The IR contrast of ships at sea is of importance for those who want to detect or identify the ship and for those who worry about this. This IR contrast is determined by a large number of parameters. Of course temperatures of the ship's structure and those of the ambient sea and air are important, but also important are the reflection properties of the sea background and the radiance distribution of the surrounding sky. Modeling of these phenomena appears to be very complicated and the accuracy of the results is rather course in many cases. Therefore, at FEL- TNO an approach has been followed, using a package of sensors on board the ship, giving radiometric or real temperature data of specific surface elements and background radiometric data. These data are taken into a PC system, providing radiant contrast data in any IR-spectral band. Taking into account atmospheric propagation effects and sensor performance, a simplified detection model provides range data in the form of polar diagrams with elevation as parameter. The commander of the ship uses the system as an IR Tactical Decision Aid (TDA), as he may decide upon countermeasures, if the vulnerability of his ship exceeds certain limits.