I intend to modify an existing depth transducer, replacing the piezo disc by a 3 MHz one. This may be the trickiest part.
The frequency generator can be programmed to send either a constant frequency pulse or a variable frequency ('chirp') one. This may be useful later on.
The STM32F4 board will control the frequency generator, and will receive and analyze the echo signals. The STM32F4 microprocessor has built-in DSP (digital signal processing) capabilities.
The total hardware cost (with 2 transducers) should be less than 500 US$. Not too bad for near-America Cup performance, as this could develop in an open software project.