Spatial interferometry is based on the kind of interferometry used for many years by radio astronomers. It uses the property known as coherence to obtain positional information from processing received signals. However, we can now take the method one stage further and can obtain range measurements.

The way this works is illustrated in the diagram shown above. We collect copies‘ of a radiated signal in at least three different places. We then compare these and determine the relative times of arrival. This tells us the differences in the path-lengths from each collecting point (port) and the signal source. Provided we also know the relative positions of the collecting points we can then use simple trigonometry to work out the direction to (the bearing) and the distance to (the range) of the source. If we use at least four ports, placed so that they are not all on the same straight line, we can make a 3D measurement of the source position.

Unlike radar, this method does not require us to beam power at the source. Instead we can rely upon its own emissions. The method can use both deliberate (unnatural) radio transmissions and the thermal radiation all natural objects emit simply becuase they have a temperature above absolute zero. Hence we can imagine using this method for a wide variety of applications, from locating transmitters on a battlefield to 3D medical thermal imaging.

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Acoustic Imaging and Interferometry.

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