The influence of the ionosphere can be one of the main obstacles to GPS carrier phase ambiguity resolution in real-time, particularly over long baselines. This is important to all users of GPS requiring sub-decimeter positioning, perhaps in real time, especially with high geomagnetic activity or close to the Solar Maximum. Therefore, it is desirable to have a precise estimation of the ionospheric delay in real-time, to correct the data. In this paper we asses a real-time tomographic model of the ionosphere created using dual-frequency phase data simultaneously collected with the receivers of a network of stations in the USA and Canada, with separations of 400--1000 km, during a period of high geomagnetic activity (Kp=6). When the tomographic ionospheric correction is included, the resolution on-the-fly (OTF) of the widelane double-differenced ambiguities at the reference stations is nearly 100% successful for satellite elevations above 20 degrees, while the resolution of the L1, L2 ambiguities at the rover is typically more than 80% successful. ¿ 2000 American Geophysical Union