We made the world's smallest sonar modem

Published on November 06, 2018

We made the world's smallest sonar modem

    Greetings to you, our dear habread reader!


    “So the Spirits have diminished, and the palace
    Holds innumerable swarms
    Spacious.”
    (C) John Milton, Paradise Lost

    Adherents of the dark kagala of underwater communications and navigation work day and night to bring this underwater communications and navigation to the masses, and today one of them, as promised, will report on the work done and the progress achieved.



    So, it seems to me this time we have reached the physical limit and most likely it is impossible to make a device for transmitting digital data by sound through the water at a distance of 1000 meters any noticeably less.

    Who was excited by the topic - forward, to the hydroacoustic abundance, under the cat!

    Traditionally realizing that the topic of hydroacoustic communication and related problems is far from familiar to everyone, I propose a set of ours (and not only)


    I will briefly mention that here we have 95% of the ocean remains unexplored, and the difficulty is not only that we can no longer live under water, but also that radio waves do not propagate under water (however adequate frequency), and accordingly all what is possible on the earth and in the sky using radio communication becomes almost impossible even under a thin layer of water, and yes, it is still mostly dark and dim.

    Humanity is trying to get out, and today the only more or less widespread alternative to radio communications under water is acoustics, or rather hydroacoustics. With the help of sound waves, which by the way, spread in the water five times faster than in the air:


    And they simply transmit digital data, while often nowadays it is good practice to combine data transmission with navigation.

    Devices with the help of which digital data is transmitted through water is called (oddly enough) hydroacoustic modem.

    Transmitting sound through water is difficult, but you can


    To understand why this is difficult, imagine that you are looking through a babbling brook at a newspaper lying on the bottom and are trying to read it. All that you can make out through the running ripples and glare - the title. Strongly straining your eyes, you may be able to sort out (even, rather guess) the text of the subtitles, but you will understand what is written in the columns with great luck, when the ripples are momentarily smoothed and a thin layer of laminar water flows between the eye and the text.

    Sound in water propagates nonlinearly, undergoing bending (refraction), reflection, addition and subtraction of reflected copies of a signal (reverberation and multipath propagation), Doppler shift as well as stretching and compression of the spectrum.

    Following our analogy, the larger the heading - the easier it is to read, that is, the lower the transmission speed - the more reliable the connection is, in general, the “guess” that is written in a smaller text can be compared with noise-resistant coding (this takes time and additional information) , and read a very small text can only be in good conditions.

    People close to radio communication will rightly notice that all the difficulties listed are inherent in varying degrees and radio channels, but the main differences are that in hydroacoustics for data transmission, a band is available somewhere from 5 to 100 kHz with a very big stretch ( in reality, it is noticeably less), and control to the head: if (plus or minus) to transmit data in the 5-15 kHz band, having worked hard for 10–20 km, then in the 80–100 kHz band, it is possible to reach a range of 1 km with a fair fractions of luck, which is associated with a strong non-uniformity of attenuation I have sound waves of different frequencies.

    Well, yes, the speed of sound is a little less than the speed of light - only 200,000 times.

    To illustrate the capabilities of modern sonar modems, here is a tablet from the dissertation(page 54 to PDF) Bridget Benson, very popular with trying in vain to make an on-the-knee acoustic modem. The label is somewhat outdated, because this work dates back to 2010, and, unfortunately, I cannot correct it based on rumors, so I bring it intact:

    Manufacturer Modem Band, kHz Power consumption (transmission), W Range, km
    Aquatec AQUAModem 8-16 20 ten
    DSPComm Aquacomm 16-30 varied 3
    Tritech Micronmodem 20-24 7.92 0.5
    WHOI Micromodem 25 50 1-10
    Benthos ATM885 16-21 28-84 2-6
    EvoLogics S2CM48 / 78 48-78 2.5-80 one
    Linkquest UWM2000H NS 1.5 0.8
    UCSD UCSDModem 40 1.3-7.0 0.4


