An apparatus, comprising: hardware processing circuitry; one or more hardware memories storing instructions that when executed configure the hardware processing circuitry to perform operations comprising: obtaining, for each of a plurality of geographic regions, a corresponding plurality of first expected phase differences of a first waveform of a first frequency transmitted from the respective region by a first wireless device to a pair of receive elements (230e, 230f, 230g, 230h) of a second wireless device; obtaining measured phase differences of a first waveform transmitted by the first wireless device to the pair of receive elements of a second wireless device; determining which of the plurality of first expected phase differences match the measured phase differences; obtaining, for each of the plurality of geographic regions, a corresponding plurality of second expected phase differences of a waveform of a second frequency, the waveform transmitted from the respective region by the first wireless device to a pair of receive elements of the second wireless device; obtaining measured second phase differences of a second waveform having the second frequency, the second waveform transmitted by the first wireless device to a pair of receive elements of the second wireless device; second determining which of the second expected phase differences of the plurality of geographic regions match the measured second phase differences; conditionally marking each of the plurality of geographic regions as a possible location of the first wireless device if the respective geographic region's expected phase differences are equivalent to the measured phase differences; and estimating a location of the first wireless device based on the marked geographic regions and the second determining. hardware processing circuitry; one or more hardware memories storing instructions that when executed configure the hardware processing circuitry to perform operations comprising: obtaining, for each of a plurality of geographic regions, a corresponding plurality of first expected phase differences of a first waveform of a first frequency transmitted from the respective region by a first wireless device to a pair of receive elements (230e, 230f, 230g, 230h) of a second wireless device; obtaining measured phase differences of a first waveform transmitted by the first wireless device to the pair of receive elements of a second wireless device; determining which of the plurality of first expected phase differences match the measured phase differences; obtaining, for each of the plurality of geographic regions, a corresponding plurality of second expected phase differences of a waveform of a second frequency, the waveform transmitted from the respective region by the first wireless device to a pair of receive elements of the second wireless device; obtaining measured second phase differences of a second waveform having the second frequency, the second waveform transmitted by the first wireless device to a pair of receive elements of the second wireless device; second determining which of the second expected phase differences of the plurality of geographic regions match the measured second phase differences; conditionally marking each of the plurality of geographic regions as a possible location of the first wireless device if the respective geographic region's expected phase differences are equivalent to the measured phase differences; and estimating a location of the first wireless device based on the marked geographic regions and the second determining. obtaining, for each of a plurality of geographic regions, a corresponding plurality of first expected phase differences of a first waveform of a first frequency transmitted from the respective region by a first wireless device to a pair of receive elements (230e, 230f, 230g, 230h) of a second wireless device; obtaining measured phase differences of a first waveform transmitted by the first wireless device to the pair of receive elements of a second wireless device; determining which of the plurality of first expected phase differences match the measured phase differences; obtaining, for each of the plurality of geographic regions, a corresponding plurality of second expected phase differences of a waveform of a second frequency, the waveform transmitted from the respective region by the first wireless device to a pair of receive elements of the second wireless device; obtaining measured second phase differences of a second waveform having the second frequency, the second waveform transmitted by the first wireless device to a pair of receive elements of the second wireless device; second determining which of the second expected phase differences of the plurality of geographic regions match the measured second phase differences; conditionally marking each of the plurality of geographic regions as a possible location of the first wireless device if the respective geographic region's expected phase differences are equivalent to the measured phase differences; and estimating a location of the first wireless device based on the marked geographic regions and the second determining.