1. A method for passive approximate localization using broadcast waves and software-defined radio, the method comprises the steps of: (A) providing a plurality of broadcasting transmitters, wherein the broadcasting transmitters are distributed throughout a region of interest; (B) providing a software-defined radio, wherein the software-defined radio is located within the region of interest; (C) dividing the region of interest into a plurality of geo-hashes, wherein each geo-hash is associated to a simulated power spectrum; (D) sensing an observed power spectrum at an unknown location with the software-defined radio, wherein the unknown location is within the region of interest; (E) executing a peak-finding process with the software-defined radio in order to identify dominant frequencies within the observed power spectrum; (F) executing a filtering process with the software-defined radio in order to identify proximal geo-hashes from the plurality of geo-hashes, wherein the simulated power spectrum for each proximal geo-hash shares similar dominant frequencies with the observed power spectrum; and (G) executing a localization process with the software-defined radio in order to identify the unknown location as a closest geo-hash from the proximal geo-hashes, wherein the simulated power spectrum for the closest geo-hash shares similar power values at the dominant frequencies of the observed power spectrum.