poll the transceiver for this message to be sent. MMSI, time, SOG, COG, longitude, latitude, true heading, ship type, dimensions. Message 24 Class B CS Static Data Report: This message is sent every 6 minutes, the same time interval as for Class A transponders. Because of its length, this message is divided into two parts, sent within one minute of each other. This message was defined after the original AIS specifications, so some Class A units may need a firmware upgrade to be able to decode this message. MMSI, boat name, ship type, call sign, dimensions, and equipment vendor id. Detailed description: AIS receivers A number of manufacturers offer AIS receivers, designed for monitoring AIS traffic. These may have two receivers, for monitoring both frequencies simultaneously, or they may switch between frequencies (thereby missing messages on the other channel, but at reduced price). In general they will output RS-232, NMEA, USB or UDP data for display on electronic chart plotters or computers. As well as dedicated radios, software defined radios can be set up to receive the signal. Technical specification RF characteristics AIS uses the globally allocated Marine Band channels 87 and 88. AIS uses the high side of the duplex from two VHF radio "channels" (87B) and (88B) Channel A 161.975 MHz (87B) Channel B 162.025 MHz (88B) The simplex channels 87A and 88A use a lower frequency so they are not affected by this allocation and can still be used as designated for the maritime mobile frequency plan. Most AIS transmissions are composed of bursts of several messages. In these cases, between messages, the AIS transmitter must change channel. Before being transmitted, AIS messages must be non-return-to-zero inverted (NRZI) encoded. AIS messages are transmitted using Gaussian minimum-shift keying (GMSK) modulation. The GMSK modulator BT-product used for transmission of data should be 0.4 maximum (highest nominal value). The GMSK coded data should frequency modulate the VHF transmitter. The modulation index should be 0.5. The transmission bit rate is 9600bit/s Ordinary VHF receivers can receive AIS with the filtering disabled (the filtering destroys the GMSK data). However, the audio output from the radio would need to be then decoded. There are several PC applications that can do this. The signal can travel a maximum of 75 kilometers Message organization As there are a multitude of automatic equipment transmitting AIS messages, to avoid conflict, the RF space is organized in frames.