Recently I was reading a vent from a cruiser describing how dangerous it can be to use an AIS to avoid close quarter situations.
From what I could work out they had not long installed an AIS and, by the sounds of it, had problems either operating it or maybe relied heavily on the data but did not know how to interpret the information. The author then continued with nothing good to say about the system at all.
I must admit I was a little taken aback by the comments. While I am sure their experience was not nice I do wonder, if they had a better understanding of the system, whether their encounter would have been totally different. The good news is they did not get hurt and I think only their pride was dented. Unfortunately they didn’t expand on vessel types nor just how close was the situation.
On the other hand we have had nothing but positive experiences over the last six years, AIS has taken the heat off what could have been a close-quarter’s situation several times. However it is not all close-quarter situations that the AIS system has helped to keep us and other vessels out
of danger, but more on this later.
I do know that AIS has become very popular with the sailing community and, when sailing offshore, it is more likely to see private vessels fitted with AIS transponders than those without.
I would like to give an overview of the AIS system, then I would like to talk about some of the errors we have seen and hopefully give an explanation how to spot these. If you can spot the errors early you may not fall into the trap like the cruiser mentioned earlier.
What is AIS?
As the acronym suggests it is a vessel’s automatic identification system. This system is possibly one of the most substantial advances in navigation safety and an aid to sailors since the introduction of global positioning system (GPS) and radar (radio detection and ranging).
Simply put, the AIS is a very high frequency (VHF) radio broadcasting system that transfers packets of data over a VHF data link (VDL). This enables AIS-equipped vessels and shore-based stations to send and receive identification information that can be displayed on an integrated display, computer or chartplotter.
Getting slightly technical, AIS operates on two dedicated channels in the marine VHF frequency band. The two dedicated VHF frequencies used for AIS are: AIS1: 161.975MHz (channel 87B) and AIS2: 162.025MHz (channel 88B).
When used with a graphical display this information can help create situational awareness and provide a means to assist in collision avoidance. Additionally, AIS can be used as an aid to navigation by providing location and additional information on buoys and lights. AIS uses a time-division multiple access (TDMA) scheme to share the VDL.
There is a multitude of automatic equipment transmitting AIS messages, to avoid conflict the radio frequency space is organised in frames. The VDL is divided into 2250 time slots that are repeated every 60 seconds (one frame) and each AIS vessel in range sends a report to one of the time slots. At the same time, every AIS vessel in range is listening to all the timeslots and can read the reported information. As transmission can happen on two channels, there are 4500 slots per minute.
What types of AIS are there?
There is one system but, as with a lot of things marine, there are different classes. There are AIS mobile units and AIS stations:
AIS class A and AIS class B: AIS carried on board vessels
AIS base stations: AIS used for shore stations
AIS AtoN: AIS aids to navigation
AIS SART: AIS search and rescue transmitters
SAR aircraft: AIS fitted to search and rescue aircraft
AIS MOB: AIS man overboard units/personal beacons/lifejacket units.
AIS is included in the International Convention for the Safety of Life at Sea (SOLAS) convention for: vessels of 300 gross tonnage and upwards engaged on international voyages; plus vessels of 500 gross tonnage and upwards not engaged on international voyages as well as passenger ships irrespective of size.
The AIS referred to in the SOLAS convention is often termed AIS class A. AIS class B is intended for use on non-SOLAS vessels. A non-SOLAS vessel is a vessel to which the SOLAS convention does not apply. These can include pleasure and domestic commercial vessels.
AIS class B units have less functionality than AIS class A units but they operate and communicate with AIS class A units and other types of AIS units. So this means you will find some smaller commercial vessels fitted with AIS class B transponders, this is something of which to be aware.
Class A transceivers are designed for large commercial vessels. The system offers many features and benefits to larger vessels and these include: 12.5 watt transmission power; comprehensive connectivity options for connecting to on board systems; large screen for viewing targets; mini keyboard and display; enhanced environmental protection; pilot plug; three receivers; flexible transmission timing depending on voyage status; enhanced transmission type ensuring
all AIS data transmits properly.
Class A devices use self organised time division multiple access (SOTDMA) to transmit data. This is a system that reserves data slots and therefore ensures the data gets through into the data stream.
