The Height Modernization Program, which has been designed and is being implemented by the National Geodetic Survey (NGS), is an ongoing operation focused on forming accurate, reliable heights using the Global Navigation Satellite System (GNSS) technology. The determination of GPS-derived ellipsoidal heights is one of the most critical components in the Height Modernization Program and subject to potentially significant error sources in GPS.
The GPS error sources may reduce the accuracy of GPS-derived coordinates and ellipsoidal heights. The height component is primarily affected by inherent geometric weakness and by un-modeled part of the neutral atmosphere (troposphere). Some recent studies have shown that tropospheric delay is one of the most challenging and essential error sources in space-based geodetic applications; especially, in the determination of ellipsoidal height, if it is not sufficiently accounted for.
This thesis focuses on possible improvements in the accuracy of the GPS-derived ellipsoidal height, and addresses the effects of tropospheric delay, pertinent to the NGS web-based GPS processing engine, OPUS-Projects, based on the national Continuously Operating Reference Station (CORS) network. This thesis validates that the effect of tropospheric delay, through the combination of the national and global permanent GNSS networks (CORS and International GNSS Service (IGS)), can be reduced, resulting in the improved accuracy of GPS-derived ellipsoidal heights. Specific experiments have been designed and performed to illustrate the improvements.
The experiments presented in this study were conducted in the State of Ohio and used the Ohio CORS stations to generate case studies with variable GPS data spans, baseline lengths, and network designs. In addition to CORS stations, IGS stations are included to improve the accuracy of the estimated tropospheric corrections.
The experimental results show that the reliability of tropospheric corrections is highly correlated to the length of the baseline, the duration of GPS data, and the network configuration. The experiments attempt to investigate the required baseline length with respect to the duration of GPS data and determine the optimal network configuration to assure proper estimation of tropospheric corrections, and, ultimately, ellipsoidal heights.
The experiments carried out in this study prove that the optimal network design is based on the multiple base approach that utilizes the IGS stations. Not only this approach improves the accuracy and consistency of tropospheric corrections, but it also improves the accuracy of the ellipsoidal heights determination.
In conclusion, this thesis demonstrates the importance of proper accommodation of the tropospheric effect in the determination of ellipsoidal heights. More importantly, this thesis illustrates that the tropospheric effect can be accurately modeled and, subsequently, correction can be applied to improve the accuracy of GPS positioning using permanent GPS networks; even with short data spans, with right network configuration.