ShipInsight Where maritime regulation and technology meet

What is AIS, VDES, SSAS and LRIT?

Posted on:


The Automatic Identification System (AIS) was initially developed purely as a response to the issue of collision avoidance and as a corollary to aid shore-based VTS operators as well as navigators on ships to properly identify radar targets. AIS consists of a transponder system in which ships continually transmit their ID, position, course, speed and other data over VHF. The data transmitted is derived from ship’s equipment as regards position, course and speed, from initial input for the ID, which comprises ship’s name and call sign, and from direct manual input for other details such as port of destination and type of cargo.

Updated information is transmitted at regular intervals of very short duration. When received by other ships, the data is decoded and displayed for the officer of the watch, who can view AIS reports from all other AIS-equipped ships within range in graphic and text format. The AIS data may optionally be fed to the ship’s integrated navigation systems and radar plotting systems to provide AIS ‘tags’ for radar targets. It can also be logged to the ship’s Voyage Data Recorder (VDR) for playback and future analysis.

In 2000, IMO adopted a new requirement as part of a revised new SOLAS Chapter V for all ships to carry AIS capable of providing information about the ship to other ships and to coastal authorities automatically. The regulation requires AIS to be fitted aboard all ships of 300gt and upwards engaged on international voyages, cargo ships of 500gt 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.

The IMO regulation requires ships fitted with AIS to maintain AIS in operation at all times except where international agreements, rules or standards provide for the protection of navigational information. The regulation requires that the AIS must provide information – including the ship’s identity, type, position, course, speed, navigational status and other safety related information – automatically to appropriately-equipped shore stations, other ships and aircraft and to receive automatically such information from similarly-fitted ships.

AIS transmitters can also be attached to navigational marks or to hazards and transmit information that will complement the sight/ sound signals that may be present. These fixed AIS transmitters can also be used to give other information such as current strength and direction.

Although initially intended only for navigation use by ships and shore authorities, AIS data is now regularly disseminated by commercial operations either to subscribers or on a gratis basis allowing almost anyone to determine any specific ship’s current whereabouts and operational status. The IMO does not condone this use but appears powerless to prevent it.

Because AIS operates on VHF radio, there is a natural limit to the distance over which it can be transmitted. However, there is a small but growing number of service providers using satellites that can receive AIS signals when ships are out of the range of shore stations. These services are generally referred to as satellite AIS or S-AIS. Most of the service providers say that their services are targeted purely at national security organisations but others make no secret of the fact that their customers are often commercial organisations including commodity traders and analysts.

It is fair to say that AIS has not been universally welcomed by navigators or ship operators. Many believe that its introduction was rushed and insufficient thought given to its use under operational circumstances. Officially AIS is an aid to navigation and not a collision avoidance method in its own right. Some seafarers believe that AIS overrides COLREGs but official advice from some flag states makes clear this is not so.

Just as with radar in its early days, AIS has been blamed for causing rather than helping to avoid collisions. One of the factors that many say was ill-considered was the fact that, by being obliged to transmit its identity, cargo and destination, a ship can easily be identified by those with criminal intent or even terrorists. Some operators address this by either transmitting false information at certain stages of a voyage or by switching the device off except when in very close traffic situations. 


As the IMO further develops the concept of e-navigation, a potential new technology that is sometimes referred to as ‘AIS on steroids’ is being explored: VHF Data Exchange System (VDES). VHF had traditionally been used for voice transmission until the advent of AIS and VDES began as a concept developed by the International Association of Lighthouse Authorities’ (IALA’s) e-NAV Committee. It was originally developed to address emerging indications of overload of the AIS VHF Data Link (VDL) and simultaneously to enable a wider seamless data exchange for the maritime community.

With VDES it will be possible to send broadband data, making it more economical for ships to maintain a data connection at sea by eliminating the need to use satellites in coastal waters, while not compromising on the global connectivity that satellites will be able to provide in the future. VDES is expected to cover up to 50km from the nearest land-based equipment, which will allow ships to benefit from modern communication and navigation methods without increasing costs. VDES is capable of facilitating numerous applications for safety and security of navigation, protection of marine environment, efficiency of shipping and others. Proponents claim it could have a significant beneficial impact on maritime information services including Aids to Navigation and VTS in the future.

Ship security alert system (SSAS)

Following the terrorist attacks in New York in September 2001, the IMO Diplomatic Conference on Maritime Security held in London in December 2002 adopted several amendments to SOLAS. These amendments include the introduction of Maritime Security in Chapter XI of SOLAS 74 and incorporated the International Ship and Port Facility Security (ISPS) Code which came into effect on 1 July 2004.

As a consequence, all passenger vessels and other ships over 500gt are required to be provided with a ship security alert system (SSAS). The requirements of the system are specified in Regulation 6 of Chapter XI-2 of SOLAS 74. In regard to Regulation 6.2.1, the ship security alert system, when activated by the ship shall:

  • initiate and transmit a ship-to-shore security alert to a competent authority designated by the Administration, which in these circumstances may
  • include the Company, identifying the ship, its location and indicating that the security of the ship is under threat or it has been compromised;
  • not send the ship security alert to any other ships;
  • not raise any alarm on board the ship; and
  • continue the ship security alert until deactivated and/or reset.

