Studying anomalous propagation over marine areas using an experimental AIS receiver set-up

Abstract

Automatic Identification System (AIS) is a wireless communication system used by vessels to exchange real-time information with each other and with coastal authorities, enhancing situational awareness and maritime safety. Consequently, safety at sea depends on reliable signal transmission, which can be disrupted by anomalous signal propagation. In particular, tropospheric ducting can extend the AIS antenna horizon, allowing messages to be received over greater distances than under standard conditions. To study the behaviour of the AIS signal under standard and anomalous propagation conditions, 1-year of AIS-observations were collected from two antennae at 7 and 30 m heights above the mean sea level on the Utö Island in the Baltic Sea. The AIS antennae were co-located with mast-mounted measurements of temperature and humidity. This allows for studying the AIS signal propagation alongside observed refractivity profiles. The AIS over-the-horizon observations occurred 34 % of the time for the 7 m antenna and 59 % of the time for the 30 m antenna, mainly during the spring and summer months. A strong diurnal cycle was observed in the Archipelago Sea, north of Utö, while no diurnal cycle was observed in the open sea region south of Utö. During periods of anomalous signal propagation, the AIS messages were received from farther away, from up to 600 km from Utö and the observed received signal strength decayed slower with distance, indicating reductions in propagation losses due to ducting. Anomalous AIS observations were also associated with stronger and higher ducts; when the duct height was 59 m, the occurrence rates were 90 % and 95 % for the 7 and 30 m antenna, respectively.