Journal of Surveying Engineering

The National Oceanic and Atmospheric Administration’s National Geodetic Survey (NGS) manages the National Continuously Operating Reference Station (CORS) system that comprises a network of over 1,350 sites, each containing a geodetic quality Global Navigation Satellite System receiver. This network is currently growing at a rate of about 15 sites per month. NGS collects, processes, and distributes data from these sites in support of high-accuracy three-dimensional positioning activities throughout the United States, its territories, and a few foreign countries. CORS data are also used by geophysicists, meteorologists, atmospheric and ionospheric scientists, and others in support of a wide variety of applications. This paper addresses the history of the CORS network, some of its applications, and plans for enhancing it within the next few years.

The city of Frisco, Tex., published state plane coordinates were “ground scaled” into surface coordinates for the purpose of establishing a hybrid coordinate system to be easily used by surveyors. Nineteen city monuments were found and collected by GPS observation using real-time kinematic (RTK) connected to the Texas RTK Cooperative Network. The network creates a virtual reference station near the GPS roving receiver, allowing the surveyor to use the rover unit by itself without the need for setting up a base station on the project site. Using a GPS manufacturer’s standard protocol, localizing ground-scaled GPS vectors to the city’s hybrid values yielded acceptable horizontal $\left(H\right)$ and vertical $\left(V\right)$ residuals over a $233{\mathrm{km}}^2$ network. Two out of 19 monuments observed explained maximum residual error ( $H=0.029\mathrm{m}$ , $V=0.082\mathrm{m}$ ) with no significant differences in the mean $\Delta N$ and $\Delta E$ between published grid, hybrid, and surface coordinates $(P=0.77)$ . This paper will illustrate and discuss the procedures and results of this project.

Online global positioning system (GPS) processing services have provided users with the opportunity to obtain freely high precision coordinates in a recognized datum. However, all services are dependent on the quality of the data and the length of data span supplied to them by the user. This study aims to assess the four globally available online GPS processing services from the point of view of a user wishing to take advantage of such a service for datum realization. Data from eight selected international GNSS service stations have been processed by each service and the results have allowed a statistically significant comparison analysis to be performed. Although users can expect a variability of results from different services when processing identical data, solutions with $24\mathrm{h}$ data are generally repeatable at the $1–2\mathrm{cm}$ level and accurate in terms of international terrestrial reference frame coordinate recovery at the $3–4\mathrm{cm}$ level. Such differences are accentuated as observation time decreases. Finally, postprocessing kinematic data, currently performed by only one online service, is a valid option for users provided they ensure data quality.

Terrestrial network solutions prepared by different institutes and/or at different epochs imply different reference frame definitions, since various reference stations and processing strategies may be involved. A combination procedure, utilizing a time-variant similarity transformation model, enables a geometric integration of multiple solutions into a common reference frame definition. Additional benefits, including an elimination of systematic bias and a cross check on the quality of each individual network solution, could also be achieved. In this study, a combination approach which takes into account complete geometric interrelations between multiple solutions is developed. With the observable dependency analysis procedure, the proposed approach guarantees a self-consistent, more meaningful, combination solution regardless of the choice of a reference solution. Numerical tests have been performed on actual International Global Navigation Satellite System (GNNS) Service (IGS) and National Geodetic Survey (NGS) solutions using the Geodetic Network Analysis Tool software developed along with this study. Results reveal potential problems if the proposed analysis procedure is not implemented in a combination solution.

By applying integrated least-squares adjustment of multicampaigns geographic positioning system (GPS) measurements, the velocity of any GPS monument can be obtained. The resulting velocities are used to estimate the strain rate tensors and their variances under infinitesimal strain assumption in a discrete triangle. In this paper, the GPS measurements of five campaigns, which were carried out in Shanghai City from 2002 to 2005, are used to determine the velocity of each GPS monument. Then the horizontal strain rate tensors are calculated with the relative velocities of the sites obtained. The analysis of these principal strain rates together with a comparison of the geological information is performed. The patterns of the principal strain rates show a significant northwest to southeast extension zone in the southwestern part of Shanghai, which agrees well with the neo-tectonic activity of an existing geological fault, the so-called “Dachang–Zhoupu” fault.