  Time-delay interferometry (TDI) is a post-processing technique used to reduce
laser noise in heterodyne interferometric measurements with unequal armlengths,
a situation characteristic of space gravitational detectors such as Laser
Interferometer Space Antenna (LISA). This technique consists in properly
time-shifting and linearly combining the interferometric measurements in order
to reduce the laser noise by several orders of magnitude and to detect
gravitational waves. In this communication, we show that the Doppler shift due
to the time evolution of the armlengths leads to an unacceptably large residual
noise when using interferometric measurements expressed in units of frequency
and standard expressions of the TDI variables. We also present a technique to
mitigate this effect by including a scaling of the interferometric measurements
in addition to the usual time-shifting operation when constructing the TDI
variables. We demonstrate analytically and using numerical simulations that
this technique allows one to recover standard laser noise suppression which is
necessary to measure gravitational waves.
