Dual-Frequency Static PPP Over Fixing Time Using Final-Product Free Online Services: CSRS-PPP Vs. PPP-WIZARD Vs. APPS

This study examines the performance of dual-frequency static Precise Point Positioning (PPP) as a function of observation time using three of the most used free online services, namely: the Canadian Spatial Reference System Precise Point Positioning (CSRS-PPP), PPP-WIZARD, and the Automatic Precise Positioning Service (APPS). The comparison has been conducted under open-sky desert conditions, where multipath effects are negligible, and under two processing scenarios: GPS-only and GPS+GLONASS in case of CSRS-PPP & PPP-WIZARD, and GPS-alone in case of APPS. Dual-frequency GNSS data has been collected from ten spatially well-distributed stations across Libya, each observed continuously for 24 hours. Static PPP solutions have been generated for fixing intervals beginning at 1 hour and incrementally increasing to 24 hours. The coordinates of these reference points, provided and approved by Libyan Survey Authority, have been used as the reference for evaluating the accuracy and stability of all time-based solutions. In terms of comparing GPS + GLONASS with GPS-alone, the results of PPP-CSRS & PPP-WIZARD services show that even under optimal open-sky conditions, where multipath is virtually absent and the number of satellite is adequate, utilizing GLONASS observations alongside GPS leads to notable improvements in solution reliability, robustness and overall positioning stability throughout the fixing intervals. These benefits are particularly clear during the first several hours, when PPP solutions are most sensitive to satellite geometry and convergence behavior. This can be shown significantly in the first hour, where the absolute errors in Easting (E), Northing (N), Height (H), 2 Dimensions (2D), and 3 Dimensions (3D) are reduced with GPS+GLONASS by (0.2, 2.2, 3, 1.8 and 2.22 cm) & (6, 6.5, 11, 9, and 13 cm) for PPP-CSRS & PPP-WIZARD, respectively. In addition, although the enhancement in mean absolute positional accuracy becomes marginal for longer observation periods, the multi-constellation configuration significantly reduces the occurrence of outliers and decreases the dispersion of coordinate residuals across all components and the 3D quality obtained via one-hour static dual-frequency PPP with GPS+GLONASS can be reached after 3 to 4 hours fixing time using GPS-alone.  As for comparing the performance of the three free online services together, CSRS-PPP ranks first and second, as it has provided the most accurate results and the fastest convergency time, with 3D absolute error of nearly 2 cm & 5 cm for GPS+GLONASS and GPS-alone solutions, respectively after only one hour fixing time. The quality of these two solutions increases with time, reaching the level of less than 1 cm after 6 and 10 hours using multi constellations and GPS-alone, respectively. PPP-WIZARD comes third and fourth, with comparable 3D quality of 7 cm for GPS+GLONASS and acceptable 3D quality of 20 cm for GPS-alone after one hour fixing time. The results show that the quality of the two PPP-WIZARD solutions may not be able to reach the level of 1 cm even after 24 hours observation period. The APPS GPS-alone solution comes last in term of the 3D quality of the first fixing hour with accuracy of 45 cm, but the performance improves quickly comparing to PPP-WIZARD GPS-alone solution and almost matches that of PPP-WIZARD GPS+GLONASS for long period before reaching the level of 1 cm after nearly 20 hours of fixing time.  Overall, the findings confirm that multi-constellation GNSS integration is a critical factor in enhancing static PPP performance, particularly for applications requiring high accuracy over short observation periods. Among the investigated services, CSRS-PPP is identified as the most reliable and efficient solution, while PPP-WIZARD and APPS remain viable alternatives for longer observation durations.