AIS Automatic Identification System for Ships is a traffic system that uses VHF transponders on vessels to periodically send out location information. It was developed by Swedish inventor Hakan Lans and is similar in concept to the ADS-B transponder system used on aircraft for position reporting.
AIS uses two VHF marine channels 87B (161.975 MHz) and 88B (162.025 MHz). Transmission is by 9.6Kbps GMSK modulation using HDLC packet protocol. VHF channel access is by SOTDMA Self Organized Time Division Multiplexing. AIS uses navigational information from external on board GPS, Inertial Navigation and Ship Control systems. AIS has several different classes of equipment such as Class A, Class B, Base Station and Aids to Navigation. Depending on the Class, different types of information are transmitted when the ship is in port, or underway.
In a previous post (Ref.1), we looked at decoding AIS using aisdeco2 and showing the results on the OpenCPN chart display. In order to use aisdeco2 software we need to determine several parameters for the RTL-SDR. The first parameter is the RF Gain and the second parameter is the Frequency Correction to be used. I used Toronto Marine Weather at 162.400MHz to make my adjustments as shown in Figure 1. Once we determine the gain & frequency correction we need to pass this information as parameters to the aisdeco2.exe program. This is shown in Figure 2. The easiest way to do this is to create a desktop shortcut as shown in Figure 3. Figure 4 shows the window that shows successful completion of the shortcut and Figure 5 shows an AIS sigcap of the Toronto Harbour showing 2 active contacts.