i was looking at omnidirectional ultrasonics - the only ones my short googling produced were very large and were for submerging!
So i guess i'll put more than one in / on the moving body, but these generally have a narrow beam (like 9 degrees) so i'll have to look for slightly wider beams, which are available, and see if I could mess with the casing or something.
They do have longer ranges than one metre, so if i were to widen their angles a bit, it shouldn't be a huge problem.
Right now the way i'm planning to go about this is to do it all relatively. I don't need measurements, but rather relative distances. With a bit of math I could obviously turn this into measurements, but that's not what i need.
The speed of the system can be slow / jerky at first, but once it's working, i'm sure it would be easy to modify it to smooth it out.
40khz and 20khz transducers are readily available for the equivalent of about 1 pound. There are others, but they are too large.
I did a search at ti.com and microchip using the keyword ultrasonics, and they both produced very similar pdf docs with application notes for integrating a transducer and two controllers. One, connected to the transducer, sends it a signal, and waits for an echo response, then sends a signal to the mcu, which processes the time taken, and gives the other controller a command to repeat the process.
Their system works on echoing, but if my transmitter object sends a continuous broken signal (beep, beep, beep, beep...), i could make comparisons using that and the time when each of the 3 or 4 (i like 4 - thanks) receivers receive the signal. What u think?