Real time kinematic or RTK is a high accuracy GNSS relative positioning technique which allows to measure positions in real time with an accuracy usually better than 1 decimeter. Ionospheric small scale variability can strongly degrade RTK accuracy. In this paper, we present a method allowing to assess in a direct quantitative way the influence of the ionospheric activity on RTK accuracy. We apply this method on two different baselines : one of 4.08 km and another of 18.58 km. Both of them are taken between Belgian stations. We show that on a 4 km baseline, the ionospheric activity have an influence on RTK accuracy as on a larger baseline and we try to analyse this thanks to different combinations of doubles differences.