Benefits and Shortfalls of AIS for Recreational Boaters
Boaters tend to regard AIS as a source of absolutely correct and valid information that they can rely on to provide them with data regarding approaching and overtaking vessels. But the fact is that AIS data can be erroneous and it can lack critical details about vessels that may pose a hazard to them.
Lets give at least two specific examples about AIS data that may be surprising. The first example is AIS data from a tug with a tow. The AIS reported position is at the location of the tug and does not depict the fact that a tow is following several hundred feet behind and that the length of the towed barge is not presented on the MFD. Thus if a boater were in bad weather or cruising at night with a tug and tow crossing perpendicular to the path of the cruising boat. If that boater follows a path that is immediately aft of the tug and does not see the trailing barge the mistake could be fatal when the cruiser hits the huge tow hawser just underwater.
The second example is when AIS data for direction, speed or location are erroneous. This author has witnessed several vessels that were traveling in directions nearly 180 degrees from the true course / heading of the vessel. The error arises when the AIS transponder uses a Heading or Course data source that has not been calibrated or validated at the time of the AIS transponder installation. This can occur because you cannot see your own AIS ICON on your own chart plotter or MFD. It requires a separate boat that has AIS receive capability to verify that the reported heading /course over ground is correct. This author has witnessed even County Sheriff Patrol boats transmitting wildly inaccurate heading or course data as shown in the image below.
In the first example the AIS broadcast is valid and is not misleading in terms of the tug itself. But your MFD / chart plotter will not show an extended target that properly indicates the full nature of the threat. In the case of large ferries and other ships, the chart plotter or MFD usually reports a special AIS vessel icon that implies a ship or other large vessel is present. But there is no display of a hybrid vessel that consists of two vessels – one powered and the other towed. The only way to know that you are crossing paths with both a tug and tow is to either physically see the vessels or to observe them via radar.
In the second case one must be lucky to visually observe that the transmitted heading / course over ground data vector (usually a line with an arrow that is 3-6 minutes long at the vessel reported speed) does not fall anywhere along the actual periodic GNSS position updates that are reported by the vessel transponder. This is very hard to detect happening when there is a busy waterway involved. A skipper assumes the plotted direction vector data is correct and plans his course accordingly. The only way to detect the AIS course data is in error is to observe the vessel on radar and being using “trails” or MARPA / ARPA target vector data that is independently measured by the radar itself.
It is an interesting thing to observe that if an ownship course or heading data is in error then radar observations of other vessels will also show incorrect course data of EVERY other vessel that is being reported by the radar. Now one would hope that an error this large would be easily detected and corrected by the vessel owner.
Using only AIS Receive
Vessels equipped with no radar and AIS receive only are certainly ahead of the game when compared with a vessel with neither capability. AIS receive equipped vessels have the benefit of seeing other threats around them and certainly this dramatically improves their chances of safe travel at night or in poor visibility conditions.
These vessels can receive emergency man over board or other vessel emergencies in order to render assistance or to steer clear of a critical situation. They can also observe special Aids to Navigation (ATN), buoys or markers that physically are not present as anchored buoys or day marks but exist only as a mark that is present only on an AIS equipped vessel.
Thus the vessel equipped with AIS receive only has gained several advantages but it is not broadcasting its own presence to other vessels. Therefore this vessel must assume other vessels may not be observing them visually or on radar and must choose to navigation to avoid collision without interaction with other vessels or by contacting other vessels on VHF radio on 16 or other Vessel Traffic System channel to avoid a hazardous crossing.
The great value of AIS is in poor visibility conditions. So the AIS receive only vessel is burdened with making sure they have been observed when crossing with other vessels. Certainly they are far better off than a vessel with no AIS and no Radar but they do have some handicap.
In the image below one can see radar returns painted in magenta (purple) for the nearby islands and a nearby AIS icon for a vessel passing well to starboard aft of the Own Ship Icon. The image shows that the actual position of the AIS target is in front of the current location. This reflects the delay in transmitting position for high speed vessels. Just to the right and below of the AIS target is another Radar target on a second vessel that is not carrying AIS. While this radar target for the second vessel shows it poses no problems for the Own Ship vessel it does point out that AIS does not report everything on the water.
By only carrying AIS receive and not using Radar a vessel still cannot detect other recreational vessels that are not equipped with an AIS transponder. This suggests that a vessel without radar and only AIS receive will be at risk in poor visibility conditions and should avoid being underway in such conditions. Certainly this is not too great a limitation in fine summer boating weather on salt water or small inland lakes where pontoon boats do not carry AIS transponders!
Low Cost AIS Protection
AIS is a great asset for safety and general situational awareness on board most recreational vessels. Given that there are now VHF communications radios that provide built in GPS receiver, AIS receive and even an AIS target display and target list even small boats can easily afford to have AIS monitoring capability even if they do not have a Chart Plotter or MFD installed. For vessels that do have a chart plotter or MFD installed these same VHF radios with built in AIS receive can provide AIS data to the Chart plotter or MFD via NMEA 2000 or NMEA 0183 High Speed data.
