Potential and Limitations of Free Online Precise Point Positioning Services for GNSS Rover-Base Surveys in Low-Density CORS Areas

In the last years, the diffusion of the Precise Point Positioning (PPP) technique has constantly increased thanks to the more precise and accurate results that it can reach. Until some years ago, this technique was limited by long measurement sessions to obtain good precisions (centimetre-level), using only one GNSS dual frequency receiver. Online PPP free services that permit broad access to PPP technique have spread. In this contribution, two PPP online services (Canadian Spatial Reference System Precise Point Positioning tool; Automatic Precise Positioning Service) are analysed as potential solutions for realising GNSS surveys in disadvantaged areas for the lack of geodetic infrastructures. The PPP online services are compared with a relative positioning online tool (AUSPOS). Their elaboration power was tested for different stationing times (three scenarios of 3 h, 1 h and 30 min, respectively). The data PPP-treated were collected in southwest Niger, along the Sirba river. The results reveal precisions and relative accuracies lower than 5 cm for three hours sessions. The short observation sessions (i.e. one hour and half hour) emerged that APPS provide the most confident solutions. The less performant service is AUSPOS, which provides 0.612 cm precision for one hour. CSRS-PPP has precision values between the ones of AUSPOS and APPS.