Apparatus for detecting the position of a mobile or immobile entity in a bounded space, comprising: - means for detecting the presence of said entity in at least two at least partially overlapping observation zones (Z1, Z2) of said space, said means being suitable for gathering at least one piece of information (Ip) on the presence of this entity independently of its movement, and one piece of information (Im) on the movement of said entity, in each of said observation zones; - processing means that are suitable for fusing, by way of logic operations, the pieces of information (Ip, Im) on the presence and movement of said entity gathered for each of said observation zones; and - deciding means that are connected to the processing means and that are suitable for commanding an action depending on a logic signal produced by the processing means, said presence-detecting means comprising: - one signal emitting/receiving device per observation zone, said device comprising at least one emitting element for emitting signals into the associated observation zone and at least one receiving element for collecting an instantaneous echo made up of reflected signals originating from said observation zone; - means for storing a reference echo for each observation zone, said reference echo being representative of reflected signals originating from each of said zones in the absence of said entity; and - means for comparing said instantaneous echo and said reference echo for each observation zone, said means being suitable for measuring amplitude and phase variations between said echoes so as to produce said pieces of information on the presence and movement of said entity in said observation zone, said apparatus being characterized in that the emitting/receiving device comprises a radiofrequency emitter/receiver circuit comprising a radiofrequency signal generator that is coupled to at least one antenna system that forms an emitting element for emitting the signals, and at least one radiofrequency sensor that forms the receiving element that is connected to an analogue-digital converter by way of an amplifier and a low-frequency filter, and in that the first observation zone corresponds to the total volume of said space and in that the second observation zone is comprised in a volume of said space, which is bounded by walls of said space and located between two substantially horizontal planes, so that the second observation zone partially overlaps the first observation zone. - means for detecting the presence of said entity in at least two at least partially overlapping observation zones (Z1, Z2) of said space, said means being suitable for gathering at least one piece of information (Ip) on the presence of this entity independently of its movement, and one piece of information (Im) on the movement of said entity, in each of said observation zones; - processing means that are suitable for fusing, by way of logic operations, the pieces of information (Ip, Im) on the presence and movement of said entity gathered for each of said observation zones; and - deciding means that are connected to the processing means and that are suitable for commanding an action depending on a logic signal produced by the processing means, - one signal emitting/receiving device per observation zone, said device comprising at least one emitting element for emitting signals into the associated observation zone and at least one receiving element for collecting an instantaneous echo made up of reflected signals originating from said observation zone; - means for storing a reference echo for each observation zone, said reference echo being representative of reflected signals originating from each of said zones in the absence of said entity; and - means for comparing said instantaneous echo and said reference echo for each observation zone, said means being suitable for measuring amplitude and phase variations between said echoes so as to produce said pieces of information on the presence and movement of said entity in said observation zone,