JP3571670B2 - Additional equipment for automatic ship identification system - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a ship automatic identification system (Universal ship-borne Automatic Identification System: hereinafter referred to as AIS) specified in "International Convention for the Safety of Life at Sea (SOLAS Convention)", and particularly to a standard AIS. The present invention relates to two types of additional devices that are desirably added to the device.
[0002]
[Prior art]
AIS stipulated in the SOLAS convention is that a plurality of ships in a common sea area autonomously transmit their own ship's position information based on GPS position signals, and information such as course and speed obtained from various instruments, In addition, the marine radio communication system aims at recognizing the presence and the positional relationship of a ship around the own ship by receiving such information of another ship. However, there is no interactive communication between the ships.
[0003]
The AIS is assigned two frequency channels in the VHF band, but sometimes dozens of ships in the same sea area share only two channels. Inevitable. However, in order to reduce the chance of collision as much as possible and to make effective use of radio waves, time distribution management by time division multiple access (TDMA) is performed.
[0004]
That is, in each channel, one minute is defined as one frame, and one frame is divided into 2250 time slots. Each ship selects an empty slot in one of the channels (referred to as "slot reservation") and transmits information on the ship. The time slots used by each ship are synchronized based on the GPS time signal.
[0005]
Even if the above-mentioned management is performed, transmission collision from each ship actually occurs. Communication collisions under TDMA occur in such a way that multiple ships accidentally reserve the same time slot. This is called "slot collision". When a slot collision occurs, a vessel attempting to receive signals from the plurality of vessels cannot receive the message content due to interference, and thus loses track of the position of another vessel.
[0006]
In order to avoid the occurrence of slot collision as much as possible, the recommended procedure for AIS is to (a) know the slot reservation status of the other ship from the received information, and select an empty slot that has not been reserved by another ship as the transmission slot of the own ship; (B) To make the slot selection random, (c) to make the reporting time short enough that the distance the vessel moves during the reporting does not matter, and (d) After a few frames even if a slot collision occurs. Specifies that a maximum reservation time (timeout) is provided so that the collision is resolved.
[0007]
In addition, since it is necessary that substantially all vessels in one sea area can recognize each other's positional relationship, the AIS provides a transmission output such that the transmission distance is about 30 NM (National Mile: 1 NM = 1.852 km). Recommended.
[0008]
However, even if the above recommended procedure is applied to the standard AIS, a malicious slot collision may occur in the case shown in FIG. That is, the two ships A and B, which are out of the transmission range of each other, cannot know the reserved slot of the other ship, and therefore may reserve an overlapping slot. In this case, the ship C, which is within the transmission range from both the ships A and B, cannot receive the information of both the ships A and B due to the slot collision.
[0009]
In addition, when many vessels are in the same sea area, even if the above recommended procedure is applied to the standard AIS, the probability of occurrence of slot collisions can be avoided because many vessels try to use slots. Absent.
[0010]
[Problems to be solved by the invention]
An object of the present invention is to reduce the number of slot collisions to an allowable range even when the sea area is crowded with a large number of ships, and to minimize the number of slot collisions in the situation shown in FIG. Is to be able to receive this information.
[0011]
[Means for Solving the Problems]
The above-described problem is caused by a first additional device including a power control calculation circuit (20) and a variable attenuator (21) indicated by a thick line in FIG. 3, and a sector receiving antenna (30, 31) indicated by a thick line in FIG. ) Are added to the standard AIS (10) respectively.
[0012]
In the first additional device, the power control calculation circuit sets an upper limit value of the slot map utilization rate corresponding to the allowable number of slot collisions, and sets the upper limit value of all the ships in the normal transmission range by the standard AIS. Variable attenuator control that calculates a transmission range that is as large as possible within a range not exceeding the upper limit of the slot map utilization rate based on the ship position information and the reporting interval information, and that provides transmission power corresponding to the transmission range. Output a signal. Further, the variable attenuator controls the transmission output of the TDMA transmitter based on a given control signal.
[0013]
The reason for employing the first additional device is as follows. As shown in FIG. 2, the average number of times of receiving the slot collision signal in one frame in one ship increases as the slot map utilization rate increases. Here, the slot map utilization rate is a ratio (%) of reserved slots per frame. When all vessels in the same sea area are transmitting to the same transmission range, the slot map utilization rate is almost proportional to the number of vessels in the transmission range. Assuming a uniform distribution of ships, the number of ships is proportional to the area of a circle whose radius is the transmission distance.
