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The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) was a limb-viewing infrared Fourier transform spectrometer that operated from 2002 to 2012 aboard the Environmental Satellite (ENVISAT). The final re-processing of the full MIPAS mission Level 2 data was performed with the ESA operational version 8 (v8) processor. This MIPAS dataset includes not only the retrieval results of pressure–temperature and the standard species H2O, O3, HNO3, CH4, N2O, and NO2 but also vertical profiles of volume mixing ratios of the more difficult-to-retrieve molecules N2O5, ClONO2, CFC-11, CFC-12 (included since v6 processing), HCFC-22, CCl4, CF4, COF2, and HCN (included since v7 processing). Finally, vertical profiles of the species C2H2, C2H6, COCl2, OCS, CH3Cl, and HDO were additionally retrieved by the v8 processor. The balloon-borne limb-emission sounder MIPAS-B was a precursor of the MIPAS satellite instrument. Several flights with MIPAS-B were carried out during the 10-year operational phase of ENVISAT at different latitudes and seasons, including both operational periods when MIPAS measured with full spectral resolution (FR mode) and with optimised spectral resolution (OR mode). All MIPAS operational products (except HDO) were compared to results inferred from dedicated validation limb sequences of MIPAS-B. To enhance the statistics of vertical profile comparisons, a trajectory match method has been applied to search for MIPAS coincidences along the 2 d forward and backward trajectories running from the MIPAS-B measurement geolocations. This study gives an overview of the validation results based on the ESA operational v8 data comprising the MIPAS FR and OR observation periods. This includes an assessment of the data agreement of both sensors, taking into account the combined errors of the instruments. The differences between the retrieved temperature profiles of both MIPAS instruments generally stays within ±2 K in the stratosphere. For most gases – namely H2O, O3, HNO3, CH4, N2O, NO2, N2O5, ClONO2, CFC-11, CFC-12, HCFC-22, CCl4, CF4, COF2, and HCN – we find a 5 %–20 % level of agreement for the retrieved vertical profiles of both MIPAS instruments in the lower stratosphere. For the species C2H2, C2H6, COCl2, OCS, and CH3Cl, however, larger differences (within 20 %–50 %) appear in this altitude range.

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an infrared imaging FTS (Fourier transform spectrometer) with a 2-D infrared detector that is operated on two high-flying research aircraft. It has flown on eight campaigns and measured along more than 300 000 km of flight track. This paper details our instrument calibration and characterization efforts, which, in particular, almost exclusively leverage in-flight data. First, we present the framework of our new calibration scheme, which uses information from all three available calibration sources (two blackbodies and upward-pointing “deep space” measurements). Part of this scheme is a new algorithm for correcting the erratically changing nonlinearity of a subset of detector pixels and the identification of the remaining bad pixels. Using this new calibration, we derive a 1σ bound of 1 % on the instrument gain error and a bound of 30 nW cm−2 sr−1 cm on the instrument offset error. We show how we can examine the noise and spectral accuracy for all measured atmospheric spectra and derive a spectral accuracy of 5 ppm on average. All these errors are compliant with the initial instrument requirements. We also discuss, for the first time, the pointing system of the GLORIA instrument. Combining laboratory calibration efforts with the measurement of astronomical bodies during the flight, we can achieve a pointing accuracy of 0.032∘, which corresponds to one detector pixel. The paper concludes with a brief study of how these newly characterized instrument parameters affect temperature and ozone retrievals. We find that the pointing uncertainty and, to a lesser extent, the instrument gain uncertainty are the main contributors to the error in the result.

