The automatic identification system (AIS) is an automatic tracking system used on ships and by vessel traffic services (VTS). When satellites are used to detect AIS signatures, the term Satellite-AIS (S-AIS) is used. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for water transport.
Information provided by AIS equipment, such as unique identification, position, course, and speed, can be displayed on a screen or an ECDIS. AIS is intended to assist a vessel's watchstanding officers and allow maritime authorities to track and monitor vessel movements. AIS integrates a standardized VHF transceiver with a positioning system such as a GPS receiver, with other electronic navigation sensors, such as a gyrocompass or rate of turn indicator. Vessels fitted with AIS transceivers can be tracked by AIS base stations located along coast lines or, when out of range of terrestrial networks, through a growing number of satellites that are fitted with special AIS receivers which are capable of deconflicting a large number of signatures.
The International Maritime Organization's International Convention for the Safety of Life at Sea requires AIS to be fitted aboard international voyaging ships with 300 or more gross tonnage (GT), and all passenger ships regardless of size.
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Viewing and using AIS data
AIS is intended, primarily, to allow ships to view marine traffic in their area and to be seen by that traffic. This requires a dedicated VHF AIS transceiver that allows local traffic to be viewed on an AIS enabled chartplotter or computer monitor while transmitting information about the ship itself to other AIS receivers. Port authorities or other shore-based facilities may be equipped with receivers only, so that they can view the local traffic without the need to transmit their own location. All AIS transceivers equipped traffic can be viewed this way very reliably but is limited to the VHF range, about 10-20 nautical miles.
If a suitable chartplotter is not available, local area AIS transceiver signals may be viewed via a computer using one of several computer applications such as ShipPlotter and Gnuais. These demodulate the signal from a modified marine VHF radiotelephone tuned to the AIS frequencies and convert into a digital format that the computer can read and display on a monitor; this data may then be shared via a local or wide area network via TCP or UDP protocols but will still be limited to the collective range of the radio receivers used in the network. Because computer AIS monitoring applications and normal VHF radio transceivers do not possess AIS transceivers, they may be used by shore-based facilities that have no need to transmit or as an inexpensive alternative to a dedicated AIS device for smaller vessels to view local traffic but, of course, the user will remain unseen by other traffic on the network.
A secondary, unplanned and emerging use for AIS data is to make it viewable publicly, on the internet, without the need for an AIS receiver. Global AIS transceiver data collected from both satellite and internet-connected shore-based stations are aggregated and made available on the internet through a number of service providers. Data aggregated this way can be viewed on any internet-capable device to provide near global, real-time position data from anywhere in the world. Typical data includes vessel name, details, location, speed and heading on a map, is searchable, has potentially unlimited, global range and the history is archived. Most of this data is free of charge but satellite data and special services such as searching the archives are usually supplied at a cost. The data is a read-only view and the users will not be seen on the AIS network itself. Shore-based AIS receivers contributing to the internet are mostly run by a large number of volunteers. AIS mobile apps are also readily available for use with Android, Windows and iOS devices. See External links below for a list of internet-based AIS service providers. Ship owners and cargo dispatchers use these services to find and track vessels and their cargoes while marine enthusiasts may add to their photograph collections.
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Deployment history
Vessel-based AIS transceivers
The 2002 IMO SOLAS Agreement included a mandate that required most vessels over 300GT on international voyages to fit a Class A type AIS transceiver. This was the first mandate for the use of AIS equipment and affected approximately 100,000 vessels.
In 2006, the AIS standards committee published the Class B type AIS transceiver specification, designed to enable a simpler and lower-cost AIS device. Low-cost Class B transceivers became available in the same year triggering mandate adoptions by numerous countries and making large-scale installation of AIS devices on vessels of all sizes commercially viable.