    Manufacturer Modem Power consumption (reception), W Modulation Baud rate, bps Price, $
    Aquatec AQUAModem 0.6 DSSS 300-2000 7600
    DSPComm Aquacomm varied DSSS / OFDM 480 6600
    Tritech Micronmodem 0.72 DSSS 40 3500
    WHOI Micromodem 0.23 / 2 FSK / PSK 80/5400 8100-9400
    Benthos ATM885 0.7 FSK / PSK 140-15360 7200-11000
    EvoLogics S2CM48 / 78 0.5 S2c 15,000 12500
    Linkquest UWM2000H NS Proprietary 9600 7,000
    UCSD UCSDModem 0.42 FSK 200 350 *
    * Component cost estimate only

    What has changed since 2010? Benthos was bought by Teledyne, Aquatec no longer does an acoustic modem (now it does only optically at a distance of 1 meter), and EvoLogics modems, according to rumors that have reached me, have dropped in price significantly. But the most global change from my point of view happened quite recently: hydroacoustic modems appeared on alibab-e. Priced around $ 1000- $ 2000 each. In China, there is technology, and most likely it has moved into the category of civilians. The gentlemen from the tablet accept new members to their ranks.
    But while slow Chinese manufacturers have just released devices priced at $ 1,000- $ 2,000 apiece, experienced DSPComm with modems from $ 6,600 have long written on their website that their goal is modems cheaper than $ 1,000.

    People try to make a cheap sonar modem.


    And the key word here is “cheap” - you just need to score in Google “Low cost underwater acoustic modem”. If we drop the pile of theses, suspiciously closely repeating the thesis of B. Benson, there will still be a sufficient number of interesting projects. An interesting generalization in my opinion was made by the Spanish comrades. In many ways, very honest, without any special attempts to embellish something.

    But back to the topic of the article.


    The mentioned Chinese products scare with their thoroughness: F120x600, weight 12 kg, 40 watts of the transmitter push data with speeds from 140 to 1200 bits / s at a distance of up to 20 kilometers. With regards to the transmission rate - I recommend always to look at the lower limit of the range.

    The smallest (and cheapest) modem from those shown in the tablet - MicronModem from the English spin-off of the New Castle University company has dimensions of F56x79 mm, dry weight is 235 grams and according to the manufacturer can transfer user data at 40 bit / s to distance up to 500 meters.

    Not bad, but in 2016 we released our RedLINEwhich, with dimensions of 64x62 mm and a dry weight of 360 grams, can transmit digital data at a speed of 80 bit / s over a distance of up to 8000 meters, while at the same time costing much less than a competitor, and even having a relay function. In many ways, so tightly packaged "spirits in the palace" was made thanks to the efforts of StDmitriev , and a little mine. Our idea then took shape in a whole patent .

    Running back


    Let us ask ourselves, what does a modem consist of? As a rule, from a transceiver antenna made of one or several piezoceramic rings, a power amplifier, a preamplifier and filters, as well as some kind of digital signal processor. It seems not rocket science? But after all, in a rocket, the cost of all materials and aggregates is only less than 10% of the cost of the rocket itself.

    While technology has not gained mass status - it remains expensive.

    And we want to buy a hydroacoustic modem was no more difficult than to buy an Arduino, GPS or radio module.

    What are we doing for this? Continuing to exploit our patent StDmitrievthis time, “pushed the spirits” into the even smaller chamber, and I, respectively, wrote down a new firmware that could take full advantage of the resulting record hardware.

    Meet this uWAVE - the smallest and cheapest sonar modem in the world.



    Now the spirits are cramped in a cylinder with the size of F40x45 millimeters, the weight of which does not exceed 160 grams. Hissing spirits allows you to transfer data at a speed of 78 bps over a distance of up to 1000 meters. And the modem itself can be powered even from USB.

    The following photo size comparison of RedLINE (yellow) and uWAVE (red):





    But this is not all: the modem supports not only the transmission of so-called. transparent channel with code division of subscribers, but also a command mode in which you can remotely request the depth, temperature (there is a built-in sensor on board) and the power supply of the remote subscriber with measurement of the propagation time of the signal. And for telecontrol tasks, 9 user code commands are provided.

