Modern Transportation March 2014, Volume 3, Issue 1, PP.8-12
The Problem of Preventing Collisions of the Ship based on AIS Yueming Lu, Yidong Xue Pilotage Section, Shanghai Pilot Station, 20F 908 Dong Daming Road Shanghai, Shanghai, 200082, China #Email: pilotyue@gmail.com
Abstract AIS can enhance the accuracy and reliability in acquiring the information about vessels. By applying AIS into ship collision avoidance, it will enhance the safety of navigation and collision avoidance. And whether the measure to be taken to avoid collision is successful mainly depend on the accuracy and reliability of AIS information broadcasted by other ships. So the mariners and pilots shall pay attention to some of the problems about the influence of AIS information on the collision avoidance of ship. Keywords: AIS; Navigation; Collision Avoidance; Problem
1 BACKGROUND According to the Article 19, Chapter 5 of SOLAS (Amendment), nowadays, most of the ships are fitted with AIS (Automatic Identification Systems). After fitting it, ship can broadcast its own AIS information and receive the AIS information of other ships nearby automatically. And the information includes the static information and the dynamic information. The static information includes the ship’s name, call sign, MMSI, IMO number (If available), the length and breadth of the ship and etc. The dynamic information includes position, speed over ground, course over ground, heading, navigation status, the turn rate and the information related to the voyage mainly including draft, dangerous cargo (type), number of the crew, destination, ETA and etc [1]. AIS is useful for ships to identify each other, ensure the safety of navigation and for MSA to improve the supervision ability [2]. With the better function, the lower price, and the potentials of enhancing the safety of navigation, more and more ships will be fitted with AIS. So the mariners and pilots shall understand and use it properly to make it play its anticipant role.
2 LIMITATIONS OF RADAR/ARPA AND VHF Before, radar/ARPAR is the most common used equipment for collision avoidance. But it is vulnerable to be affected by weather and sea and is limited by range resolution and bearing resolution. And it maybe tracks by mistake or misses the target sometimes in the congested waters, narrow channel, or when it is affected by rain, snow or heavy sea. In addition, the radar cannot get the information such as the heading, ship’s name, ship’s type, navigational status and etc. Although radar / ARPA have automatic collision warning, trial, the relative motion and true motion display conversion functions, the limited information does not meet the requirement of collision avoidance. If we use the VHF, there may be some identification mistakes, language barrier, disturbance and misunderstanding. Especially sometimes in the water area of high traffic density, the VHF is noisy and chaotic, and it is difficult to call the target ship correctly.
3 FUNCTIONS AND ADVANTAGES OF AIS AIS will continually broadcast its information to the ships nearby automatically. At the same time, it can receive the information broadcasted by the ships nearby fitted with AIS. And the position is more accurate provided by GPS. In addition, AIS broadcasts information with a certain VHF radio waves that can pass the obstacle and eliminate blind spots caused by the terrain. And it is little affected by weather and unaffected by sea, rain and snow. And it will not miss the small target. So AIS get the information more accurate and further. And it can get the static and dynamic information of the ship nearby as soon as possible, avoiding such traffic accidents caused by which find the ship too -8www.ivypub.org/mt
late or difficultly identifying. So it brings conveniences to navigate safely and avoid collision. At the same time, it improves the safety of navigation [3].
