Go do a search for RDF  (Radio direction finding)  or in Ham radio we call it
fox hunting.. This is a much broader scale than you are looking for, but it
may give you some good ideas as to where to start if you are designing your
own. In fox hunting you have a reciever with 4 antennas and swithing circuit to
switch between the antenna,s giving you a signal from each..

You may be able to do the same with the ultra sonic sensors using a time
variable to determine the distance. Just a thought

Eddie turner, kc4awz

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