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) was operated on board the German High Altitude and Long Range Research Aircraft (HALO) during the PGS (POLSTRACC/GW-LCYCLE/SALSA) aircraft campaigns in the Arctic winter 2015/2016. Research flights were conducted from 17 December 2015 until 18 March 2016 within 25–87° N, 80° W–30° E. From the GLORIA infrared limb-emission measurements, two-dimensional cross sections of temperature, HNO3, O3, ClONO2, H2O and CFC-12 are retrieved. During 15 scientific flights of the PGS campaigns the GLORIA instrument measured more than 15 000 atmospheric profiles at high spectral resolution. Dependent on flight altitude and tropospheric cloud cover, the profiles retrieved from the measurements typically range between 5 and 14 km, and vertical resolutions between 400 and 1000 m are achieved. The estimated total (random and systematic) 1σ errors are in the range of 1 to 2 K for temperature and 10 % to 20 % relative error for the discussed trace gases. Comparisons to in situ instruments deployed on board HALO have been performed. Over all flights of this campaign the median differences and median absolute deviations between in situ and GLORIA observations are −0.75 K ± 0.88 K for temperature, −0.03 ppbv ± 0.85 ppbv for HNO3, −3.5 ppbv ± 116.8 ppbv for O3, −15.4 pptv ± 102.8 pptv for ClONO2, −0.13 ppmv ± 0.63 ppmv for H2O and −19.8 pptv ± 46.9 pptv for CFC-12. Seventy-three percent of these differences are within twice the combined estimated errors of the cross-compared instruments. Events with larger deviations are explained by atmospheric variability and different sampling characteristics of the instruments. Additionally, comparisons of GLORIA HNO3 and O3 with measurements of the Aura Microwave Limb Sounder (MLS) instrument show highly consistent structures in trace gas distributions and illustrate the potential of the high-spectral-resolution limb-imaging GLORIA observations for resolving narrow mesoscale structures in the upper troposphere and lower stratosphere (UTLS).

The Geostationary Emission Explorer for Europe (G3E) is a concept for a geostationary satellite sounder that aims to constrain the sources and sinks of greenhouse gases carbon dioxide (CO2) and methane (CH4) for continental-scale regions. Its primary focus is on central Europe. G3E carries a spectrometer system that collects sunlight backscattered from the Earth's surface and atmosphere in the near-infrared (NIR) and shortwave-infrared (SWIR) spectral range. Solar absorption spectra allow for spatiotemporally dense observations of the column-average concentrations of carbon dioxide (XCO2), methane (XCH4), and carbon monoxide (XCO). The mission concept in particular facilitates sampling of the diurnal variation with several measurements per day during summer. Here, we present the mission concept and carry out an initial performance assessment of the retrieval capabilities. The radiometric performance of the 4 grating spectrometers is tuned to reconcile small ground-pixel sizes (~2 km × 3 km at 50° latitude) with short single-shot exposures (~2.9 s) that allow for sampling continental regions such as central Europe within 2 h while providing a sufficient signal-to-noise ratio. The noise errors to be expected for XCO2, XCH4, and XCO are assessed through retrieval simulations for a European trial ensemble. Generally, single-shot precision for the targeted XCO2 and XCH4 is better than 0.5 % with some exception for scenes with low infrared surface albedo observed under low sun conditions in winter. For XCO, precision is generally better than 10 %. Performance for aerosol and cirrus loaded atmospheres is assessed by mimicking G3E's slant view on Europe for an ensemble of atmospheric scattering properties used previously for evaluating nadir-viewing low-Earth-orbit (LEO) satellites. While retrieval concepts developed for LEO configurations generally succeed in mitigating aerosol- and cirrus-induced retrieval errors for G3E's setup, residual errors are somewhat greater in geostationary orbit (GEO) than in LEO. G3E's deployment in the vicinity of the Meteosat Third Generation (MTG) satellites has the potential to make synergistic use of MTG's sounding capabilities e.g. with respect to characterization of aerosol and cloud properties or with respect to enhancing carbon monoxide retrievals by combining G3E's solar and MTG's thermal infrared spectra.

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) presents new opportunities for the retrieval of trace gases in the upper troposphere and lower stratosphere. The radiometric calibration of the measured signal is achieved using in-flight measurements of reference blackbody and upward-pointing "deep space" scenes. In this paper, we present techniques developed specifically to calibrate GLORIA data exploiting the instrument's imaging capability. The algorithms discussed here make use of the spatial correlation of parameters across GLORIA's detector pixels in order to mitigate the noise levels and artefacts in the calibration measurements. This is achieved by combining a priori and empirical knowledge about the instrument background radiation with noise-mitigating compression methods, specifically low-pass filtering and principal component analysis (PCA). In addition, a new software package for the processing of GLORIA data is introduced which allows us to generate calibrated spectra from raw measurements in a semi-automated data processing chain.

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA), a Fourier-transform-spectrometer-based limb spectral imager, operates on high-altitude research aircraft to study the transit region between the troposphere and the stratosphere. It is one of the most sophisticated systems to be flown on research aircraft in Europe, requiring constant monitoring and human intervention in addition to an automation system. To ensure proper functionality and interoperability on multiple platforms, a flexible control and communication system was laid out. The architectures of the communication system as well as the protocols used are reviewed. The integration of this architecture in the automation process as well as the scientific campaign flight application context are discussed.

