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Sailing with AIS

Jan 5, 2007

To the editor: Recently, I sailed my Pearson 424 Ketch, Sarah, from the island of Sao Miguel in the Azores to Cascais, Portugal. This is a journey of about 750 nm and was the final leg of our trans-Atlantic sail from Florida to Europe. While berthed in Cascais, I thought about the difficulties I might have the next spring sailing down the coasts of Portugal and Spain, through the Strait of Gibraltar and into the Mediterranean. Around this time I first became aware of the automatic identification system (AIS) transceivers equipped on nearly all large commercial craft. Also at this time DigiBOAT, the publisher of Software On Board (SOB), the primary navigation package I use on my navigation PC, released a version of the software that plotted AIS targets using the NASA AIS Engine.

The NASA AIS Engine is basically a single-purpose VHF radio receiver that captures AIS broadcasts from ships and converts the broadcast data into a stream of standard NMEA sentences. The engine connects to a computer or chartplotter via a standard PC data cable, just like a GPS. A software package, like SOB, then plots the target information sent by the AIS engine on an electronic chart.

I had no real knowledge of how useful this AIS data would be on my sail to the Med the next spring, but I decided to invest a little money in the system and find out. While at the London Boat Show that winter I purchased the NASA AIS Engine and installed it on Sarah. My PC became an AIS plotter using SOB in addition to a backup chartplotter to the C120.

On April 19, 2006, along with my international crew of Chris Behrman (German) and Martin Morgan (Welsh), we departed Cascais en route to Gibraltar with short stops in Sines and Lagos, Portugal.

Upon departure from Cascais I immediately had my first opportunity to use AIS to monitor an approaching ship while underway on Sarah. As we approached the traffic lane leading into the Rio Tejo, a medium-sized freighter was steaming in our direction.

AIS Figure 1 shows the SOB display of this situation. Sarah's position is the yellow circle near the center of the screen. The 331-foot cargo ship OPDR Cartagena was approaching the Rio Tejo just as we were about to cross the range line used by large ships (the thin black line from the center of the screen to the upper right corner). If neither Sarah nor Cartagena altered course or speed a collision was possible. The SOB software recognized this situation and plotted the icon (triangle) for Cartagena in red.

In the target panel (upper right corner of the screen) SOB displayed detailed information on the ship and the collision potential. The center of this panel displays the collision potential information. The data here is relative to the closest point of approach (CPA) position calculated by SOB. This is the point where Sarah will be when it is closest to Cartagena. TCPA is the time when Sarah is at the CPA, which SOB has calculated to be in less than 17 minutes. DCPA is the distance at CPA (the distance between Sarah and Cartagena when Sarah is at the CPA), which SOB has calculated to be 2,929 feet. This is the value that caused SOB to flag Cartagena as a high potential for collision. I had set the CPA Safe Distance value in SOB to 1.0 nm. Any target that is projected to come within that safe distance will be considered to have a high potential for collision.

So the DCPA for Cartagena is more than half a nautical mile, not really a threatening course intersection, especially since this was on a sunny morning with little other traffic in the area.

A few days later we had departed Lagos and were headed for the Strait of Gibraltar, where I expected to get a good workout of the AIS system. En route we encountered an AIS target that confirmed to the old computer adage of garbage in/garbage out, or GIGO. That is, if the target vessel is transmitting bad information, the AIS system will not correct it. Just south of Faro we received target information on MSY Wind Spirit.

This is one of those cruise ships with sails. It appeared to have departed Portimoa, Portugal, and was headed for Tangier, Morocco. However, the AIS data transmitted by the ship and displayed by SOB said it was heading due north at more than 100 knots. Obviously something was wrong with the information being fed by Wind Spirit's instruments to its AIS transceiver.

This is one of the few times I've seen this sort of bad dynamic information from AIS. Normally the errors are in the static data that is entered manually by one of the deck officers or the navigator. The static data includes the ship's name, call sign, MMSI, ship type and length, status, destination and ETA. Many times the crews fail to update this information before they depart for a new destination. In this case, SOB has plotted a track for Wind Spirit based on the dynamic data previously received. That track clearly shows Wind Spirit heading SE, and the Tangier destination is probably correct. The static data and dynamic data are separate broadcast messages in AIS. The dynamic data (which includes position, course and speed data) are transmitted much more frequently than the static data. The frequency of the dynamic data broadcast increases with the speed of the ship to as often as every two seconds for a ship moving in excess of 20 knots. Static data are broadcast once every few minutes, or even less often.

