Source: AIS

What is AIS?

Automatic Identification System - AIS - is a communication tool primarily used by marine vessels to avoid collision with other vessels.

• Most vessels have a range of communications tools available to them, such as radar and very high frequency (VHF) handheld radio. AIS is also a type of VHF radio, but it operates on different frequencies and is an automated messaging system with pre-programmed fields that does not require someone speaking into a microphone like handheld radio.

• AIS transponders are physical devices on vessels that automatically send information about those vessels to all other AIS receivers in the vicinity. The receivers may be transponders on other vessels, shore-based receivers, and satellite receivers. Transponders send two types of messages

  • “Positional” information, which takes the feed of data from a vessel’s multiple sensors (e.g. its GPS unit, navigational charts) and converts the data into compact messages about the vessel’s position, speed, heading, next port of call, etc.

  • “Static” information, which sends information about the vessel itself. Some of this information is filled out in the transponder during set-up and can be changed, such as the vessel name, call sign, IMO number, etc.

  • “Positional” and “Static” information can be “joined” by the key field, Maritime Mobile Service Identity (MMSI). The MMSI is supposed to be a unique number for every AIS transponder.

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AIS Transponder

Can we see every vessel on AIS?

No. The International Maritime Organization (IMO) requires only certain vessels to carry AIS.

AIS must be fitted aboard all ships of 300 gross tonnage and upwards engaged on international voyages, cargo ships of 500 gross tonnage and upwards not engaged on international voyages and all passenger ships irrespective of size. The requirement became effective for all ships by 31 December 2004 (source).

AIS messages can reach more vessels than other communication systems because of a global network of terrestrial and satellite AIS stations that receive messages and "reflect" them back out. An unexpected outcome of this system has been that the public can now also access and see AIS messages from around the world.

How does Skylight use AIS?

We use AIS as a foundational, real-time source of vessel positional information. It is important to know that most non-industrial fishing vessels are not carrying AIS. This is rarely illegal because most fishing vessels do not meet the IMO’s criteria. Local fishing vessels almost never meet the criteria; it is primarily large, industrial vessels that do. Additionally, even if artisanal vessels wanted to voluntarily carry AIS for safety or other purposes, the equipment is too costly for the vast majority to be worth their while.

Skylight takes in AIS anyway for these 3 reasons:

• We can detect the fishing activity of some commercial fishing vessels directly, who are likely to have other fishing vessels around them even if they are not carrying AIS

• The interactions fishing vessels have with other vessels can be captured on AIS (e.g. transshipments from fishing vessels to fish carriers, fishing vessels selling fish or bycatch to other fishing vessels, tankers re-fueling fishing vessels at-sea

• AIS can help filter out those vessels that are not of-interest, making it easier to find vessels that are “dark” when layering on other data feeds

Who is Skylight’s AIS provider?

Our AIS provider is Spire. We subscribe to their network of satellite, terrestrial, and ship-based receivers, which gives us comprehensive coverage over the globe. The average revisit rate for the constellation we subscribe to is 5-10 minutes. Read about Spire’s Constellation coverage in the Spire Help Center. See slides 4, 5, and 6 from Spire’s presentation for the 2021 CubeSat Developers Workshop (link to outside source PDF) for more information on Spire’s constellation coverage.

Is AIS a reliable source of data to monitor vessels?

Yes and no. AIS transponders are generally configured correctly and regularly monitored by flag states to help vessels stay safe in the sea - the original and main purpose of AIS. In many countries, Class A devices can only be configured by the manufacturer/at port and require a password to change static information. However, issues can arise a few unintentional and intentional ways:

• Some AIS transponders are more powerful than others. For example Class B transponders are generally weaker than Class A transponders (see Class A and Class B differentiation on the US Coast Guard website). The signal may be strong enough to be picked up by vessels nearby, but not by satellite receivers. As a result in some areas, vessels appear to not be transmitting AIS when they actually are.

• Some vessels intentionally and unintentionally use duplicate MMSI numbers which makes it very difficult to create tracks for these vessels on a global map when they are operating at the same time

• Some vessels may turn off their AIS in order to stay safe in areas of high risk of attacks or robberies. This is an ongoing, genuine concern in the Gulf of Guinea.

• Some vessels may turn off their AIS just before entering port to reduce noise because it is not required by the IMO or by that particular port authority.

• Some vessels may turn off their AIS to hide their activity, particularly while out at-sea. Because they are operating in distant waters, their flag state has hundreds of vessels to monitor, and turning off the AIS is difficult to distinguish from technical signal issues, these vessels are rarely reprimanded.

• It is very easy for vessels to change their static information in some transponders, particularly Class B transponders, to obscure their identity. This is one type of spoofing.