Regional Ionosphere VTEC Modelling and Application for Single-Frequency Positioning in the Western Balkans

The ionospheric delay is one of the main error-sources in applications that rely on the Global Navigation Satellite System (GNSS) observations. Dual-frequency receivers allow the elimination of the major part of the ionospheric range error by forming an ionosphere-free linear combination (L3). However, although global models broadcasted by the satellite systems are available, single-frequency mass-market receivers are not able to correct the signal’s delay with sufficient accuracy and precise regional ionosphere models are necessary. Today no regional ionosphere models, based on the national GNSS/GPS infrastructure, are available in the Western Balkans countries.

In this study, an ionosphere vertical total electron content (VTEC) model IONO_WB is derived from dual-frequency GPS observations of Continuously Operating Reference Stations (CORS) belonging to the following positioning networks: ALPBOS and IGEWE (Albania), BIHPOS (Bosnia and Herzegovina), CROPOS (Croatia), MAKPOS (North Macedonia), and SIGNAL (Slovenia). In addition, observations from 8 permanent stations of the EUREF Permanent Network (EPN) in this region are used. The chosen network comprises in total about 70 CORS and EPN stations in the range from about 40⁰ N to 47⁰ N and 13⁰ E to 23⁰ E. The estimation of the ionosphere VTEC model parameters is based on the geometry-free (L4) linear combination of phase (zero-difference) observations. The ionosphere is approximated by a single-layer model at a height of 450 km. TEC modelling is performed by two-dimensional Taylor series expansions in a Sun-fixed reference frame with a degree and order of 2 and a temporal resolution of 1 hour. Corrections for positioning with a single frequency (L1) are estimated and evaluated in positioning application. Data processing, model estimation and positioning evaluation are performed in the Bernese GNSS Software v.5.2

The developed ionosphere IONO_WB model is tested for periods of the solar maximum (March 2014) and the St. Patrick´s geomagnetic storm (March 2015). For validation purposes, the model is compared to Global Ionosphere Maps (GIM) issued by the IGS Associate Analysis Centers (CODE, ESA/ESOC, JPL, gAGE/UPC) and the regional high-resolution VTEC maps from DGFI-TUM realized as multi-scale B-spline representations. The model`s applicability is evaluated with single-frequency positioning, where selected EPN and CORS stations are processed applying the corrections estimated from the regional model IONO_WB. Resulting 3D position errors (RMS) were in most cases at least 20% to 50% lower compared to CODE ionosphere products during high solar activity and severe geomagnetic storm.