4 THE INFLUENCE OF AIS ON THE NAVIGATION AND COLLISION AVOIDANCE From the basic principle of collision avoidance, it is the precondition/premise of collision avoidance successfully that the ships find each other and get the information including ship’s name, type, navigational status, position, distance, course, speed, and other information effectively, or it is difficult to do so. But there are many limitations in communication and getting the information by RADAR or VHF. So it is difficult to take measures cooperatively to avoid collision. And the collision avoidance mainly depends on the lookout and the experience of the mariners and pilots. If they do not keep proper lookout or short of experience, there may be accident or urgent situation. Because taking avoidance measures depends on the information such as course, speed, ship’s type and navigational status of related ships, or it may take uncoordinated action. Radar/ARPA can provide deck officer with visual and continuous on-site situation, and can read out the target ship’s position, distance, course, speed, DCPA, TCPA and other parameters at the same time. When the ship’s DCPA or TCPA is less than the set of DCPA or TCPA, it means that the two ships involve in risk of collision. ARPA will sound the alarm through lighting, sound, etc. The mariners and pilots judge and take avoidance measures according to the regulation, their experience or usual practice. But there are many limitations in the use of radar / ARPA for collision avoidance. The main problem is that the information provided by radar / ARPA is not reliable, such as the small target, with stealth capabilities of the ships, waves, rain and snow interference, or the target in the shadow of the blind spot of radar. At present, there are two kinds of display modes for AIS [4]. One is plot mode. It shows the around ships with its own ship as the center of the map. The mariners and pilots can identify the ship depending on the relationship between the two ships’ positions. AS shown in Figure 1. The other way is arranged based on the distance. As shown in Figure 2. When you move the cursor to the image or the ship on the list, and then confirm, you can get the related information of the ship, DCPA and TCPA and so on. And you can correctly identify the target, and determine if risk of collision exists correctly. If there is a risk of collision, we can call the target correctly and exchange the information related with safety of navigation, according to the information such as the ship’s name, call sign provided by AIS.
FIGURE 1. PLOTTER MODE
FIGURE 2. TARGET LIST MODE
When the two ships take uncoordinated actions to avoid collision, the two ships cannot pass safely. This is mainly due to the following reasons:(1) The give-way ship hesitates or the measures to avoid collision is not large enough, and the stand-on ship misunderstands and give up the right to keep her course and speed.(2) The give-way ship and stand-on ship do not exchange information correctly and judge the wrong situation.(3) One ship does not understand the avoidance measures taken by another ship.(4) The vessel restricted in her ability to maneuver or the vessel not under command does not take proper action according to the regulation. But if the ship is fitted with AIS, he can find the ship which involves risk of collision in advance, according to DCPA and TCPA and the information including the ship’s name and take the most effective avoidance measures. But the drawback is that the LCD screen is too -9www.ivypub.org/mt
small, as shown in Figure 1. Sometimes, the marks of the ship’s AIS (acute isosceles triangle) may overlap. And they cannot identify effectively. In figure 2, it only shows a few ships. It is difficult to find the ship involving risk of collision in the congested waters.
5 THE INTEGRATION OF AIS, RADAR AND ECDIS 5.1 The Integration of AIS and Radar AIS only can be detected by the ship fitted with AIS, but the radar can detect all the targets, according to the reflection waves from the target. And radar has more functions such as the relative motion / true motion vector line, trail, off center and etc. But the speed and course of the target measured by radar/ARPAR is not very accurate. As a result, there is large error in DCPA and TCPA calculated by radar/ARPAR. And DCPA and TCPA of the target provided by AIS relatively accurate and update very quickly. After integration of AIS and radar, the mariners and pilots can recognize the ship fitted with AIS and read out the related information. And they can complement each other, so it shall enhance the safety of navigation. But there are some problems on the integration of AIS and radar. The main problem is that the mark of the AIS does not overlap the radar image on some ships fitted with AIS. Because the position of the ship broadcast by AIS is GPS antenna location. And GPS accuracy is affected by coordinate system used by GPS receiver, set of the horizontal position accuracy factor (HDOP), GPS antenna position, accuracy provided by the government, GPS receiver noise and quantization error, sunspot activity, severe weather and other factors. If the scan rate of radar image is inconsistent of “report rate” of AIS information also cause the problem. So When a ship has two “ship’s position”, the radar target is more accurate.
