Diagram showing the basic layout of the spacecraft and payload. The spacecraft is about 2m in diameter and 0.5m high.
The 'Y'-shaped payload has two identical optical assemblies with transmit/receive telescopes and optical benches carrying the inertial sensor and the interferometry optics. Because of slight changes in the spacecraft formation, the two optical assemblies must be articulated to accommodate a changing angle between the two arms.
The inertial sensor consists of a free-falling proof mass inside a reference housing, which is fixed to the spacecraft. the relative positions and orientations of the reference housing and the proof mass are monitored by capacitive sensors. These signals in effect measure the spacecraft trajectory relative to the proof mass which falls in a more nearly inertial orbit.
The spacecrafts position is controlled using micronewton thrusters to keep the spacecraft centered on the proof mass, so called 'drag-free' operation. Minimizing the relative motion of the spacecraft reduces disturbances of the proof masses caused by the spacecraft's thermal, electromagnetic and gravitational gradients.
The spacecraft and payload require careful layout and thermal design. Sensitive payload components must be separated from noisier spacecraft systems. There are very few movable elements. Power dissipation is kept constant. Materials and construction details are selected to thermally isolate the inertial sensor and optical components from varying power sources, notably the Sun. Thermal shields closing the top and bottom of the spacecraft cylinder are not shown for clarity.