A system comprising a first interrogator device that includes: a first antenna configured to transmit, to a target device, a first radio-frequency (RF) signal having a first center frequency; a second antenna configured to receive, from the target device, a second RF signal having a second center frequency that is a harmonic of the first frequency; and first circuitry configured to obtain, using the first RF signal and the second RF signal, a first mixed RF signal indicative of a first distance between the first interrogator and the target device. The system further comprises at least one processor configured to determine the first distance based, at least in part, on the first mixed RF signal, and determine a location of the target device based, at least in part, on the determined first distance.