  The current design of space-based gravitational wave detectors utilizes
heterodyne laser interferometry in inter-satellite science measurements.
Frequency variations of the heterodyne beatnotes are predominantly caused by
the Doppler effect from relative satellite motion along lines of sight.
Generally considered to be outside the measurement band, this Doppler frequency
shift appears to have been overlooked in numerical simulations of time-delay
interferometry (TDI). However, the potential impact on the implementation of
TDI should be assessed. The issue is particularly relevant to TianQin that
features geocentric orbits, because of strong gravity disturbances from the
Earth-Moon system at frequencies $<1\times 10^{-4}$ Hz. In this
proof-of-principle study, based on high-precision orbital data obtained from
detailed gravity field modeling, we incorporate the Doppler shift in the
generation of TianQin's beatnote phase signals. To remove the large-scale
Doppler phase drift at frequencies $<1\times 10^{-4}$ Hz, we develop a
high-performance high-pass filter and consider two possible processing
sequences, i.e., applying the filter before or after TDI combinations. Our
simulation results favor the former and demonstrate successful removal of the
low-frequency gravity disturbances for TianQin without degrading the TDI
performance, assuming 10 m pseudo-ranging uncertainty. The filtering scheme can
be used in developing the initial noise-reduction pipeline for TianQin.