We report first chemistry mode retrieval results from the new airborne limb-imaging infrared FTS (Fourier transform spectrometer) GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) and comparisons with observations by the conventional airborne limb-scanning infrared FTS MIPAS-STR (Michelson Interferometer for Passive Atmospheric Sounding – STRatospheric aircraft). For GLORIA, the flights aboard the high-altitude research aircraft M55 Geophysica during the ESSenCe campaign (ESa Sounder Campaign 2011) were the very first in field deployment after several years of development. The simultaneous observations of GLORIA and MIPAS-STR during the flight on 16 December 2011 inside the polar vortex and under conditions of optically partially transparent polar stratospheric clouds (PSCs) provided us the first opportunity to compare the observations by two different infrared FTS generations directly. We validate the GLORIA results with MIPAS-STR based on the lower vertical resolution of MIPAS-STR and compare the vertical resolutions of the instruments derived from their averaging kernels. The retrieval results of temperature, HNO3, O3, H2O, CFC-11 and CFC-12 show reasonable agreement of GLORIA with MIPAS-STR and collocated in situ observations. For the horizontally binned hyperspectral limb images, the GLORIA sampling outnumbered the horizontal cross-track sampling of MIPAS-STR by up to 1 order of magnitude. Depending on the target parameter, typical vertical resolutions of 0.5 to 2.0 km were obtained for GLORIA and are typically a factor of 2 to 4 better compared to MIPAS-STR. While the improvement of the performance, characterization and data processing of GLORIA are the subject of ongoing work, the presented first results already demonstrate the considerable gain in sampling and vertical resolution achieved with GLORIA.

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an airborne infrared limb imager combining a two-dimensional infrared detector with a Fourier transform spectrometer. It was operated aboard the new German Gulfstream G550 High Altitude LOng Range (HALO) research aircraft during the Transport And Composition in the upper Troposphere/lowermost Stratosphere (TACTS) and Earth System Model Validation (ESMVAL) campaigns in summer 2012. This paper describes the retrieval of temperature and trace gas (H2O, O3, HNO3) volume mixing ratios from GLORIA dynamics mode spectra that are spectrally sampled every 0.625 cm−1. A total of 26 integrated spectral windows are employed in a joint fit to retrieve seven targets using consecutively a fast and an accurate tabulated radiative transfer model. Typical diagnostic quantities are provided including effects of uncertainties in the calibration and horizontal resolution along the line of sight. Simultaneous in situ observations by the Basic Halo Measurement and Sensor System (BAHAMAS), the Fast In-situ Stratospheric Hygrometer (FISH), an ozone detector named Fairo, and the Atmospheric chemical Ionization Mass Spectrometer (AIMS) allow a validation of retrieved values for three flights in the upper troposphere/lowermost stratosphere region spanning polar and sub-tropical latitudes. A high correlation is achieved between the remote sensing and the in situ trace gas data, and discrepancies can to a large extent be attributed to differences in the probed air masses caused by different sampling characteristics of the instruments. This 1-D processing of GLORIA dynamics mode spectra provides the basis for future tomographic inversions from circular and linear flight paths to better understand selected dynamical processes of the upper troposphere and lowermost stratosphere.

The novel airborne Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) measures infrared emission of atmospheric trace constituents. GLORIA comprises a cooled imaging Fourier transform spectrometer, which is operated in unpressurized aircraft compartments at ambient temperature. The whole spectrometer is pointed by the gimbal towards the atmospheric target. In order to reach the required sensitivity for atmospheric emission measurements, the spectrometer optics needs to operate at a temperature below 220 K. A lightweight and compact design is mandatory due to limited space and high agility requirements. The cooled optical system needs to withstand high pressure and temperature gradients, humidity, and vibrations. A new cooling system based on carbon dioxide and liquid nitrogen combined with high-performance insulation has been developed to meet the mechanical, thermal, and logistical demands. The challenging mechanical and spatial requirements lead to the development of a novel rigid linear slide design in order to achieve the large optical path difference for high spectral resolution. This paper describes the mechanical and thermal setup of GLORIA and presents the performance results on two different research aircrafts.

