Boffins at Blighty's National Physical Laboratory (NPL) - birthplace among other things of the internet* and the Dambusters' bouncing bomb - say they have validated a cunning super-accurate, laser-assisted space formation flying tactic. This will allow the detection of ripples in the very fabric of space-time, allowing top brainboxes to probe the deepest secrets of the cosmos.
It seems that if you want to make a group of spacecraft fly in a really, really accurate formation the way to do it is by the use of "femtosecond comb lasers". The very short, 0.000000000000001 second laser pulses are sent between the satellites or whatever, and their brevity means that accurate timing of their passage can measure the distance covered with an accuracy "of microns".
More: http://www.theregister.co.uk/2009/10/05/lisa_lasers_go/