AIS & Non-SOLAS
ships
With the introduction date of the mandatory carrying of Automatic Identification Systems (AIS) drawing near, manufacturers are speeding up their R & D departments to design a system complying with the now accepted standards (IMO res. MSC.74(69), annex 3 & ITU-R M.1371-1). Many trials have been conducted worldwide involving VTS systems and ships of different types and duties. Recommendations from these trials have been studied, some adopted and included in official standards, others remain on the wish list of operators.
AIS is a shipboard transponder system that makes it possible to monitor ships from other ships and from shore based stations. Ships equipped with AIS will continually transmit their position, course, speed and other relevant data (see attached) via dedicated VHF frequencies. Other AIS equipped ships will receive the vessel’s information, anywhere on earth, without shore side interference, and will be able to identify and track it on the electronic navigation chart (ECDIS), radar (ARPA) or voyage data recorder (VDR). VTS systems worldwide will receive the same information from AIS equipped ships, within their area.
The ship’s AIS unit has two dedicated VHF receivers and one transmitter to which the International Telecommunications Union (ITU) has allocated two dedicated frequencies, these are 161.975 MHz and 162.025 MHz (VHF 87B & 88B), however they may not be available in every part of the world. In the United States for example, VHF 87A & 87B were recently allocated for AIS communications instead. In addition the transponder unit includes a GPS system (GLONASS or GPS) to determine the ship’s own position and for time synchronization, as well as the means for displaying the received information from other AIS equipped ships.
The operation of AIS depends on the Self Organizing Time Division Multiple Access (SOTDMA) data communication technology, which was developed in the 1980’s. It allows for large numbers of transmitters to share one single narrowband radio channel, by synchronizing their data transmission to an exact timing standard. Under SOTDMA each minute of time is divided into 2250 timeslots or 26.67 ms each timeslot. With a transmission speed of 9.6 kbps this translates into 256 Bits/time-slot, sufficient for one AIS report. The exact timing signal of the GPS receiver is essential to synchronize the timeslots of communicating AIS ships, as well as providing the position data for each ship. When a ship first “talks” to another ship, it takes up an unoccupied timeslot, and it reserves a timeslot for its next contact, depending on the status of the vessel according to the standards[1] (e.g. a ship sailing at 23 knots updates its information every two seconds and therefore “reserves” a timeslot 75 slots on from the first contact (2250/30=75) and so on). The range of the system is the VHF horizon for each AIS-ship, with the ship in the centre of its own communication “cell”. The size of this cell will adjust to the traffic density, if slot capacity starts to run out, the system will automatically discard targets at a greater distance and assign those timeslots to targets of greater importance.
In the early 1990’s, the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) initiated the installation of DSC transponders for the primary use by shore based VTS areas, as part of the Global Maritime Distress and Safety System (GMDSS). Shortly after this initiative, Scandinavian countries proposed a more comprehensive system that would use the marine VHF frequencies, would be automatic and would be suitable for both ship-to-ship and ship-to-shore communication. In 1996 the VTS sub-committee of IALA, incorporating the expanded system, prepared a draft recommendation. This became IMO’s basis for the performance standards on Universal AIS, adopted in May 1998. Moving at a fast pace, IMO has made carriage of AIS mandatory in the recently revised SOLAS chapter V, for all new ships over 300 GRT, from July 1, 2002, and existing ships to follow in a tight schedule there after (see attached).
The standards to which the AIS system must adhere to, were developed mainly by IALA, and their VTS committee. Driven by the revised IMO “Guidelines for Vessel Traffic Services” which enters into force on the same date as the AIS carriage requirements, on 1st July 2002, Governments worldwide have been reviewing their VTS centres. IMO’s intentions are to improve vessel safety, however Governments are expected to take advantage of the ship’s AIS broadcasted information in VTS areas, to gather evidence leading up to nautical disasters, such as collisions and pollution incidents, and use this for prosecution. In addition this information could be used for the charging of dues and fees.
Currently radar is the main tool to monitor vessel movements along the coast, in estuaries and in harbours. The VTS radar has the same limitations as those used onboard: no identification of targets; large blind spots behind islands, capes and at turns in rivers & canals; target swapping to land, beacons, bridges and to other ships; and the coverage is limited to the line of sight. The establishment of a shore-based AIS network would remove these limitations, while the installation of additional base stations along the coast, would greatly enlarge the coverage area, at a fraction of the price of a VTS radar station. This would then open the way for control of vessel traffic movement from shore stations, by VTS operators without seafaring experience and/or certification. According to the guidelines, a VTS centre may include a traffic organization service concerned with the operational management of traffic, however these must be result orientated, leaving the details of the action to the master onboard[2].