Since 2006, the AIS technical standard committees have continued to evolve the AIS standard and product types to cover a wide range of applications from the largest vessel to small fishing vessels and life boats. In parallel, governments and authorities have instigated projects to fit varying classes of vessels with an AIS device to improve safety and security. Most mandates are focused on commercial vessels, with leisure vessels selectively choosing to fit. In 2010 most commercial vessels operating on the European Inland Waterways were required to fit an Inland waterway certified Class A, all EU fishing boats over 15m will have to have a Class A by May 2014 , and the US has a long-pending extension to their existing AIS fit rules which is expected to come into force during 2013. It is estimated that as of 2012, some 250,000 vessels have fitted an AIS transceiver of some type, with a further 1 million required to do so in the near future and even larger projects under consideration.
Satellite-based AIS (S-AIS)
AIS was developed in the 1990s as a high intensity, short-range identification and tracking network and, at the time, it was not anticipated to be detectable from space. Nevertheless, since 2005, various entities have been experimenting with detecting AIS transmissions using satellite-based receivers and, since 2008, companies such as exactEarth, ORBCOMM, Spacequest, Spire and also government programs have deployed AIS receivers on satellites. The Time Division Multiple Access (TDMA) radio access scheme used by the AIS system creates significant technical issues for the reliable reception of AIS messages from all types of transceivers: Class A, Class B, Identifier, AtoN and SART. However, the industry is seeking to address these issues through the development of new technologies and over the coming years the current restriction of satellite AIS systems to Class A messages is likely to dramatically improve with the addition of Class B and Identifier messages.
The fundamental challenge for AIS satellite operators is the ability to receive very large numbers of AIS messages simultaneously from a satellite's large reception footprint. There is an inherent issue within the AIS standard; the TDMA radio access scheme defined in the AIS standard creates 4,500 available time-slots in each minute but this can be easily overwhelmed by the large satellite reception footprints and the increasing numbers of AIS transceivers, resulting in message collisions, which the satellite receiver cannot process. Companies such as exactEarth are developing new technologies such as ABSEA, that will be embedded within terrestrial and satellite-based transceivers, which will assist the reliable detection of Class B messages from space without affecting the performance of terrestrial AIS.
The addition of satellite-based Class A and B messages could enable truly global AIS coverage but, because the satellite-based TDMA limitations will never match the reception performance of the terrestrial-based network, satellites will augment rather than replace the terrestrial system.
Shipboard AIS transceivers have a horizontal range that is highly variable, but typically only up to about 74 kilometres (46 mi). They reach much further vertically - up to the 400 km orbit of the International Space Station (ISS).
In November 2009, the STS-129 space shuttle mission attached two antennas--an AIS VHF antenna, and an Amateur Radio antenna to the Columbus module of the ISS. Both antennas were built in cooperation between ESA and the ARISS team (Amateur Radio on ISS). Starting from May 2010 the European Space Agency is testing an AIS receiver from Kongsberg Seatex (Norway) in a consortium led by the Norwegian Defence Research Establishment in the frame of technology demonstration for space-based ship monitoring. This is a first step towards a satellite-based AIS-monitoring service.
In 2009, ORBCOMM launched AIS enabled satellites in conjunction with a US Coast Guard contract to demonstrate the ability to collect AIS messages from space. In 2009, Luxspace, a Luxembourg-based company, launched the RUBIN-9.1 satellite (AIS Pathfinder 2). The satellite is operated in cooperation with SES and REDU Space Services. In late 2011 and early 2012, ORBCOMM and Luxspace launched the Vesselsat AIS microsatellites, one in an equatorial orbit and the other in a polar orbit. (VesselSat-2 and VesselSat-1)
In 2007, the U.S. tested space-based AIS tracking with the TacSat-2 satellite. However, the received signals were corrupted because of the simultaneous receipt of many signals from the satellite footprint.
In July 2009, SpaceQuest launched AprizeSat-3 and AprizeSat-4 with AIS receivers. These receivers were successfully able to receive the U.S. Coast Guard's SART test beacons off of Hawaii in 2010. In July 2010, SpaceQuest and exactEarth of Canada announced an arrangement whereby data from AprizeSat-3 and AprizeSat-4 would be incorporated into the exactEarth system and made available worldwide as part of their exactAIS(TM)service.
