Radio interferometry is a powerful tool that can be used for a number of diverse applications. A radio interferometer consists of more than one antenna tuned to receive radio emissions from the desired frequency. The signals from the receivers are then cross-correlated to produce a "fringe pattern". This fringe pattern can then be analysed to produce a result ranging from an image of a distant astronomical object to the location of a nearby terrestrial or extra-terrestrial radio emitter.
Links:
Fringe Dwellers - Simple Radio Interferometry
History of astronomical interferometry
