  Heterodyne laser phase measurements in a space-based gravitational wave
interferometer are degraded by the phase fluctuations of the onboard clocks,
resulting in unacceptable sensitivity performance levels of the interferometric
data. In order to calibrate out the clock phase noises it has been previously
suggested that additional inter-spacecraft phase measurements must be performed
by modulating the laser beams. This technique, however, considerably increases
system complexity and probability of subsystem failure.
  With the advent of self-referenced optical frequency combs, it is possible to
generate the heterodyne microwave signal that is coherently referenced to the
onboard laser. We show in this case that the microwave noise can be cancelled
directly by applying modified second-generation Time-Delay Interferometric
combinations to the heterodyne phase measurements. This approach avoids use of
modulated laser beams as well as the need of additional ultra-stable oscillator
clocks.
