VLBI (Very long baseline interferometry)
A method of interconnecting two or even more radio telescopes over long distances (very large baseline distances between telescopes) in such a way that they can be used, in some respects, like a single large telescope. This is done by exploiting the wave properties of the received radio signals, in particular the ability to superimpose.
On the one hand, VLBI can be used to obtain detailed images of distant cosmic objects (such as radio galaxies). On the other hand, it is possible to determine with high precision how far apart two radio sources are in the sky; this can be used, for example, to measure the relativistic deflection of light in the Sun’s gravitational field with great accuracy.