GPS - VTEC in the American sector during a high solar activity year: Observations and IRI predictions

VTEC measurements obtained with GPS satellite signals during year 1999 are used to check the validity of IRI to predict this ionospheric variable in the American sector. Measurements obtained during June solstice and September equinox at 9 stations are considered. The considered latitude range extends from 18.4 to -64.7 and the longitude ranges from 281.3 to 297.7. The deviation between modelled and measured values was obtained. The results show that for solstice the model overestimates VTEC at nighttime, sunrise and sunset hours and underestimates VTEC for daylight hours at northern stations. For the South, IRI overestimates the VTEC for all hours of the day during solstice. Good agreement was observed for some cases during equinox. Additional studies covering more stations and conditions and using ionosonde data will be useful to complete the IRI validation.