AIS

According to Wikipedia, the Automatic Identification System (AIS) is an automated tracking system that uses transponders fitted on vessels and monitored by Vessel Traffic Services (VTS). The equipment operates in the VHF maritime mobile band using frequencies 161.975 MHz (channel 87B) and 162.025 MHz (channel 88B). It integrates a VHF transceiver with a positioning system such as GPS to regularly broadcast data.

Practical navigators must distinguish between AIS receivers (which only intercept signals) and transponders (transceivers that actively transmit). Transponders are categorized into Class A (12.5 W output, mandatory under SOLAS for ships over 300 gross tonnage) and Class B (2 W CSDMA or 5 W SOTDMA output, used on recreational yachts). While a traditional marine compass indicates Heading, AIS delivers precise Course Over Ground (COG), Speed Over Ground (SOG), unique MMSI numbers, and vessel dimensions.

In modern electronic navigation, chartplotters process AIS data to calculate Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA), generating alarms for potential collision hazards. Navigators should note that AIS is a cooperative tracking system; unequipped wooden craft, fishing buoys, and land obstacles will not appear on AIS displays, making visual watchkeeping and radar essential companions.