The advantages of AIS for ship operators are more directly related to the improvement of safety and the avoidance of collisions. Other than the afore mentioned identification of targets, the quoted radar limitations are not of great importance to the seafarer, however s/he is interested in the recognition of collision danger. When comparing AIS with ARPA the major advantages are the time with which course and speed alterations of other vessels are recognized, and the detection of small targets that are otherwise hidden in sea and rain clutter. Typically the ARPA will track the most reflective part of the vessel, on a VLCC this would be the superstructure. Before a change of heading of such a ship is detected on the ARPA, the VLCC will probably have turned 30-40 degrees, equivalent to 4 or 5 minutes. Another 1 to 2 minutes are needed to obtain a steady vector of the target, for smaller vessels this is somewhat less, however an average of five to six minutes between wheel-over and recognition of this fact on ARPA is common. In contrast, the data of an AIS-ship, sailing at a speed greater than 14 knots and turning, will be updated every 2 seconds[3], showing the course alteration virtually from the moment that the wheel was put over.
The second major advantage of the system however, is not included in the IMO recommendations and is therefore not talked about much. This is the installation of an AIS transponder (or a simplified AIS transmitter not unlike a radar reflector) on non-SOLAS ships and in particular those under 20 m in length. When these small vessels are equipped with an AIS transponder they will clearly appear on ECDIS/ARPA displays, no longer unrecognizable in the sea- or rain clutter, or only show on the last minute because of its size. This would greatly enhance the navigational safety for both the professional seafarers as well as for the small boat operators, as these are the areas in which most collisions and close quarter situations occur. The arguments against implementing AIS on these smaller vessels and including this in the IMO recommendations, are the lack of capacity (bandwidth) on the two available AIS frequencies and the purchase price of the units. Whereas to the latter, the manufacturers expect the unit price to drop below $1000, the capacity issue is a matter of setting priorities. When information bursts would be limited to fit into one or two timeslots, combined with the IMO proposed update schedule this should leave ample timeslots available for a large number of small vessels (up to 800 – depending on transmission rate[4]). However this is not the direction in which the system seems to be heading at this moment. In order to be of greater use for VTS centres and to generate income, more features are added to the AIS unit. These features include obtaining information of the ship’s destination, its size and draft, and interrogation of the ship for cargo & other pertinent information, including copies of the current part of the ship’s planned route, as well as the broadcasting of navigational- weather- and traffic information, and certain commercial e-mail facilities. These added features quickly fill-up the limited timeslots, severely limiting the timeslots available for the basic AIS static and dynamic information reports, while raising questions with regards to security and illegal use of the system by pirates, to obtain detailed information on the ship’s cargo. Currently only two areas worldwide have expressed concerns with regards to available capacity, Malaka Strait and northeast/south of UK. Because of the cellular aspects of the system, adding base stations effectively reduces the size of the cells and therefore, like a cellular telephone system, increases the capacity by each station handling less “calls” than previously.
When correctly implemented, this slot-reuse capability and scrutiny with regards to the permitted information exchange, the bandwidth/capacity issue is of no concern. Additional concerns with regards to the cluttering of the VDU and message garbling are based on unfamiliarity with the system and skepticism of its reliability, similar to those uttered when radar was first introduced.
The introduction of AIS is large step towards improved navigational safety worldwide, with significant benefits to the watch standers both on the bridge of a ship as well as in the VTS centre. Improved situational awareness, near real time data exchange and reduced VHF voice traffic will transform the manner in which ships interact with each other and the shore based VTS. When properly implemented, AIS will virtually eliminate the need for VHF traffic communication, uncertainty of other ship’s actions and prevent degrading of information when in adverse weather conditions. However care must be taken that the primary needs of the seafarer are addressed over commercial interests. Current IMO requirements are for mandatory carriage on all ships subject to the SOLAS convention, but what about the smaller vessels, such as yachts and small fishing boats? The need for these vessels to carry at least a scaled down AIS unit is as important, if not more so, to reduce loss of life at sea. The technology to satisfy both parties in all areas worldwide is available, and the onus is on the responsible coastal States to implement local regulations to make carriage of AIS on small ships mandatory, ahead of further IMO legislation.
Target presentation:
The most common means to display the AIS information will be the ARPA display, it is crucial to set clear standards for the consolidation of ARPA and AIS targets. This was discussed extensively on the UAIS website by Ir F.W.Pot in his review of major trials conducted in South-East Alaska, and with Master Mariners who attended STCW upgrade classes recently.
