Real Time Kinematic (RTK) is a system that utilises Global Navigation Satellite Systems (GNSS) to provide accurate positioning in real time. The contribution of the troposphere, the ionosphere and local effects, such as receiver noise and multipath are the most significant error sources affecting network RTK measurements We show how measurements with network RTK are affected by these different error sources under varying circumstances such as time of year or time of the day, network infrastructure, satellite systems and processing techniques We find that, for Scandinavian conditions, the effect of the ionospheric spatial variability on network RTK measurements is greater during nighttime than during daytime. The effect is also largest in the months October and November and smallest in the months of June and July. A densification of the reference network from 70 km to 35 km between the reference stations results in improved measurements. The error in the measured vertical position coordinate is reduced from 26 mm to 17 mm. The access to new GNSS reduces error in the measured vertical position coordinate from 26 to 21 mm. By using the L3-combination, the contribution from the ionosphere is reduced to virtually zero. However, this has been at the expense of the local errors