A procedure for resolving carrier phase ambiguities, even under unfavorable geomagnetic conditions, has been tested. Precise ionospheric corrections have been made using dual-frequency data from control stations. Successful results will be presented, based on data collected during a period of high geomagnetic activity from an area straddling the North West of the United States and the South West of Canada. The corrections have been obtained modelling the ionosphere by computed tomography. This technique could enable wide area GNSS networks to support sub-decimeter differential surveying and navigation in real time.