TROPOMI - the Instrument
Long-term monitoring is essential for climat research and for monitoring the effect of the measures of the Montreal Protocol. In order to prepare for the period after 2010, when OMI on Aura and SCIAMACHY on ENVISAT have reached their end-of-life, the OMI and SCIAMACHY participating parties in the Netherlands are designing an innovative UV-VIS-NIR-SWIR imaging spectrometer. The instrument concept has been proposed for two missions:
- TROPOMI on TRAQ, a mission proposed in response to ESA’s 2005 EOEP call.
- TROPI on CAMEO, as specified in a proposal for the U.S. NRC Decadal Study on Earth Science and Applications from Space.
Apart from these proposals, the instrument concept fits closely to the formulation of ESA’s CAPACITY study to realize a LEO mission with a UV-VIS-SWIR nadir viewing spectrometer (Sentinel 5).
TROPOMI: the best of two worlds
TROPOMI is a Dutch national inititive that builds on the successes of SCIAMACHY, GOME and OMI. TROPOMI combines the large viewing angle of OMI with the large wavelength range of SCIAMACHY. TROPOMI will play a key role for future climat research and air quality measurements.
TROPOMI data products
TROPOMI is an instrument concept based on the GOME, OMI and SCIAMACHY instruments. The range of the TROPOMI spectrometer covers the range as the OMI instrument and is meant to measure ozone (O3), nitrogendioxide (NO2), formaldehyde (HCHO), Sulpherdioxide (SO2) and aerosols. In addition a NIR channel and a SWIR-module are added for enhanced cloud detection, aerosol height distribution and detection of carbonmonoxide (CO) and methan (CH4) (greenhouse gasses).
Level 2 data products:
- total ozone column
- total column and tropospheric NO2 (nitrogendioxide)
- cloud height
- surface UVB
- CO (carbonmonoxide)
- CH4 (methan)
- HCHO (formaldehyde)
- SO2 (sulpherdioxide)
- BrO (brominemonoxide)
- OClO (chlorinedioxide)
- Ozone profile
In order to improve with respect to the SCIAMACHY and OMI capabilities, the instrument has been designed with smaller ground pixels (optimizing the chance for cloud-free observations), improved signal-to-noise performance, and multiple overpasses per day to follow diurnal cycles.
TROPOMI is a non-scanning imaging absorption spectrometer. For an optimal thermical design there are independent UV/VIS/NIR and SWIR modules. An improved design and advanced detectors offer a larger viewing angle (115^o) and an ultra high resolution (10 x 10 km^2). TROPOMI has even smaller groundpixels than OMI but despite that fact a better signal-noise ratio. The SWIR-module uses unique immersed grating technology resulting in a reduction of the size of the instrument by a factor 40 compared to conventional grating technology. In addition the SWIR-possibilities of TROPOMI are a lot beter than those of SCIAMOACHY, both where the spacial resolution is concerned as well as the signal-noise ratio.
- non-scanning imaging absorption spectrometer
- spectral coverage UV/VIS/NIR/SWIR
- spectral resolution 0,45 nm (UV/VIS/NIR), 0,25 nm (SWIR)
- viewing angle 114° ('spoorbreedte' 2600 km at a height of 720 km)
- daily mondial coverage in a sun synchronus polar orbit (From a inclined orbit, as proposed for the TRAQ mission, a location ot an average lattitude will by observed 5 times a day)
- high resolution 10 km × 10 km
The design of the TROPOMI instrument is based on that of OMI and uses two dimensional (2D) detector technology. With this technique the complete 'spoorbreedte' of 2600 km is 'afgebeeld' along one axis of the 2D focal plane array. The spectral information wil be projected along the other axis. TROPOMI uses state-of-the-art 2D radiation hard detectors.
To continue and improve on the SCIAMACHY data record of CO and CH4 a SWIR channel is included in the design studies. The SWIR channel is the most innovative part of the instrument as it combines for the first time immersed grating technology, a polarization scrambler, a hybrid-CMOS focal plane array (FPA), and SWIR LED’s for in-flight detector calibration. The immersed grating technology is mandatory to transfer the push-broom imaging concept to the SWIR spectral domain without a huge increase of instrument size compared to the SCIAMACHY SWIR channels. The TROPOMI-SWIR instrument study will test essential elements of the TROPOMI / TROPI SWIR channel on component level and will join them together with the immersed grating to realize a breadboard model. The feasibility of the immersed grating-based SWIR channel will be demonstrated on instrument level by performing gas cell absorption measurements on CH4 and CO. The silicon-based immersed grating technology developed by SRON and TNO offers a factor 3.42 more diffraction than a conventional grating.
Measurements of sources of spectral lines
First tests with the SWIR breadboard have shown the feasability of the extension of the OMI technology to the SWIR spectral band by using the immersed grating and 2D CMOS detector technology. The measured dispersion is as expected and the resolution of 0.25 nm is within the secifications.