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Very Long Baseline Interferometry

Figure 3 - VLBI concept (Figure courtesy of IVS)

Very Long Baseline Interferometry (VLBI) consists of two antennas separated by any distance on Earth, pointing at the same radio source and collecting the signal in the same frequency bands. These sources emit radio waves which are propagating through the universe like an expanding sphere. When these wave-fronts reach the Earth's surface, they arrive as a plane wave perpendicular to the antenna's pointing direction. The emitted signal consists of a continuous sequence of waves. First the reference antenna is reached and after a certain time delay the wave front strikes the second (remote) antenna. This time-of-arrival difference is called time delay and is the main measurement type for geodetic and astrometric applications. Figure 3 depicts the VLBI concept schematically (IVS, 2012) (http://ivscc.gsfc.nasa.gov/).



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