Abstract
Abstract. The shortwave infrared (SWIR) spectrometer module of the Tropospheric Monitoring Instrument (TROPOMI), on board the ESA Copernicus Sentinel-5 Precursor satellite, is used to measure atmospheric CO and methane columns. For this purpose, calibrated radiance measurements are needed that are minimally contaminated by instrumental stray light. Therefore, a method has been developed and applied in an on-ground calibration campaign to characterize stray light in detail using a monochromatic quasi-point light source. The dynamic range of the signal was extended to more than 7 orders of magnitude by performing measurements with different exposure times, saturating detector pixels at the longer exposure times. Analysis of the stray light indicates about 4.4 % of the detected light is correctable stray light. An algorithm was then devised and implemented in the operational data processor to correct in-flight SWIR observations in near-real time, based on Van Cittert deconvolution. The stray light is approximated by a far-field kernel independent of position and wavelength and an additional kernel representing the main reflection. Applying this correction significantly reduces the stray-light signal, for example in a simulated dark forest scene close to bright clouds by a factor of about 10. Simulations indicate that this reduces the stray-light error sufficiently for accurate gas-column retrievals. In addition, the instrument contains five SWIR diode lasers that enable long-term, in-flight monitoring of the stray-light distribution.
Highlights
The Tropospheric Monitoring Instrument (TROPOMI) is the only instrument on board the ESA Copernicus Sentinel-5 Precursor satellite, which was launched on 13 October 2017 (Veefkind et al, 2012)
The shortwave infrared (SWIR) band is used for the retrieval of atmospheric CO and methane columns
To achieve the required accuracy of the spectral-radiance measurements, an accurate correction for the stray light must be included in the data processing
Summary
The Tropospheric Monitoring Instrument (TROPOMI) is the only instrument on board the ESA Copernicus Sentinel-5 Precursor satellite, which was launched on 13 October 2017 (Veefkind et al, 2012). The two effects are probably caused by the very large signal currents in the light peak (200× the highest value in nominal operations) and an exposure time at the limit of the detector specification. To examine the data more closely, all frames at a given wavelength are shifted vertically until the peaks overlap within a pixel.
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