5.2 Integration of AIS and ECDIS There are more advantages on the integration of AIS and ECDIS. It can display all the ships fitted with AIS on the ECDIS, we can learn about all the ships in the related waters directly and effectively. At the same time, they can provide the detailed information of the related ships and get the DCPA and TCPA correctly. It is effective to determine if risk of collision exists. There are some possible problems on the integration of AIS and ECDIS, due to the same problem as that with GPS above, some ships fitted with AIS will have deviation for their AIS marks which displayed on the ECDIS, the mariners and pilots should not take measures to avoid collision just based on the AIS position of other ships. Somehow some ships input the MMSI incorrectly, which makes two or more ships have the same MMSI at the same water area, these ships can identify, track and supervise other ships without any problem, but the position and distance of other ships which is calculated from different ships will be changeable, and there will be a great difficulty for other ships to identify, track and supervise these ships with incorrect MMSI, the broadcasted information will change from one to another between these ships, particularly will be displayed as a unstable signal, even as a AIS mark without the real ship. At last, some ships will not be found in ECDIS because there is not AIS on ship or AIS is out of work, partly some reason from AIS appliance itself, and also some ships can receive information but can’t broadcast.
6 THE PROBLEM ON THE SHIPBOARD AIS 6.1 Speed Broadcasting Based On AIS The construction of AIS is showed in the figure 3. There are special interfaces to connect with sensor log and GPS/DGPS or GNSS. When they are both connected, the speed broadcast by AIS is log speed. When it is only connected with GPS/DGPS or GNSS, the speed broadcast by AIS is GPS/DGPS or GNSS speed. So there may be some speed error broadcast by AIS. In addition, there are two kinds of log. One is relative log, and the other is absolute log. They separately provide relative speed and absolute speed. When the ships navigate in the rapid current, the speed error provided by the relative log is large. And then the DCPA and TCPA are not correct. So, sometimes the speed broadcast by AIS may be wrong [5]. The Doppler log is absolute log. When the depth under the keel is less than design standard, it shows speed over the ground and is more accurate. But when the depth under the keel is more than the design standard, it displays the speed over the water. The standard of some foreign well-known brands is 100 meters. So it always displays the speed - 10 www.ivypub.org/mt
over the ground in the port. But the standard of some Doppler log is relatively small. It is only 3 meters. When the depth under the keel is more than 3 meters, it will show the speed over the water.
FIGURE3.THE FRAME OF AIS
The new ships usually check the log speed before delivery, so the speed error is small. But it will become larger, if it is not checked, used or maintained properly. In addition, because the speed of ultrasonic changes in the water, and affected by the current and other factors, there is some speed error. In fact, AIS relies on GPS. No GPS, there is no ship’s position. It can not be synchronized timing, and can not use self-organizing time division multiple access (SOTDMA) technology．And AIS is out of the question. Although there are kinds of ship’s position error in the GPS receiver resulting in the speed error, the speed over the ground is more accurate provided by the GPS receiver than the absolute log. If we can find that the target ship’s AIS connecting with is GPS or DGPS from our own ship’s AIS, we can learn about the target ship’s position accuracy. But there is no way to know that the speed broadcasted by other ships’ AIS is GPS speed or log speed from our own AIS. If we learn about this, there is a practical significance to prevent collision when two ships are close to cross or overtake.
6.2 No Heading, Wrong Heading or Some Problem on the Input Equipment The heading broadcast by AIS is from the compass. If there is compass error, there is heading error broadcast by AIS. So the mariners and pilots cannot depend on the headings broadcast by AIS to determine that the two ships are cross or overtaking situation. If there is no input heading or some problem on the input equipment, the course will be the same as the heading. So we cannot recognize the drift. When the ship is turning, the course lags behind the heading. So we cannot get the true heading. And the heading will continuously change with the latitude and longitude of the ship’s position received by the GPS receiver changing for an anchored ship or moored ship. And it is different with other ships which input the heading correctly, and other ships maybe misunderstand.
6.3 The Problem with Manual Input for AIS Due to different culture, there are some difference between the name broadcast by AIS and the actual pronunciation, such as Chinese “LV”—green, “NV”—female, “XI” —west and etc. The foreigners cannot read them correctly. At the same time, the ship in some non-English speaking countries will also encounter similar problem when input in English. Therefore the static information and the dynamic information about ship should be inputted manually. The static information include ship name, call sign, MMSI, IMO number (if available), GPS antenna position (from which we can know the length and breadth), ship’s type, etc. The dynamic data refer to the navigation status and the information related with the voyage, such as draft, dangerous goods (type), number of crew, destination and ETA. So there may be some problem about the information. As a result, there are some differences between the actual information and the information got by AIS [6].