Class B transceivers offer vessels most of the AIS features of class A devices but without some of the requirements needed for larger vessels. Class B devices offer: lower power (2W) transmission output; multiple connectivity option outputs to satisfy different display protocols; same number of transmitters and receivers as class A, but used differently; active GPS antenna/receiver incorporated in the device; not interfaced to a compass/gyro, (brand dependant) so heading information rarely transmitted; not interfaced to a computer system so voyage status cannot be input and
so is not transmitted.
Class B units operate using carrier sense time division multiple access (CSTDMA).
Fishing vessels may be required to carry AIS, but any vessel may switch off its AIS if the master/captain believes continual operation may compromise the safety or security of the vessel.
Base stations offer a link between vessels sailing within range and are able to relay information from AtoN’s to a monitoring or command centre that may be located inland. Shore-based AIS transceiver operates using SOTDMA. Base stations have a complex set of features and functions.
Due to these features and functions base stations are able to control the AIS system and all devices operating therein. There is the ability to interrogate individual transceivers for status reports and or transmit frequency changes. Fixed access time division multiple access (FATDMA) is also used by base stations, FATDMA allocated slots are used for repetitive messages.
AIS base stations offer the chance to form an AIS network along a country’s coastline that can enhance national security. Base station equipment can offer many different connections and data formats, allowing connections for many different display systems, including PCs, chart plotter displays, or wireless enabled devices.
AtoN units can be deployed on land or out at sea and have many benefits to both mariners and coastal monitoring centres. The key difference between the two types of AtoN is the power consumption. Keep in mind the AtoN stationed out at sea must rely on the power generated at the buoy, mostly done by using solar panels.
An AtoN will use one of the two different types of transmission: random access time division multiple access (RATDMA) or FATDMA. Devices that use the FATDMA transmission type have the slots on the AIS slot map controlled by an AIS base station(s) ashore. This ensures the transmissions get through, while also keeping power consumption down.
AtoN using RATDMA-type transmission will use more power as it behaves like a class B device and must be powered up and running prior to sending so the unit can scan for available space on the slot map. To increase the range of an AtoN, ‘chaining’ can be used and this also has the added benefit of an increase in the range of base stations that monitor traffic along the coastline. These AtoN buoys/fixed beacons/lighthouses have the possibility of also housing sensor units that monitor metrological and hydrological data along with the AIS AtoN unit.
In an emergency situation, mariners have predominantly used a radar SART as a means to alert ships of the location if they are in distress. Unfortunately the radar SART has to be in the beam of the search and rescue ship’s radar to display a position on its screen.
An AIS SART can offer many benefits, including the ability to be seen around headlands, up rivers or behind islands. It does not need a radar beam to activate it. AIS SART offer enhanced location finding in an emergency, by transmitting the position, heading and speed of the ship in distress, or liferaft, with the AIS data eight times in a minute. A safety message ‘SART active’ is sent every four minutes. This is the primary benefit over other technology as it allows SAR vessels/aircraft to track and plan a course to intersect and rescue the vessel’s crew.
An AIS SART will use pre-announced time division multiple access (PATDMA) as it continuously transmits but does not receive. A SART unit, on a man overboard device/personal beacon/lifejacket, sends the information often to ensure that the signal is at the peak of a wave on at least one of the transmissions, helping to maximise the range of the one watt transmitter.
AIS fitted to SAR aircraft aid search and rescue tracking. Most systems display a SART once activated, I know both my shipboard display units come up with a warning that a SART has been activated, then the system will ask if
I want to lay a course to steer to an active SART.
There are personal locating beacons (PLBs) and AIS PLBs. Make sure you get to know the difference between an AIS MOB beacon, also known as an AIS PLB, and a PLB so you purchase the correct one for your needs.
While these AIS units are not new on the market, before purchasing a beacon check if the one you are thinking of purchasing is authorised with AMSA and Australian Communications and Media Authority (ACMA) to be on the AIS network. Some are not due to the transmission type.
