Investigation into the accuracy of single frequency precise point positioning (PPP)
Creators: name: Choy, Suelynn
Publisher: name: RMIT University
Year: 2024

description: This thesis investigates the major errors and processes affecting the performance of a viable, standalone point positioning technique known as single frequency Precise Point Positioning (PPP). The PPP processing utilises both single frequency code and carrier phase GPS observables. The mathematical model implemented is known as the code and quasi-phase combination. Effective measures to improve the quality of the positioning solutions are assessed and proposed.<br><br>The a priori observations sigma (or standard deviation) ratio in the sequential least squares adjustment model plays a significant role in determining the accuracy and precision of the estimated solutions, as well as the solutions convergence time. An 'optimal' observations sigma ratio is found using an empirical approach, whereby different sigma ratios are tested and evaluated. It is concluded that an a priori code and quasi-phase sigma ratio of 1:50 provides optimal performance irrespective of the ionospheric conditions and the location of the GPS receiver. This is an innovative attribute of the research.<br><br>The feasibility of using Regional Ionosphere Maps (RIMs) to improve the accuracy of the single frequency PPP solutions is also examined. The performance of the RIMs is evaluated as a function of geographical locations and different ionospheric conditions. The quality of the estimated positioning solutions based on the RIMs is then compared to those using the Broadcast model and the Global Ionosphere Maps. It is concluded that the RIMs are advantageous for GPS stations located in the low latitude regions and also during periods of high ionospheric activity.<br><br>The single frequency PPP solutions convergence is investigated with respect to i) satellite clock corrections at different sampling rates, ii) varying observation sampling intervals, and iii) the different tropospheric delay mitigation methods. It is found that the clock corrections and observations sampling intervals have minimal impacts on the solutions convergence time. However, in order to improve the time of convergence, the use of a modelled tropospheric delay (instead of estimating the tropospheric delay as part of the solutions) is recommended.<br><br>The viability of using the various International GNSS Service (IGS) satellite orbit and clock corrections in single frequency PPP processing, particularly the near real-time and real-time products, is evaluated. The outcomes of this study demonstrate the potential benefits of the near real-time and real-time corrections for high accuracy point positioning. Numerical validations have been carried out using GPS data collected from different receiver types and qualities, i.e. geodetic grade, medium-cost, and low-cost receivers. The results suggest that single frequency PPP has the potential to provide 0.1m to 0.9m point positioning accuracy in post-processing mode. For real-time scenario, point positioning accuracy of about 1m to 2m can be expected. Despite the encouraging results, PPP is a challenging positioning technique and users should be aware of its limitations.<br><br>The accuracy of the PPP solutions is dependent on the quality of the GPS measurements and corrections products used, as well as the capacity of the processing engine. It is anticipated this research will provide valuable guidelines for high accuracy point positioning using a single frequency GPS receiver. <br>