Patent ID: 7693177

Claim:
A navigational aid for use as an AIS apparatus, said navigational aid comprising: a receiver for receiving signals transmitted from other stations; a reference timing generator for generating a synchronization timing signal used for controlling timing of communication with the other stations; a memory for storing information about previous use of each time slot specified in accordance with the synchronization timing signal; a signal detector for judging whether an information signal exists in a time slot specified in accordance with the synchronization timing signal by using the signal received by said receiver and the information stored in said memory; and a transmitter for transmitting information about own station in a time slot which has been judged by said signal detector to be free of an information signal transmitted from any of the other stations; wherein the signal detector sets a threshold, for judging whether an information signal exists in the specified time slot, based on a probability of a presence of an information signal during previous use of the specified time slot.