An active device includes a receiving antenna (6) that receives an electromagnetic (2) signal coming from a localisation system (3) and at least one emitting antenna (8) that 5 resends said signal (2) to said localisation system (3), and is characterised in that said device (4) includes a plurality of said emitting antennae (8) connected to at least one receiving antenna (6) in such a way that said emitting antennae (8) receive the electromagnetic (2) signal coming 10 from said at least one receiving antenna (6), with each of said emitting antennae (8) being the ones that resend the signal (2) to said localisation system (3). This offers the possibility of creating artificial measuring points that are retained over time easily, cheaply and simply, 15 whether in wooded zones, snow-covered zones or at sea.