Well the basic idea for the thing i am trying to build is to keep track of the distance a moving object is from the point of origin. From approx 18m to lets say around 3m
The easy way is to buy a $150 laser range finder as used by golfers. (These have a minimum range of 5m.)
The more interesting way....
Vehicle has a wireless "camera remote shutter control", Arduino, microphone and 40kHz band pass filter.
The origin has the corresponding wireless receiver, another Arduino to generate a 40kHz square wave bust, a cheap 100W public address amplifier turned up full, and a 50W piezo tweeter.
Fit ear plugs, (the tweeter will probably make loud clicking sounds.)
Trigger a pulse from the vehicle and measure the time for the sound to be received.
Calibrate for 3 and 18m.
Reduce the transmit power until it just works at 18m and remove ear plugs.