Swiss ionospheric gradient threat model revisited during solar maximum 2025

This study investigates ionospheric slant gradients over Switzerland during the ongoing solar maximum, focusing on their implications for Ground Based Augmentation System (GBAS) performance. Using Global Navigation Satellite System data collected between January and March 2025, gradients were calculated for three station pairs with varying baseline lengths: ZHAW - ETH2, ZHAW - ZIM2, and ZHAW - AJAC. Gradients were computed on both L1 and L5 frequencies using a code-based approach with 600 s code-carrier smoothing, and were analyzed in relation to satellite elevation, diurnal variation, baseline geometry, and geomagnetic activity as indicated by the Kp index. The results show that gradient magnitude decreases with increasing baseline length and satellite elevation, with the shortest baseline (ZHAW– ETH2) exhibiting the largest gradient variability. L5 gradients consistently show higher magnitudes than those on L1, and peak values occur around local noon. While the majority of the gradients remained within the bounds of the German GBAS threat model, up to 33 % of values for L1 exceeded the model limits at low elevations. Similarly, the benchmark of ±50 mm/km used by the Swiss ionospheric model was exceeded in 23 % and 16 % of the cases for the ZHAW– ETH2 and ZHAW– ZIM2 pairs, respectively. In contrast, the observed long-baseline gradients of station pair ZHAW- AJAC remained well within the gradients of both the German and Swiss model. It should be noted that in this study, a code-based approach was used whereas both the German and the Swiss model are based on carrier phase measurements. Correlation analysis with the Kp index revealed weak or no association, suggesting that global geomagnetic indices may not adequately capture local ionospheric gradient variability. For future research, comparisons with high-resolution Total Electron Content maps and a transition to carrier-phase-based gradient estimation are recommended to improve accuracy and compatibility with previous studies.