Comparison of Free and Open PPP Services for Master-base Positioning in Geodetic Disadvantaged Areas: Case Study along the Sirba River in Sub-Saharan Africa

The Precise Point Positioning (PPP) is a Global Navigation Satellite System (GNSS technique) for post- and real-time processing. PPP has recently spread thanks to the high precision and accuracy positioning results that it provides. Until some years ago, this technique was limited by long sessions of measurements and professional software to obtain results with centimetre-level precision, using only one GNSS dual frequency receiver. Nowadays, the PPP technique is well established among GNSS experts, and many software exists to perform it. The PPP technique uses data from continuously operating reference stations (CORS) to process the pseudo-range and carrier phase measurements. CORS can also be located very far from the surveyed area. This makes PPP particularly suitable for GNSS surveys in areas considered disadvantaged for the lack of geodetic infrastructures, such as CORS and known-coordinates points. Recently, PPP online free and open tools have been made available by national agencies worldwide. This contribution analyses the PPP online services as potential solutions for realising GNSS surveys in geodetic disadvantaged areas. Specifically, it compares two PPP online services: the Canadian Spatial Reference System Precise Point Positioning tool (CSRS-PPP) and Automatic Precise Positioning Service (APPS). In the analysis, the AUSPOS Online GPS processing service that applies relative positioning technique based on the closest CORS was considered a non-PPP post-processing online tool to compare results. Data were collected in South-West Niger, along Sirba River, 900 km away from the closest CORS and 250 km far from the closest known-coordinates point. The estimated coordinates' precision was tested for different session lengths (three scenarios of 3 hours, 1 hour, and 30 minutes) over 17 sessions. Then, the precision vas validated by analysing the one-year daily acquisitions dataset (from CORS). The results revealed precisions and relative accuracies lower than 5 cm for three hours' sessions. From the analysis of the short stationing sessions (i.e., one hour and half hour) emerged that APPS provides the most confident solutions. As expected, the less performant service in CORS-depressed areas is the relative positioning service AUSPOS.