Thanks for your response. If the tag didn't need to be passive, what would be the lowest power option to achieve this? Such that the battery would last some time. Wifi, bluetooth, rfid?
Apart from perhaps Pozyx the other suggested methods cannot provide the resolution you need.
You would have trouble telling the difference between 1m and 10m with RF signal strength methods, even after calibraration.
This question comes up almost daily on here, if there was some cunning wizard and cheap way of doing this there would be gazillions of tutorials to be found on the Internet.