AIS TideMet

Integrated Tide and Meteorological AIS Transponder

Developed to allow ships and coastal stations to accurately locate and identify one another, AIS is the mariner’s most significant development in navigation and safety since the introduction of radar. AIS is a marine technology based around positions and timing derived from the GPS network. Primarily the system is used as a form of visual tracking whereby a vessel transmits its position and basic information while at the same time view all other vessels transmitting their information within its reception range. An auxiliary application of AIS is as an Aid To Navigation (AtoN), this is defined as "a device or system external to vessels that is designed and operated to enhance the safe and efficient navigation of vessel traffic". These AtoNs consist of a variety of devices such as buoys and shore stations designed to promote and enhance safety and efficiency of navigation, the function described as 'TideMet' is the provision of weather and tidal data over an AIS AtoN station.

In recent years the hydrographic survey profession has moved away from tide gauges in favour of using RTK GPS elevations as their preferred vertical datum. This is not a practical solution for the reliability and all weather requirements of a navigational user such as shipping or ports and harbours. At present the marine user is restricted to requesting local tide and weather value over speech channels from VTS or by reading some form of visible tide value such as a tide board, as they will certainly not have access to a survey quality receiver for deriving tidal heights from GPS. Some experiments have been tried for posting live values on websites which can be viewed using Internet access, however, the reliability of these services has not been adequate, access has been difficult for a live marine situation such as a vessel berthing and message latency is a commonplace issue on websites. The introduction of AIS has provided a radio frequency standard together with the provision of message structures to allow the reliable transmission of tide and met data within the local area of measurements. Furthermore, AIS allows the majority of vessels, of all sizes, to receive such data without the need to invest in any additional hardware, or (in the most basic of realisations) even any additional software.

The AIS AtoN is box of electronics programmed to transmit three messages (type 21, 6 and 8) at predefined timings and sequence over the AIS frequencies. The transmit functions amd MMSI of the AtoN must be configured prior to installation. The configuration parameters specify what messages are transmitted over the air, how often the transmissions occur and  sometimes the slot on the VHF data link (VDL) they are transmitted over. Message 6 is defined as an Addressed Binary Message, data from a message 6 may contain a status report on the health of the AtoN or other status information. Message 21 is defined as an Aids-to-Navigation Report and is usually transmitted every 3 minutes containing the origination MMSI, name of the AtoN (if applicable), type of AtoN (fixed or floating), position of the AtoN and the positional accuracy. Unlike the message 6, this report is broadcast and meant to be seen by all AIS transponders. Binary Message 8 is a broadcast message containing a binary payload which may vary depending on its parameters, in the case of the Tide and Metrological data message the packet is formed as in the table below

Receiving Software

A major current obstacle with TideMet data transmitted over AIS using binary messages is that, as yet, very few of the software packages used commercially can 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 used is in itself a potential hazard to navigation,  the data is not live or corrected for local atmospheric conditions so could easily be up to 1m or more in error.

However, this situation is already beginning to change, as navigators and Pilots begin to demand more accurate information, and are becoming more familiar with modern technology, in particular electronic chart systems. A number of software providers have added the potential to decode AIS Message 8, and many more will do so as the use of the message becomes more widespread.

Standalone ‘Real’ AtoN TideMet station

The AIS TideMet system has been designed around a standard AtoN device developed for the marine industry by L3 Communications Inc. This Transponder unit is a fully automated system, once it has been installed and turned on, no maintenance is required to keep it operational. The only time the user needs to perform any function on the transponder is to change the AtoN’s settings and local offset data as required. The AtoN unit is fully compliant with the IMO specifications defined in IMO MSC.74(69) Annex 3, IEC 61993-2, and ITU.R M.1371-1. The device is currently the only one that has been fully tested and certified to comply with the recommendations laid down in IALA A-126. This AtoN engine can be found within many derived AtoN products on the market. Ohmex have taken this device and added electronics to measure tide and weather conditions, this data is processed and the message passed to the AtoN for transmission using the allocated time slot.

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