The invention relates to generating correction information to be used to correct observations coming from a navigation satellite system (NSS) receiver in a region of interest. For each of a plurality of reference stations in said region, raw observations obtained by the reference station observing NSS multiple-frequency signals from a plurality of satellites over multiple epochs are received. Then, precise satellite information on the orbit position, clock offset, and biases of each satellite is obtained. For each reference station, ambiguities in the carrier phase of the received raw observations are estimated, using the precise satellite information and the position coordinates of the reference station. Geometric-free phase linear combination values are then computed based on the received raw observations together with the estimated ambiguities. The correction information is generated based on the computed geometric-free phase linear combination values. The correction information comprises, for each NSS satellite, a regional ionospheric delay function and its coefficients, those representing, per NSS satellite, the ionospheric delay in said region. The correction information is sent, for example to facilitate position determination, to the NSS receiver or to a server in charge of processing observations from the NSS receiver.