At least three sensors are arranged in {HEAD ~ a substantially regular manner} essentially along {TAIL ~ a closed curve} 
At least three sensors are arranged in { HEAD ~ a substantially regular manner } essentially along { TAIL ~ a closed curve }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL