A navigational aid for use as an AIS apparatus includes a memory for storing information about previous use of individual time slots synchronized with transmission schedules of other stations and a signal detector for judging whether an information signal exists in a time slot specified in accordance with a synchronization timing signal based on the information stored in the memory and a result of monitoring of the behavior of a baseband signal obtained from a received signal on an IQ-plane. The navigational aid transmits information about own station based on a result of judgment by the signal detector. The monitoring of the behavior of the received baseband signal plotted on the IQ-plane can be accomplished by performing pattern recognition operation, for which a carrier sense technique, such as a support vector machine, subspace method or neural network, can be used.