Histogram of the frequency shift between the pulsar and Earth terms, in units of the frequency bin T⁻¹), where T is the observing time (assumed 10 yr in all cases), for the current IPTA (left-hand panels), SKA1 (central panels) and SKA2 (right-hand panels). The upper plots assume a typical observed time delay consistent with the mean estimated pulsar distance in the current IPTA (≈1.5 kpc). The lower plot assumes a typical time delay consistent with the maximum pulsar distance in the current IPTA (≈6 kpc).