It is imperative to know how your boat’s electronics will react to an AIS MOB transmission. If your AIS system/chartplotter will not display the AIS PLB then it could turn out to be a costly exercise upgrading the AIS display unit. The primary reason for the purchase in the first place is self-rescue, unfortunately not all AIS display devices do it correctly yet.
The positive side of testing is that it is important to know how to activate the device manually before you find yourself trying to figure it out in desperate circumstances. Remember that even if it is supposed to activate automatically, sometimes things go wrong with the automatic function for
a multitude of reasons.
The similarities between AIS and RADAR
The good thing is AIS has been designed and manufactured to work automatically and continuously, regardless of where a vessel is located.
In the early advertisements AIS was touted as being radar-like, however this is not the case at all. The only real similarities between AIS and radar is that the displays can be configured to look similar. Perhaps this is what the advertisements meant when they describe AIS radar.
The reality is radar transmits a pulse of extremely high frequency electrical energy that reflects off objects returning to the point of origin, after several pulses have been received and after undergoing signal processing, the blip or returned pulse is displayed on a radar screen.
While radar is good the information that can be received from an AIS target can be better in some respects. In most cases we are able to read data sent from the AIS target on the display device by various means and get a host of information.
The process to read the data transmitted from the target will be dependent on the chartplotter brand. On some display devices just moving the cursor over the target will display a list of data. Different brands of devices will require more or less button pressing and cursor movements, but usually the important information is there at your fingertips.
But do not stop using the radar once you have AIS, just keep in mind it is very good at double-checking the AIS target is transmitting its correct position. Or perhaps check for vessels out there that are not transmitting an AIS signal at all.
For some of you headed to Asia, a word of warning: at night be on the lookout for tugs, the lights are usually correct for the type of tow but mostly they do not transmit AIS. The good news is that the extremely large barges they tow can be good radar reflectors.
AIS information that can be called up and displayed includes: the target vessels, identity, position, current course, heading and speed. If your vessel is equipped with an AIS transceiver all this happens while also providing your information in real time to the other AIS system users.
VHF frequencies have a longer wavelength than radar, AIS signals can be received from behind breakwaters, headlands, up river bends. It is also good for seeing vessels behind other ships in close quarters situations where a radar signal will not see or is blocked. So, just this feature of the AIS signal alone can add to safer navigation by detecting the position(s) of a ship(s), even when tucked behind an island or hidden from view behind another larger ship.
If you are busy hanging on and find it is getting difficult to check the vessels around you as the weather deteriorates, one of the features on most chartplotters, or navigation software, with integrated on-board AIS systems is the function to set alarms. These alarms can warn you if and when you will get too close to another vessel.
These collision avoidance alarms are an advantage and can save you a lot of time and anxiety when navigating across busy shipping lanes or in bad weather. Unfortunately this information is only as good as the data the AIS on your vessel is receiving and still requires a human for the interpretation/crosschecking of the data.
Two AIS vessels moving along at two knots may actually turn out to be one towing another vessel on a massively long cable.
The two alarms that can be set up are: closest point of approach (CPA) and time to closest point of approach (TCPA). It will depend on the brand and setup of the AIS system as to how these alarms are set. The best thing to do is to read the manual.
Most systems will allow you to enter your own CPA and TCPA values for when the alarm will be triggered. So you need to decide how close you want to let other vessels get to you (CPA) before an alarm sounds and also how much time you need to take avoiding action (TCPA).
Remember: give yourself time for an ‘out’, especially when the vessels you may need to avoid are over three hundred metres long and travelling at twenty five knots. You might decide that one nautical mile is plenty close enough for a large ship to pass and you want to have at least twenty minutes notice so that you have time to take avoiding action and/or call them on the radio.
Careful use of the CPA and TCPA alarm can certainly make your trips safer; however, as I said before, read the manual and familiarise yourself with how your particular system works.
The downside to all this automated system process is that when entering a harbour or anchorage, you may want to know how to turn off the alarms temporarily otherwise they are likely to drive you crazy when they keep going off reporting a moored or anchored vessel
is a dangerous target.
While it is all well and good to set CPA and TCPA alarms, in most cases you will still need to interpret the data and make the call on whether you will be crossing ahead or behind the vessel you have been tracking. It can make a big difference. Crossing a quarter of a mile astern of a vessel moving at twenty five knots is not a problem, but I would not think about trying to cross the bow at this range.
