Figure 7: Schematic transfer functions of noises to the two-way Doppler link, adapted from [46,
125]. For each type of disturbance a separate diagram (space vertically, time horizontally) is
shown. Radio waves propagate continuously up to and down from the spacecraft; some of these
are represented as the dashed lines to illustrate the indicated Doppler frequency perturbations. For
example, a momentary glitch in the FTS affects the frequency reference for both the receiver and
the transmitter. This shows up as an immediate effect in the received Doppler (difference between
the transmitted and received frequency). Because the glitch also affects the transmitted frequency, it
shows up again – but with the opposite sense – in the Doppler after a two-way light time. These various
noise responses contrast with the three-pulse GW response, shown in Figure 1; these differences are
exploited in the signal processing.