7 CONCLUSION The advantage of AIS is obvious. The integration of AIS with radar and ECDIS provides for the mariners and pilots - 11 www.ivypub.org/mt
a clear piece of the traffic map including depth, navigable waters, underwater navigation obstacles and etc [7]. But International Maritime Organization (IMO) in the first part of “GUIDELINES FOR THE ONBOARD OPERATIONAL USE OF SHIPBORNE AUTOMATIC IDENTIFICATION SYSTEMS (AIS)” points out: The officer of the watch (OOW) should always be aware that other ships, in particular leisure craft, fishing boats and warships, and some coastal shore stations including Vessel Traffic Service (VTS) centers, might not be fitted with AIS. The OOW should always be aware that AIS fitted on other ships as a mandatory carriage requirement might, under certain circumstances, be switched off on the master's professional judgment [8]. So it is only one kind of identification system. You can identify the ships nearby. And you can coordinate the avoidance action if you use the information provided by AIS properly. And it plays an important role in collision avoidance, especially in restricted visibility. And it is the same as “The International Regulation for Preventing Collision at Sea, 1972” towards to the radar information-Assumptions shall not be made on the basis of scanty information, especially scanty radar information. So the mariners and pilots shall not partly depend on the navigational aids, and they shall make flexible use of them to guarantee the navigation safety and avoid collision.
REFERENCES [1] IALA. GUIDELINES ON UNIVERSAL SHIPBORNE AUTOMATIC IDENTIFICATION SYSTEM (AIS) 2001.VERSION 1.0 [2] HUANG Jianwei.LU Yueming.LI Yibin. New tasks faced by VTS at age of AIS[J]. Journal of Shanghai Maritime University, 2007(3), 19-23. [3] IALA. Recommendation Automatic Identification System (AIS) Shore Station and Networking aspects relating to the AIS Service.2002. [4] ITU-R Radio communication Study Group 8. Technical Characteristics for a Universal Shipborne Automatic Identification System Using Time Division Multiple Access in the VHF Maritime Mobile band[R]. Draft Revision of RECOMMENDATION ITU-R M 1371, 2001. [5] LU Yueming.MEI Yingqun.LI Yibin. The Problems that we should put more attention when AIS is installed setup and used[J]. Marine Technology, 2010(3).11-13. [6] LU Yueming. ZHOU Yizong. Some Issues and Considerations Related To the Carriage and Use of AIS Equipment on Board Chinese Ships[C]. Proceedings of Asia Navigation Conference 2008.198-207. [7] ZHU Jinfa, et. a1.The Manual of Automatic Identification System[M]. Beijing: China Communications Press 2005. [8] GUIDELINES FOR THE ONBOARD OPERATIONAL USE OF SHIPBORNE AUTOMATIC IDENTIFICATION SYSTEMS (AIS).IMO Resolution A.917 (22). Adopted on 29 November 2001
AUTHORS 1Yueming
LU(1963- ), male, Han, He obtained the Master
2Yidong
Xue(1966- ), male, Han, He obtained the Master degree
degree from Shanghai Maritime University. He is a senior pilot
from Shanghai Maritime University. He is a senior pilot working
working Shanghai pilot station. He engaged in the pilot work
Shanghai pilot station. He engaged in the pilot work more than
more than 25 years. He is researching Ship pilotage, the
25 years. He is researching Ship pilotage, safty and Management
exchange of information between captain and pilots, navigation
pilotage.
equipment, etc. Contributed more than thirty articles to various publications, such as Navigation of China, Journal of Shanghai
Email: xyd2009@hotmail.com.
Maritime University, China Harbor Engineering, Marine Technology, etc. Email: pilotyue@gmail.com.
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