On July 12, 2010, The Norwegian AISSat-1 satellite was successfully launched into polar orbit. The purpose of the satellite is to improve surveillance of maritime activities in the High North. AISSat-1 is a nano-satellite, measuring only 20x20x20 cm, with an AIS receiver made by Kongsberg Seatex. It weighs six kilograms and is shaped like a cube.
On 20 April 2011, Indian Space Research Organisation launched Resourcesat-2 containing a S-AIS payload for monitoring maritime traffic in the Indian Ocean Search & Rescue (SAR) zone. AIS data is processed at National Remote Sensing Centre and archived at Indian Space Science Data Centre.
On February 25, 2013--after one year launch delay--Aalborg University launched AAUSAT3. It is a 1U cubesat, weights 800 grams, solely developed by students from Department of Electronic Systems. It carries two AIS receivers--a traditional and a SDR-based receiver. The project was proposed and sponsored by the Danish Safety Maritime Organisation. It has been a huge success and has in the first 100 days downloaded more than 800,000 AIS messages and several 1 MHz raw samples of radio signal. It receives both AIS channels simultaneously and has received class A as well as class B messages. Cost including launch was less than EUR200,000.
Canadian-based exactEarth's AIS satellite network provides global coverage using 8 satellites. This network will be significantly expanded with the announcement of a partnership with Harris Corp to utilize 58 hosted payloads on the Iridium NEXT constellation. Additionally exactEarth is involved in the development of ABSEA technology which will enable its network to reliably detect a high proportion of Class B type messages, as well as Class A.
ORBCOMM operates a global satellite network that includes 18 AIS-enabled satellites. ORBCOMM's OG2 (ORBCOMM Generation 2) satellites are equipped with an Automatic Identification System (AIS) payload to receive and report transmissions from AIS-equipped vessels for ship tracking and other maritime navigational and safety efforts, and download at ORBCOMM's 16 existing earth stations around the globe.
In July 2014, ORBCOMM launched the first 6 OG2 satellites aboard a Spacex Falcon 9 rocket from Cape Canaveral, Florida. Each OG2 satellite carries an AIS receiver payload. All 6 OG2 satellites were successfully deployed into orbit and started sending telemetry to ORBCOMM soon after launch. In December 2015, the company launched eleven additional AIS-enabled OG2 satellites aboard the SpaceX Falcon 9 rocket. This dedicated launch marked ORBCOMM's second and final OG2 mission to complete its next generation satellite constellation. Compared to its current OG1 satellites, ORBCOMM's OG2 satellites are designed for faster message delivery, larger message sizes and better coverage at higher latitudes, while increasing network capacity.
In August, 2017, Spire Global Inc. released an API that delivers S-AIS data enhanced with machine learning (Vessels and Predict) backed by it's 40+ constellation of nano-satellites.
Correlation of data sources
Correlating optical and radar imagery with S-AIS signatures enables the end-user to rapidly identify all types of vessel. A great strength of S-AIS is the ease with which it can be correlated with additional information from other sources such as radar, optical, ESM, and more SAR related tools such as GMDSS SARSAT and AMVER. Satellite-based radar and other sources can contribute to maritime surveillance by detecting all vessels in specific maritime areas of interest, a particularly useful attribute when trying to co-ordinate a long-range rescue effort or when dealing with VTS issues.
Applications
The original purpose of AIS was solely collision avoidance but many other applications have since developed and continue to be developed. AIS is currently used for:
How AIS works
Basic overview
AIS transceivers automatically broadcast information, such as their position, speed, and navigational status, at regular intervals via a VHF transmitter built into the transceiver. The information originates from the ship's navigational sensors, typically its global navigation satellite system (GNSS) receiver and gyrocompass. Other information, such as the vessel name and VHF call sign, is programmed when installing the equipment and is also transmitted regularly. The signals are received by AIS transceivers fitted on other ships or on land based systems, such as VTS systems. The received information can be displayed on a screen or chart plotter, showing the other vessels' positions in much the same manner as a radar display. Data is transmitted via a tracking system which makes use of a Self-Organized Time Division Multiple Access (SOTDMA) datalink designed by Swedish inventor Håkan Lans.
