Abstract: A device and method for detecting the presence of nearby aircraft which have a strobe-type, anticollision signalling device with respect to a monitoring aircraft. Incoming radiation is detected, including (i) background radiation (ii) pulse-like radiation occurring in random manner, and (iii) pulse-like radiation characterized by uniform periodic pulse rate. Background radiation is filtered, leaving a signal representing pulse-like radiation occurring either in random manner and/or periodic regularity. Those signals having periodic regularity are identified and recorded as pulse trains which provide the basis for generating an alarm indicating detection of a strobe anticollision device. Additional devices and procedures are applied with respect to the invention to define its accuracy and economy as part of a passive optical traffic alert system for small aircraft.