Normalised correlation strength vs. frequency difference for all four coarse channels. Here the frequency difference (Δf) is the difference between the reference frequency (f_ref) and each of the other frequencies for the dot-product cross correlation in MHz. For each channel, f_ref is chosen to be the highest frequency and each circle point represents the correlation strength between f_ref and each of the other frequencies in that channel. The correlation strength decreases with increasing Δf.