Interferometry is based on wireless propagation
Wireless propagation in space is very useful for location tracking - RADAR is a good example. However, RADAR is, by definition, a two-way propagation paradigm, A to B to A, measuring propagation time at the speed of light. Since the speed of light is enormous, even the slightest timing errors are catastrophic.
Interferometry measures angles, not distances
• Interferometry is not a direct function of the speed of light, but rather of wavelength. In typical wireless applications such as Wi-Fi, the wavelength is 5-12cm, a very reasonable size to design access points for.
• Interferometry is by nature passive: B transmits, A receives. This has huge advantages as transmitter B is not modified in any way for tracking. A receives the signal from B devices, the existing infrastructure. A can operate on foreign B signals, and no consent from B is required, while B is never aware of A. Devices never interfere with each other, at any number.
• Interferometry is based on measuring the phase difference between two coherent wave fronts. In the famous slit experiment, two slits in an opaque plate let light from a single source propagate concurrently through both slits, interfering with one another in a very deterministic way, sensitive to the angle between the direction perpendicular to the plate and the line of sight from the slits to the observer. Given the spacing between the slits, in wavelength units, the measured phase difference depends on the Angle of Departure. The very same phenomenon applies to light signals and wireless signals. When using coherent wireless transmitters such as access points, the slits are replaced by antenna elements, the coherence is achieved by generating the signal in a single chip in the AP, driving all antenna elements in the array.