A sonar receiver detects sound.
There are 2 things that make it a 'SONAR' receiver:
1). The part that gives it the ability to recognize one very specific sound, and ignore all other sounds.
2). The part that's able to very accurately and precisely measure the amount of TIME that passes, between when it's told to start its clock, and when it hears the special sound that it recognizes.
Here's how the SONAR system works:
-- A powerful sound generator sends out a very short burst of very loud sound, focused in one direction. At the same time, it tell the SONAR receiver to "START YOUR CLOCK".
-- If there's anything big and solid out in that direction from the sound generator, then part of the sound energy that hits it, bounces back (reflects) in the direction it came from, and eventually gets detected back by the SONAR receiver.
-- When the SONAR receiver hears the special sound, it stops its clock, and reads the amount of time that passed since the sound was sent out.
That's exactly the amount of time it took the sound to travel out, bounce, and travel back. That's DOUBLE the time it took to reach whatever it reflected from. If you know the speed of the sound, it's easy to calculate the distance to whatever it was that bounced some of the sound back to you.
