SUBMILLIMETER CRYOGENIC RADIOMETER FOR ICE ACTIVITY OBSERVATIONS

D. Belousov1, A. Murk², J. Fritschi¹, T. Plüss², L. Stöckli¹, N. Thomas¹

¹University of Bern, Physics Institute, Space Research & Planetary Science Division, Bern, 3012, Switzerland, ²University of Bern, Institute of Applied Physics, Bern, 3012, Switzerland

Passive rotational spectroscopy under space conditions is a powerful tool in remote sensing, particularly for planetary atmosphere observations such as Earth limb measurements. During limb observations, a radiometer detects emission from hot gas molecules against the cold microwave background. Some bodies in the Solar System exhibit active gas ejection on their surfaces, known as plumes. The same limb technique can be applied to plume observations. The Microwave Instrument for the Rosetta Orbiter (MIRO), onboard the Rosetta mission, investigated comet 67P/Churyumov-Gerasimenko during its perihelion passage [1]. MIRO was equipped with a submillimeter channel radiometer to probe the comet atmosphere, probing e.g., water rotation line 556.96 GHz, thereby providing insight into the comet’s structure and mechanisms of activity. Passive remote sensing of gases in low concentrations (e.g. plumes or tenuous icy moons atmospheres) requires low-noise receivers with high spectral resolution. Here, we present a 557 GHz double-sideband cryogenic heterodyne radiometer and its initial calibration tests. The radiometer is the part of the WEEVIL (the Water Emission of Vapour from Ice in the Laboratory) experimental setup dedicated to studying the spectroscopy of gas plumes arising from the sublimation of icy, porous and dusty media in the controlled laboratory environment.

The 557 GHz heterodyne double sideband radiometer setup comprises a heterodyne radiometer with a subharmonic mixer, cryogenic LNA, local oscillator chain, an FFT spectrometer, calibration targets, and refocusing optics, following the approach in [2]. The local oscillator (LO) chain, which drives the subharmonic mixer, consists of an active multiplier chain (AMC) with a multiplication factor of six, a broadband tripler, and a frequency synthesizer with an external 10 MHz reference. The test setup was designed to evaluate the radiometer under vacuum conditions with cryogenically cooled components. To achieve high sensitivity, the subharmonic mixer, LNA and a tripler are actively cooled by a cryocooler. The receiver, operating at 60 K, achieves a double sideband noise temperature as low as 550 K at 560 GHz. Allan deviation measurements indicate an Allan time of 100 s at 24.4 kHz bandwidth. The frequency-switching baseline offset was characterized at various frequency steps and local oscillator settings, showing a strong dependence on the chosen local oscillator frequency.

The phase noise and subharmonics of the signal generator used in the LO of the radiometer are key parameters that affect the system noise temperature and spectral purity. Based on a case study on our 557 GHz radiometer, we characterized the effect of phase noise of the local oscillator on the IF spectrum of the radiometer. For this, we compare the system noise temperature and the IF spectra of the same radiometer, when it is operated using different signal sources, and we also analyzed and compared the phase noise spectra of those sources.

For double sideband receivers, knowledge of the sideband ratio is crucial. To determine this, we performed spectroscopy measurements, fully characterizing the 556.936 GHz water rotational line at different gas pressures using a temperature-controlled water-vapor gas cell.

Literature:

1. Gulkis, S., Frerking, M., Crovisier, J. et al. MIRO: Microwave Instrument for Rosetta Orbiter. Space Sci Rev 128, 561–597 (2007).

2. O. Auriacombe et al., “TeraHertz desorption emission spectroscopy (THz DES) of space relevant ices,” Monthly Notices of the Royal Astronomical Society, vol. 515, no. 2, Sep., pp. 2698-2709, 2022.