allanhurst:
The coding could be as simple as Morse - made of short/long pulses. Say every transmission signal consists of (eg) 8 pulses of short or long - that allow you to address each of the outstations.
The general outline you've presented makes sense, but I think using pulse width ala Morse code is going to be problematic for several reasons.
One is that multipath echoes may overlap the direct path signal making it appear longer than the intended transmission. Second, if one uses the HC-SR04 electronics, it only detects the leading edge of the pulse and ignores what follows. Finally, the piezo transducers are resonant at the transmit frequency (~40 kHz for HC-SR04) so they have an exponentially decaying tail after the driving signal is stopped, thus there's not a clean transmission termination, even if one were operating in an anechoic environment.
Some variant of pulse position modulation is probably dictated by the physical constraints of the problem. My conclusion in the thread linked in the earlier post was that one could get something on the order of 100 bits per second out of an HC-SR04 based ultrasonic communication system so the position updates for the original poster's scenario are likely to be some small fraction of that.