Observed line of SiS J = 1–0 towards IRC+10216. The abscissa in the bottom axis is rest frequency in MHz adopting a local standard of rest velocity of −26.5 km s⁻¹ (Cernicharo et al. 2000). The upper abscissa axis represents the local standard of rest velocity adopting a frequency for the 1–0 line of SiS of 18154.8861 MHz. The ordinate is the antenna temperature in milli-Kelvin. The line is in the middle of a heavy RFI frequency zone. As a consequence, the measured noise is 7 mK while its expected value should be 0.4 mK. Moreover, this extra noise does not look like a white noise but appears with spectral structures arising from the RFI signals.