Conflict probes will be important components of air traffic control tools in the next years. They appear in almost every project (CINCAT, ERATO, HIPS, PHARE, and so on). To be useful, they have to fulfill two goals : reliability (they have to detect all conflicts) and efficiency (they must minimize the number of false alarms). Conflict probes rely on trajectory prediction, and their reliability and efficiency highly depend on the accuracy of trajectory prediction. In this paper, we present a quick mathematical insight of the influence of ground speed errors on trajectory prediction, results of arithmetic simulations on real traffic both on ground and vertical speed errors, and a statistical analysis of these results to model the influence of vertical and ground speed errors on conflict probe.