This is necessarily a short answer, I don't doubt there will be other more extensive answers later.
The location of a spacecraft in space is changing due to gravity forces and perturbations from drag and solar pressure.
The location of DSN stations in inertial reference frame is not constant either. Earth's orbital motion is the first component to consider.
Over very long time intervals, DSN stations drift with the continents they are located on, thus their longitude and latitude may be changing.
Attitude of the Earth relative to far away stars includes three more components:
- rotation (length of the day and Universal Time - UT1) is essentially a random process, highly correlated with Atmospheric Angular Momentum.
- nutation (with main oscillatory periods of 18.61 years, 183 days and so on)
- precession (oscillation with a dominant period of 26 thousand years)
DSN maintains a highly accurate and up-to-date model of Earth's orientation in space (based on Very Large Base Interferometry - observations of differences in arrival of signals from distant quasars to different DSN stations).
Range and range rate (Doppler) data acquired from spacecraft are dominated by the rotation of the Earth and errors in UT1 are the most critical.
Diurnal signature of the Earth is thus the mathematical model of our Earth's orientation/rotational speed relative to an inertial reference frame with all its wobbly components that should be subtracted from raw Doppler shift observations to get true spacecraft coordinates after a few more calculations.