Novel payload architectures for LISA

The present LISA (Laser Interferometer Space Antenna) gravitational wave detector concept features three satellites in individual earth trailing helio-centric orbits, which are linked by bi-directional monostatic laser interferometry between free-falling inertial reference masses inside the payload. The spacecrafts are maintaining an equilateral triangular constellation with 5 Million km armlength. The optical payload consists in the present configuration out of two assemblies, each one comprising a telescope, an optical bench and an inertial sensor and serving one arm of the adjacent interferometers. Due to orbital distortions, the constellation triangle is not perfectly maintained, but the line of sights offset angle is slowly changing during a one year revolution by 60°±0.75°. This variation is far beyond the diffraction limited beam width (2.5 μrad) and hence requires active compensation presently done by actuation of the complete assemblies. While allowing almost stationary on-axis operation of the optics, the arrangement requires two separate active inertial sensors, a rather sophisticated optical interfacing between the interferometer arms and active electrostatic suspension of the test masses in all but one degree of freedom. We identified an alternative architecture, characterized by a single operational inertial sensor and a single optical bench serving both adjacent interferometer arms. Both telescopes are rigidly fixed to the optical bench and the angular breathing is accommodated by in-field of view pointing of transmit and receive beams via on-bench actuation mechanisms. Only attitude electrostatic actuation of the test mass is required, which can be kept otherwise in free fall. Such an architecture requires a decoupled inter- and intraspacecraft metrology in two steps linked via optical bench fiducial points (strap-down). Peculiar technical challenges are the actuation mechanism and the inherent metrology to calibrate or compensate within the LISA measurement band –at pm and nrad resolution- for laser phase and pointing changes, respectively, inside the optical assembly.