In essence, the HIFI intensity calibration approach inherits from the chopper wheel method introduced by Penzias and Burrus ([14]), which consists of relating backend counts of a differential (on-off) source observation to the output of a hot (also called the chopper) stable temperature load compared to that of a colder one (originally, the blank sky). Compared to this standard scheme, HIFI faces the simplification that all contributions and instabilities from the atmosphere can be neglected. Also, many of the approximations used by the standard approach are not valid for systems with large IF frequencies, such as HIFI, and do not allow the high calibration accuracy required by HIFI to be met. Moreover, they do not exploit the full capabilities of an instrument with two thermal loads for calibrating the spectral bandpass. Finally, the standard calibration scheme contains no particular means to treat standing waves (seen as "ripples" on the baseline of a spectrum) that are created by reflections between the telescope structure and the receiver.

The HIFI intensity calibration uses a new calibration scheme for the planning and reduction of HIFI observations that takes advantage of the lack of an atmosphere and corrects for the effects of standing waves in the combined observations of lines and continuum with HIFI.