What is Very Lone Baseline Interferometry, VLBI?

The principle of “interferometry" is that if signals scattered over a large area are combined together, multiple antennas will be able to work together like an interferometer to simulate a giant telescope called an “array telescope”. The array will collect signals in digital form through optical fibers to a central location for processing by a “correlator” (a supercomputer with specialized purposes).

Very Long Baseline Interferometry (abbreviated as “VLBI”) as its name suggests applies interferometric techniques on “particularly long baselines”. This technique uses multiple telescopes separated by hundreds or even thousands of kilometers around the world to detect the same signal source in space simultaneously. In practice, data from telescopes in different places can be recorded separately by each telescope and stored in on-site devices at each telescope station. Then all storage devices are transported by ship or plane to one place where all data are played back together through computers for data synthesis. The precision of time stamps in the data will therefore be crucial in such post-synthesis. VLBI adopts hydrogen masers, a type of atomic clock to serve as precision frequency reference, at each telescope station to ensure the array is synchronized.

In other words, telescopes do not need to be physically connected; we only need to allow signals recorded by each telescope to be subsequently replayed in a correlator for calculation, which will equivalently combine resolution and data from telescopes around the world to create a virtual and huge radio telescope. The maximum aperture can be equal to Earth’s diameter.