The AIS standard comprises several substandards called "types" that specify individual product types. The specification for each product type provides a detailed technical specification which ensures the overall integrity of the global AIS system within which all the product types must operate. The major product types described in the AIS system standards are:
AIS receivers are not specified in the AIS standards, because they do not transmit. The main threat to the integrity of any AIS system are non-compliant AIS transmissions, hence careful specifications of all transmitting AIS devices. However, it is well to note that AIS transceivers all transmit on multiple channels as required by the AIS standards. As such single-channel, or multiplexed, receivers will not receive all AIS messages. Only dual-channel receivers will receive all AIS messages.
Type testing and approval
AIS is a technology which has been developed under the auspices of the IMO by its technical committees. The technical committees have developed and published a series of AIS product specifications. Each specification defines a specific AIS product which has been carefully created to work in a precise way with all the other defined AIS devices, thus ensuring AIS system interoperability worldwide. Maintenance of the specification integrity is deemed critical for the performance of the AIS system and the safety of vessels and authorities using the technology. As such most countries require that AIS products are independently tested and certified to comply with a specific published specification. Products that have not been tested and certified by a competent authority, may not conform to the required AIS published specification and therefore may not operate as expected in the field. The most widely recognized and accepted certifications are the R&TTE Directive, the U.S. Federal Communications Commission, and Industry Canada, all of which require independent verification by a qualified and independent testing agency.
Message types
There are 27 different types of top level messages defined in ITU M.1371-4 (out of a possibility of 64) that can be sent by AIS transceivers.
AIS messages 6, 8, 25, and 26 provide "Application Specific Messages" (ASM), that allow "competent authorities" to define additional AIS message subtypes. There are both "addressed" (ABM) and "broadcast" (BBM) variants of the message. Addressed messages, while containing a destination MMSI, are not private and may be decoded by any receiver.
One of the first uses of ASMs was the Saint Lawrence Seaway use of AIS binary messages (message type 8) to provide information about water levels, lock orders, and weather. The Panama Canal uses AIS type 8 messages to provide information about rain along the canal and wind in the locks. In 2010, the International Maritime Organization issued Circular 289 that defines the next iteration of ASMs for type 6 and 8 messages. Alexander, Schwehr and Zetterberg proposed that the community of competent authorities work together to maintain a regional register of these messages and their locations of use. The International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA-AISM) now established a process for collection of regional application-specific messages.
Detailed description: Class A units
Each AIS transceiver consists of one VHF transmitter, two VHF TDMA receivers, one VHF Digital Selective Calling (DSC) receiver, and links to shipboard display and sensor systems via standard marine electronic communications (such as NMEA 0183, also known as IEC 61162). Timing is vital to the proper synchronization and slot mapping (transmission scheduling) for a Class A unit. Therefore, every unit is required to have an internal time base, synchronized to a global navigation satellite system (e.g. GPS) receiver. This internal receiver may also be used for position information. However, position is typically provided by an external receiver such as GPS, LORAN-C or an inertial navigation system and the internal receiver is only used as a backup for position information. Other information broadcast by the AIS, if available, is electronically obtained from shipboard equipment through standard marine data connections. Heading information, position (latitude and longitude), "speed over ground", and rate of turn are normally provided by all ships equipped with AIS. Other information, such as destination, and ETA may also be provided.
An AIS transceiver normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas. AIS transceivers use two different frequencies, VHF maritime channels 87B (161.975 MHz) and 88B (162.025 MHz), and use 9.6 kbit/s Gaussian minimum shift keying (GMSK) modulation over 25 kHz channels using the High-level Data Link Control (HDLC) packet protocol. Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss from other ships. The system provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations.
