In the context of the European Space Agency/European Space Operations Centrefunded Study “GNSS Contribution to Next Generation Global Ionospheric Monitoring,”four ionospheric models based on GNSS data (the Electron Density AssimilativeModel, EDAM; the Ionosphere Monitoring Facility, IONMON v2; the TomographicIonosphere model, TOMION; and the Neustrelitz TEC Models, NTCM) have been runusing a controlled set of input data. Each model output has been tested against differentialslant TEC (dSTEC) truth data for high (May 2002) and low (December 2006) sunspotperiods. Three of the models (EDAM, TOMION, and NTCM) produce dSTEC standarddeviation results that are broadly consistent with each other and with standard deviationspreads of ∼1 TECu for December 2006 and ∼1.5 TECu for May 2002. The lowestreported standard deviation across all models and all stations was 0.99 TECu (EDAM,TLSE station for December 2006 night). However, the model with the best overall dSTECperformance was TOMION which has the lowest standard deviation in 28 out of 52 testcases (13 stations, two test periods, day and night). This is probably related to theinterpolation techniques used in TOMION exploiting the spatial stationarity of verticalTEC error decorrelation.