IONO4HAS, a Real-Time Ionospheric Model for Galileo High Accuracy Service, SL2. Results and Validation

The Galileo High Accuracy Positioning Service (HAS) is an existing capability of Galileo, the European Global Navigation Satellite System (GNSS), to offer user positioning with decimeter-level accuracy, employing multiple constellations. Available since January 2023, Galileo HAS is a global Precise Point Positioning (PPP) service, to be deployed in two service levels. Service Level 1 (SL1) comprises satellite orbit and clock corrections (i.e. non-dispersive effects), and dispersive effects such as code and phase biases. Service Level 2 (SL2) incorporates ionospheric corrections for Fast-PPP navigation (at least over Europe). Funded by the European Space Agency (ESA) under the ESA Real-Time Ionospheric Continental Caster (eRTICC) project, and also supported by the EU GNSS Agency (now EUSPA) and European Commission in its early stages, gAGE/UPC has developed and deployed the IONO4HAS tool: A Central Processing Facility (CPF) to compute, in real-time, Galileo HAS SL2 corrections for Fast-PPP, encompassing precise clocks, code and phase biases, and a two-layer ionospheric model. IONO4HAS tool's basic input are GNSS observations from a worldwide distributed network of permanent stations, collected in real-time using the Networked Transport of RTCM via Internet Protocol (NTRIP). The routinely assessment of the ionospheric model is done in parallel using a novel position-domain real-time test that directly links the instantaneous (snapshot) position error with the error of the ionospheric corrections, a key point for a HAS. The test involves several GNSS receivers in Europe acting as user receivers at various latitudes, with distances to the nearest reference receivers ranging from 80 to 300 kilometers. The results of this test reveal that the 95th percentile of the instantaneous position error depends on the user to nearest reference receiver distance, as expected, ranging in the horizontal and vertical components from 17 to 28 cm and from 21 to 57 cm, respectively, which are several times smaller than the corresponding errors obtained when using other ionospheric models. Several of these results meet the Galileo HAS SL2 requirements and even exceed them by enabling “instantaneous positioning”. We conclude that formal errors of transmitted ionospheric corrections, typically at the decimeter level (1 sigma), play a key role in safeguarding users against misleading information by appropriately weighing their measurements in the navigation filter. For such purpose, the internal algorithms working in the CPF also involve different ionospheric activity indicators to drive the process noise model and trigger alarm flags.