The next morning we began to encounter heavy shipping traffic. Sarah's position was southeast of Trafalgar heading for Tarifa, Spain. SOB had plotted five ships outbound in the strait and one inbound ship. There was also a cluster of ships in the harbor at Tangier. Many of those targets were ferries that run between Spain and Morocco. While the nominal range of my AIS installation is less than 20 nm, I have picked up targets from as far away as 100 nm, but those must have been the result of unusual radio propagation and a very big ship.

There is one other aspect of AIS that differentiates it from radar. The line of sight from Sarah to the ships passing just south of Algeciras Bay and entering the outbound traffic lane goes through some pretty high mountains. Radar could not have picked up those targets because the cliffs would block the radar signals. The mountains, however, did not stop VHF radio signals. It is almost like being able to see around a corner.

In AIS Figure 2 Sarah is in the Strait heading for Algeciras Bay and Gibraltar. Europa Point and "the Rock" have just become visible around the Spanish headlands. Now we have a current boost of more than 2 knots, and our speed over the ground is more than 8 knots. We topped 9 knots several times in this area. The currents in the Strait can be very tricky, but normally there is a favorable current when entering, as there is a nearly constant inflow of water at the surface from the Atlantic into the Mediterranean. So entering the Med is comparatively easy.

In AIS Figure 2 SOB has displayed the data on one of the large, high-speed ferries (Alcantara) that cross the Strait between Africa and Spain or Gibraltar. This data shows the 280-foot long ferry is moving at nearly 30 knots.

By this time it was clear that AIS made the passage very interesting as we could identify the ships we were seeing in the traffic lanes and many that were over the horizon; however, our passage through the Strait would not have been any more difficult without AIS. We made our passage in daylight in good weather and did not have to enter the shipping lanes at any time. So it was really not an acid test for my AIS setup. If our transit had been at night or if the weather had reduced visibility I believe AIS would have been a significant benefit to our passage. As it was, AIS just kept us entertained.

Finally, in AIS Figure 3 Sarah had entered Algeciras Bay and was heading toward Gibraltar. Gibraltar and the Spanish port of Algeciras are very busy ports. Most of the targets in this screen are anchored, but are still being displayed in red as high-potential collision targets. This is because GPS will record a speed over ground (SOG) value greater than zero for even moored vessels, usually less than a quarter of a knot. When SOB projects that small speed and the course over ground (COG) it can find a potential intersection. As you can see, the target icons take up a lot of space on the displayed chart. It is difficult to identify the moving ships from the moored ones with AIS. We steered Sarah visually around the moving ships, including a large containership that approached us from starboard, and only used the AIS to confirm what we were seeing.

After I provided this screen capture to DigiBOAT, they added a feature to allow the user to specify a minimum target speed, below which the target will not be flagged as a high-potential collision. This feature would have allowed me to filter out all the red icons in screen three except those actually underway and any real collision threat.

Gibraltar is undergoing major redevelopment along its waterfront, which includes a number of high-rise condominiums and resorts. This has resulted in the closing of one of only three marinas in Gibraltar and a very tight situation for transient berths. We were lucky to get the last available berth in Marina Bay the night we arrived.

For this trip AIS proved to be more entertaining and educational than essential in navigating Sarah through the shipping traffic in the Strait of Gibraltar. If our transit had been at a different time of day or in different weather, AIS might have been more useful. Now that I have the Raymarine firmware that supports AIS I will be adding it to the C120 chartplotter display as well. It is one more tool to navigate safely in an area of heavy ship traffic. I actually hope AIS never becomes an essential tool for my navigation as that would mean I am having many close encounters with large, fast ships. My hope is that it remains part of the onboard entertainment systems on Sarah.

- John Stevenson is a retired computer professional who has owned a series of boats in Chesapeake Bay. He sailed his Pearson 424, Sarah, trans-Atlantic and voyaged in the Mediterranean in the summer of 2006. He is currently living aboard Sarah in Lagos, Portugal, and will return to the Chesapeake next spring.