W.T.F is A.I.S?
Here at Shrunk, we have been wrangling freight transport and logistics datasets, looking for potential optimisation and decarbonisation strategies. Freight transport networks underpin the global production systems, and contribute significantly to global GHG emissions. However freight networks are somewhat abstract to us in our everyday lives; freight rail yards and port terminals are not places we often visit.
One such freight dataset we are currently working with is AIS ship position data. AIS (Automatic Identification System) was originally developed as a collision avoidance system, to prevent ships colliding at night or in bad weather.?AIS was originally designed for sending signals ship-to-ship or ship-to-shore, with ranges of approximately 20 nautical miles between ships or 40 nautical miles for shore based receivers.?Land based stations are operated by Port and Maritime Authorities as well as by private operators and hobbyists.
Around 2010, researchers realised AIS message traffic could be received by Low Earth Orbit (LEO) satellites. This came as a surprise as LEO satellites can be hundreds of kilometres above the earth, however the thinner atmosphere increases the range of the VHF signals. Currently a constellation of about 60 satellites are used to monitor global AIS message traffic.?
AIS operates over a number of VHF channels worldwide. To avoid message conflicts, the RF space is divided into discrete time frames. A number of schemes are then used to coordinate channel-access for transmitters, one of these is the SOTDMA (Self-Organised Time-Division Multiple Access) method. SOTDMA is ‘self-organising’ in that no base stations are involved in allocating transmission slots.?Global AIS traffic is on the order of 100 million daily messages and these channel-access schemes can result in dropped messages if the system is overloaded with traffic.
AIS data is not without some inherent security and reliability issues. It’s possible to spoof AIS signals, so that a vessel appears to be somewhere it's not. It’s also possible to jam AIS signals, preventing a transmission from a vessel reaching any receivers . While public disclosure of AIS positions is seen as an aid to illegal fishing operations, in high-risk piracy areas it can be dangerous for a ship to publish its location. In both these cases it is possible for the ships to stop transmitting their position by AIS to avoid detection.?
These AIS datasets allow for the investigation of lots of interesting questions, for example how sub-networks of ports form as container ships follow regular routes. Furthermore, AIS is now used for more applications than simply safety, for example fisheries management (monitoring of illegal fishing), oil spill monitoring, shipping noise pollution monitoring and the study of environmental impacts of shipping.?
领英推荐
Jacob - Shrunk Dreamweaver
Further reading
Fournier, 2017, Past, present, and future of the satellite-based automatic identification system: areas of applications (2004–2016), WMU Journal of Maritime Affairs,?https://doi.org/10.1007/s13437-018-0151-6
Goudossis et al., 2017, Towards a secure automatic identification system (AIS), Journal of Marine Science and Technology,?https://doi.org/10.1007/s00773-018-0561-3
Kaluza et al, 2010, The complex network of global cargo ship movements, R. Soc. Interface,?https://doi.org/10.1098/rsif.2009.0495