Performance Assessment of Dual-Frequency Static GPS Positioning Using OPUS as a Function of Observation Duration

This study investigates the performance of dual-frequency static positioning as a function of observation length using the free Online Positioning User Service (OPUS) under a GPS-only processing configuration. OPUS is a publicly accessible online GPS processing service developed by the National Geodetic Survey (NGS). It offers free access to highly accurate coordinates within the National Spatial Reference System (NSRS). The service employs the same processing software used to determine coordinates for the nation’s geodetic control points and the NOAA Continuously Operating Reference Stations (CORS) Network. OPUS is capable of processing GPS data collected anywhere in the world. The evaluation is conducted using ten GNSS datasets acquired from various sites across Libya with dual-frequency receivers operating in static mode under open-sky desert environments. Each dataset contains continuous 24-hour observations, and processing was carried out at fixed observation intervals ranging from 1 to 24 hours. The complete 24-hour solution was considered the reference standard for evaluating accuracy in the Easting, Northing, Height, 2D, and 3D components. The findings indicate that the performance of relative positioning is highly influenced by the observation duration. At one hour of data collection, comparatively large errors are recorded, reaching about 22 cm, 15 cm, and 15 cm in the Easting, Northing, and Height components, respectively, corresponding to 26 cm and 31 cm in 2D and 3D accuracy. Following two hours of observation, the accuracy improves to approximately 2 cm, 4 cm, and 7 cm in the Easting, Northing, and Height components, respectively, equivalent to 5 cm and 8 cm in 2D and 3D accuracy. Nevertheless, a substantial enhancement is achieved as the observation period increases, with centimeter-level accuracy (~1 cm) attained in all coordinate components after three hours of processing. Extending the observation duration further results in progressive refinement of the solution, reaching millimeter-level accuracy after nearly 10, 16, and 17 hours in the Easting, Northing, and Height components, respectively, beyond which the solution becomes stable. These outcomes demonstrate that GPS-only static positioning using OPUS can provide high-precision coordinate estimates under open-sky conditions; however, its convergence rate remains relatively slow and strongly dependent on the observation period. The inclusion of datasets from geographically dispersed locations also illustrates the impact of satellite geometry and visibility on positioning accuracy and stability. In general, the results highlight the importance of adequate observation duration for achieving dependable centimeter- to millimeter-level accuracy in GPS-only static double-difference relative positioning.