The un-consolidated ARPA targets should be represented by a round dot and the un-consolidated AIS targets by a triangle, the colour of which is commensurate with their status according to the RoR*. The icon of acquired radar targets of which the data is not (yet) accurate and AIS targets with an estimate position error (EPE) of more than 50m, should be hollow. Those radar targets that have not been acquired (manual or automatic) will be represented on the radar screen only, by their echo.
Consolidated ARPA/AIS targets should be shown as diamonds, the colour of which is commensurate with their status according to RoR. All consolidated and un-consolidated targets should show a two-digit identification number and a curved vector, the OOW should be able to select the length of the curved vector.
The diamond or triangle should be replaced with a simplified silhouette of the ship that it represents, as soon as the LOA of the ship to the scale of the active chart is greater than the icon. The silhouette should be aligned with the targets heading, and the scale be equal to the scale of the active chart.
* Status according to the Rule of the Road (RoR):
Power Driven Vessels
Vessels under 20m in length (additionally)
Sailing vessels
Fishing vessels
Vessels Restricted in Ability to Manoeuvre
Vessels Not Under Command
For the ARPA and AIS targets to be consolidated or fused, they must meet the following strict criteria:
The difference between their ranges is within 2.5% of the average range
The difference between their bearings is less than 3 degrees
The difference between their relative speeds is equal or less than 0.2 knots
The difference between their relative courses is less than 10 degrees
The estimated position error of the AIS target is less than 50m
The above criteria are met for 3 consecutive AIS updates
To eliminate any own-ship errors, the target's relative course and speed should be considered. If the above criteria are met, then the fused target should be positioned on the line connecting the ARPA and AIS targets on a distance from the AIS target equal to the EPE of the AIS target. If the EPE is greater than the distance between the two targets, the fused target should be placed on the ARPA target. The true course and speed should be set to the AIS target values.
ARPA acquisition may be manual or automatic. A facility should be available to the OOW to suppress automatic ARPA acquisition and AIS target display in certain areas. On any range scale where ARPA acquisition and AIS target display is suppressed in certain area(s), the area(s) should be indicated on the VDU. An option should be available to suppress the display of AIS targets of vessels with LOA less than 20m, when the distance to own ship is greater than 5Nm or TCPA greater than 15 minutes.
The target data should be available off the active screen, the displayed data should be dynamically updated and when an unconsolidated ARPA target is selected, the unknown values should remain blanc. The data should display the typical anti-collision data (range, bearing, course, speed, CPA and TCPA), additional data should be available to the preference of the OOW. This should include the option to alter the target identification number for the ship's name.
The information that is broadcasted by the AIS unit is sorted into three independent data reports and transmitted at set schedules according to the agreed standards. The seafarer is interested in the vessels name, its status according the Rules of the Road, its course and speed, rate of turn (RoT) and what the CPA and TCPA are. This information is packed in the so-called Static and Dynamic data packages, all other data is for the primary use of the maritime authorities.
Static data (pre-programmed and does not change):
o Ship’s name and call sign
o IMO number
o Length & beam
o Location of antenna
o Ship’s type
Voyage related data (to be inputted every new voyage):
o Draft
o Cargo information
o Destination and ETA
o Other relevant information
These two reports are updated every 6 minutes.
Dynamic data (automatically derived from ship’s interfaces):
o Time
o Ship’s position
o Course over ground
o Speed over ground
o Gyro heading and Rate of Turn
o Navigational status (according to Rules of the Road)
The Dynamic reports are updated depending on ship’s speed and status, as follows:
o At anchor 3 minutes
o 0 – 14 knots 10 seconds
o 0 – 14 knots & changing course 3.3 seconds
o 14 – 23 knots 6 seconds
o 14 – 23 knots & changing course 2 seconds
o 23+ knots 2 seconds
o 23+ knots & changing course 2 seconds
AIS introduction schedule:
Mandatory carriage requirements are set for all ships of 300 GRT and over engaged on international voyages, and those of 500 GRT and over not on international voyages as well as all passenger vessels irrespective of size.
All ships constructed on or after July 1st, 2002 must be fitted with AIS. Existing ships engaged in international voyages must be fitted with AIS as follows:
o all passenger ships not later than July 1st, 2003
o all tankers not later than the first safety equipment survey after July 1st, 2003
o other vessels of 50,000 GRT and over not later than July 1st, 2004
o other vessels of 10,000 GRT to 50,000 GRT not later than July 1st, 2005
o other vessels of 3,000 GRT to 10,000 GRT not later than July 1st, 2006
o other vessels of 300 GRT to 3000 GRT not later than July 1st, 2007
Existing ships not engaged on international voyages, constructed before July 1st, 2002, must be fitted not later than July 1st, 2008.