One thing to keep in mind when coming up on large ships is its positioning antenna placement. You may find that a large ship has almost three hundred metres (0.16nm) of ship ahead of the position reported on the AIS.
Due to the great safety benefits offered by AIS, the network has continued to mature and expand to fulfil the design brief. Now, combined with shore stations, this system offers port authorities and fully equipped maritime safety bodies the capability to manage maritime traffic and reduce the hazards of marine navigation.
A safety feature of the AIS is that AtoN can be transmitted over AIS. These can be physical aids like beacons, buoys, lighthouses fitted with an AIS transceiver; or they can be computer-generated (virtual) used to mark a new or temporary danger such as a shoaling bank or wreck. The computer generated AtoN can be set up and working quickly and is all done without the need for a marker buoy or transmitter to be physically placed on the dangerous point.
AIS can also be used to identify if navigational aids have moved from a charted position. As an example, if a beacon or floating buoy fitted with an AIS transmitter, is noticed as not being in the correct location corrective action can be put into motion quickly.
Maritime safety bodies can send safety messages that might include developing storms, other weather events or search and rescue information. Additionally, safety messages can be issued from either a ship or maritime safety stations. A ship that is underway but not able to manoeuvre properly due to some exceptional circumstance may issue a broadcast warning that they are now ‘not under command’.
The AIS system has many features that have the ability to bring immense benefits and collision avoidance to sailors. Important when on coastal passages and entering or traversing shipping lanes, these features are especially useful for shorthanded crews: e.g. cruising couples.
AIS does not replace a proper look-out but it can enhance situational awareness for the person on watch. Because the AIS can handle 2,250 reports per minute and may update information as often as every two seconds, real-time changes of another ship’s movements can usually be immediately recognised.
Here is the COL regulation rule 5: every vessel shall at all times maintain a proper look‑out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision.
Like all things electronic there is a possibility for errors to creep into the system when things malfunction without totally failing. If you can spot the errors you may not fall into the trap like the cruiser mentioned earlier. Remember, the accuracy of AIS information received is only as good as the accuracy of the AIS information transmitted in the first place.
It is not prudent for the crew on watch to assume that the positioning information received from other ships is as accurate and of the same precision as that available on their own vessel.
Operational problems encountered
Do not always accept the ship type listed in the AIS information to be one hundred percent correct.
We were heading out from Tioman Island, Malaysia, across the busy South China Sea shipping lane to the Anambas Island group. We had been tracking what we believed at the time a fishing vessel travelling along at two knots. Possibly trawling, or so we thought at the time.
They were several miles off at the time we started to pick it up on the AIS and track its course. Due to the current, weather and our sailing angle we were on a near collision course.
OK this will not be a problem, we will just point a little more on the wind and go astern of the vessel. Then, as time progressed we got a glimpse of a very dim light astern of the supposed fishing vessel.
With the radar set at the highest gain range, we got a very intermittent and weak return from something tracking along at the same speed as the AIS target. This is where it got silly.
His lights were not right for a tug with tow so we started using our spotlight to try and get a glimpse of what was trailing behind our AIS target. This resulted in the tug/vessel blacking out all lights and turning off its AIS. Well, we now had a fair idea of what we were dealing with and then it was not long before the dim light on the tow also went off. So we started the motor, changed tack and were able to pass ahead of the tug.
It would appear that, perhaps, the tug/vessel skipper was paranoid about being boarded by pirates. So we think he had set the vessel type to fishing vessel then went into hiding when a small boat (us) ried to shine a light his way. If only he knew, even with no lights on, his vessel was still very visible through the binoculars.
Sometimes we do not always see the vessels name dimensions or other data. This problem is due to the operation of the system, shipboard AIS units separately broadcast different AIS messages at different times; see sidebar box. So it is normal for an AIS to receive numerous position reports from a vessel before the vessel name, type and other data associated with its voyage reports.
A fault with the AIS can also result in missing data; one vessel we sailed with for some time had an ongoing problem and it was rare we would see any of the static vessel data blanks filled in.
