Receiver Antenna and Empirical Multipath Correction Models for GNSS Solutions

The advent of the Global Navigation Satellite Systems (GNSSs), in particular, the U.S. Global Positioning System (GPS) and the Russia's GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) have revolutionized geodesy by enabling a cheap and robust way of providing precise and continuous position estimates to users. Moreover, GNSSs have been shown to be extremely useful for a wide variety of other applications, in particular, geophysical, atmospheric, oceanographic studies, as well as industrial applications. Although the last two decades of GNSS exploitation were marked by great advances in accuracy and precision of the involved techniques, improvements can still be made. This thesis addresses the topic of receiver antenna and empirical multipath correction models, aiming to further improve GNSS solutions. GNSS utilizes measurement of ranges between satellites orbiting the Earth and receivers located on the Earth's surface through modulated electromagnetic signals. However, the actual point where the signal is received and which is denoted as a phase centre of an antenna, is not fixed, but varies depending on many parameters. Therefore, high-precision GNSS fundamentally depends on antenna phase centre corrections (PCC) and failing to accurately apply the latter results in biases and elevated uncertainties of estimated GNSS solutions. Additionally, due to repeating satellite-receiver geometry, these phase centre modelling deficiencies may lead to the generation of harmonic signals in the time series of the estimated parameters. In turn, identifying geophysical signals in the time series may be compromised by the presence of these artificial signals, resulting in inaccuracies in derived models. The geodetic community employs averaged (type-mean) PCC to estimate GNSS orbits, clock biases, tropospheric delays and other parameters, as well as to realize and provide access to the terrestrial reference frame. However, the use of individual PCC is beneficial for GNSS solutions, as it allows for more accurate estimation of satellite orbits and ground station coordinates. The latter is demonstrated using a regional network of 55 GNSS stations and processing the GPS data over a period of 10 years. Another topic addressed in this thesis concerns development of empirical site models (ESMs) using post-fit phase residuals accumulated over a period of time. These models are aimed to mitigate multipath and other unmodelled site effects that have a negative impact on GNSS solutions. Using a global network of stations the derived ESMs are evaluated for their capability to improve the GPS orbit determination as well as to increase the accuracy of ground station coordinate estimation