To fulfil the scientific objectives noted in Section 3.1, “What Science Is Possible With HIFI?”, the HIFI instrument has been designed with the following important characteristics:
complete coverage of 480-1250 and 1410-1910 GHz (625-240 and 213-157 microns), to allow multiples lines of important molecules, such as H2O, to be sampled, and to allow broad, unbiased spectral surveys;
a resolving power of up to 107, corresponding to a velocity resolution up to 0.03 km/s (requiring a narrow local oscillator line-width and an Intermediate Frequency (IF) spectrometer -- measuring the frequency difference between signal and local oscillator signals -- with a resolution of up to 125 kHz);
a receiver sensitivity of 3-4 times the quantum limit, to make maximum use of the limited satellite lifetime (requiring low-noise mixers and IF amplifiers);
a large instantaneous band-width (4 GHz in each sideband) to increase spectral survey speeds, to minimize the risk of spectral coverage gaps, and to observe broad features (requiring mixers, amplifiers, and a spectrometer with 4 GHz of IF bandwidth);
dual-polarization operation to make maximum use of the energy collected by the HIFI optical beam; and
at least 10% calibration accuracy (with a goal of 3%)
NOTES:
1. The time needed to observe a weak spectral line scales inversely with the square of the receiver noise temperature.
2. The bandwidth is only 2.4 GHz in Bands 6 and 7 (due to a bandwidth limitation in the state-of-the-art HEB mixers that are used at these high frequencies).
3. For bands 5, 6 and 7, the receiver temperatures are rather 10-20 times the quantum limit.