The New Gravitational wave Observatory (NGO) mission consists of three spacecraft, separated by 106 km, orbiting the Sun in a configuration that forms a near-equilateral triangle.
To measure the strain, or deformation, on the fabric of spacetime caused by gravitational waves, the mission uses precision laser interferometry.
The constellation of the three NGO spacecraft itself forms the science instrument. The two sides of the equilateral triangle, from the apex to the two vertices with a spacecraft at each end, form two measurement arms. Each spacecraft carries the interferometry equipment needed to measure changes in arm length arising from the passage of gravitational waves. The spacecraft at the apex - the 'mother' spacecraft - houses two send/receive laser ranging terminals and two free-falling test masses, forming one set of endpoints of the two measurement arms. The other two spacecraft at the vertices of the triangle - the 'daughter' spacecraft - house one laser ranging terminal each and one free-floating test mass, and form the two other endpoints of the measurement arms. The mother and daughter spacecraft are identical but for the number of payloads they carry. The laser in the daughter spacecraft is phase locked to the incoming laser light.
For practical purposes, the measurements are split up into three distinct parts: the measurement between the spacecraft (that is, the measurement between the optical benches that are fixed to the spacecraft), and the measurement between each of the test masses and its respective optical bench. By combining these three measurements, the distance between the test masses is reconstructed and kept unaffected by the noise in the position of the spacecraft with respect to the test masses.
Splitting up the measurements in this manner is normally avoided as it increases the noise due to the number of detectors involved. However, as the NGO detector noise is generally negligible, splitting the measurements does not significantly degrade the overall sensitivity.
The blue dots indicate where interferometric measurements are taken, the yellow blocks indicate test masses and the light green blocks indicate the laser. The sketch leaves out the test mass interferometers for clarity.