The present disclosure relates generally to a method and apparatus for determining a presence of a concealed object and, in particular, to a method of determining the presence of the concealed object from a depth profile generated from backscattered electromagnetic energy using range-gated detection windows. 
In various law enforcement and/or military applications, there is a need to determine the presence of certain objects such as contraband, explosives, etc.,  which may be concealed from view within or by various obscurants, such as a container, a backpack, a garbage can, a box, a wall or other concealing structure, etc.  In some instances, the concealed objects have been known to be stored or concealed within a person, either by ingesting the item or by surgically implanting and/or injecting the item underneath the skin.  Most methods of volume interrogation are suited to a laboratory environment, where the interrogator is able to move the object to a location with respect to the incident beam and wherein a source and detector may be placed at arbitrary locations and standoffs.  However, for explosive material in a field of combat or civilian location, the interrogator does not have these abilities.  Bringing the test equipment close to a suspected explosive opens up the possibility of having an explosion destroy the testing equipment or harm the interrogator.  Additionally, explosives and other contraband are generally hidden in locations at which there is little possibility of safely placing a beam source and detector on opposite sides of the volume.  Methods for testing for such objects outside of a laboratory therefore must address such obstacles.