Galileo, and in general existing GNSS, are based on the broadcasting of electromagnetic ranging signals in the L frequency band. These satellite signals are subject to distortions when propagated through the Earth’s atmosphere. Earth’s atmosphere can be subdivided into the troposphere and the ionosphere, which is the ionised part of the atmosphere, inducing a dispersive group delay that is several orders of magnitude larger than the one from the troposphere.

Single frequency receivers will be able to counteract the errors introduced by the ionospheric propagation delay using the Galileo single-frequency ionospheric correction algorithms described below:

• NeQuick G, is a reference ionospheric correction algorithm, based on a three-dimensional representation of the electron density using an adaptation of the NeQuick ionospheric electron density model for quasi-real-time corrections and driven by three broadcast coefficients in the navigation message. Additional details can be found in the Reference Ionospheric Correction Algorithm for Galileo Single Frequency Users (v1.2).
• NTCM G (Neustrelitz Total Electron Content Model for Galileo), an alternative to NeQuick G, is also based on the broadcast coefficients in the navigation message for Galileo single-frequency users. NTCM G brings reduced complexity and runtime, making it more computational-efficient than NeQuick G. Additional details can be found in the Alternate Ionospheric Correction Algorithm for Galileo Single Frequency Users (v1.0).