The prototype of an underwater navigation beacon was dispensed with without batteries

At the top - the path of the sound signal in water, at the bottom-a comparison of the code of zeros-ones in shallow water and in deep water / Ghaffarivardavagh et al. / HotNets ’20

Engineers have developed an underwater navigation beacon that does not require power supply. It is based on a transponder that responds to sonar signals, and simultaneously feeds on the energy of its sound waves. The system calculates the distance to the transponder based on the delay time of the signal and its content, and to overcome the echo, signals in multiple frequency ranges are integrated. The article is published in the collection of conference materials HotNets ’20: the 19th ACM Workshop on Hot Topics in Networks.

The water is opaque to short and medium-range radio waves, and therefore GPS navigation does not work underwater. A few years ago it was reported about the development of acoustic GPS for American submarines, but it is unclear whether it will be available to civilians. In addition, it does not imply the possibility of creating an analog of a GPS tracker-a small, lightweight device that transmits data about its position to the operator. However, ichthyologists really need an analog of such a device for the aquatic environment, since it will allow you to track the movements of individual fish. In addition, a convenient navigation tool will be useful for underwater robots.

As an alternative solution, over the past few years, a group of engineers led by Reza Ghaffarivardavagh from the Massachusetts Institute of technology has been developing an echolocation beacon-a transponder that responds to sonar signals from a navigation station. The station listens to his answers, and based on their delay and the speed of sound in the water, calculates the distance to the object. The disadvantage of such a system is that the transponder requires a large battery for long operation, which, again, limits mobility.

Last year, developers tested a transponder that doesn't require a battery, but instead uses a piezoelectric the generator converts sonar sound into electricity. However, the test sample was not navigable by itself, because before issuing an answer, it had to accumulate electricity for a time that varies depending on environmental conditions (within a hundred milliseconds). In addition, the sound is repeatedly reflected from the bottom and from the surface of the water, creating an echo, and therefore, when the transponder finally responds, it will be impossible to tell whether it has responded to a direct signal or to a reflected one.

Now the same research team has tested a non-volatile transponder and developed a prototype of an underwater navigation system based on it. The high accuracy of this system is ensured by the accumulation and analysis of a large amount of data. At first, the tracking station emits a pulse at a certain frequency for a few seconds, which is modulated by a conditional code. Then, when the transponder accumulates power, its response will depend on where it started reading the code. After recognizing this location, the station will calculate the delay in collecting energy, and make a correction for it.

But the exact position of the transponder after this is still unknown, because the signal is interfered with by an echo. Therefore, after receiving the first response, the station changes frequency, and the cycle repeats. Having accumulated data in a wide frequency range, the system calculates the true distance to the device based on a large array of data. When the echo is too strong, such as in shallow water, after receiving the first signal, the station sends a code with extended character gaps. This increases the time to update the position, but gives you an exact win.

The system has successfully demonstrated its operability during tests in the river at a distance of several tens of meters. The operating frequency varied in the range from 7.5 to 15 kilohertz in 75 Hertz increments, each pulse lasting six seconds, which was enough to position slow-moving objects with an accuracy of ten centimeters. However, before using these developments to create a full-fledged underwater tracker comparable to GPS in accuracy, a number of problems will have to be solved, including learning to calculate not only the distance to the transponder, but also its three-dimensional position in the water column. In addition, it is not known what problems engineers will face when trying to scale the system to work at multi-kilometer distances.

In case of successful development, underwater transponders will greatly help ichthyologists, because on land and in the air, zoologists have long used GPS trackers. For example, they were able to find out how false vampires survive the dry period, and also that Mediterranean gulls have adapted to humans and prefer to live in economic areas.

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