The next, and the major, milestone for the mission is to find interference fringes. This is critical to operating the spacecraft as part of a high-resolution telescope.
Interference fringes arise from the wave nature of light. When superimposed, two wave trains cancel or reinforce. Beats between two very close notes, in music, is an example of this: where the waves are in phase they produce a strong note, and when out of phase they produce silence. For light, when two wave trains of the same frequency traveling in slightly different directions meet at a screen, they produce alternating bands of bright and dark, where the waves are in phase and out of phase, respectively. Those bands might look like the fringe on the edge of a piece of fabric -- so they are called "fringes".
Light -- and many other waves such as ocean waves -- always take the fastest path. Transparent, refracting materials such as glass slow light down. Light will bend, so as to spend less time in the slower material: even though the overall path has greater length, travel time is less. It's kind of like flying from Los Angeles to Washington DC, and then driving to Gaithersburg MD: even though you retrace part of the your path, it's far faster than driving the direct route all the way!
Interference is responsible for the ability of a lens to focus light. Refraction by the glass of the lens bends the light, so that waves from all points over the surface of the lens reinforce at the focus, and cancel elsewhere. In effect, the lens acts as a circular analog computer to form the image. Lenses are very cool in this way.
Radio waves can easily be recorded and played back using video recorders -- so the process of aligning the phases along different paths can be done by recording signals at different points, then playing them back so that they combine with exactly the correct lag. This used to be done by aligning video tapes with exactly the right offset. Nowadays, it's done by reading the data streams from the hard disk of a computer with exactly the right lag, as in a solid-state DVR.
The trick is to find exactly the right lag. That requires knowledge of the exact positions of all the radio telescopes used for the experiment. That requires knowledge of their positions to a fraction of a radio wavelength: a few cm. Or, you can perform a search of all possible positions, look for the appearance of interference, and then know the positions to that accuracy. This allows the measurement of distances between radio telescopes to a few cm, and demonstration that the continents are still drifting today. GPS works via a closely related principle: but instead of one source and many radio telescopes, it has many sources (the GPS satellites) and one (tiny) radio telescope inside the receiver.
Finding the exact position of the Radioastron satellite will be harder. Space is big and Radioastron moves fast. Various techniques are being used to track the satellite, including optical telescopes, radio delay-Doppler ranging, and laser ranging; but the test will be to find that first interference fringe on some astrophysical source. Then the scientific work can begin.
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