Assessment Of Single-Difference Ionospheric Residuals In A Regional Network For Gbas

During approach and landing of airplanes, the Ground Based Augmentation System (GBAS) is used to augment the Global Positioning System (GPS) positioning by delivering differential corrections. The ionospheric delays are typically considered to be removed by generating differential observations if the baseline between the GBAS station and airplane is relatively short. However, the situation may change in case of strong or stormy ionospheric activities. The remaining differential ionospheric residuals that are not eliminated might cause large slant pseudorange errors and degrade the positioning results significantly, especially in the vertical direction. In this work the Global Positioning System (GPS) data of the swisstopo's Automated GNSS Network for Switzerland (AGNES) were processed over 15 years from 1999 to 2013. We used the double-difference phase observations with reliably resolved ambiguities to establish the station-pair single-difference ionosphere residuals in the slant and vertical direction. An epoch-wise zero-mean condition over all satellites and per baseline was applied to avoid singularities and the estimated single-difference slant ionospheric residual was corrected with the 2-hour global ionosphere maps provided by the Center of Orbit Determination in Europe (CODE). The spatial gradients were calculated using the Ionosphere Pierce-Point (IPP) distances or the baseline length depending on the concrete case. It was found that the absolute maximum slant ionosphere gradients calculated from the slant differential ionospheric residuals and the baseline length during these 15 years are below the slant ionosphere gradient bound of the Conterminous United States (CONUS) ionospheric anomaly threat model.At last, the so-called overbounding vertical ionosphere gradients sigma(overbound)(vig), which confine all the non-Gaussian tails on a daily basis, were calculated for all days from 1999 to 2013 except the days with very stormy ionosphere activities. Ignoring the gradients with an IPP distance shorter than 20 km, over 96% of the. values are smaller than 4 mm/km.