What is this?

How to use it?

Press the start button, and make sure that sound output is enabled on your computer. Don't use headphones. By default, the sonar emits sound at near-ultrasound frequencies, so you may not hear anything, but you should see the 'peak input level' value increase. You may have to adjust volume and microphone sensitivity to get good results. Currently only the left audio channel is used.

Warning: there seems to be a problem with some browsers (confirmed on Firefox for Android). Try using a different browser if the canvas stays blank after pressing the start button.

Moving your hand towards the speaker should produce something like this:

The range-doppler plot has two axes: bistatic range and bistatic velocity. Bistatic range of a target is:

(distance from the speaker to the target) + (distance from the target to the microphone) - (distance from the speaker to the microphone)

The range-doppler plot shows the intensity of echo received for each combination of bistatic range and velocity.

Cool things to try

• Move your hand around (not too fast) and see its echo appear.
• Compare the sonar's performance for constant-velocity targets and accelerating targets (spoiler: it doesn't work well for accelerating ones)
• Stand far away from the computer and move a flat object back and forth, facing the sonar.
• See what happens when a target moves so fast that it crosses the top or bottom edge of the range-doppler map.
• Move one hand forward towards the microphone, and the other hand backwards, and see if you can detect two echos at the same range, with different velocities.
• If you're using a laptop, try connecting it to an external speaker. This way you should have more control over the direction of the transmitter beam. First experiment with the speaker and microphone pointing in the same direction.
• Move away far enough that your echo will loop back to zero range on the range-doppler map.
• Point the speaker at the ceiling or a wall and try to detect an indirect echo (one that bounces off the ceiling and then off a moving object).
• Try using different objects as reflectors. Try to make a corner reflector.
• Make a small pendulum and see the echo that it generates when oscillating
• Experiment with bandwidth. More bandwidth gives better range resolution.
• Use the sonar in a big room or an open space. Compare with a small room. You should see fewer indirect echos in big rooms.
• Disable clutter filtering and audio delay compensation, connect a small speaker (or headphones) to the computer, and observe the direct-path signal between the speaker and the microphone while moving the speaker around.
• Set 'chirps per CPI' to 1 to effectively disable Doppler processing. The sonar now works in Moving Target Indication (MTI) mode. Use the two-pulse canceller for best results.
• Try to find the best possible parameters for the sonar. In general, you won't be able to find a set of settings that is satisfactory in all aspects. It's always a matter of compromise between range ambiguity, velocity ambiguity, and length of the coherent processing interval.

How does it work?

Soon there will be a blog post discussing this sonar in greater detail. You can also read about pulse-doppler radar in books like M. Richards, Fundamentals of Radar Signal Processing, or M. Skolnik, Introduction to Radar Systems.

The source code is available on Github