Tropomi measuring air quality, carbon monoxide, methane, nitrogen oxide, and ozone with unprecedented accuracy
On Friday 13 October 2017, the Sentinel-5P satellite bearing the Dutch TROPOMI space instrument was successfully launched from Plesetsk, in Siberia. From space, TROPOMI will monitor the air quality and distribution of greenhouse gases worldwide. This groundbreaking instrument will strengthen the position of the Netherlands as a supplier of high-end aerospace instruments.
TROPOMI, which stands for TROPOspheric Monitoring Instrument, is a satellite instrument that will carry out measurements on the troposphere, the lowest layer of our atmosphere. It is capable of measuring air quality to an unprecedented degree of accuracy, and of identifying carbon monoxide, methane, nitrogen oxide, and ozone, among other things. “The launch of TROPOMI means a new standard is being set in the field of atmospheric research,” says Nick van der Valk, a senior system engineer at TNO.
From province to city
TROPOMI has four detectors that together are able to observe wavelengths in infrared, visible, and ultraviolet light. The device takes a picture of the earth every second, as it rotates below, with a resolution of seven by seven kilometres. This means that TROPOMI is able to assess air quality at city level, whereas its predecessor was only able to do so at province level. It is able to distinguish between different types of pollution, so that measures can be taken accordingly. TROPOMI gives us an accurate picture of the current state of our changing climate, helping us to answer the most pressing questions about climate change.
“TROPOMI gives us an accurate picture of the current state of our changing climate, helping us to answer the most pressing questions about climate change”
The development of TROPOMI has taken place in a partnership between various parties, including the Royal Netherlands Meteorological Institute (KNMI), Airbus Defence and Space Netherlands, and the Netherlands Institute for Space Research (SRON). TNO is the designer of the optomechanical heart of the instrument – the ultraviolet visible near-infrared optical bench module (UVN-OBM). It consists of the joint telescope, calibration unit, and three of the four spectrometers that TROPOMI has.
The various parties worked very closely throughout the project. “And that was important because an instrument like TROPOMI has never been built before, and nobody knew beforehand what exactly was, and was not, possible. More than fifty TNO employees worked on TROPOMI. This included people from a range of design disciplines, optical and mechanical alike, as well as experts in the field of making and integrating mirrors and lenses. Everyone performed outstandingly; it is fantastic to work in a team of this kind,” emphasizes Van der Valk.
“An instrument like TROPOMI had never been built before, so nobody knew beforehand what exactly was, and was not, possible”
At the very limits of what is possible
The development of an instrument with which we can measure a wide range of wavelengths in high resolution in a compact housing is an enormous challenge. “Fortunately, with the help of smart optical and mechanical inventions, we have succeeded in meeting the requirements that we were set. Developing an aerospace instrument is necessarily complex as it has to survive the launch, and because at a later stage you cannot get to it. Also, a great deal of functionality has to be able to fit into a small space. That makes even the easy tasks, like tightening a bolt, difficult. The smallest possible bolts should be used, even though they have to be very strong, and extensive tests are needed to see how far they should be tightened. In addition, we were making something that had not previously existed, and it was sometimes a real puzzle. We used groundbreaking new technologies,” explains Van der Valk.
Free-form optics is an example of the type of innovative technology that was used. Mirrors with a variable curvature were used for the telescope, and this makes the high resolution of TROPOMI possible. With conventional round lenses and mirrors, it would not have been possible to meet the tough requirements. Van der Valk continues, “We designed, had produced, and tested distorting mirrors with exact shapes. It means that TROPOMI is many times better than its predecessors. In just a second, it is able to capture a clear image of an area measuring 2.6 kilometres by 3.5 kilometres.”
“In just a second, TROPOMI is able to capture a clear image of an area measuring 2.6 kilometres by 3.5 kilometres”
How does it work? TROPOMI measures direct sunlight and compares it to the light reflected by the earth. The difference is caused by the gases in the atmosphere, which each absorb light of a certain wavelength. TROPOMI is able to determine what gases are located in the atmosphere, and in what concentration.
The UVN-OBM was constructed and tested at TNO, before being relocated to Airbus Netherlands where it was integrated with the other TROPOMI modules. TROPOMI was then extensively tested and calibrated by Airbus Netherlands, with TNO having a supporting role. “The remarkable and enjoyable thing about this project is that it is primarily a Dutch partnership. That makes the lines shorter, and it is easier to meet up as well. Other space travel projects are much more international, in which case we tend to do just our bit and have a less clear idea of the project as a whole,” claims Van der Valk.
Innovative satellite instruments like SCIAMACHY and OMI, the predecessors of TROPOMI, have been developed in the Netherlands since the 1950s. “We have an excellent reputation in that area. Thanks to TROPOMI, we are strengthening the leading role played by the Netherlands in space and climate research,” concludes Van der Valk.
The development of TROPOMI is a collaboration project between Airbus Defence and Space Netherlands, the Royal Netherlands Meteorological Institute (KNMI), the Netherlands Institute for Space Research (SRON), and TNO and is funded by the Ministry of Economic Affairs the Ministry of Education, Culture and Science, and the Ministry of Infrastructure and the Environment. Airbus Netherlands is the main contractor, with the KNMI and SRON being in charge of the scientific aspects.
Based on the development a further step is taken by TNO to measure even smaller ground pixels of 1×1 km2 while in the same time miniaturizing the instrument to fit in a cubesat. For this the instrument functionality was limited to measure NO2.
A bread board of this instrument called TROPOLITE has flown on an airplane above Berlin demonstrating feasibility. Further development of this instrument is ongoing together with several Dutch industrial partners (AIRBUS DS NL, ISIS, S&T). In parallel new calibration methods are developed together with VSL in the frame of the EU MetEOC3 program to simplify and improve the calibration method for these kind of instruments.