Thermal Design of the Sentinel 5 precursor TROPOMI Instrument

Abstract

The Tropospheric Monitoring Instrument (TROPOMI) is being developed for launch in Q1 2015 on ESAs Sentinel 5 Precursor spacecraft. The mission of this instrument is to perform observations on air quality and on sources and sinks of air quality and climate related gases and aerosols. TROPOMI is a passive Solar backscatter imaging spectrometer sensitive in several spectral bands from ultraviolet (UV) to Short Wave Infrared (SWIR), allowing deep penetration into the atmosphere, observing scattered radiation close to the Earth’s surface. The instrument’s spatial resolution of 7 x 7 km2 results in a high fraction of cloud-free observations. The wide swath range allows daily Earth coverage. TROPOMI contains two dispersive modules: the UVN spectrometer covering UV, Visible and Near Infrared spectral bands and the SWIR spectrometer. The instrument builds on heritage gained with the OMI instrument (Ozone Monitoring Instrument) on NASAs AURA satellite and the SCIAMACHY instrument on ESAs ENVISAT. The push broom observation geometry is a heritage from OMI; the observations in the SWIR spectral range and the use of a sophisticated passive cooler are a derivative from SCIAMACHY. The instrument’s performance requirements lead to several derived thermal requirements. The four 2D detectors in the instrument (SWIR, NIR, U-VIS, UV) are cooled down to temperature levels varying from 140 K to 220 K to reduce the dark current output. Also the detectors are temperature stabilized to guarantee spectral and radiometric accuracy. The UVN optical bench is operated at 293 K, while the SWIR optical bench is cooled down to a 200 K temperature level to reduce thermal background signal on the SWIR detector. Spatial and temporal gradients in the optical benches and instrument support structure are minimized to meet the wavelength stability and co-registration requirements. Cooling of the instrument is performed by a passive radiant cooler based on the SCIAMACHY design concept, which includes several radiator patches, a reflector to optimise the cold stage performance and an Earth shield to suppress Earth fluxes. A thermal bus unit consisting of constant conducting heat pipes and flexible thermal links brings the heat from the instrument towards the radiant cooler. The TROPOMI project has entered the C/D phase of the design. This paper gives an overview of the current status of thermal design of both the instrument and radiant cooler.