It turned out the wire used for the cable run from the navigation panel to the AIS was inadequate and the AIS unit would be in a continuous start-up loop. It would transmit a position report and, while doing so, shut down due to lack of voltage; then restart after transmitting. So it never ran long enough to transmit the static information.
How can we see vessels reporting they are at anchor but travelling at speed? This would most likely be a class A unit.
Class A units allow an input of voyage-related information, in this case the person in charge of inputting the information has not done the job or forgot to hit the return key. The other voyage information transmitted from this vessel may or may not be correct. Usually a call to the vessel in question will result in a hasty update.
From time to time we see the ‘Inland blue flag’ set, it is a part of the European, inland AIS standard. The blue flag signal, commonly seen on inland waters, indicates that the vessel requests a ‘stbd-stbd’ passage or crossing. This blue flag signal is manually switched on/off, by the target. We can only assume these are not turned off when leaving Europe, or have been set on by untrained operators.
Sometimes radar and AIS do not correspond, this error can be caused by a multitude of reasons.
The speed of the vessel you are tracking may be the cause of what you are seeing. The slower the vessel is travelling the more time between the position reports. I have seen a few vessels that did have a fault and, even at speed, the position reports were several minutes apart. The best thing to do is call the ship and let them know they may have a problem with the AIS and explain the fault.
We have seen vessels in the data list notated as being in the northern hemisphere just in front of us coming up the NSW coast. So obviously its AIS had a hang-up or the connection to GPS or ships navigation system had a fault.
What else could cause an error? AIS position data received from another ship might not be referenced to the WGS84 datum; if the data on your vessel is, then there will be a discrepancy in the displayed position and that ship’s real position. The discrepancy will be distance. How much distance? This will all depend on the datum they have selected the system to run on.
Sometimes the CPA and TCPA does not look correct for a ship pointed straight at us. Check the vessel’s course and heading matches up to within a couple of degrees, the closer the better. More than likely this is a set-up error or the gyro connection to the AIS unit was not wired in correctly when the system was installed.
Ships gyros use different systems (formats) to output course data, a crossed-over wire at the interface or an open circuit on one pair can cause an incorrect output without causing a total failure or error condition at the AIS unit. Be aware of this condition, a lot will depend on the brand of chartplotter or display device on your vessel.
Some chartplotters do not show targets as anything other than a triangle. If this is the case then scan your targets data when they first come into range.
We have seen several ships with this error, unfortunately once we contact them and let them know of the failure they brush it aside or do not understand. A very well-spoken watch officer on a cruise ship brushed it aside, treating us as if we were being a nuisance.
The screen shot of the MV Menang Jaya gives a good example of this fault, the vessels COG is 299 degrees and the heading is listed as 186 degrees. His COG more than likely is being calculated by the GPS device in the AIS or the vessels navigation system, from what we saw this was correct, the heading information will most likely be coming from the on board gyro and was wrong.
We recently sailed in Indonesia with a couple whose AIS reported the vessel was heading 000 degrees. However, the COG would change to report the vessels direction of travel at the time.
This turned out to be a fault with a gyro card fitted in the all in one chartplotter. This fault did give several ship’s watch-keepers a fright when they thought they were going to run them down.
Similar error symptoms can occur, but the COG and heading match up. Check the age of the report, this is very important. Some AIS units will hold a target static for some time when data has not been received. I have seen ghosts where the data is several minutes old. Large ships travelling at 25 knots can travel a lot of distance in this several minutes.
AIS does not relieve you of watch-keeping duties, use your eyes to first scan the horizon then the AIS data on the chartplotter information page. As I have said earlier the accuracy of AIS information received is only as good as the accuracy of the AIS information transmitted.
When I am looking at a website for AIS sometimes you may not see all the vessels I can see on the water. Under SOLAS and the relevant International Maritime Organisation (IMO) guidelines, the master of any vessel has the discretion to turn off the AIS unit if its continual operation might compromise the vessel’s safety or security.
The publicly available AIS websites (such as www.marinetraffic.com) use a crowd-based approach to gather AIS information. The IMO has condemned the display of AIS data on public websites. The AIS receivers used for this purpose are not certified AIS base stations and may not provide accurate or valid data.
