The probability that an intrusion detection sensor will sense an unauthorized action (Garcia 298).

The probability of sensing (PS) is the probability that a sensor will alarm for a particular type of intrusion under certain conditions.  The PS value is a measured value and should be associated with a statistical confidence level. The design of detection systems should take into account certain factors that affect P_S such as environmental conditions (e.g., fog, rain, snow, etc.), installation conditions, and the condition of the equipment (a sensor's P_S value may degrade over time). In addition, the design needs to account for the type of action by an intruder to be detected (e.g., running, walking, jumping, crawling, tunneling, breaching, etc.).