The three satellites, separated by a distance of 1 Mio km, will form a high precision interferometer that senses gravitational waves by monitoring the changes in distance between free falling test masses inside the spacecraft.
A key feature of the eLISA/NGO concept is a set of three orbits maintaining a near-equilateral triangular formation, without the need for station-keeping. Depending on the initial conditions of the spacecraft, the formation can be kept in an almost constant distance to the Earth or be allowed to slowly drift away to about 70 x 106 km, the outer limit for communication purposes.
The centre of the formation is in the ecliptic plane 1 AU form the Sun and 20° behind the Earth. The plane of the triangle is inclined by 60 deg with respect to the ecliptic. These particular heliocentric orbits for the three spacecraft were chosen such that the triangular formation is maintained throughout the year, with the triangle appearing to rotate about the centre of the formation.
eLISA/NGO will coherently measure the stretching and squeezing of spacetime, including frequency, phase, and polarisation. Hence it will shed light on the origin of gravitational waves - large-scale violent cosmic events - and trace the motions of distant matter directly.