On the use of partial interferograms for GHG measurement using a solar occultation geometry

International audienceImSPOC which acronym means Imaging Spectrometer On Chip is a concept of compact Fourier imaging-spectrometer developed at ONERA and IPAG since a few years [French patent registered the 30th, June 2016 under n°16 56162]. It is based on a static micro-interferometer array that provides one-shot retrieval of both spatial and spectral information without scanning component. The first onboard prototypes operate in visible and in near infrared with a mid-spectral resolution (R from 10 to 100) and a few square dozens of imaging pixels, for geophysics or drone-based gas detection purposes. We studied the capabilities of the concept for high spectral resolution monitoring of GreenHouse Gases (1,000<R<10,000) on the atmospheric column. The goal is to measure the main GHG (CO2, CO, CH4 and N2O) with a statistical error less than 1% of the averaged column density. For these resolutions, the small number of available interferometric samples requires to use partial interferogram and to retrieve geophysical and atmospheric parameters directly in the Fourier domain. Such an approach has been already considered for the IASI space instrument and demonstrated a gain in terms of bias impact reduction [Grieco et al. 2011, Serio et al. 2012]. We must now assess the ImSPOC concept performances with its own instrumental specificities (radiometry, spectral channel, sampling), and demonstrate the breakthrough regarding weigh and volume for this kind of instrument. The work we present is a numerical simulation of main GHG column amount retrieval (CO2, CO, CH4 and N2O) from an ImSPOC-based imaging spectrometer in clear sky condition using a solar occultation geometry. We have developed an inverse model based on the line-by-line radiative transfer code LBLRTM and a Levenberg-Marquardt algorithm. This tool allows deducing atmospheric variables from partial interferograms using the ImSPOC instrumental function. Firstly, we used LBLRTM downwelling radiances (calculated with the US standard description of the atmosphere) as input of our instrumental model to generate synthetic measured partial interferograms. After chosen the optimal interferometric samples, we studied the accuracy on the various gases retrieval by varying atmospheric composition, molecular column amount and temperature profile. For an example, some promising preliminary results have been obtained on the 1.6 µm CO2 band. We chose 100 joints interferometric samples (four sample per fringe) at the maximum of instrumental sensitivity for the CO2 on this band. We introduced a bias over the atmospheric composition in our model of 10% for all the molecular amounts (by profile scaling), and we kept only three free parameters for the inversion (H2O, CO2 and O2 molecular total amount). As a result, we retrieved the introduced CO2 concentration over the column density with an estimation error less than 10-3%. We generalize and discuss this preliminary result to the other species and bands by considering more error sources (concentration and temperature profile). Then, real observed spectral radiances will be used to validate the model. In this paper, after a brief presentation of the ImSPOC concept and a description of the method, the first results of this study will be presented and discussed

On the use of partial interferograms for GHG measurement using a solar occultation geometry

International audienceImSPOC which acronym means Imaging Spectrometer On Chip is a concept of compact Fourier imaging-spectrometer developed at ONERA and IPAG since a few years [French patent registered the 30th, June 2016 under n°16 56162]. It is based on a static micro-interferometer array that provides one-shot retrieval of both spatial and spectral information without scanning component. The first onboard prototypes operate in visible and in near infrared with a mid-spectral resolution (R from 10 to 100) and a few square dozens of imaging pixels, for geophysics or drone-based gas detection purposes. We studied the capabilities of the concept for high spectral resolution monitoring of GreenHouse Gases (1,000<R<10,000) on the atmospheric column. The goal is to measure the main GHG (CO2, CO, CH4 and N2O) with a statistical error less than 1% of the averaged column density. For these resolutions, the small number of available interferometric samples requires to use partial interferogram and to retrieve geophysical and atmospheric parameters directly in the Fourier domain. Such an approach has been already considered for the IASI space instrument and demonstrated a gain in terms of bias impact reduction [Grieco et al. 2011, Serio et al. 2012]. We must now assess the ImSPOC concept performances with its own instrumental specificities (radiometry, spectral channel, sampling), and demonstrate the breakthrough regarding weigh and volume for this kind of instrument. The work we present is a numerical simulation of main GHG column amount retrieval (CO2, CO, CH4 and N2O) from an ImSPOC-based imaging spectrometer in clear sky condition using a solar occultation geometry. We have developed an inverse model based on the line-by-line radiative transfer code LBLRTM and a Levenberg-Marquardt algorithm. This tool allows deducing atmospheric variables from partial interferograms using the ImSPOC instrumental function. Firstly, we used LBLRTM downwelling radiances (calculated with the US standard description of the atmosphere) as input of our instrumental model to generate synthetic measured partial interferograms. After chosen the optimal interferometric samples, we studied the accuracy on the various gases retrieval by varying atmospheric composition, molecular column amount and temperature profile. For an example, some promising preliminary results have been obtained on the 1.6 µm CO2 band. We chose 100 joints interferometric samples (four sample per fringe) at the maximum of instrumental sensitivity for the CO2 on this band. We introduced a bias over the atmospheric composition in our model of 10% for all the molecular amounts (by profile scaling), and we kept only three free parameters for the inversion (H2O, CO2 and O2 molecular total amount). As a result, we retrieved the introduced CO2 concentration over the column density with an estimation error less than 10-3%. We generalize and discuss this preliminary result to the other species and bands by considering more error sources (concentration and temperature profile). Then, real observed spectral radiances will be used to validate the model. In this paper, after a brief presentation of the ImSPOC concept and a description of the method, the first results of this study will be presented and discussed

