Anti-ship missiles, especially seaskimmers, are severe threats for the ships of naval forces. Infrared Search and Track (IRST) systems offer the passive detection of such targets at long ranges. The performance requirements for such a warning system are very tough: the false alarm rate (FAR) shall not exceed one false alarm per day. To assess the performance of an IRST system (FAR, detection probability, range) a very large data set must be processed. For this purpose an IRST data acquisition and assessment system has been built. By means of this system a large amount of long IR image sequences can be measured, stored digitally, and processed in near real time. In 1997 two measurement campaigns with this system have been conducted, delivering a huge amount of image data with and without targets. Several sequences of this data have already been analyzed. The essential components of the built IRST data acquisition and assessment system are described. Details about one of the measurement campaigns are reported and samples of measured image data and first results are presented.