Automatic identification systems (AIS) were primarily used as a form of visual tracking (see Figure 1). A vessel transmits its position and basic information about itself and at the same time it can view all other vessels transmitting their information within its reception range, with the information usually presented on electronic navigation chart (ENC) displays. An auxiliary application of AIS technology is as an aid to navigation (AtoN). This normally consists of a transponder transmitter/receiver device or system on a variety of platforms, such as buoys and shore-based stations, designed to transmit a variety of messages to enhance the safe and efficient navigation of vessel traffic.

Due to the great safety benefits offered by AIS, this technology was made compulsory throughout the world in 2002 for all passenger ferries, workboats and vessels over 300 gross tonnes. In addition to conventional marine navigation, the real-time tide and meteorological data are of direct use in a wide variety of marine civil engineering applications. This environmental data source can be used in the design, planning, construction and operational phases of many marine projects.

The roots of AIS technology lie in radio data links over the marine very high frequency (VHF) band, designed for reliable transmission using message formats such as those in Figure 2, as defined by the International Maritime Organization (IMO). The actual data link is referred to as the VHF data link (VDL), which transports data messages (VDM) sent in an encoded text format resembling the National Marine Electronics Association (NMEA) data strings, familiar to most GPS and marine data users. This technology assumes that there may be potentially 2,250 vessel slots on each of two VHF radio channels within a radius of about 25km from the AIS receiver.

The AIS TideMet system has been designed around a standard AtoN device, developed for the marine industry by L-3 Communications Inc. This type 3 transponder unit can transmit a type 8 binary message and is a fully automated system; once it has been installed and turned on, no maintenance is required to keep it operational (see Figure 3). The only time the user needs to perform any function on the transponder is to change the local offset parameter data as required. The AtoN unit is fully compliant with IMO specifications and is currently the only one to have been fully tested and certified to comply with the recommendations laid down in the International Association of Lighthouse Authorities (IALA) A-126. Ohmex has taken this device and added electronics and transducers to measure tide and weather conditions; these data are processed and the message is passed to the AtoN for transmission using the allocated time slot. The AIS TideMet system makes use of an automatic slot-allocation mechanism. The AIS transponder device must meet very strict radio, message format and electrical specification standards, governed by the IMO. The standard AIS class A transmitter used by most large vessels or AtoNs is rated at 12.5 watt output power. This provides broadcast ranges of up to 35 kilometres from the transmitted source, more than adequate for the environmental data applicable to the immediate area of measurement.

AIS is an omnidirectional broadcast, and can therefore be received both offshore and by inland receivers such as those in sheltered river harbours or shore-based monitoring stations. Some experiments involving posting real-time environmental values on websites for use by work vessels with internet access were carried out. However, the reliability of these services proved to be inadequate, access was difficult for live marine situations such as vessel manoeuvres, and message latency was a common issue on websites. For civil engineering or dredging applications the TideMet data can be easily captured on a low-cost receiver used on all work vessels, without the need for dedicated radio telemetry links.

All maritime navigational aids, AIS and radar must be licensed and meet strict specifications. The UK regulations governing maritime radio are, in general, derived from the International Telecommunication Union (ITU) Radio Regulations, the relevant EU Directives (99/5/EC) and the Wireless Telegraphy Act 2006. All radio equipment must meet certain essential criteria - this is a legal requirement under the Radio and Telecommunications Terminal Equipment (R&TTE) Regulations. In the UK, the government body OFCOM (Office of Communications) issues licences for the use of the AIS radio frequencies as well as a unique Maritime Mobile Service Identity (MMSI) number for each transmitting AIS site. Unfortunately, the implementation of this is a little disjointed as current AIS licenses really only apply to an MMSI used by vessels and the international nature of shipping means that a local national licensing system is difficult to enforce. Static, temporary or moored AIS transmission equipment such as AtoNs also need to be issued with an individual MMSI, but unfortunately this license assumes the equipment to always be at the same location as the license, which becomes a problem in the case of a surveying or dredging company that wishes to deploy a TideMet for short periods of time at different locations, and probably even in different countries.

A major obstacle regarding TideMet data transmitted over AIS using binary messages is that, as yet, very few of the low-cost software packages used commercially are able to decode and display the relevant data contained within the message. In fact, the emphasis on predicted tide values shown on most of the chart displays is in itself a potential hazard to navigation - the data are not real-time and are uncorrected for local atmospheric conditions so could easily contain an error of 1 metre or more. However, this situation is already beginning to change, as navigators and pilots begin to demand more accurate information and become more familiar with modern technology, in particular the use of electronic chart systems. A number of software providers have added the potential to decode AIS binary messages, and many more will do so as use of the messages becomes more widespread (see Figure 4).

Thanks to Peggy Browning, Nick Ward, Keith Oliver, Kaimes Beasley, Steve Brown, Paul Fonseka, Charles Amah, Carl Ainley, Stephen Hames, Per Resen Steenstrup and Kurt Schwehr.

Read, E.F., Heaps, W.S., 2008. Tidal & Meteorological data over AIS. Hydrographic Society. Hydro8 proceedings.