Realisation and characterization of large silicon cavities for interferometric applications

International audienc

NanoCarb part 2: Performance assessment for total column CO2 monitoring from a nano-satellite

International audienceNanoCarb is an innovative Fourier Transform imaging spectrometer dedicated to the measurement of CO2 and CH4. Both its unusual optical principle and sampling strategy allows to reach a compact design, ideal for small satellite constellation as investigated by the European project SCARBO. The NanoCarb performance assessment as well as a proof of concept are required in this framework. A strategy of design is developed to optimize the performances and reach the sensitivity target of the space mission, demonstrating the potential of the concept without drastic complexity gain. A preliminary bias mitigation in the retrieval strategy is presented concerning water for CO2 measurement, illustrating the efficiency and the flexibility of the NanoCarb partial interferogram sampling technic. The presented design reaches a random error sub-ppm for CO2 and sub-10ppb for CH4, considering a 128 to 192 km swath, respectively, for 2 or 3 km of resolution at ground. A full mitigation of the water bias is performed on CO2 band thanks to partial interferograms

NanoCarb partie 1 : interféomètre imageur compact pour la surveillance du CO2 depuis l'espace

International audienceNanocarb is a snapshot imaging interferometer concept dedicated to the measurement of CO2. Thanks to its very compact design, it is a good candidate for small satellites, which would pave the way to a constellation of satellites to monitor emission of anthropogenic greenhouse gases from space. Nanocarb is based on a multiaperture design, with a stepcase interferometric plate to measure partial interferograms. It is developed in the framework of the Scarbo project, an European project involving several partners, the aim of which being to design and analyse the performances of the Nanocarb concept, complemented by collocated compact aerosol sensor and CO2 reference instruments. The overall measurement concept will be experimentally validated through a dedicated airborne campaign featuring instrument prototypes

A novel method to enable Black Silicon to absorb in the SWIR range for spectral imagers applications

International audienc

NanoCarb hyperspectral sensor: on performance optimization and analysis for greenhouse gas monitoring from a constellation of small satellites

International audienceNanoCarb is an innovative Fourier-transform imaging spectrometer dedicated to the measurement of CO 2 and CH 4. Both its unusual optical principle and sampling strategy allow to reach a compact design, ideal for small satellite constellation as investigated by the European project SCARBO. The NanoCarb performance assessment as well as a proof of concept is required in this framework. We have developed a design strategy to optimize the performances. We demonstrate the potential of the concept through an estimation of the sensitivity, compliant with the space mission target. We also present a preliminary mitigation of the bias induced by water on CO 2 and CH 4 retrieval, illustrating the efficiency and the flexibility of the NanoCarb partial interferogram sampling technique. The presented design reaches a sub-ppm random error for CO 2 and sub-10 ppb random error for CH 4 , considering 128 km swath and 2 by 2 km 2 ground resolution. Design optimization and more systematic performances are discussed

Le projet Nanocarb pour la mesure des gaz à effet de serre

International audienceNanoCarb is a compact snapshot imaging spectrometer in the shortwave infrared, dedicated to the measurement of greenhouse gases, like CO2 orCH4. It was developed in the framework of the H2020 Scarbo project by Onera and UGA, and two avenues of research were investigated: a preliminary design for a space mission, and the development of two airborne prototypes. The former proved the relevance of Nanocarb as a compact payload for a constellation of minisatellites dedicated to GHG monitoring with a sub-daily revisit, while the airborne prototypes proved the manufacturability of such a microspectrometer. Even though more experiments and signal processing improvements are still to be done to quantitatively prove the performance of Nanocarb GHG sensor, the current results confirm the interest to pursue the development of Nanocarb