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Maritime Security vessel's course over ground with 180 degree ambiguity. Hence, direction is between 0 and 180 degrees, counted clockwise from the north. Time of day, given in hours, is computed as the absolute value of the hour of day relative to noon. Hence, AIS timings at noon are assigned a time of day around 0 hours while timings early in the morning and late at night are assigned a time of day of about 12 hours. With regard to the first three indicators, a correlation between vessel speed and vessel length may be expected because larger vessels are usually designed to travel at higher speed. Hence, large vessels moving at low speed or small vessels moving at high speed may indicate suspicious behavior, especially when headed towards the coast. Results To learn about the normal behavior of vessels along the Dutch coast, about one week of coastal AIS data was analyzed. The AIS receiver was located at TNO Defence, Security and Safety on a 50 m high tower near the sea. In one week, about 2.5 million AIS messages were received originating from approximately 1,100 vessels. The vessels were grouped into five classes using the vessel type field in AIS: pilot vessels, fishing vessels, passenger ships, tankers, and cargo vessels. The number of vessels of each class from which the messages were received are: 38, 44, 20, 256 and 706, respectively. The relatively small number of fishing vessels is explained by the fact that AIS is not yet mandatory on board these vessels. For each class, the indicators discussed in the previous section were computed. In figures 1-5, the investigated indicators are shown for all five-vessel classes. The values in these figures have been computed for each class by averaging the speed over ground, direction over ground, vessel length, and time of day, in the received AIS messages over a grid of about five km cell size. In the right half of figures 1-5, the Dutch coastline is marked in red. On the left, part of the English coast can be seen. The main ports of Amsterdam and Rotterdam in the Netherlands are indicated, as well as the port of Antwerp in Belgium. Note from figures 1-5 that under favourable weather conditions most of the Dutch North Sea can be covered by one coastal AIS receiver on a 50 m high tower. From figure 1 it can be seen that pilot vessels have a typical length of about 30 m and a typical speed of 812 knots. As expected, their boarding operations are mainly confined to the entrance of Rotterdam port to take cargo vessels and tankers into port. Hence, most pilot vessels move in a direction of 110 to 130 degrees. With regard to the time of day, values between 0 and 12 hours were found which means that boarding activities take place all through the day. Most of the smaller vessels with a typical length of 50 m are seen to leave from and return to Scheveningen, which is a small harbor some 20 km to the north of Rotterdam. Their fishing grounds are near the Dutch coast, in between Rotterdam and Amsterdam, as indicated by the small vessel speeds in that area. Larger vessels of about 100 m size are seen to head for fishing grounds in deeper seas. Figure 2 shows that passenger ships have an average length of 150-200 m and typically travel in shipping lanes connecting Rotterdam and Amsterdam with other ports. Note that further inside the Rotterdam and 28 Amsterdam port areas, the vessel length decreases to 50-100 m. Typically, such smaller ships provide transport to passengers between these ports and other Dutch cities. With tankers, figure 3 shows that the vessel length varies between 100 m and more than 200 m. At least two important shipping lanes can be seen in figure 4. One with a direction of 20-30 degrees used by the larger tankers, and one with a direction of 50 degrees mainly used by smaller tankers. As expected, most tankers have Rotterdam and Amsterdam as destination harbors. Note the correlation between vessel speed and vessel length in open sea. Also note that tankers enter or leave the harbor of Antwerp at a time of day of about 12 hours, i.e. around midnight. A likely explanation is that Antwerp is connected to the sea by a complex delta of estuaries and shoals, which means that larger vessels may have to move with the tide. Some cargo vessels vary significantly in length and that they use a complex net of shipping lanes. Originating from Amsterdam, Rotterdam and Antwerp, a net of at least five lanes can be seen, mainly to harbors on the east coast of England. To the north of the Dutch coast, shipping lanes are seen to converge into a number of routes to Scandinavian countries. Similar to tankers, the larger cargo vessels likely have to wait for the tide to enter or leave the harbour of Antwerp. Also similar to tankers, vessel speed and vessel length are seen to be correlated in open sea. From the above discussion it is obvious that vessel behavior in the North Sea is complex as it varies significantly with position and a relatively large number of indicators will be needed to discriminate suspect from fiducial vessels. For surveillance of the entire Dutch coast, this likely means that computer based classification algorithms have to be developed as providing border patrols with the basic indicators that were discussed seems impractical. However, the basic indicators may be used to derive a limited number of new indicators (typically one or two) that show less spatial variation while each indicator may take approximately the same value for all vessel types. If such a set of indicators can be derived is the subject of ongoing research at TNO Defence, Security and Safety. About one week of coastal AIS data was used to investigate the normal behavior of vessels along the Dutch North Sea coast. Even from this limited amount of data, good agreement was found between the spatial distribution of the indicators for different vessel classes with e.g. shipping lanes and fishing grounds. Hence, AIS contains valuable information to derive vessel behavior. Possibly, the basic indicators or combinations of these indicators, may be used in classification algorithms or by border patrols to remotely detect suspect vessels. Figure 1: Indicators for pilot vessels. Shown are: speed over ground in knots (top left), direction over ground in degrees (top right), vessel length in meters (bottom left), and time of day in hours (bottom right). Figure 2: Indicators for passenger ships. Shown are: speed over ground in knots (top left), direction over ground in degrees (top right), vessel length in meters (bottom left), and time of day in hours (bottom right). Arthur Smith received the M.S. (1992) and Ph.D. (1999) degrees in Aerospace Engineering from Delft University of Technology, Netherlands. Since 1997 he has been with the Radar and Electronic Warfare Group of TNO Defence, Security and Safety, in the Netherlands. His research interests include various aspects of coastal, airborne and spaceborne radar systems with emphasis on maritime security and safety applications. Figure 3: Indicators for tankers. Shown are: speed over ground in knots (top left), direction over ground in degrees (top right), vessel length in meters (bottom left), and time of day in hours (bottom right). Maritime Reporter & Engineering News