Pre-launch calibration results of the TROPOMI payload on-board the Sentinel 5 Precursor satellite

Abstract

Abstract. The Sentinel 5 precursor satellite was successfully launched on 13th October 2017, carrying the Tropospheric Monitoring Instrument TROPOMI as its single payload. TROPOMI is the next generation atmospheric sounding instrument, continuing the successes of GOME, SCIAMACHY, OMI and OMPs, with higher spatial resolution, improved sensitivity and extended wavelength range. The instrument contains four spectrometers, divided over two modules sharing a common telescope, measuring the ultraviolet, visible, near-infrared and shortwave infrared reflectance of the Earth. The imaging system 5 enables daily global coverage using a push-broom configuration, with a spatial resolution as low as 7 x 3.5 km2 in nadir from a Sun-synchronous orbit at 824 km and an equator crossing time of 13:30 local solar time.This article reports the pre-launch calibration status of the TROPOMI payload as derived from the on-ground calibration effort. Stringent requirements are imposed on the quality of on-ground calibration in order to match the high sensitivity of the instrument. In case that the systematic errors that originate from the calibration exceed the random errors in the observations, 10 the scientific products may be compromised. A new methodology has been employed during the analysis of the obtained calibration measurements to ensure the consistency and validity of the calibration. This was achieved by using the production grade Level 0 to 1b data processor in a closed-loop validation setup. Using this approach the consistency between the calibration and the L1b product could be established, as well as confidence in the obtained calibration result.This article introduces this novel calibration approach, and describes all relevant calibrated instrument properties as they 15 were derived before launch of the mission. For most of the relevant properties compliance with the requirements could be established, including the knowledge of the instrument spectral and spatial response functions, and the absolute radiometric calibration. Partial compliance was established for the straylight correction; especially the out-of-spectral-band correction for the NIR channel needs further validation. Incompliance was reported for the relative radiometric calibration of the Sun port diffusers. These latter two subjects will be addressed during the in-flight commissioning phase in the first 6 months following

Light source for stray light characterisation of EnMAP spectrometers

“…For a full overview of different types of stray light see Ref. 3. Here, we are only concerned with measuring diffuse, in-band, in-field stray light.…”

Determination of the TROPOMI-SWIR instrument spectral response function

“…The three measurement data sets (irradiance, radiance and diode lasers) are corrected for detector features such as memory and pixel response nonuniformity (PRNU), using the operational level-1b processor developed by KNMI (Kleipool et al, 2018). Changes of the background signal are removed by dedicated background measurements, which include offset, detector dark current and thermal background signal.…”

“…The ISRF measurements are part of the on-ground calibration campaign performed at the Centre Spatial de Liège (CSL) in Belgium (Kleipool et al, 2018). The limited time slot did not allow us to perform the measurements with a very accurate wavelength meter and wait for a stable laser at a given wavelength.…”

“…For the later OMI instrument on board NASA's EOS Aura satellite, a white light source followed by a highresolution monochromator was used to create a comb of narrow spectral lines (Dobber et al, 2006). A similar approach was followed for the other three bands of TROPOMI (Kleipool et al, 2018). The NASA OCO instruments had the ISRF measured with a monochromatic line source (Day et al, 2011;Sun et al, 2017).…”

New method to determine the instrument spectral response function, applied to TROPOMI-SWIR

“…In this section, we describe the setup used during the ISRF calibration measurements, performed at the Centre Spatial de Liège (CSL) in Belgium, during the on-ground calibration campaign (Kleipool et al, 2017). In this setup, the light source employed is a 2 W continuous-wave optical parametric oscillator (OPO), custom-built by VSL (Delft, the Netherlands).…”