Assisted Code Point Positioning at Sub-meter Accuracy Level with Ionospheric Corrections Estimated in a Local GNSS Permanent Network

It is well know that GNSS permanent networks for real-time positioning with stations spaced at few tens of kilometers in the average were mainly designed to generate and transmit products for RTK (or Network-RTK) positioning. In this context, RTK products are restricted to users equipped with geodetic-class receivers which are continuously linked to the network processing center through Internet plus mobile phone. This work is a first step toward using a local network of permanent GNSS stations to generate and make available products devoted to ionospheric delay correction that could remarkably improve positioning accuracy for C/A receiver users, without forcing them to keep a continuous link with the network. A simple experiment was carried out based on data from the RESNAP-GPS network (w3.uniroma1.it/resnap-gps), located in the Lazio Region (Central Italy) and managed by DITS-Area di Geodesia e Geomatica, University of Rome "Sapienza". C/A raw observations were processed with Bernese 5.0 CODSPP module (single point positioning based on code measurements) using IGS precise ephemeris and clocks. Further, the RINEX files were corrected for the Differential Code Biases (DCBs) according to IGS recommendations. One position per epoch (every 30 s) was estimated from C/A code; the vertical coordinate errors showed a typical signature due to the ionospheric activity: higher errors for day-time (up to 5 m) and smaller ones for night-time (around 1.5 m). In order to improve the accuracy of the solution, ionospheric corrections were estimated using the La Plata Ionospheric Model, based on the dual-frequency observations from the RESNAP-GPS network. This procedure allowed to reduce horizontal and vertical errors within 0.5 m (CE95) and 1 m (LE95) respectively. Finally, the possibility to predict the ionospheric model for few hours was preliminary checked. Our approach shows the possibility of a novel use of the measurements collected by GNSS permanent networks designed for real-time positioning services, which can assist and remarkably improve the C/A code real-time positioning supplying off-line predicted ionospheric corrections, acting as a local Ground Based Augmentation System.