Journal of Surveying Engineering

Centimeter-to-decimeter-level positioning accuracy from the global positioning system (GPS) requires the use of carrier-phase measurements. The system is generally operated in a double differential mode in which a nearby reference station is used to calibrate for errors in the satellite differential measurements. For large-scale applications, a network of multiple differential reference stations is necessary. This paper describes the major errors affecting differential GPS (DGPS) applications, how a network of reference stations can be used to estimate these errors, and one method of implementing carrier-phase network differential GPS (CP-NDGPS) using partial derivative algorithms (PDAs). PDAs can be implemented by a network service provider and are used to estimate spatial and nonspatial signal errors that cannot be measured by a single GPS reference station. For networks having numerous reference stations, a PDA is an efficient method of transmitting information though a data link to the network users. Such a system is also capable of reducing DGPS errors. Of the networks studied during this project, DGPS errors were reduced 30 to 90%.

In areas that are subject to tidal waters, projects such as bridge construction and property boundary location require the determination of the water level or water datum. Of the different water data that can be established, the mean high water (MHW) is of prime importance. To establish the MHW line at a project site, it is necessary to either make long-term (19 years) continuous tide observations or to perform simultaneous short-term tide observations at points in the vicinity of the project and at a tidal control station. Of the three short-term observation and computation methods for determining the MHW, the amplitude ratio method is applicable in most general circumstances. The current computation procedure of the amplitude ratio method utilizes graphic means to determine the tide amplitude. A new analytical solution for this method was developed using least squares principles. The advantages of the proposed solution are simplicity of computation, suitability for automatic data processing, and usability even with incomplete observation data sets.