Underwater Positioning × SDGs
Fish in the ocean often migrate in schools of the same species. We need to know where and what species and sizes of fish are swimming. If we know these, we will be able to fish more efficiently and manage resources better. If we can accurately detect fish schools using ultrasonic waves, we can avoid excessive fishing. I would like to contribute to the SDGs by protecting the abundance of ocean and making it possible to supply food sustainably.
The United Nations has designated the decade starting in 2021 as the "Decade of Ocean Science," with the aim of contributing to SDGs. Ocean science, as defined by the UN, includes the field of fisheries.
Identifying the position of an object in the water
Identification of “position” is one of the important parameters in oceanographic observations.
For example, information on when and where the target organism has been is essential for understanding its movement and behavior.
In addition, when we observe under the water from a ship with observation equipment, it is important for the equipment to know where the observation target is, even if it is not a direct observation target.
Although GPS can obtain location information on land or the water surface, it cannot be used for underwater positioning because radio waves do not reach far underwater. Therefore, sound waves are used as a means to identify the position of an object in the water.
What information do I need to identify the position?
How can I find out where I am while looking at the map? Perhaps the location is estimated by relying on the landmarks (objects) that are easy to understand on the map.
Information needed to identify the positional relationship between yourself and the landmark includes ① the distance to the landmark and ② the direction of the landmark.
You can identify your position by combining these. This also applies to underwater.
A transponder is an acoustic equipment used for underwater positioning. When the transponder receives a specific acoustic signal (interrogation signal), it sends a response signal.
Let c be the speed of sound, and t be the time from the transmission of the interrogation signal until the return of the response signal, then the distance r to the transponder can be calculated as .
Positioning using a transponder
If you think of a transponder as a landmark, the transmission of an interrogation signal from an object whose position you would like to identify (for example, observation equipment to observe underwater) allows you to determine the distance between them. In reality, the position can be identified by installing transponders at three locations and measuring the distance to each transponder.
A circle can be drawn if the distance to one location is known. In other words, if you know that the object is located somewhere on this circle. If the distance to another location is known, then another circle can be drawn, and you know that the object is located at one of the two intersections of the circles. Furthermore, if the distance to another location is known, there will be one intersection of the three circles, allowing the identification of the position of the object.
Identifying the direction from which sound comes
Identifying the direction from which sound comes involves the use of an apparatus (receiver) that receives multiple sounds. When two receivers are installed as shown in the figure, the time for each receiver to receive a sound differs depending on the direction from which the sound comes. This difference in the time （t2 - t1） and direction of the sound arrival （θ） can be expressed as using the installation interval （d） between receiver ① and receiver ②.
Since there are two values of θ that satisfy this condition, the direction of sound arrival can be determined by installing two additional receivers and measuring the difference in time.
Positioning using direction and distance
The positional relationship between two points is known if the mutual distance and direction of two points are identified simultaneously. A positioning method utilizing this relationship has also been used.
When a transponder is used, the position of the transponder can be identified by receiving its response signal with multiple receivers (receiver arrays) and calculating the direction of the sound arrival.