Analysis of the Effect of Different Combinations of Observation Satellites on the Resolving Accuracy of GNSS Observation Data

: In this paper, 24 C-level control points under different terrain conditions were selected to be the testing points. The binary-satellite system (GPS+GLONASS) and the triple-satellite system with BeiDou Navigation Satellite System (BDS) (BDS+GPS+GLONASS) were adopted for static measurement; and the observation data from BeiDou Ground-based Augumentation System (GBAS) base stations in Guangxi were collected for solution. By comparing the residuals of GPS tri-dimensional baseline vectors and the internal accord accuracy of each control point under the binary and triple-satellite systems, the effect of data collected by different satellite systems under different terrain conditions on measurement accuracy was studied. According to the results, (1) the triple-satellite system with BDS showed more stable measurement accuracy; (2) in plane, the two systems were of equivalent measurement accuracy in mountainous and flat areas; in elevation, the triple-satellite system showed higher and more stable measurement accuracy.


Introduction
China has launched the last of its domestically developed Beidou (BDS) 3 global networking satellites, the networking was completed, which constituted four major global navigation satellite systems (GNSS) [5 ， 7] with the U.S. global positioning system (GPS) , the Russian Glonass positioning system (GLONASS), and the European Galileo system (GALILEO) [3][4] . Successful networking of BDS marked large-scale application of high-precision real-time navigation and positioning technology across the country supported by BeiDou base stations. At the end of 2018, Guangxi completed upgrading of 102 BeiDou base stations of the whole region based on the original continuous operational reference system (CORS) for satellite positioning (hereinafter referred to as the binary system) [6] , introducing BeiDou satellite receivers and building GBAS covering the whole area (hereinafter referred to as the triple-satellite system) to provide users with highprecision, round-the-clock and real-time dynamic positioning service [2] . To study the positioning accuracy of the upgraded triple-satellite system, in this paper, 24 C-level control points were included and observed under different system modes; observed data then went through baseline computing and adjustment with Trimble Business Center (TBC) and CORSGPS V6.0 to study the effect of the introduction of BeiDou on positioning accuracy under different satellite navigation systems.
Served data from 24 testing points were collected for baseline computing and adjustment with TBC and CORSGPS V6.0. The resolving process included data import, automatic elimination of gross baseline errors, automatic baseline computing, automatic adjustment, etc., as shown in Figure 1. After baseline resolving, the residuals of GPS tri-dimensional baseline vectors were compared under the binary and the triple-satellite systems; formula (1) [1] shows the evaluation precision: In the formula, is the X-direction residual of the tri-dimensional vector, the Y-direction residual, and the Z-direction residual; σ is the baseline accuracy for this level.
Based on difference between the resolved threedimensional coordinates and the known true values of 24 C-level control points, the internal accord accuracy of the control points was obtained; formula (2) [1] is the computing formula: In which, is internal accord accuracy; n is the number of control points; is the square of the difference between the resolved value and the true value.

Accuracy Analysis
To study the effect of the number of observation satellites on resolving accuracy under different system modes, after the independent-baseline GNSS space vector network consisting of the 24 C-level testing points passed the quality check of TBC, data from 24 testing points under different modes were adjusted with CORSGPS V6.0 in CGCS2000 coordinates, thus obtaining the statistical chart of residuals of GPS tridimensional baseline vectors under the binary and triplesatellite systems, as shown in Figure 2: According to Figure 2 (a), (b) and (c), residuals of GPS tri-dimensional baseline vectors under the triplesatellite system showed higher precision than under the binary system; the precision of residuals was higher in X and Y directions than in Z direction in both systems; the maximum residual of the binary and triple-satellite systems in X and Y directions was 0.15cm approx., and 0.25cm approx. in H direction.
Based on difference between the adjusted threedimensional coordinates and the known true values of 24 control points, the three-dimensional deviation of each control point in different areas was obtained as shown in Figure 3 (a) and Figure 3 (b). The internal accord accuracy of each point was then calculated and collected according to different accuracy ranges, as in Table 1. According to Figure 3 (a) and Figure 3 (b), in plane, measured values of the two systems showed little difference from the true values, and received little impact from terrain conditions, i.e. the plane coordinates were close in flat and mountainous areas; in elevation, the triple-satellite system showed significantly smaller deviation than the binary system, i.e. the triple-satellite system better matched the true values, even more obvious in complex mountainous areas. In low relief areas, the maximum elevation was 5.51cm under the binary system, and 3.21cm under the triple-satellite system; in mountainous areas, the maximum elevation was 6.11cm under the binary system, and 4.70cm under the triple-satellite system. Elevation deviation of the triple-satellite system in low relief areas was ±2-3.5cm, the maximum difference between the elevation coordinates of the binary system being 3.2cm.