So I understand that a typical ultrasonic range finder works as below:

My idea is to essentially take away the returning wave and basically have a one way signal that will track a moving object with the transmitting sensor.
The transmitter and receiver are independent of each other, however, the challenge is to overcome calibration. This can be resolved by having a fixed starting position for the transmitter. The transmitter simply needs to be powered and maintain a consistent ping interval, and since we know the starting position of the transmitter relative to the receivers when the transmitter moves we can figure out what direction it is moving because we can track the different delays to each receiver. I don't necessarily need to know the exact distance of the object, I just need to know the position relative to the starting point.
In theory this sounds like it could work, but does this sound like it is doable? Would anyone be able to tell me a good ultrasonic independent transmitter and receiver that could work with arduino?
Progress:I have found a good
independent ultrasonic transmitter and receiver that seems like it could fit my needs. If you look at some of the videos at the bottom of the page (primarily the one with the oscilloscope) you can see the 40Khz wave that is being received. Now I just need to figure out how to determine the distance if the transmitter is always on...
Does anyone have any ideas on how I can measure the distance of the transmitter to the receiver?
- Transmitter will have a fixed starting position x cm away from the receiver. x will be known
- The transmitter will be always on at 40Khz when it is moving, however thinking about it I may need to have it pulse - am I correct in thinking this? if so how should I go about achieving this?