A method of selectively waking up a plurality of RFID tags includes (a) providing a first RFID tag; (b) programming the first RFID tag to respond and wake up only to signals at a first sub-carrier frequency or with a first code; (c) providing a second RFID tag; and (d) programming the second RFID tag to respond and wake up only to signals at a second sub-carrier frequency or with a second code. A method of operating an RFID tag positioning system includes (a) positioning two real time location tracking active RFID readers in two respective locations; (b) positioning a first calibrating RFID tag at a fixed and known position; (c) moving a second calibrating RFID tag along a pre-determined path; (d) obtaining the first RFID reader's readings of location-related information of the RFID tags at a point in time; (e) obtaining the second RFID reader's readings of location-related information of the RFID tags at that point in time; and (f) comparing the readings with the known positions of the first and second calibrating tags.