A system and method for monitoring a spatial position of a mobile transmitter is provided. In particular, the mobile transmitter may be attached to or included in an object of interest. By analyzing the signal strengths of radio frequency signals emitted by the transmitter, a spatial position of the mobile transmitter can be determined, and it is possible to detect whether or not the spatial position of the mobile transmitter is outside an allowable area. By applying the monitoring of the spatial position to a radio frequency system on a vessel, a reliable man-over-board detection can be achieved.