Abstract: A ranging device is disclosed that monitors for changes in a scene of interest. The ranging device includes a transmitter that sends pulses into the scene, a receiver that generates pulse samples in response to pulse returned from the scene, and a target discriminator that analyzes the pulse samples. The target discriminator detects targets entering the scene, targets leaving the scene, and targets moving in the scene through the use of at least one non-averaging statistical characteristic of the pulse samples. The target discriminator may generate short term and long term high order statistical samples in response to the pulse samples as well as detector samples consisting of the ratio of high order statistical samples. The detector samples are highly indicative of the presence or absence of a target in the scene.