The<i>Herschel</i>-Heterodyne Instrument for the Far-Infrared (HIFI)

<i>Aims. <i/>This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) that was launched onboard ESA's <i>Herschel<i/> Space Observatory in May 2009. <i>Methods. <i/>The instrument is a set of 7 heterodyne receivers that are electronically tuneable, covering 480–1250 GHz with SIS mixers and the 1410–1910 GHz range with hot electron bolometer (HEB) mixers. The local oscillator (LO) subsystem comprises a Ka-band synthesizer followed by 14 chains of frequency multipliers and 2 chains for each frequency band. A pair of auto-correlators and a pair of acousto-optical spectrometers process the two IF signals from the dual-polarization, single-pixel front-ends to provide instantaneous frequency coverage of 2 × 4 GHz, with a set of resolutions (125 kHz to 1 MHz) that are better than 0.1 km s<sup>-1<sup/>.<i>Results. <i/>After a successful qualification and a pre-launch TB/TV test program, the flight instrument is now in-orbit and completed successfully the commissioning and performance verification phase. The in-orbit performance of the receivers matches the pre-launch sensitivities. We also report on the in-orbit performance of the receivers and some first results of HIFI's operations.