The regulation goes on to say that the SSAS shall be capable of being activated from the navigation bridge and in at least one other location and conform to performance standards not inferior to those adopted by the Organization. The SSAS activation points must be designed so as to prevent the inadvertent initiation of the ship security alert.

When a flag state receives notification of a ship security alert it is obliged to immediately notify the state(s) in the vicinity of which the ship is presently operating. If the alert is received by any state other than the flag state, that Contracting Government should immediately notify the relevant flag state and, if appropriate, the state(s) in the vicinity of which the ship is presently operating.

The exact type of equipment that can be used to satisfy the regulation is not specified but some operators have chosen to make use of the GMDSS radio station while many others have installed separate and dedicated satellite equipment. By the standards of modern communications technology, SSAS is quite basic, consisting of a GPS receiver linked to a transmitter, a power supply, some software and activation buttons.

Because the technology is simple and the market huge, a sizeable number of manufacturers have come up with SSAS products. They can all be expected to meet the necessary legal obligations but the ways in which they do this vary somewhat.

The principal differences centre on the methods used to transmit the alarm message, but there are also a number of other features and benefits. As an example, some suppliers will act as a co-ordinating centre and when an alert signal is received they will contact selected personnel of the owner or manager. At least one has developed an app for smart phones that will give all the information to designated persons whenever an alert is made.

Long range identification & tracking (LRIT)

After the use of AIS for reasons of security (rather than its intended aim of an aid to navigation) was found to be inefficient, at MSC 81in 2006 the IMO adopted proposals for long-range tracking and identification (LRIT) of ships, to form part of SOLAS Chapter V.

The obligations of ships to transmit LRIT information and the rights and obligations of SOLAS Contracting Governments and of Search and rescue services to receive LRIT information are established in regulation V/19-1 of SOLAS. Provisions of the amendment came into force in 2009 after which all internationally-trading vessels over 300gt operating outside of GMDSS Sea Areas A1 were required to install the necessary equipment and transmit via satellite technology their identity, location, date and time of position to shoreside bodies authorised to receive it. Ships operating exclusively in coastal Sea Area A1 and fitted with an AIS are exempt.

LRIT requires ships to make regular transmissions of identification and position every six hours to a tracking service which can only release the information with the authority of the vessel’s flag state. Other states with an interest in particular ships may make applications to the flag state for access to the information. If security levels are raised, or if a particular ship becomes of special interest, then the regularity of transmissions and monitoring may be stepped up to as much as once every 15 minutes. The operating standards for LRIT demand that the transmissions can be controlled remotely, without intervention on board. Effectively this means that the transmitter must be of a type that can be polled by a service nominated by the flag state.

Unusually for a new IMO regulation, the vast majority of ships were not required to install any new equipment but only to have certification proving that whatever was on board and intended for use was in compliance with the LRIT equipment requirements.

For most vessels the Inmarsat C GMDSS system is acceptable, as are some SSAS devices. Some Iridium systems are also approved for LRIT compliance. Whatever equipment is used must either have its own in-built GPS system or be connected to an external GPS.

Ensuring that ships comply with the LRIT regulations is the responsibility of the flag state. Under the LRIT framework, each state can either establish a national data centre (DC) or join with others to form a regional or co-operative data centre. Flag states have appointed Application Service Providers (ASP) to manage communications between the ship, the Communication Service Provider (CSP) and the DC. All information is stored by the various DCs and passed to other centres and states when authorized by a ship’s flag state. The choice of ASP is down to the ship operator from any appointed by the flag state. Many ASPs have been appointed by several flag states.

The LRIT system has now been in operation for several years and while it appears to be functioning as intended there is not universal satisfaction with it. The costs of the six-hourly transmissions are borne by the flag state and some are finding this a heavy financial burden, especially as most of the data is simply received and stored and never requested by any other contracting government.

It has been proposed that AIS and S-AIS could provide an alternative because there is no immediate cost involved in any transmission. The question is under discussion but whether all flag states are happy for information collected by commercial enterprises from vessels on the high seas to be sold to other governments and organisations and individuals prepared to subscribe remains to be seen. Another option proposed at MSC 95 in 2015 was to reduce the number of daily transmissions from four to two. There was support for this proposition but no action has yet been put in place to implement a change. At NCSR4 in 2017, two options were proposed for further consideration but the matter is still to be finalised.

Another issue with LRIT is that, since the structure involves several commercial organisations and governments acting as DCs all gathering and passing data, continual operations can be subject to disruptions. For some time now, the IMO has been concerned about this and is attempting to improve continual service by developing and testing means of recovery for the network when individual components are affected by factors such as maintenance, loss of service or cyberattack. The matter is continually under review at the NCSR sub-committee and at MSC with reporting at each meeting up to and including those taking place in 2019.

