A new method of real-time GNSS user positioning availability control for transportation applications is considered. The positioning availability value is defined as a full probability of event when we get a personal GNSS user’s required navigation parameters (RNP) taking in account the current Positioning Delusion of Precision (PDOP), ranging errors and positioning errors. On a basis of GNSS/LAAS network dataset we get current GNSS performance statistics, such as positioning error standard mean and standard deviation and a number of positioning failures within the observation period. The PDOP values are used in order to compute an alarm index of a probable sudden positioning deterioration under poor GNSS satellite vehicles (SV) geometry. The method can be recommended for some transportation applications which have strong RNP. Especially, the method can be recommended for transportation applications such as aviation landing systems and automatic railroad traffic control.