The Atmospheric Model for GNSS (the component which describes the propagation of the radio signals in the atmosphere) consists of a tropospheric and an ionospheric part.

Troposphere: The pressure, temperature and humidity fields are provided from a global Numerical Weather Model (NWM), the Global Forecast System (GFS) of the National Centers for Environmental Prediction (NCEP) (https://www.nco.ncep.noaa.gov/pmb/products/gfs/). We use a ray-trace algorithm (Zus et al., 2014) and compute zenith hydrostatic (wet) delays, derive the hydrostatic (wet) mapping function coefficients and the horizontal delay gradient components. In post-processing mode we use the NWM analysis whereas in real-time mode we use NWM short-range forecasts. On the importance of the tropospheric delay model in the GNSS analysis the reader is referred to Zus et al., 2021.

Ionosphere: The electron density field is derived from a climatological model, the latest version of the International Reference Ionosphere (IRI) (https://iri.gsfc.nasa.gov/), or derived from data provided by the Space Weather Prediction Center (SWPC) of the National Oceanic and Atmospheric Administration (NOAA) (https://www.swpc.noaa.gov/). Earth's magnetic field is provided from the 13th generation of the International Geomagnetic Reference Field (IGRF) (https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html). We use a ray-trace algorithm to derive ionospheric delay parameters which can then be used to perform higher-order ionospheric corrections in precise applications (Zus et al., 2017).