Light and sound signalling

Although we are in a digital age and electronic systems now make up the majority of navigation equipment there is still a need for simpler and more conventional methods to signal intentions to others. Not all communication between ships is done by radio or satellite and this is unlikely to change so long as the redundancy that comes from older methods that add to safety and permit communication with leisure and non-SOLAS vessels is considered desirable. Many of the older communication means are not located on the bridge itself but are controlled from there and it falls to the navigation team to interpret the signals given by others.

Conceivably the oldest and most important are navigation lights. The types of lights required and their use are specified in The International Regulation for Preventing Collisions at Sea (COLREGs 1972). Navigation lights must be displayed from sunset to sunrise and during time of low or poor visibility so that nearby ships can navigate safely after seeing the navigation lights.

Five separate lights are fitted at different positions on the ship according to the requirements of the rules and these allow easy identification of the displaying ship’s size, direction of travel or to indicate the ship is at anchor. Navigators learn the use of lights at an early stage in their education. The lights are:

  • Foremast – Bright White with a horizontal arc range of 225 degrees.
  • Mainmast – Bright White all-round light with a horizontal arc visibility of 360 degrees.
  • Port side – Bright Red with a horizontal arc visibility of 112.5 degrees.
  • Starboard side – Bright Green with a horizontal arc visibility of 112.5 degrees
  • Stern of the ship- Bright White with a horizontal arc visibility of 135 degrees.

In addition to the above, two anchor lights are fitted forward and aft and are bright white in colour. The power for the navigational lights must be supplied from a separate distribution board which has no other supplies attached to it. This is done so that they cannot be extinguished by inadvertent operation of a wrong switch. The masthead lights must be visible from at least six nautical miles away for vessels over 50m in length and from three nautical miles for smaller vessels.

Due to the critical nature and essential safety requirement of navigational lights, they are fitted in duplex manner at each position with two separate lamps or a lamp holder with dual fitting. All lights are switched from the bridge and any fuses must also be able to be changed on the bridge. A control panel must have indicator lamps and an alarm in case of failure of any light.

Despite being a fixture of ships for a considerable length of time, navigation lights are kept under review. At NCSR 4 in March 2017, the Sub-Committee agreed an IACS draft Unified Interpretation on application of the COLREGs with respect to the placement of side lights as an interim measure, ahead of a future revision of the COLREGs.

LED Navigation Lights

In this age of energy efficiency, modern ships are often fitted with LED lights with a much lower power consumption. LED lamps are used in all areas of the ships and this includes for navigation lights.

As LEDs replace incandescent lights for navigation, new issues arise for monitoring the performance and compliance of LED navigation lights. COLREGs requires that navigation lights maintain a certain luminous intensity, a measure of brightness. IMO Resolution MSC.253 (83) 4.3 provides two options for monitoring compliance of LED lights: ships can either install an alarm system that will alert the Officer of the Watch when intensity falls out of compliance or LEDs can only be used for a set ‘lifespan’ before being replaced.

Due to the difficulties in monitoring intensity of LEDs compared to traditional incandescent bulbs, most manufacturers and vessel operators use the second standard and replace LED navigation lights after 50,000 hours. However, using lifespan standards does not actually guarantee that LEDs are compliant with COLREGs, as an LED can diminish in its intensity while still within its lifespan.

Signalling Lamps and Signs

Another visual aid that must be available and used from the bridge is the Daylight Signalling Lamp which is mandatory on all ships above 150gt and all passenger vessels. The lamp must have its own emergency power source and usually this will be a battery kept in a constant state of charge. It will be used to send Morse signals to other vessels when appropriate.

The use of the signalling lamp is now one of only two instances where Morse code remains in shipping (the other being the ship’s whistle) following its phase-out for radio communications under GMDSS.

Another requirement of COLREGs is that during daylight hours, ships that are at anchor, being towed or not under control should display an appropriate shape from the fo’c’sle. These are known as the Black Ball or Black Diamond, the former being for ships at anchor and the latter for towed or Not Under Command (NUC) vessels.

As far as visual aids and signals go, the final requirement for ships that harks back to an earlier era but which is still relevant and mandatory is the carriage and occasional use of signal flags. The 26 alphabet flags and the substitute and numerical pennants can be used to signal a range of information with each of the letter flags having a unique meaning assigned in accordance with The International Code of Signals 2003. More complex messages can be sent using combinations of letters and numbers. Navigators are expected to know the meaning of the signal flags but a copy of the code is also required on the bridge.

In cases of needs it is also possible to signal using semaphore flags but the use of VHF radio has made most such forms of signalling somewhat outmoded.

Ship’s Whistle

SOLAS requires all ships to have a means of signalling by sound and for merchant vessels this is covered by the requirement to be equipped with a whistle often erroneously referred to as a foghorn. Sound signals are made using Morse and the meanings attributed by the Signals Code. Ships generally have two whistles, one electric and the other powered by compressed air.

Sound signals are only of use if they can be heard and in the fully enclosed bridge of modern ships that would not always be possible, therefore it is a requirement of SOLAS that such vessels should be equipped with a Sound Reception System that enables the navigating officer inside the cabin to listen to the sound signals and horns from other ships.

Copy link
Powered by Social Snap