For a traffic control of ships passing the control region arbitrarily, optimization techniques can be utilized to get smooth and safe courses avoiding dangerous encounters where miss distance and destination error for each ship are evaluated as a criterion function. To realize such optimal course selection, however, reliable communication systems are indispensable, because ships may take the nearest courses just beyond the miss distance. In the paper, a communication system using conventional marine radars is proposed to realize mutual communication between ships and/or the control station. In the proposed system, control information such as position or velocity of each ship is broadcast to surrounding ships by PPM (pulse position modulation) of radar pulse with dual-k convolutional coding. The information is broadcast, divided into 4 subframes corresponding to each scan of the radar antenna, and those subframes of different radars are identified by detecting the preamble uniquely specified to each radar. The data of 80 bit length can be broadcast within 20 seconds using conventional radars. Use of the coding technique as well as the PPM ensures reliability of the communication, enabling each receiver to separately extract correct information of other ships in heavy radio interference environment. The applicability of the system is evaluated for the situation investigated in the previous papers. From the investigation, it is shown that the overall error rate of the communication system, taking account of false preamble error and decoding error, is less than 10^-5 over 99.99999% of the total time, which is regarded sufficient for such purposes.