We have also found that the opposite can also be true when vessels are still shown as being live on the website even when they have not had a received transmission for a day or more. However, an even scarier feature is the ability for people to call their vessel position in on a mobile phone, without even having an AIS transceiver. Be very careful if you want to go down the AIS over the internet approach, good luck with that, but it is a recipe for disaster.
To ensure good coverage, check your antenna connection. The AIS unit can have its own antenna or, by using a splitter, can use the same antenna as the VHF radio. The loss in sensitivity to the VHF radio by adding a splitter is marginal and should not be a problem.
However, some older VHF antenna may not be suitable for AIS as the frequencies used are on the edge of the marine band and some aerials do not work well at the band edges. If your VHF voice transmissions are getting out and you have good range, then perhaps it is time to look at the antenna you are using.
Another thing to check is your cable runs. In most cases the VHF antenna is at the top of the mast, this can help with extending the range. The down-side is the cable run is long and, if the cable run up the mast is not low loss coaxial cable, then this could be the source of some signal loss.
All said and done, a check by a proper qualified technician should be able to give an answer on whether to use the radio, splitter, cable or antenna.
If you are using a splitter, check to see if it has any status lights on the front. I have seen active splitters malfunction from time to time. Splitters appear to be especially sensitive to lightning storms and the storm does not need to be close.
A power reset will sometimes do the trick, turning it off for a little while and then powering it back on.
If you are using an AIS transceiver, the splitter must be suitable for transmit use; this is certainly worth checking if you are upgrading from a receive only unit.
Keep in mind, if the VHF radio is using the same antenna as the AIS, sometimes the AIS targets will disappear from your chartplotter when using the VHF radio. This usually depends on the length of time you have been, or are on, the VHF.
When I was using a passive antenna splitter I had problems when in heavy shipping areas. Voice communications were interrupted by the AIS as it was still sending during a voice transmission.
To get around this I installed an active splitter, this always gives priority to the VHF radio transmissions and also has a fail-safe feature where it is designed to protect/maintain the VHF connection in the event of the splitter circuit failing.
I know from experience the AIS system has aided rescue or has helped keep people out of trouble.
The first was when a catamaran was moving through the Great Barrier Reef area and a ship contacted them by name and let them know they must take evasive action now or they would be run down. They took evasive action and were able to power out of the ship’s way quickly.
Another time when moving through the Barrier Reef area up near Flinders Island, an EPIRB had been activated and through the aid of AIS the reef volunteer rescue was able to contact the two closest boats to the activated beacon. As it turned out that was ourselves and a vessel we were travelling with at the time, but other boats were slightly closer and able to find the distressed boat, a broken down tinnie, then tow them to safety.
Before heading into the channel between the two main islands of the Kai group in Indonesia I was tracking the progress of a vessel already in the passage. As I was watching its progress, thanks to the AIS, I noticed they were headed for a shoal area at seven knots. I called them on the VHF to let them know my charts showed they were possibly sailing into trouble.
As it turned out the older chartplotter chip they were using in the cockpit did not have the shoals marked, but the newer chip showed the rocks and shoals clearly. If it was not for the fact I was able to track its progress, thanks to the AIS, a disaster was averted.
While moving through a crowded ship anchorage we were coming up to a large tanker, thanks to the AIS we were able to see a dredge was about to come out from behind the tanker at speed. Because of the proximity of the moored tanker and the lack of manoeuvrability in the anchorage, the dredge was shielded from both our vision and the radar.
Remember: the AIS only improves the safety of navigation by assisting the person on watch.
As with all navigational and/or electronic equipment, AIS has limitations. Always keep in mind that the accuracy of AIS information received is only as good as the accuracy of the AIS information transmitted in the first instance.
Users must be aware that erroneous information might be transmitted by the AIS from another ship. Try not to become over-dependant on the AIS as this can cause complacency on the part of the watch-keeper.
If the person on watch is not keeping a vigilant watch out on the real world, there could be developing close quarters situations when ships where the AIS, if fitted, is switched off.
While there is talk of errors with the system, analyse the data available from several sources. By using the AIS as another tool, not the only tool, good decisions can be made to keep your
vessel safe. ≈