Single-frequency, single receiver terrestrial and spaceborne point positioning
High-accuracy, point positioning has been an attractive research topic in the GPS community for a number of years. The overall quality of precise point positioning results is also dependent on the quality of the GPS measurements and the user’s processing software. Dual-frequency, geodetic-quality GPS receivers are routinely used both in static and kinematic applications for high-accuracy point positioning. However, use of lowcost, single-frequency GPS receivers in similar applications creates a challenge because of difficulty of handling the ionosphere, multipath and other measurement error sources. Potential use of such receivers to provide horizontal positioning accuracies of a few decimetres, and vertical accuracies of less than two metres, will be examined in this dissertation. Practical applications of post-processed, high-accuracy, single-frequency point positioning include a myriad of terrestrial and space-borne applications, where the size and cost of the GPS unit is an issue. The processing technique uses pseudorange and time-differenced carrier-phase measurements in a sequential least-squares filter. In developing the approach, different techniques were investigated. Ionospheric delay grid maps are used to remove the bulk of the ionospheric error, while tropospheric error is handled by a prediction model. Pseudorange multipath errors are mitigated by means of stochastic modelling and carrier phase cycle slips are detected and corrupted measurements are removed in a quality control algorithm. The technique was first tested on L1 measurements extracted from datasets from static, high-quality GPS receivers. Accuracies better than two-decimetres in horizontal components (northing and easting r.m.s.), and three-decimetre accuracies in the vertical component (up-component r.m.s.), were obtained. A test dataset from a stationary lowcost GPS receiver has been processed to demonstrate the difference in data quality. Positioning results obtained are worse than those of a high-quality GPS receiver, but they are still within the few decimetre accuracy level (northing and easting r.m.s.) and less than two metre vertical accuracy level. The use of the technique is not restricted to static applications, and the results of kinematic experiments are also presented. These experiments consist of terrestrial data processing and spaceborne data processing. The kinematic terrestrial tests include processing of single-frequency data from geodeticquality GPS receiver and low-cost GPS receiver from a moving vehicle. The spaceborne kinematic tests include processing of dual-frequency data from a geodetic-quality GPS receiver on board of a low Earth orbit (LEO) satellite, and processing of the simulated single-frequency data from a low-cost GPS receiver for a future satellite mission. The question whether it is possible to use low-cost GPS receivers for high accuracy GPS positioning has been answered. Contributions to the leading edge research in the area of high precision GPS point positioning have been made. The software that was developed is the only software capable of reliable pseudorange and carrier-phase data processing from low-cost GPS receivers. Its reliability is accomplished through data quality control based on residual outlier detection theory. The implemented algorithm is capable to detect 95% of outliers. Despite the encouraging results the limitations of this technique were found. During the static terrestrial data testing it was found that the presence of multipath has negative impact on the positioning results from low-cost GPS receivers. The kinematic terrestrial data testing is limited to short periods of time when a reliable reference solution is available. The majority of the test results are from terrestrial platforms, because the spaceborne single-frequency point positioning requires more sophisticated ionospheric models than the terrestrial single-frequency point positioning. One example of sophisticated ionospheric model is a global 3D ionospheric model which was tested in this dissertation.