According to section 7.3.6.2.3 of the datasheet of PGA460-Q1 (SLASEC8B –FEBRUARY 2017–REVISED JANUARY 2019), the "zero" of the ultrasonic measurement result for the BURST/LISTEN command begins after the burst stage is complete (see the attached picture (Figure 32 of the datasheet)).
Why is this so? It seems a little odd to me, because the time when the echo signal rises above the time-varying threshold should be (ideally)
2 * (distance to the object)/(speed of sound) + time when the burst begins
The second term seems to be replaced by "time when the burst ends" in the model the actual implementation assumes. I'd like to know the rationale for this.
Thank you.