a method is provided for tracking an object , including positioning a radio frequency driver to radiate an rf driving field toward the object , and fixing to the object a wireless transponder that includes a power coil and at least one sensor coil . the method also includes receiving the rf driving field using the power coil and storing electrical energy derived therefrom . a plurality of field generators are driven to generate electromagnetic fields at respective frequencies in a vicinity of the object that induce a voltage drop across the at least one sensor coil . a digital output signal is generated at the wireless transponder indicative of the voltage drop across the sensor coil , and the generation of the digital output signal is powered using the stored electrical energy . the digital output signal is transmitted from the wireless transponder using the power coil , and the transmission of the digital output signal is powered using the stored electrical energy . the digital output signal is received and processed to determine coordinates of the object .