Eagle files for ultrasonic transmitter and receiver boards now in Github (https://github.com/jtrezzo/acoustic-location-system). See Hardware folder. Carey has corrected the wiring error on the original version of the receiver - see “40khz_ultrasonic_receiver_0.1.brd”.
I am still using the hand wired receiver for my testing, along with the three transmitters based on the Eagle files.
A quick update on our approach: Project is going well. I spent a few days with Frikk when he visited SF/OpenROV HQ and we worked to simplify the approach used in a paper on time difference of arrival for uw sensors (see reference). A key gain is that no time sync is needed with the transmitter as with ToA. If we use three separate frequencies (seems to work so far) then we can transmit all three at the same time (independent channels if separation is OK) and filter things out to get TDoA. Each individual transmitter is identified by it’s frequency and we know it’s location. Since we have the depth sensor on the ROV, it becomes a 2D problem followed by a 3D projection using the depth data.
Another simplification is to have the three transmitters in a fixed orientation (two sides of a
vessel plus the stern). Location relative to the transmitters would be calculated and if GPS/heading of one of the transmitters are known, location can be transformed to lat/long.
We are developing the algorithms in Matlab and will convert to C code that can run on the ARM Cortex M4 (Teensy 3.1 at the moment). We think the critical DSP filters which separate the frequencies and give us TDoA data will be able to run on the ARM micro-controller during the time between transmissions (every second or so), but this needs to be verified.
I have collected some data signals in the OpenROV test tank that we will run through our algorithms and software to verify. Once working I will test with more realistic deployments at greater distances.
I will give a more detailed update (possibly a paper) on this once we get the math and DSP calculations worked out. For reference look at this paper (Silent Positioning in UnderwaterAcoustic Sensor Networks - IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 57, NO. 3, MAY 2008). It lays out a much more complex scenario that we were able to simplify for our purposes.
Note that I will be away on holiday in Argentina until the end of October - my wife and I are celebrating our anniversary. So limited e-mail for a while.