In order to ensure that the VHF transmissions of different transceivers do not occur at the same time, the signals are time multiplexed using a technology called Self-Organized Time Division Multiple Access (SOTDMA). The design of this technology is patented, and whether this patent has been waived for use by SOLAS vessels is a matter of debate between the manufacturers of AIS systems and the patent holder, Håkan Lans. Moreover, the United States Patent and Trademark Office (USPTO) canceled all claims in the original patent on March 30, 2010.
In order to make the most efficient use of the bandwidth available, vessels that are anchored or moving slowly transmit less frequently than those that are moving faster or are maneuvering. The update rate ranges from 3 minutes for anchored or moored vessels, to 2 seconds for fast moving or maneuvering vessels, the latter being similar to that of conventional marine radar.
Each AIS station determines its own transmission schedule (slot), based upon data link traffic history and an awareness of probable future actions by other stations. A position report from one station fits into one of 2,250 time slots established every 60 seconds on each frequency. AIS stations continuously synchronize themselves to each other, to avoid overlap of slot transmissions. Slot selection by an AIS station is randomized within a defined interval and tagged with a random timeout of between 4 and 8 minutes. When a station changes its slot assignment, it announces both the new location and the timeout for that location. In this way new stations, including those stations which suddenly come within radio range close to other vessels, will always be received by those vessels.
The required ship reporting capacity according to the IMO performance standard is a minimum of 2,000 time slots per minute, though the system provides 4,500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% through sharing of slots, and still provides nearly 100% throughput for ships closer than 8 to 10 nmi to each other in a ship to ship mode. In the event of system overload, only targets further away will be subject to drop-out, in order to give preference to nearer targets, which are of greater concern to ship operators. In practice, the capacity of the system is nearly unlimited, allowing for a great number of ships to be accommodated at the same time.
The system coverage range is similar to other VHF applications. The range of any VHF radio is determined by multiple factors, the primary factors are: the height and quality of the transmitting antenna and the height and quality of the receiving antenna. Its propagation is better than that of radar, due to the longer wavelength, so it is possible to reach around bends and behind islands if the land masses are not too high. The look-ahead distance at sea is nominally 20 nmi (37 km). With the help of repeater stations, the coverage for both ship and VTS stations can be improved considerably.
The system is backward compatible with digital selective calling systems, allowing shore-based GMDSS systems to inexpensively establish AIS operating channels and identify and track AIS-equipped vessels, and is intended to fully replace existing DSC-based transceiver systems.
Shore-based AIS network systems are now being built up around the world. One of the biggest fully operational, real time systems with full routing capability is in China. This system was built between 2003 and 2007 and was delivered by Saab TranceiverTech. The entire Chinese coastline is covered with approximately 250 base stations in hot-standby configurations including seventy computer servers in three main regions. Hundreds of shore based users, including about twenty-five vessel traffic service (VTS) centers, are connected to the network and are able to see the maritime picture, and can also communicate with each ship using SRM's (Safety Related Messages). All data are in real time. The system was designed to improve the safety and security of ships and port facilities. It is also designed according to an SOA architecture with socket based connection and using IEC AIS standardized protocol all the way to the VTS users. The base stations have hot-standby units (IEC 62320-1) and the network is the third generation network solution.
By the beginning of 2007, a new worldwide standard for AIS base stations was approved, the IEC 62320-1 standard. The old IALA recommendation and the new IEC 62320-1 standard are in some functions incompatible, and therefore attached network solutions have to be upgraded. This will not affect users, but system builders need to upgrade software to accommodate the new standard. A standard for AIS base stations has been long-awaited. Currently ad-hoc networks exist with class A mobiles. Base stations can control the AIS message traffic in a region, which will hopefully reduce the number of packet collisions.
