Here the problem of the impact of space weather and ionospheric conditions on ionospheric delay of the satellite navigation system signals is addressed. First, the space weather term is defined, along with the parameters describing its conditions. The process of ionospheric layer generation is presented, and related parameters describing the conditions of the ionosphere defined. The ionospheric models describing the relationship between the space weather and the ionosphere are discussed. The existing standard GPS ionospheric delay model is presented. Limitations and drawbacks of existing models in relation to the impact of the conditions and dynamics of space weather are analysed. The impact of extreme space weather and ionospheric disturbances on GPS ionospheric delay is analysed, based on particular case of extreme space weather and ionospheric disturbances observed during October and November 2003, as experienced in Republic of Croatia. The true values of GPS ionospheric delays for the period in question are extracted from raw GPS pseudorange measurements taken at the reference station Osijek, Croatia. The GPS ionospheric delay dynamics is analysed, and the correlation comparison with the space weather and the ionospheric conditions development examined. The results of the analysis form the foundation for a new GPS ionospheric delay model definition, established upon the monitoring of true GPS ionospheric delay dynamics. Finally, the general composite model for GPS ionospheric delay estimation is proposed, which takes into account the local and the regional characteristics of the ionosphere. Proposed model uses a present state of the space weather conditions as an input, and is completely applicable in all space weather conditions.