That'll be because you're thinking ultrasonics, whereas I was thinking magnetic fields. :)
The ultrasonic method should work but ultrasonic signals don't carry well. Their power is dissipated excedingly quickly over distance. An array of 3 antennae with a very short wavelength radio signal could possibly do phase comparison to get directionality, but it's no mean feet.
Yes difficult to get the information there.
figures I have seen comparing sonar.
active sonar range 1 km
passive sonar range 6 km
for same equipment.
ultrasonic tape measure range 12 m so if it all compares well I would expect an active transponder system to have a range of 50 m or so.
did op say what range he wanted
main problem I see is getting the transponders.
smallest I have are about 2 cm dia and the maths says they should be spaced under 1 wavelength apart
Have you started building the bat traps yet :)
needs work, keep getting this annoyed bloke in black underpants
Scary home work here
Chip that does it all here