AIS receive capability is a tremendous help to displacement or semi-displacement vessels like sailboats and trawlers that travel at relatively slow speeds and therefore have limited ability to make meaningful course changes in the event of a close encounter with a high speed vessel like a ferry (330+ ft and 17+Kts in Pacific NW of USA) or standard shipping that can be 1000ft and traveling close to 20+kts.
If a small slow vessel does not carry Radar, the least expensive alternative to provide the most cost effective collision avoidance is a modest VHF Communications radio with AIS receive and display built in. These systems typically sell for under $500 and offer emergency distress calling when registered with an MMSI number in addition to providing AIS information. Examples are the Standard Horizon GX2400 and GX6000 that both retail for under $500 (The GPS Store prices May, 2022).
In congested waterways like the beautiful San Juan Islands of Washington State and The Gulf Islands of Canadian British Columbia AIS is a huge help in detecting the many ferries, ships and tugs that ply these waters. Being able to know that there is a high speed ferry approaching from behind an island is a great asset indeed! Being caught in summer time fog in these channels not only poses a navigation hazard but is a real threat when commercial ferries traveling close to 20Kts can be expected at any time.
Adding Marine Radar to Compliment AIS at Sea
In the image below we highlight a busy water way in the San Juan Islands of Washington State in the US. In this image the tow tug Island Mist AIS Icon can be seen with a radar detection that follows the AIS Icon. This is a direct indication that there is a TOW barge present that does not have a corresponding AIS Icon. While it appears that the Radar return (red blob) is very close to the reported AIS Icon, note that the image scale shows 1 Nmi is about 1/2″ on the display! So the red radar image is actually a long way aft of the reported AIS Icon for Island Mist at this scale. Zooming on the chart plotter would readily show the actual separation distance.
You can also notice that the “Own Ship” Vessel Icon in the center of the image is being followed close behind by an AIS Icon from another trawler and intends to pass to Starboard. At this range the radar return for the very close vessel may have been suppressed but would appear when zoomed in to shorter range for tactical information.
In the image above we notice that there is a large AIS target (designated as shipping by the AIS Icon type) just to the right of Island Mist. There is no radar detection of this vessel because it is just behind the high point of Cypress Island. The island is blocking the direct illumination of the vessel by the radar. This shows that while AIS signals can leapfrog over most nearby land masses radar cannot “see around corners”.
If you look closely at this image you will note that there appears to be a number areas that are missing radar detections, such as the left side of Guemes Island and no detections of Sinclair Island. In all of these cases the issue is “radar shadowing”. If the radar does not have a direct line of sight to the object because of a large object in the foreground the radar cannot see that target.
AIS can receive signals that are behind many (not all) other land masses because it does not depend on illuminating the target with a transmitted signal. But it can receive a signal that is either refracted (bent) or bounced from other land masses to the AIS receiver. These bounced or bent signals may flicker or come and go as the own ship changes position but they are at least observable part of the time.
Another significant problem for radar is angular resolution of multiple closely spaced targets. When it comes to radar systems, size does matter. A small radome radar that encloses a hidden rotating antenna that is barely 18″ across will produce a very wide beamwidth that can be as much as 6.5 degrees wide. Consider that at 3000ft (1/2 Nmi) a 6.5 degree radar beam will be 331ft wide! Clearly a couple of 100ft long boats could be within 100ft of each other and so far as the small radar is concerned there is just ONE vessel present because it cannot measure less than 331ft.
A 6ft long open array on the other hand will have a 1.2 degree beamwidth and can resolve 61ft of separation between targets. It will clearly show the two 100ft vessels placed 100ft apart and report them as both being present.
Finally the most challenging conditions for a radar system are very heavy seas or very heavy rain conditions. In these conditions waves themselves generate radar reflections that directly compete with the reflections from small vessels. Attempting to attenuate “sea clutter” or reflections from wave tops can also eliminate detection of small vessels in the same wave structure.
Rain can pose a similar problem when it is very heavy (1 – 2″ inches per hour – a very heavy storm by any measure) by generating large areas of “volume detections” – meaning that rain can fill the entire 22Degrees of vertical extent and say 1.2 degrees of azimuth extent with rain that is readily detected. This competing “rain clutter” can be attenuated but it will also cause small targets in the same area as the rain to be eliminated as well. Heavy rain (>1″ an hour) will also directly attenuate the radar signals themselves on both the transmit and receive paths.
While attenuation of typical X Band Marine Radar signals is virtually hill in heavy fog conditions, heavy rain poses a threat of total loss of visibility and attenuation of radar detections as well. When combined with heavy seas and wind, heavy rain conditions pose the most threatening conditions on the water so far as collision avoidance is concerned. Commercial shipping carry C Band radars that employ very large antennas of 6 -12 feet so that under heavy rain and wind conditions they can still maintain radar visibility. C Band radar incurs much lower attenuation of radar signals and can penetrate rain with less rain reflectivity. But C Band radars operate at 3 GHz and will require antennas that are on the order of 2- 3 times larger than a standard X band antenna to achieve the same angular resolution. Carrying a 12f – 18ft open array antenna on recreational vessels is totally impractical and is reserved only for large ships for that reason.
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