  The GRACE Follow-On satellite mission measures distance variations between
the two satellites in order to derive monthly gravity field maps, indicating
mass variability on Earth on a few 100 km scale due to hydrology, seismology,
climatology and others. This mission hosts two ranging instruments, a
conventional microwave system based on K(a)-band ranging (KBR) and a novel
laser ranging instrument (LRI), both relying on interferometric phase readout.
In this paper we show how the phase measurements can be converted into range
data using a time-dependent carrier frequency (or wavelength) that takes
potential intraday variability in the microwave or laser frequency into
account. Moreover, we analyze the KBR-LRI residuals and discuss which error and
noise contributors limit the residuals at high and low Fourier frequencies. It
turns out that the agreement between KBR and LRI biased range observations can
be slightly improved by considering intraday carrier frequency variations in
the processing. Although the effect is probably small enough to have little
relevance for gravity field determination at the current precision level, the
analysis is of relevance for detailed instrument characterization and
potentially for future more precise missions.