Broadcast information
An AIS transceiver sends the following data every 2 to 10 seconds depending on a vessel's speed while underway, and every 3 minutes while a vessel is at anchor:
- The vessel's Maritime Mobile Service Identity (MMSI) - a unique nine digit identification number.
- Navigation status - "at anchor", "under way using engine(s)", "not under command", etc.
- Rate of turn - right or left, from 0 to 720 degrees per minute
- Speed over ground - 0.1-knot (0.19 km/h) resolution from 0 to 102 knots (189 km/h)
- Positional accuracy:
- Longitude - to 0.0001 minutes
- Latitude - to 0.0001 minutes
- Course over ground - relative to true north to 0.1°
- True heading - 0 to 359 degrees (for example from a gyro compass)
- True bearing at own position. 0 to 359 degrees
- UTC Seconds - The seconds field of the UTC time when these data were generated. A complete timestamp is not present.
In addition, the following data are broadcast every 6 minutes:
- IMO ship identification number - a seven digit number that remains unchanged upon transfer of the ship's registration to another country
- Radio call sign - international radio call sign, up to seven characters, assigned to the vessel by its country of registry
- Name - 20 characters to represent the name of the vessel
- Type of ship/cargo
- Dimensions of ship - to nearest meter
- Location of positioning system's (e.g., GPS) antenna on board the vessel - in meters aft of bow and meters port or starboard
- Type of positioning system - such as GPS, DGPS or LORAN-C.
- Draught of ship - 0.1 meter to 25.5 meters
- Destination - max. 20 characters
- ETA (estimated time of arrival) at destination - UTC month/date hour:minute
- optional : high precision time request, a vessel can request other vessels provide a high precision UTC time and datestamp
Detailed description: Class B units
Class B transceivers are smaller, simpler and lower cost than Class A transceivers. Each consists of one VHF transmitter, two VHF Carrier Sense Time Division Multiple Access (CSTDMA) receivers, both alternating as the VHF Digital Selective Calling (DSC) receiver, and a GPS active antenna. Although the data output format supports heading information, in general units are not interfaced to a compass, so this data is seldom transmitted. Output is the standard AIS data stream at 38.400 kbit/s, as RS232 and/or NMEA formats. To prevent overloading of the available bandwidth, transmission power is restricted to 2 W, giving a range of about 5-10 mi.
Four messages are defined for class B units:
Detailed description: AIS receivers
A number of manufacturers offer AIS receivers, designed for monitoring AIS traffic. These may have two receivers, for monitoring both frequencies simultaneously, or they may switch between frequencies (thereby missing messages on the other channel, but at reduced price). In general they will output RS232, NMEA, USB or UDP data for display on electronic chart plotters or computers.
Technical specification
RF characteristics
AIS uses the globally allocated Marine Band channels 87 & 88.
AIS uses the high side of the duplex from two VHF radio "channels" (87B) and (88B)
- Channel A 161.975 MHz (87B)
- Channel B 162.025 MHz (88B)
The simplex channels 87A and 88A use a lower frequency so they are not affected by this allocation and can still be used as designated for the maritime mobile frequency plan.
Most AIS transmissions are composed of bursts of several messages. In these cases, between messages, the AIS transmitter must change channel.
Before being transmitted, AIS messages must be NRZI encoded.
AIS messages are transmitted using GMSK modulation. The GMSK modulator BT-product used for transmission of data should be 0.4 maximum (highest nominal value).
The GMSK coded data should frequency modulate the VHF transmitter. The modulation index should be 0.5.
The transmission bit rate is 9600bit/s
Ordinary VHF receivers can receive AIS with the filtering disabled (the filtering destroys the GMSK data). However, the audio output from the radio would need to be then decoded. There are several PC applications that can do this.
