal resolutions and in real time. In addition, new Global Navigation Satellite Systems (GNSS) are under development quickly. This has the potential to improve the retrieval of PWV, leading GPS-MET research to a new stage of GNSS-MET. This study aims to take these aspects into account and investigates the retrieval of zenith total delay (ZTD) and PWV using real-time precise point positioning (PPP) approach. The PPP processing in this study is conducted using the BKG (the Federal Agency for Cartography and Geodesy) NTRIP Client (BNC) software platform which is substantially modified. The modifications include the modelling of tropospheric delay in which GPT2 is implemented and the corrections of error sources such as solid Earth tides, ocean tide loading and the antenna-related. The retrieved ZTD is then converted into PWV by multiplying a dimensionless proportionality which is derived from the Forecast Vienna Mapping Functions 1 (VMF1-FC) model. The retrievals of ZTD and PWV are validated using GPS observations in a one-month period at 20 globally distributed stations. The derived real-time ZTDs at most stations agree well with the tropospheric products from the International GNSS Service (IGS) and the root mean square (RMS) errors are <12 mm. The RMS errors of the PWVs in comparison with the radiosonde data are ≤3 mm. Note that 15 mm accuracy is the threshold if ZTDs are input to Numerical Weather Prediction (NWP) models and 3 mm accuracy is the threshold if PWVs are inputs to weather nowcasting according to the document by World Meteorological Organization (WMO). Furthermore, the theoretical accuracy of PWVs in various conditions is analysed. The RMS error of PWV is proved to be a strictly increasing function of zenith wet delay (ZWD) and weighted mean temperature. Hence the retrieval of PWV is more challenging in higher temperature and humidity conditions. This research proves that even in poor retrieval conditions, i.e., high humidity and temperature, an accuracy of PWV at 3 mm level is still achievable using the real-time ZTD from PPP and the empirical models for the determination of weighted mean temperature. A preliminary study of the ZTD retrieval using multi-GNSS data is also conducted in this study. The addition of GLONASS (GLObal NAvigation Satellite System) observations will significantly increase the number of visible satellites and improve the Dilution of Precision (DOP) indices like Positional DOP (PDOP) and Geometric DOP (GDOP). However, a test of ZTD retrieval at 12 global IGS stations shows that adding GLONASS data degrades the accuracy of ZTD. A further analysis implies that the multi-GNSS processing can be improved by the refinement of functional model and real-time GLONASS orbits and clocks. This research realised the real-time, high-accuracy, high temporal- and spatial-resolution retrievals of ZTD and PWV in a context of multiple GNSS constellations. The implemented PPP approach demonstrates its high accuracy in those retrievals. The retrieved real-time ZTD and PWV potentially have a wide range of applications in meteorology such as improving the NWP models and weather nowcasting