The three-dimensional quantification of small-scale processes in the upper troposphere and lower stratosphere is one of the challenges of current atmospheric research and requires the development of new measurement strategies. This work presents the first results from the newly developed Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) obtained during the ESSenCe (ESa Sounder Campaign) and TACTS/ESMVal (TACTS: Transport and composition in the upper troposphere/lowermost stratosphere, ESMVal: Earth System Model Validation) aircraft campaigns. The focus of this work is on the so-called dynamics-mode data characterized by a medium-spectral and a very-high-spatial resolution. The retrieval strategy for the derivation of two- and three-dimensional constituent fields in the upper troposphere and lower stratosphere is presented. Uncertainties of the main retrieval targets (temperature, O3, HNO3, and CFC-12) and their spatial resolution are discussed. During ESSenCe, high-resolution two-dimensional cross-sections have been obtained. Comparisons to collocated remote-sensing and in situ data indicate a good agreement between the data sets. During TACTS/ESMVal, a tomographic flight pattern to sense an intrusion of stratospheric air deep into the troposphere was performed. It was possible to reconstruct this filament at an unprecedented spatial resolution of better than 500 m vertically and 20 × 20 km horizontally.

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an imaging Fourier transform spectrometer that is capable of operating on various high-altitude research aircraft. It measures the atmospheric emission in the thermal infrared spectral region in limb and nadir geometry. GLORIA consists of a classical Michelson interferometer combined with an infrared camera. The infrared detector has a usable area of 128 × 128 pixels, measuring up to 16 384 interferograms simultaneously. Imaging Fourier transform spectrometers impose a number of challenges with respect to instrument calibration and algorithm development. The optical setup with extremely high optical throughput requires the development of new methods and algorithms for spectral and radiometric calibration. Due to the vast amount of data there is a high demand for scientifically intelligent optimisation of the data processing. This paper outlines the characterisation and processing steps required for the generation of radiometrically and spectrally calibrated spectra. Methods for performance optimisation of the processing algorithm are presented. The performance of the data processing and the quality of the calibrated spectra are demonstrated for measurements collected during the first deployments of GLORIA on aircraft.

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an imaging limb emission sounder operating in the thermal infrared region. It is designed to provide measurements of the upper troposphere/lower stratosphere with high spatial and high spectral resolution. The instrument consists of an imaging Fourier transform spectrometer integrated into a gimbal. The assembly can be mounted in the belly pod of the German High Altitude and Long Range research aircraft (HALO) and in instrument bays of the Russian M55 Geophysica. Measurements are made in two distinct modes: the chemistry mode emphasises chemical analysis with high spectral resolution, and the dynamics mode focuses on dynamical processes of the atmosphere with very high spatial resolution. In addition, the instrument allows tomographic analyses of air volumes. The first measurement campaigns have shown compliance with key performance and operational requirements.

The upper troposphere/lower stratosphere (UTLS) represents an important part of the climate system. Even small changes in the composition and dynamic structure of this region have significant radiative effects. Quantifying the underlying physical and chemical processes therefore represents a crucial task. Currently, there is a lack of UTLS observations with sufficient three-dimensional resolution. The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) aircraft instrument addresses this observational lack by providing observations of numerous trace constituents as well as temperature and cloud structures with an unprecedented combination of vertical resolution (up to 300 m) and horizontal resolution (about 30 km &times 30 km). As a result, important scientific questions concerning stratosphere–troposphere exchange, the occurrence of subvisible cirrus clouds in the lowermost stratosphere (LMS), polar chemistry, and gravity wave processes can be addressed, as reviewed in this paper.

