Abstract: A system and method for the accurate determination of a time to fire high energy radiation for security inspection of a cargo vehicle in a drive-through inspection portal. The system includes at least two sensors, one of which is positioned at an entry to the portal, and the other is positioned just after beamline center (BCL). As a driver of the vehicle activates a button at the entry to the portal, the system takes a measurement using one sensor to determine a distance from the driver to a front of the vehicle. As the vehicle reaches the BCL, a measurement is taken by the other sensor in real time and compared with the measurement taken at the entry. A user defined offset is then applied to determine how far behind the driver should the high energy radiation be fired.