CSRS-PPP and Trimble RTX-PP Static GNSS Solutions under GPS-Only and GPS+GLONASS Configurations

This study presents a comparative evaluation of two widely used free online GNSS post-processing services, namely the Canadian Spatial Reference System Precise Point Positioning (CSRS-PPP) service and Trimble RTX Post-Processing (RTX-PP), using dual-frequency static observations collected under true open-sky desert conditions. The analysis investigates the influence of fixing time and satellite constellation configuration on positioning accuracy, convergence behavior, and solution stability. Two processing scenarios were examined for both services: GPS-only and combined GPS+GLONASS. Ten spatially well-distributed datasets collected across Libya were processed using fixing intervals beginning at 1 hour and incrementally increasing to 24 hours. The 24-hour solution was treated as the reference benchmark for evaluating all shorter observation intervals. The results demonstrate substantial differences between the two processing strategies in terms of convergence behavior and achievable positioning accuracy.
The Trimble RTX-PP service achieved centimeter-level accuracy after only one hour of observation under both processing configurations. GPS-only produced accuracies of 0.5 cm, 0.7 cm, and 1.8 cm in the Easting, Northing, and Height components, corresponding to 0.9 cm and 2 cm in 2D and 3D positioning, respectively. The integration of GLONASS improved the solution to 0.5 cm, 0.5 cm, and 1.2 cm in E, N, and H, with 0.7 cm and 1.4 cm in 2D and 3D, respectively. Furthermore, GPS+GLONASS reduced convergence time and enabled millimeter-level positioning to be achieved faster than GPS-only. In contrast, CSRS-PPP demonstrated larger positioning errors during the initial convergence period but showed significant improvement when GLONASS observations were incorporated. During the first observation hour, the absolute errors in the Northing, Height, 2D, and 3D components were reduced by approximately 2.2 cm, 3 cm, 1.8 cm, and 2.22 cm, respectively, when GPS+GLONASS was used instead of GPS-only. Although the average positional improvement became marginal after longer observation periods, the inclusion of GLONASS significantly improved solution robustness and reduced residual dispersion.
The comparative analysis confirms that both services benefit substantially from multi-constellation integration, particularly during short observation durations where satellite geometry and convergence behavior dominate solution quality. However, Trimble RTX-PP consistently demonstrated faster convergence and higher short-term accuracy than CSRS-PPP under identical observing conditions. Meanwhile, CSRS-PPP exhibited strong long-term stability and robustness, especially after convergence was achieved. The results demonstrate that integrating GLONASS with GPS improves convergence speed, solution reliability, and positioning stability for both PPP- and DGNSS-based services, even under optimal open-sky conditions with negligible multipath effects. The findings also highlight the importance of selecting an appropriate online processing strategy according to the required observation duration and target positioning accuracy.