Radar interface to help prove identification systems’ worth

Following an inauspicious start-up phase, AIS may be about to prove its full value to mariners on the bridge, following the introduction of new display standards

Automatic Identification Systems (AIS) represent a concept that has received its share of criticism, not all of it fair. After all, AIS was at an embryonic stage when it was effectively hijacked and pushed ahead more rapidly than originally envisaged by the perceived need to develop tools in the war on terror. This added further confusion to its primary purpose, which had already experienced a significant change, from being primarily a tool to assist vessel traffic management services (VTMS) to a collision avoidance aid.

As a result of the altered priorities, as well as the accelerated development and roll out, AIS was initially, in many respects, less than impressive and did not always inspire confidence amongst its onboard users. While the technology was fundamentally sound, in practice it often suffered from poor shipboard installation and sensor interface problems. A lack of training resulted in the failure of ships’ crews to appreciate the system's potential and limitations, and the input required from them for successful operation.

Given the covert nature of anti-terrorist activities, it will always be difficult to determine the real value of AIS in that role, though it does seem to be all too easy to arrange to transmit erroneous data. In the VTMS role for which it was initially envisaged, however, AIS is now highly regarded and has become an important source of information.

A major obstacle to optimising functionality and winning acceptance by mariners was – and for the most part still remains – the lack of an effective human-machine interface. Many radars and ecdis currently in service lack the ability to display AIS information, while the functionality of the monochrome minimum keyboard display is very limited. Furthermore, the small graphical display can be confusing in high traffic density and is not always conveniently positioned. Consequently, valuable information derived from AIS may be available on the bridge, but can be easily overlooked because it is not displayed in an accessible manner.

Recognising that a radar interface is necessary for AIS to fulfil its onboard potential, commercial marine radar performance specifications adopted in 2004, and which come into effect in July this year, include a requirement for this function. However, an overloaded radar display can be a serious obstacle to safe operation, so the new standard includes measures designed to provide the necessary functions while, as far as possible, maintaining a display free of unwanted graphical clutter.

For safety reasons it will not be possible to remove individual AIS targets from the display. Nonetheless, it will be possible to filter out sleeping AIS targets.

It is considered undesirable to present two target symbols for each physical target. Consequently, the automatic target association function will in future avoid this dual presentation for the same physical target, so long as the necessary association conditions are fulfilled.

If position and motion information derived from AIS and radar target tracking (TT) coincide sufficiently, then the AIS and TT information will be considered as relating to one physical target. In this case, the activated AIS target symbol and the alphanumeric AIS target data will be automatically displayed as the default condition. The operator will still be able to change the default condition to display TT information.

Should the two sets of data subsequently diverge to the point of exceeding the target association parameters, the system will revert to displaying both AIS and TT target symbols, but there will be no alarm to indicate that this has happened. The new radar standards also allow for the presentation of electronic navigational charts on the display.

Interfacing with radar in this way not only provides the mariner with AIS data in a very accessible manner, but also goes a long way to removing concerns about the reliability and validity of such data, because it can be instantly compared with radar-derived information. The mature practice of navigation and collision avoidance has always involved the informed use of information from a variety of sources, and AIS should take its place along with more traditional inputs.

The onboard situation is not going to change overnight. While new vessels must fit radars complying with the new standards, it will be some time before retrofitting and upgrades see AIS displayed on radar as the norm. However, AIS is seen as a major element of e-navigation, and it cannot fulfil this role in the absence of a sensible display platform, so there remains the possibility of mandatory radar/AIS interface at some point.

The rapid roll out of AIS to commercial shipping has been followed by the introduction of class B AIS for smaller craft not subject to mandatory carriage requirements. There were concerns that AIS could effectively crash in a particular area if swamped with transmissions from large numbers of small craft. Although technically possible, this is very unlikely. In any case, this danger was removed by the adoption of Carrier Sense Time Division Multiple Access (CSTDMA) transmission protocols for class B equipment, rather than the Self Organising Time Division Multiple Access (SOTDMA) employed for Class A.

This means that Class B AIS will only transmit if there is a time slot unreserved by a Class A transmission from a major vessel. In addition, this slot will not be reserved for subsequent transmissions, and Class B AIS has to search for a vacant slot each time it wishes to transmit, ensuring the integrity of the system is preserved, particularly from the perspective of Class A operation. Furthermore, class B transmission power is approximately 2W, while class A transmits at 12.5W.

For a number of reasons, including the low update rate for class B data and the display considerations on class A equipped vessels, there is a danger that leisure craft users may place unreasonable faith in their class B equipment, believing that they will always be detected and noted by larger vessels. It is important that they understand the limitations of the system and their equipment and do not adopt an unwarranted attitude of security. Class B users should not assume that their equipment raises the navigational profile of their vessels, while seafarers operating Class A equipment must bear in mind the limitations of Class B AIS data, if they choose to make use of it. CS