The Twin ANthropogenic Greenhouse Gas Observers (TANGO) mission will monitor and quantify greenhouse gas emissions at the level of individual facilities. A consortium consisting of ISISpace, TNO, SRON and KNMI are developing the TANGO mission for the ESA Scout program. ISISpace is the prime contractor and responsible for the spacecraft, SRON and KNMI are responsible for the atmospheric science, while TNO is developing the instruments. The TANGO space segment consists of two agile 16U CubeSat satellites flying closely in tandem, each equipped with an imaging spectrometer. TANGO Carbon measures the emission of CH₄ and CO₂ in the SWIR1 spectral band (1590-1675 nm at 0.45-nm spectral resolution), while TANGO Nitro measures the emission of NO₂ in the visible spectral range (405- 490 nm at 0.6-nm spectral resolution). Both instruments are reflective pushbroom spectrometers, made almost entirely from aluminum, and will cover a 30-km swath from a 500-km altitude with a spatial resolution of 300 m. The instruments share a similar architecture, using freeform mirrors to achieve high optical performance in a compact 8U envelope. In this paper, we will present the design and performance of the Carbon instrument, where a key engineering challenge is to achieve the desired spatial resolution and SNR from the limited instrument volume (8U). A tight integration of optical and mechanical design, coupled to detailed tolerance, alignment, straylight and STOP (structural thermal optical performance) analyses, allow us to reach that goal.
While CO2 is the main greenhouse gas responsible for climate change, it is a well-mixed gas in the atmosphere, meaning that its concentration is relatively uniform and does not vary much over short distances. This makes it difficult to monitor CO2 levels in specific regions or to detect changes in CO2 concentrations at small scales. On the other hand, NO2 is emitted from combustion sources that also emit CO2, and its concentration varies greatly depending on proximity, making it a useful tracer for identifying emissions sources. Therefore, NO2 is widely assumed to be a robust proxy for combustion CO2 and provides additional, valuable information for CO2 monitoring such as plume detection. The combination of NO2 and CO2 observations is useful in determining the exact locations and intensities of anthropogenic CO2 emissions. The idea of the present study is to design a very compact instrument for NO2 plume detection that allows easy accommodation of both CO2 and NO2 sensors on a single platform. Conceding on the need to explore internal dynamics of plumes and focusing more on the spatial evolution, NO2 detection can then rely on a relaxed set of requirements, which has a beneficial impact on instrument size, thus leading to a miniaturized instrument. In the present paper the driving requirements of such a miniaturized instrument will be introduced, and a compact design will be presented. Benefits and complexities of a compact design will be discussed.
Remote sensing of aerosols requires spectropolarimetric information over different viewing angles with demanding polarimetric accuracy and growing interest for smaller designs. In this paper, we investigate an instrument design that implements a metasurface filter which enables both functions simultaneously, allowing further miniaturization and integrability. The instrument offers 1 km ground sampling distance over its entire field of view, in Low Earth Orbit, and the concept makes use of six modules to cover the wide field of view requested for aerosol retrieval with a total of seven radiation-resistant lenses. This choice enables an optical volume per module under 50mm × 50mm × 150mm and smaller relative angles of incidence. The filters are designed to cover six spectral bands from 443 nm to 870 nm with a spectral resolution of 2 nm to 5 nm. The wide spectral band is achieved by using three distributed Bragg reflectors with bandpass filters, integrated in one double-cavity structure that can be glued on a CCD/CMOS sensor. The two cavities, operating as a metasurface, contain scatterers of different dimensions enabling the fine-tuning of the spectral resonance and the polarization filtering. Multiscale forward modeling techniques are employed for the estimation of the polarimetric accuracy with optical aberrations and realistic coatings. Using radiance values from the PACE mission, polarimetric errors and SNR at each pixel are estimated and compared to requirements of state-of-the-art missions.
The current paper introduces the Twin ANthropogenic Greenhouse Gas Observers (TANGO) instruments and mission. The purpose of TANGO is monitoring and quantifying greenhouse gas emissions, with a focus on characterizing emission sources down to the level of individual facilities. The TANGO mission was developed for the ESA-Scout program by a consortium consisting of ISISpace, TNO, SRON and KNMI. It consists of two agile CubeSat satellites that fly in tandem, with less than 1 minute between observations of the same target, each satellite equipped with a spectrometer of the TNO Spectrolite family of instruments that observes a different part of the spectrum. TANGO-Carbon measures emission of CH4 and CO2 in the SWIR1 spectral band. TANGO-Nitro measures emission of NO2 in the visible spectral range. The Nitro instrument has a multifunctional role, using the NO2 measurements to improve the detection of (anthropogenic) CO2 plumes, deriving historic CO2 emission trends based on available global NO2 observations, and quantifying the possible CO2 contribution in mixed CH4-CO2 sources. Each TANGO instrument fits in an 8U volume (on a 16U platform) and are all-aluminium, reflective pushbroom spectrometers covering a 30-km swath from a 500-km altitude, with a ground sampling distance of 300 m × 300 m. In this paper we will present the mission, the shared instrument concept, as well as the design and performance of both Carbon and Nitro instruments.
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The impact of NO2 and other atmospheric trace gases on health and the environment is now acknowledged by governments around the world. The sources, both natural and anthropogenic, have been shown to affect the quality of life due to low air quality in densely populated areas. Consequently, the need for accurate global NO2 measurements with high spatial- and temporal resolution to monitor NO2 is becoming ever more important. Through an ESA study, TNO and KNMI have been evaluating measurement requirements and an instrument design for a ‘Compact NO2 Spectrometer’, based on a hyperspectral imaging instrument operating in the VIS (405-490nm] spectral range and aimed at combining the performance of state-of-the-art instruments with fine spatial sampling (0.5x0.5 km2). By use of a novel free-form optics a very compact low volume and low mass design has been achieved. Combining this with other small satellite design approaches for components the aim is to create a low cost instrument capable of being flown on a wide variety of space platforms. Global daily coverage can then be achieved with a relatively small constellation of instruments. The key design features are described for a ‘Compact NO2 Spectrometer’, such as the optical design approach, the use of free-form optics, an ‘athermal’ all aluminium approach. An overview of the development and airborne results from a breadboard of a small prototype system (Spectrolite) developed by TNO which uses many of the design features envisaged for this new instrument is given.
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