Resolution of a Radio Telescope

The resolution means how close things can be together and still be distinguished as separate objects. The angular resolution of a telescope depends on the diameter ($D$) of the dish and the wavelength ($\lambda$) at which the measurements are being made. The angular resolution of the telescope is given as $\sim$ $\lambda/D$. Radio waves have a large wavelength and hence have poor resolution. So, to improve the resolution of the radio telescopes, they are made as large as possible.

To increase the collecting area, the radio telescopes are made in the form of arrays. In an array two or more telescopes are used and their signals are combined. The distance between two telescopes can be large. Some arrays have telescopes situated on opposite ends of the earth (called, Very Long Baseline Array (VLBI)); in this case the resolution angle becomes very small. The resolution can be further increased by placing an antenna at a great distance from the earth, in space and signals combined later. This technique, which gives micro arc-second resolution is called Space Very Long Baseline Interferometry (SVLBI).

(Ref : An Introduction to Radio Astronomy, Luke Woolfenden.
http://www.astro.cardiff.ac.uk/observatory/radioback.html)