TEST OF GPS FOR PERMANENT IONOSPHERIC TEC MONITORING AT HIGH-LATITUDES

The Global Positioning System (GPS) observables are affected by the io nosphere. The dispersive nature of this effect and the use of two freq uencies in the GPS observations make possible to measure the ionospher ic total electron content (TEC) from dual frequency GPS data. In this work we test the concept of permanent monitoring of TEC using a networ k of GPS receivers at high latitudes. We have used GPS data from five permanent receivers in Scandinavia, from 1-30 January 1994, with geogr aphic latitudes ranging from 57.4 degrees N to 78.9 degrees N. The res ults show the capability of the method to monitor the evolution of TEC as a function of time and geographical location. We have detected nig ht-time enhancements almost every night for some of the stations, and we have also been able to produce maps of the instantaneous TEC as a f unction of both latitude and longitude around the GPS network. We also present some of the current limitations in the use of GPS for estimat ing TEC at high latitudes such as the difficulties in solving for cycl e-slips, and the necessity of reliable values for the receiver and sat ellite differential instrumental biases.