Responsivity estimation using the sky simulator. Top panel: frequency sweep of a portion of one particular feedline operating at 260 GHz. Bottom panel: zoom on three typical resonances. In both panels we plot the S21 transmission parameter (dB) against the frequency. Blue lines: cold sky simulator (T_SS ~ 80 K). Red lines: 300 K background. The measured average responsivity, that is, the shift in frequency per unit temperature background variation, is around 2 kHz/K for the 260 GHz arrays and 1 kHz/K in the case of the 150 GHz array.