Radiometric performance of the 530 to 625 GHz receiver unit of the submillimetre wave instrument on juice

The upcoming Submillimeter Wave Instrument on the JUICE spacecraft is a passive radiometer/spectrometer instrument with two heterodyne receivers which are independently tunable in the frequency bands 530 to 625 GHz and 1080 to 1275 GHz. It will study Jupiter’s atmosphere as well as the atmospheres and surface properties of the Galilean moons. This work presents the results of first radiometric tests with a prototype of the 600 GHz receiver. In this context, the baseline ripples caused by the internal calibration target have been characterized using two conical prototypes with a linear and an exponential absorber coating profile. A significant reduction of the baseline ripple amplitude has been measured with the target having the exponential cone profile. The spectroscopic baseline has been characterized for various frequency steps when applying frequency switching as an alternative calibration mode. At some operating frequencies a very flat switching baseline has been measured for frequency throws up to 90 MHz, while at other frequencies significant spectral distortions are measured even with a step size of 22.5 MHz. The first radiometric tests of the sideband gain ratio with a passive Fourier Transform Spectroscopy method demonstrate the general applicability in the 530 to 625 GHz band.