  SAGE (SagnAc interferometer for Gravitational wavE) is a project for a space
observatory based on multiple 12-U CubeSats in geosynchronous orbit. The
objective is a fast track mission which would fill the observational gap
between LISA and ground based observatories. With albeit a lower sensitivity,
it would allow early investigation of the nature and event rate of
intermediate-mass black hole (IMBH) mergers, constraining our understanding of
the universe formation by probing the building up of IMBH up to supermassive
black holes. Technically, the CubeSats would create a triangular Sagnac
interferometer with 140.000km roundtrip arm length, optimized to be sensitive
to gravitational waves at frequencies between 10mHz and 2Hz. The nature of the
Sagnac measurement makes it almost insensitive to position error, enabling the
use of spacecrafts in ballistic trajectories. The light source and
recombination units of the interferometer are based on compact fibered
technologies without bulk optics. A peak sensitivity of 23 pm/sqrt(Hz) is
expected at 1Hz assuming a 200mW internal laser source and 10-centimeter
diameter apertures. Because of the absence of a test mass, the main limitation
would come from the non-gravitational forces applied on the spacecrafts.
However, conditionally upon our ability to partially post-process the effect of
solar wind and solar pressure, SAGE would allow detection of gravitational
waves with strains as low as a few 1e-19 within the 0.1 to 1Hz range. Averaged
over the entire sky, and including the antenna gain of the Sagnac
interferometer, the SAGE observatory would sense equal mass black hole mergers
in the 1e4 to 1e6 solar masses range up to a luminosity distance of 800Mpc.
Additionally, coalescence of stellar black holes (10Msun) around SMBH (IMBH)
forming extreme (intermediate) mass ratio inspirals could be detected within a
sphere of radius 200Mpc.