[0014]
Therefore, even when the sea area is crowded with a large number of ships, if all the ships reduce the transmission power and reduce the transmission distance at the same time, the slot map utilization rate decreases as a whole, and as a result, slot collisions occur. The probability of occurrence can be reduced. However, since it is desirable to maintain the transmission distance as long as the situation allows, an upper limit is set for the slot map utilization rate, and a transmission distance as long as possible within a range not exceeding the upper limit is adopted. Once the desired transmission range is determined in this way, the output of the transmitter may be controlled by a variable attenuator so that radio waves do not reach beyond the transmission range.
[0015]
Next, the second additional device is configured by replacing a standard AIS receiving antenna with a sector receiving antenna. This sector antenna can receive a signal for each sector obtained by dividing the entire circumference at an equal angle, and can select a specific sector by a built-in sector switch (not shown) and connect it to a receiver.
[0016]
The reason for employing the second additional device is as follows. Since slot collision is a phenomenon in which a signal arrives at one antenna from multiple directions at the same time, rather than indiscriminately receiving signals arriving from all directions, only signals arriving from a certain direction are preferentially received. By ignoring others, it is possible to avoid interference and receive information on at least one ship even during a slot collision.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
The following is conceivable as an embodiment relating to the first additional device.
One frame is divided into a first mode period and a second mode period, and during the first mode period, the vessels simultaneously transmit at normal transmission power. On the other hand, in order to keep the number of times of slot collisions below a certain limit, the upper limit of the slot map utilization rate is set for all ships. During the first mode, information from other ships is widely received by the standard AIS, and based on the obtained positional relationship between all ships including the own ship and data on the reporting time interval of each ship, The maximum transmission range from the own ship is calculated by calculation within the range not exceeding the upper limit of the slot map utilization rate. Next, during the second mode period, the information of the own ship is transmitted while controlling the transmission power so that the transmission range from the own ship becomes the value obtained above.
[0018]
The second mode period is a transmission power control mode period in which each ship performs transmission in an effective state in which slot collisions are suppressed as much as possible, whereas the first mode period is a preparation period for the second mode period. It can be said that this is a wide-area ship information acquisition mode period. Therefore, it is desirable to shorten the first mode period as much as possible and to allocate sufficient time to the second mode period.
[0019]
An embodiment relating to the second adding device will be described. For example, the additional device in which the number of sectors of the sector antenna is 6, and one sector faces at an angle of ± 30 ° with respect to the bow direction. Since an angle of plus or minus 30 ° with respect to the bow direction is most important for safety, it is desirable to cover this range with one sector.
[0020]
The second additional device configured as described above is operated under the following rules. That is, (a) when only one sector is receiving a signal, the signal of that sector is received. (B) Two or more sectors are receiving signals due to slot collision. If there is a difference in signal strength, the signal of the sector having the highest signal strength is selected and received. (C) When the signal intensities of two or more sectors are almost the same at the time of the slot collision, the forward (bow direction), starboard forward, port forward, starboard rear, port rear, rear (stern direction) of each sector The priorities are given in order, and the signal of the sector with the highest priority is selected and received.
[0021]
It is unnecessary to explain (a) in the above rule. Regarding (b), a large signal strength means that the other ship is nearby, and such a signal should be received with priority. With regard to (c), when another ship is approaching the ship in progress from the front or starboard ahead, the ship's ship must take an avoidance action to the starboard side. In particular, it is necessary to monitor the front and the starboard side.
[0022]
【Example】
The performance of the AIS to which the first and second additional devices were added was confirmed by computer simulation. The performance evaluation of the operation by the first additional device (hereinafter, referred to as “independent transmission power control method”) was performed based on the number of times of slot collision, the slot map utilization rate, and the blind movement rate. Here, the "blind movement rate" is the total distance (blind distance) traveled by the two ships during the time during which the position of the other ship could not be recognized (blind time) with respect to the distance between the two ships at the time of the slot collision. Percentage. The blind movement rate becomes larger as the distance between the two ships is shorter, indicating that the degree of risk is greater.
[0023]
Further, the performance evaluation of the operation by the second additional device (hereinafter, referred to as “sector antenna system”) was performed only by the blind movement rate. This is because the sector antenna method eliminates the slot collision itself, and cannot be evaluated based on the number of slot collisions. Furthermore, the evaluation when the autonomous transmission power control scheme and the sector antenna scheme were combined was also performed with the blind mobility.