Message organization
As there are a multitude of automatic equipment transmitting AIS messages, to avoid conflict, the RF space is organized in frames. Each frame lasts exactly 1 minute and starts on each minute boundary. Each frame is divided into 2250 slots. As transmission can happen on 2 channels, there are 4500 available slots per minute. Depending on the type and status of equipment and the status of the AIS slot map, each AIS transmitter will send out messages using one of the following schemes:
- ITDMA (Incremental time division multiple access)
- RATDMA (Random access time division multiple access)
- FATDMA (Fixed access time division multiple access)
- SOTDMA (Self-organizing time division multiple access)
The ITDMA access scheme allows a device to pre-announce transmission slots of non-repeatable character, ITDMA slots should be marked so that they are reserved for one additional frame. This allows a device to pre-announce its allocations for autonomous and continuous operation.
ITDMA is used on three occasions:
- data link network entry;
- temporary changes and transitions in periodical reporting intervals;
- pre-announcement of safety related messages.
RATDMA is used when a device needs to allocate a slot, which has not been pre-announced. This is generally done for the first transmission slot, or for messages of a non-repeatable character.
FATDMA is used by base stations only. FATDMA allocated slots are used for repetitive messages.
SOTDMA is used by mobile devices operating in autonomous and continuous mode. The purpose of the access scheme is to offer an access algorithm which quickly resolves conflicts without intervention from controlling stations.
Message format
An AIS slot is 26.66 ms long. The data modulation is 9600 bit/s, so each slot has a maximum capacity of 256 bits. The framing is derived from the HLDC standard, described in ISO/IEC 13239:2002.
Each slot is structured as such: <8 bit ramp up><24 bit preamble><8 bit start flag><168 bit 6-bit ASCII encoded data><16 bit CRC><8 bit stop flag><24 bit buffer>
- 24 bit preamble: this is a sequence of 01...
- Start flag: 0x7e
- 168 bit 6-bit ASCII encoded data, this is the body of an AIS message. For messages requiring more data, several slots (Maximum of 5) must be used.
- 16 bit CRC-16-CCITT: 16-bit polynomial to calculate the checksum.
- Stop flag: 0x7e
- 24 bit buffer used for bit stuffing, synchronization jitter and distance delay.
Note that the signal on the VHF carrier is NRZI encoded and uses bit-stuffing to avoid unintentional stop-flags which may otherwise occur in the data. As such, the raw bits must first be decoded, and the stuffing bits removed, to arrive at the actual usable message format described above.
AIS messages
Messages sent and received over the air
All AIS messages transmit 3 basic elements of information:
- The MMSI number of the ship or equipment that holds the transmitter (base station, buoy, etc.)
- The identification of the message being transmitted (See below table)
- A repeat indicator that was designed to be used for repeating messages over obstacles by relay devices.
The following table gives a summary of all the currently used AIS messages.
Messages sent to other equipment in the ship
AIS Equipment exchange information with other equipment using NMEA 0183 sentences.
The NMEA 0183 standard uses two primary sentences for AIS data
- !AIVDM (received data from other vessels)
- !AIVDO (own vessel's information)
Typical NMEA 0183 standard AIS message: !AIVDM,1,1,,A,14eG;o@034o8sd<L9i:a;WF>062D,0*7D
In order:
!AIVDM: The NMEA message type, other NMEA device messages are restricted 1 Number of sentences (some messages need more than one, maximum generally is 9) 1 Sentence number (1 unless it is a multi-sentence message) The blank is the sequential message ID (for multi-sentence messages) A The AIS channel (A or B), for dual channel transponders it must match the channel used 14eG;... The encoded AIS data, using AIS-ASCII6 0* End of data, always 0 for parsing and error correction 7D NMEA checksum (NMEA 0183 Standard CRC16)
Security
Due to the unauthenticated and unencrypted nature of AIS, recently Balduzzi, Pasta, Wilhoit et al. showed that AIS is vulnerable to different threats like spoofing, hijacking and availability disruption. These threats affect both the implementation in online providers and the protocol specification, which make the problems relevant to all transponder installations (estimated at 300,000+).
Source of the article : Wikipedia
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