Radio interferometry is a powerful tool that can be used for a number of diverse applications. A radio interferometer consists of a pair of directional antennas that are tuned to receive radio emissions from a source in a desired RF band. The signals from the two receivers are then cross-correlated (multiplied and accumulated) to produce a cross-correlation "fringe pattern". This fringe pattern can then be analyzed to produce a result ranging from an image of a distant astronomical object to the precise location of a nearby terrestrial or extra-terrestrial radio emitter.

The following diagram shows an arrangement consisting of three steerable antennas, forming three distinct interferometers.

The antennas are pointed at the radio source of interest and are set up to receive the signal and process it into a form suitable for transmission to the correlator. If the antennas are in close proximity to each other (connected element interferometry), the signals are sent to the correlator and processed immediately. If the antennas are separated by long distances (Very Long Baseline Interferometry -- VLBI), then the signals are recorded on magnetic tape and the tapes are shipped to the correlator for processing at a convenient time.

With the arrangement shown it is possible to do the following:

• Produce a radio image of an astronomical object using a technique called aperture synthesis.
• Precisely determine the relative position of the antennas if the radio emission is distant and stable (geodesy).
• Determine the cross-power spectra of the radio emission. This can provide critical information on the fundamental makeup and velocity of the radio source.
• Precisely determine the position of a "nearby" ground or space radio source if the positions of the antennas are known.

Modern radio interferometers have the following critical signal processing elements:

• Steerable receiving antennas, often microwave "dish" antennas with strong directional capabilities.
• RF receiver, stable local oscillator, and baseband converter. This circuitry is necessary to receive the signal and down-convert it to a baseband frequency for further processing. For VLBI, all oscillators on each antenna system are locked to a highly stable hydrogen maser "clock".
• High speed digital sampler. The analog signal is digitally sampled to simplify processing by the correlator.
• Transmission system to the correlator. In VLBI, the data is recorded on magnetic tape along with very accurate time information and then shipped to a correlation center.
• High speed digital correlator. The correlator takes the digital data, removes known geometric delay and Doppler shift due to the motion of the antennas and cross-correlates the data. In VLBI, the data is recovered from magnetic tape using special tape playback machines.
• Post-processing software. This software processes the correlator output data and extracts the desired information.

Information provided by: http://www.drao.nrc.ca