[0024]
(1) Operational Performance of Voluntary Transmission Power Control Scheme FIG. 6 shows that the upper limit of the slot map utilization rate is set to 10%, the first mode period is set to (１ ／) frame (= 12 seconds), The time distribution state of the number of times of slot collision and the slot map utilization rate in a certain ship when the mode period is the remaining (4/5) frame (= 48 seconds) is shown. From this figure, it was confirmed that the slot map utilization rate could be substantially maintained at the set value of 10%, and the number of slot collisions was reduced. FIG. 4 is a conceptual diagram showing an image of the transmission range at this time.
[0025]
(2) Frequency distribution of slot collisions with respect to inter-ship distance when using the autonomous transmission power control scheme, sector antenna scheme, and both schemes Fig. 7 shows the relationship between the distance between the transmitting vessel and the receiving vessel when a slot collision occurs. The slot collision frequency distribution is shown for each system. As can be seen from this figure, a slot collision occurs at any distance if the two ships are within the transmission range. It can be seen that the autonomous transmission power control method avoids slot collisions because the transmitting side does not transmit to vessels at long distances.
[0026]
In the sector antenna system, the receiving side preferentially receives a signal with a high power level and ignores a signal at a long distance, so that a short-range slot collision can be considerably reduced. In addition, in the combination system, the characteristics of each system were utilized, and the slot collision could be reduced over the entire distance of the ship.
[0027]
【The invention's effect】
Even when the recommended procedure (protocol) is applied to the standard AIS, it is inevitable that a malicious slot collision will occur from the viewpoint of navigational safety, depending on the relative position conditions of each ship, but between ships that are close to each other Voluntary transmission power control method (by the first additional device of the present invention) that gives priority to the notification of a message, and a sector antenna method that receives a message from a nearby ship or a ship that is considered to be dangerous for navigation (the present invention). ), The number of slot collisions can be reduced without altering the standard system configuration and protocol of AIS, and at least one signal can be received without interference even when a slot collision occurs. , Safety can be ensured.
[0028]
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a situation in which a malicious slot collision occurs due to a position of a ship.
FIG. 2 is a graph showing the relationship between slot map utilization and slot collision.
FIG. 3 is a block diagram showing an example (1) of an additional system configuration of AIS.
FIG. 4 is a conceptual diagram showing an image of power control by the additional system shown in FIG.
FIG. 5 is a block diagram showing an example (2) of an additional system configuration of AIS.
FIG. 6 is a graph showing the lapse of time between the number of slot collisions (upper) and the slot map utilization rate (lower) of a certain vessel that has performed transmission power control.
FIG. 7 is a graph showing the relationship between the distance between ships and the number of slot collisions according to the conventional method and each method of the present invention.
[Explanation of symbols]
10 ... Standard AIS
Reference numeral 20: power control calculation circuit 21: variable attenuator 30, 31: sector receiving antenna

Claims (4)

A first additional device added to a standard automatic ship identification system (AIS), comprising a power control calculation circuit and a variable attenuator,
The power control calculation circuit sets an upper limit of the slot map utilization rate corresponding to the allowable number of slot collisions, position information of each ship obtained by standard AIS for all ships in a normal transmission range, and Based on data such as transmission time intervals, calculate a transmission range as large as possible within a range that does not exceed the upper limit of the slot map utilization rate, and a variable attenuator control signal that provides transmission power corresponding to the transmission range. Output,
The first additional device, wherein the variable attenuator controls an AIS transmitter output based on the control signal. A second additional device added to the standard AIS, comprising: a sector antenna capable of receiving a signal for each sector obtained by dividing the entire circumference at an equal angle; and a sector changeover switch built in the sector antenna.
The sector number of the sector antenna is 6, and one sector is oriented at an angle of ± 30 ° with respect to the bow direction,
When only one sector of the sector antenna is receiving a signal, the signal of the sector is received,
When two or more sectors are receiving signals due to slot collision, but there is a difference in signal strength, the signal of the sector with the highest signal strength is received,
When the signal strengths of two or more sectors are almost the same at the time of the slot collision, priority is given to each sector in the following order: forward (bow direction), starboard forward, port forward, starboard backward, port backward, backward (stern direction). And the second addition device selects and receives a signal of a sector having the highest priority. Dividing one frame into a first mode period and a second mode period,
In the first mode period, the AIS transmitter transmits its own information at normal transmission power, and the power control calculation circuit determines a transmission range as large as possible without exceeding the upper limit of the slot map utilization. Calculate,
The first additional device according to claim 1, wherein the AIS transmitter whose transmission power is controlled by the variable attenuator transmits information of the own ship during the second mode period. The first addition device according to claim 3, wherein the second mode period is set longer than the first mode period.

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