Method, carried out by a computer or set of computers, for generating correction information, wherein the correction information comprises information for correcting observations of at least one global or regional navigation satellite system receiver, hereinafter abbreviated as NSS receiver, the NSS receiver being in a region of interest under a region of the ionosphere, the method comprising: for each of a plurality of reference stations in the region of interest, receiving raw observations obtained by the reference station observing NSS multiple-frequency signals from a plurality of NSS satellites over multiple epochs; obtaining information, hereinafter referred to as "precise satellite information", on: the precise orbit position of each one of the plurality of NSS satellites, a precise clock offset of each one of the plurality of NSS satellites, and a set of biases associated with each one of the plurality of NSS satellites, or information derived from the precise satellite information; for each of the plurality of reference stations, estimating ambiguities in the carrier phase of the received raw observations for the reference station, using the precise satellite information or the information derived from the precise satellite information, and the known position coordinates of the reference station; for each of the plurality of reference stations, computing geometric-free phase linear combination values based on the received raw observations together with the estimated ambiguities; generating the correction information, based on the computed geometric-free phase linear combination values, wherein the correction information comprises, for each of the plurality of NSS satellites, at least one mathematical function, each of which being hereinafter referred to as "regional ionospheric delay function", and its coefficients, hereinafter referred to as "regional ionospheric delay coefficients", the at least one regional ionospheric delay function and its regional ionospheric delay coefficients representing, per NSS satellite, the ionospheric delay in the region of interest, wherein generating the correction information comprises: computing satellite-to-satellite single-differenced combination values based on the computed geometric-free phase linear combination values and estimating an ionosphere single layer model common to the plurality of NSS satellites based on the computed satellite-to-satellite single-differenced combination values; and sending the generated correction information. for each of a plurality of reference stations in the region of interest, receiving raw observations obtained by the reference station observing NSS multiple-frequency signals from a plurality of NSS satellites over multiple epochs; obtaining information, hereinafter referred to as "precise satellite information", on: the precise orbit position of each one of the plurality of NSS satellites, a precise clock offset of each one of the plurality of NSS satellites, and a set of biases associated with each one of the plurality of NSS satellites, or information derived from the precise satellite information; the precise orbit position of each one of the plurality of NSS satellites, a precise clock offset of each one of the plurality of NSS satellites, and a set of biases associated with each one of the plurality of NSS satellites, or for each of the plurality of reference stations, estimating ambiguities in the carrier phase of the received raw observations for the reference station, using the precise satellite information or the information derived from the precise satellite information, and the known position coordinates of the reference station; the precise satellite information or the information derived from the precise satellite information, and the known position coordinates of the reference station; for each of the plurality of reference stations, computing geometric-free phase linear combination values based on the received raw observations together with the estimated ambiguities; generating the correction information, based on the computed geometric-free phase linear combination values, wherein the correction information comprises, for each of the plurality of NSS satellites, at least one mathematical function, each of which being hereinafter referred to as "regional ionospheric delay function", and its coefficients, hereinafter referred to as "regional ionospheric delay coefficients", the at least one regional ionospheric delay function and its regional ionospheric delay coefficients representing, per NSS satellite, the ionospheric delay in the region of interest, wherein generating the correction information comprises: computing satellite-to-satellite single-differenced combination values based on the computed geometric-free phase linear combination values and estimating an ionosphere single layer model common to the plurality of NSS satellites based on the computed satellite-to-satellite single-differenced combination values; and at least one mathematical function, each of which being hereinafter referred to as "regional ionospheric delay function", and its coefficients, hereinafter referred to as "regional ionospheric delay coefficients", sending the generated correction information.