Error on the 3d 21 cm power spectrum at z = 20.25 () assuming an SKA type experiment. Solid black – signal for simulations S1+S2; dashed green – noise power spectrum (equation 12) for the full SKA collecting area, 20% and 10% (increasing amplitude). The expected error taking into account the available number of modes (Eq. (7)) is shown with error bars in blue, red and yellow respectively. Measurement of the large scale power spectrum should be possible as long as the signal amplitude is not 100 times smaller than current value (this is controlled by x_α/(1 + x_α)(1 − T_γ/T_S) so it should be fine for x_α > 0.01). On large scales it should be possible to measure the signal even with only 10% of the collecting area.