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 antenna must be spaced more than 10 lambdas (baseline) apart East to West and following the natural rotation of the earth. The signals from the antenna is then cross-correlated in a Summing Amplifier at the input of the main radio receiver. As a radio emitting object passes above it produces a "fringe pattern" in the receivers measured signal strength, as the wave front of the radio emitting object passes in and out phase between each antenna.
From the Hans Michlmayr - Amateur Radio Astronomy Website
This fringe pattern can then in theory 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.
The main limitation to the Amateur Radio Astronomer is not really the electronics but more the available space on your property. This is because in order to obtain a suitable fringe pattern the two antenna must be spaced at least 10 wave lengths (or 10 lambdas) apart. For example at 74Mhz VHF the distance between the to receiving antenna is more then 40 metres.