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is a prototype of an imaging Fourier Transform Spectrometer (FTS) for PREMIER, a former candidate mission for ESA's Earth Explorer 7. GLORIA is deployed on board various research aircraft such as the Russian M55 Geophysica or the German HALO. The instrument provides detailed infrared images of the Upper Troposphere/Lower Stratosphere (UTLS) region, which plays a crucial role in the climate system. GLORIA uses a two-dimensional detector array for infrared limb observations in emission and therefore needs large-area blackbody radiation sources (126 mm × 126 mm) for calibration. In order to meet the highly demanding uncertainty requirements for the scientific objectives of the GLORIA missions and due to the sophisticated tomographic evaluation scheme, the spatial distribution of the radiance temperature of the blackbody calibration sources has to be determined with an uncertainty of about 0.1 K. Since GLORIA is exposed to the hostile environment of the UTLS with mutable low temperature and pressure, an in-flight calibration system has to be carefully designed to cope with those adverse circumstances. The GLORIA in-flight calibration system consists of two identical weight-optimised high-precision blackbody radiation sources, which are independently stabilised at two different temperatures. The two point calibration is in the range of the observed atmospheric infrared radiance emissions with 10 K below and 30 K above ambient temperature, respectively. Thermo-Electric Coolers are used to control the temperature of the blackbody radiation sources offering the advantage of avoiding cryogens and mechanical coolers. The design and performance of the GLORIA in-flight calibration system is presented. The blackbody calibration sources have been comprehensively characterised for their spatially (full aperture) and spectrally (7 to 13 μm) resolved radiation properties in terms of radiance temperatures traceable to the International Temperature Scale (ITS-90) at the Physikalisch-Technische Bundesanstalt (PTB), the national metrology institute of Germany.

The first profile retrieval results of the Terahertz and submillimeter Limb Sounder (TELIS) balloon instrument are presented. The spectra are recorded during a 13-h balloon flight on 24 January 2010 from Kiruna, Sweden. The TELIS instrument was mounted on the MIPAS-B2 gondola and shared this platform with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and the mini-Differential Optical Absorption Spectroscopy (mini-DOAS) instruments. The flight took place within the Arctic vortex at an altitude of ≈34 km in chlorine activated air, and both active (ClO) and inactive chlorine (HCl) were measured over an altitude range of respectively ≈16–32 km and ≈10–32 km. In this altitude range, the increase of ClO concentration levels during sunrise has been recorded with a temporal resolution of one minute. During the daytime equilibrium, a maximum ClO level of 2.1 ± 0.3 ppbv has been observed at an altitude of 23.5 km. This equilibrium profile is validated against the ClO profile by the satellite instrument Microwave Limb Sounder (MLS) aboard EOS Aura. HCl profiles have been determined from two different isotopes – H35Cl and H37Cl – and are also validated against MLS. The precision of all profiles is well below 0.01 ppbv and the overall accuracy is therefore governed by systematic effects. The total uncertainty of these effects is estimated to be maximal 0.3 ppbv for ClO around its peak value at 23.5 km during the daytime equilibrium, and for HCl it ranges from 0.05 to 0.4 ppbv, depending on altitude. In both cases the main uncertainty stems from a largely unknown non-linear response in the detector.

GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) is a new remote sensing instrument essentially combining a Fourier transform infrared spectrometer with a two-dimensional (2-D) detector array in combination with a highly flexible gimbal mount. It will be housed in the belly pod of the German research aircraft HALO (High Altitude and Long Range Research Aircraft). It is unique in its combination of high spatial and state-of-the art spectral resolution. Furthermore, the horizontal view angle with respect to the aircraft flight direction can be varied from 45° to 135°. This allows for tomographic measurements of mesoscale events for a wide variety of atmospheric constituents. In this paper, a tomographic retrieval scheme is presented, which is able to fully exploit the manifold radiance observations of the GLORIA limb sounder. The algorithm is optimized for massive 3-D retrievals of several hundred thousands of measurements and atmospheric constituents on common hardware. The new scheme is used to explore the capabilities of GLORIA to sound the atmosphere in full 3-D with respect to the choice of the flightpath and to different measurement modes of the instrument using ozone as a test species. It is demonstrated that the achievable resolution should approach 200 m vertically and 20 km–30 km horizontally. Finally, a comparison of the 3-D inversion with conventional 1-D inversions using the assumption of a horizontally homogeneous atmosphere is performed.

Gravity wave (GW) remote sensing from space now has reached a stage of maturity that some first confinements for GW modeling can be deduced. This is in particular due to global distributions of absolute values of GW momentum flux from infrared limb sounders and due to 2-D maps of the horizontal wave field provided by nadir viewing instruments. The logical step forward is an infrared limb imager (ILI) which combines the good vertical resolution of limb sounding with horizontal mapping capabilities and provides 3-D images of the GW temperature structures. In this paper we investigate 1) how an ILI advances measurements of GW momentum flux, 2) which additional benefits are achieved by limb imaging of GWs, and 3) how an ILI compares to other GW momentum flux measurements, in-situ, ground-based, and from space. In particular, the large advance made by gaining regular 3-D sampling is demonstrated.