A passive system is described for detecting radar emissions from vessels, receiving the radar emissions and analysing the data using a series of algorithms and software manipulation to extract radar signatures representative of the identity of the vessel. The data output is capable of comparison with a stored set of data enabling accurate identification of the vessel. The resulting output is displayed on a suitable display. A system having a library of vessel emission signatures can either be created within the operator's library through measurement made, or it can be supplied from a central database. The system is capable of installation on sea, land or air-based platforms.