CORS Station Research Articles

Assessing the quality of GEOID12B model through field surveys

AbstractElevation differences have been determined through conventional ground surveying techniques for over a century. Since the mid-80s GPS, GLONASS and other satellite systems have modernized the means by which elevation differences are observed. In this article, we assessed the quality of GEIOD12B through long-occupation GNSS static surveys. A set of NGS benchmarks was occupied for at least one hour using dual-frequency GNSS receivers. Collected measurements were processed using a single CORS station at most 24 kilometers from the benchmarks. Geoid undulation values were driven by subtracting measured ellipsoidal heights from the orthometric heights posted on the NGS website. To assess the quality of GEOID12B, we compared our computed vertical shifts at the benchmarks with those estimated from GEOID12B published by NGS. In addition, Kriging model was used to interpolate local maps for the geoid undulations from the benchmark heights. The maps were compared with corresponding parts of GEOID12B. No biases were detected in the results and only shifts due to random errors were found. Discrepancies in the range of ten centimetres were noticed between our geoid undulation and the values available from NGS.

Precision comparison and analysis of four online free PPP services in static positioning and tropospheric delay estimation

Observation data sets from three different periods and 23 International GNSS Service (IGS) stations spread over the world were processed in static mode using four online free precise point positioning (PPP) services: Automatic Precise Positioning Service (APPS), GPS Analysis and Positioning Software, Canadian Spatial Reference System precise point positioning service, and Magic-PPP. The estimated positions of the 23 IGS stations were compared with the published values. The estimated zenith tropospheric delays (ZTDs) at these stations were compared with the corresponding IGS troposphere products published on the IGS Web site. Furthermore, in order to analyze PPP precision for short observation periods, GPS observation data sets with a sampling rate of 1 s collected by one CORS station were also processed by the four online PPP services. Calculation shows that the precision of daily solutions for north and east (N/E) components estimated by the four online PPP services can reach millimeter level, the precision of ellipsoid elevation (H) can reach 1---2 cm, and precision of ZTDs estimation results is approximately 1---2 cm. Compared with the corresponding values published on the IGS Web site, the precision of ZTDs estimated by APPS is higher than those estimated by the other three PPP online services; the differences between ZTDs by APPS and IGS values are mostly within ±1 cm. For PPP with short observation periods, the precision of N/E components within 2, 1, and 0.5 h observation periods can reach about 2---3, 2---7, and 3---8 cm, respectively, while the precision of H components is about 3---5, 5---12, and 10---18 cm, respectively.

Evaluation of GPS Precise Point Positioning for Geoinformatics Community

Precise Geodetic GPS applications has evolved during the last two decades as the GPS processing and analysis techniques become more robotics and complicated. These progresses cover both the traditional relative positioning methods where more than one receiver are used simultaneously and the single positioning technique. In this study, GPS Precise Point Positioning (PPP) method, that does not require data from any other GPS receiver, was tested and evaluated through various computations environments. The un-differenced dual-frequency pseudo range and carrier phase observations were downloaded from one permanent international GNSS Service (IGS) station and one permanent CORS station. The GPS data were processed using various IGS orbit data in a post-processing manner using the ESA/UPC GNSS-Lab (gLAB) software. The results for various observations scenarios of the PPP solutions were investigated and analyzed. The conclusion and suggested future works are outlined in the research.

A zenith tropospheric delay correction model based on the regional CORS network

Abstract: Tropospheric delay is a primary error source in earth observations and a variety of radio navigation technologies. In this paper, the relationship between zenith tropospheric delays and the elevation and longitude of stations is analyzed using the zenith tropospheric delay final products of International GNSS Service (IGS) stations from 2011. Two new models are proposed for estimating zenith tropospheric delays from regional CORS data without meteorological data. The proposed models are compared with the direct interpolation method and the remove-restore method using data from Guangxi CORS. The results show that the new models significantly improve the calculated precision. Finally, the root mean square (RMS) errors of the new models were used to estimate the surface precipitable water vapor (PWV) value at CORS station, which was determined to be less than 2 mm.

Datum Definition in the Long Range Instantaneous RTK GPS Network Solution

This paper presents the multiple reference station approach to wide area (and regional) instantaneous RTK GPS, implemented in the MPGPSTM (Multi Purpose GPS Processing Software) software, developed at the Ohio State University. A weighted free-net approach (WFN) was applied in the instantaneous RTK software module, which enabled optimal estimation of the rover coordinates, properly reflecting the accuracies of the observations and the coordinates of the CORS stations. The effect of using the distance-dependent weighting scheme in the WFN approach on the final rover solution is analyzed. The influence of the different weights was studied by introducing the distance-dependent weights as a function of the CORS station separation L to the rover (1/L2). The results show that almost 100% of the differences between the computed horizontal rover coordinates and the known reference coordinates are below a decimeter, and 95% of the differences in the vertical coordinate are below 20 cm, when using the suggested approach. In addition, the accuracy analysis of two other solutions, with different datum definition (minimum constraint and over-constrained), is presented. This analysis verifies the suitability of the stochastic models used in the RTK module and the rigorous approach, taking into account inter-baseline correlation as well as the correlation in-between each baseline component.