Introduction In the article titled Converting your laptop into a JANUS modem using Unet audio, we saw how UnetStack, a Software-Defined Open Architecture Modem (SDOAM), can be used to turn your laptop into an acoustic modem. A true SDOAM can easily be extended to run on various hardware platforms with different types of physical layers (e.g. Acoustic, optical, RF) with minimal efforts to address different use cases. In this article, we will show how you can build a low-cost (< USD 300) , DIY underwater acoustic modem using only Commercial Off-The-Shelf (COTS) components and Unet audio. The goal of this article is to demonstrate the approach and basic steps one can use to build a low cost acoustic modem using Unet audio. We encourage the reader to build on these principles to build your own versions of the DIY modem.

All UnetStack enabled modems have a Real-time Clock (RTC) to keep track of modem’s system time. A separate microsecond counter is provided by phy.time for Physical agent in UnetStack.

UnetStack powered acoustic modems provide extreme flexibility to the user to automate processes such as the transmission of data frames (e.g. position updates) or signals, decision making after the reception, etc. enabling a hands-off approach to test various deployment scenarios. If you are using a program or a script to transmit and receive from your software-defined open architecture acoustic modem (SDOAM), it is often good to know your location (latitude, longitude, and depth), to make decisions such as when to transmit, what power level to use, etc. This is not a big problem if your acoustic modem is deployed in a fixed location. However, if the modem is installed in a mobile underwater asset like an autonomous underwater vehicle (AUV), such decisions are crucial.

The Unet simulator supports various ways of simulating the motion of the simulated nodes, from simple dynamics models to completely custom functions that can generate motion updates. Let’s look at how one can go about simulating the motion of nodes in the Unet Simulator.

Acoustic modems transmit physical sound waves via a transducer, typically a piezoelectric device. Such sound emitters have an ideal resonance frequency F and a Q factor of the order of 0.3. This means that the efficient region of the frequencies it can transmit centers around the resonance frequency with a bandwidth of about 0.3 x F. Q may be higher or lower depending on the exact transducer.