    But even this is not the main thing, but the main thing is that we managed to beat our own price record and the retail price for the uWAVE modem is $ 480. This is 6 (SIX, KARL!) Times cheaper than the nearest English equivalent, with twice the range, and two times smaller. We neatly hope that with an increase in production volumes, the price can be further reduced.

    As you can see, we met the expectations of 7.6% of those who voted in the previous article , and this price range ranks second in the vote after the still unattainable $ 100- $ 200.

    If someone can now less and cheaper with the same characteristics - I give my word: I leave the profession right away!

    When I said that we had reached the bottom of the physical limit in size, it was meant that even if we could fit all the electronics in a smaller volume, the size of the ring used would not allow to work effectively in this frequency band, and in energy it would not work specified characteristics. The smaller the piezoelectric element, the worse it radiates low frequencies, the higher the frequencies, the smaller the communication distance. So it goes.

    And why is all this necessary?


    “Didn’t billions of unknown bones in unknown graves called to us, demanded or reproached? I see billions of past human lives, in which, like sand between fingers, instantly flowed away youth, beauty and the joys of life - they demand to solve the great mystery of time, to join the fight with it! The victory over space is the victory over time - that is why I am sure that I was right and in the greatness of the planned business! ”(C) I. Efremov, Andromeda Nebula


    “First of all, this is simply beautiful ...” (C)

    Seriously, recently the topic of communication with a flock of autonomous vehicles, the underwater Internet of things, and other incredible things has been particularly in demand. Here, colleagues make autonomous vehicles (gliders) of a very different size, on which to place a product of Chinese industry weighing 12 kilograms is not that problematic, but a little impossible, and it is especially hard to feed it with the forces of this device, for example, take a look at this kid

    image

    On yummy


    I want to tell how we repeated the experience from the previous article , where we determined the geographical location of one of the RedGTR modems using another similar modem, GPS-module and Chinese radio bait boat.

    I quickly rewrote the application so that it could work with uWAVE modems, and the onboard firmware of the boat did not even have to be changed. The interface protocol with modems made even easier.

    Due to lack of time, we did not go to our beloved Pichugu , but were content with a backup option - the South pond , a familiar reservoir, with a depth of 2 to 1.5 meters.

    This time the “lighthouse” was located on a raft of solid thermal insulation, and was powered by Power Bank, as can be seen in the photo:



    Loading the raft with the lighthouse onto the boat:



    Working moment:


    In general, everything worked as expected and as a result we got a couple of tracks with accuracy definitions better than 1 meter. Like this:



    And like this:



    The photo above



    shows an enlarged part of the track, and the whole one means the length of the reservoir in the following photo: The green track shows the measurements - points where the distance to the respondent was measured, and where it lags behind the blue (boat track) the acoustic connection was broken. The reason for this is quite simple and banal - the hydrological conditions are such that hardly anyone other than urban madmenwe would have thought to dip such equipment into such a reservoir and try to transfer data there.

    In both cases, we got a deviation from the real position of the lighthouse in the region of 70-90 centimeters.

    This is how it looks on the console application screen: The



    modem on the boat is powered by a lead battery, its voltage as well as the depth and propagation time are displayed on the upper left, the modem - the "respondent" was powered by Power bank, its voltage is expected 5 Volt, and the depth is 80 see the water temperature according to the requesting 14.7 ° C, and according to the respondent 13.8 ° C.

    From the prospects associated with this branch of devices


    • we plan to make a direction finding antenna for working with uWAVE;
    • The firmware for the long-base system is already ready, in which the uWAVE modem will act as a pinger, and its location can be determined with the help of four small buoys. As soon as we get rid of the current tasks, we will develop the enclosures and boards.

    Finally


    Thank you for your attention, as always, reasonable criticism and suggestions are welcome!

    PS


    And he is so cool, small =)

    We got real pleasure in the process of developing our very modem itself, this is one of those moments for which we work.

    Pps


    The success of the company noted tea drinking, not getting out of the kayak.