Space-based Spectrometer Research Articles

Compact, high-resolution spectrometers with grating–metasurface coupling for CO2 detection

Off-axis reflective optical systems are commonly used to minimize the dimensions of spectrometers. However, despite sophisticated off-axis reflective designs, constructing systems with excellent spectral and spatial resolutions, particularly those that can be used for CO2 gas detection in space-based spectrometers, necessitates maintaining a significant overall instrument size. The metasurface is an innovative optical device that can enable spectrometer miniaturization. For CO2 spectral detection in the near-infrared spectral range of 1590–1620 nm, this study proposes a device that integrates a grating with a metasurface for off-axis convergence within the relevant wavelength range. The grating facilitates primary spectral dispersion, whereas the metasurface enables off-axis convergence at different wavelengths. Grating–metasurface coupling effectively enhances the spectral resolution of the spectrometer, mitigating any efficiency losses caused by excessive off-axis angles. The proposed strategy exhibits high potential for on-chip integrated photonics and the fabrication of small-size spaceborne spectrometers.

Ozone Monitoring Spectrometer-Limb observation (OMSL) on-orbit polarization correction for atmospheric radiation measurements

OMSL is a passive, space-based spectrometer designed to obtain the vertical profiles of trace gases such as ozone, by measuring atmospheric radiation in limb-viewing mode. Due to the presence of internal components such as gratings and dichroic mirror, OMSL is sensitive to polarization of incident light, which can lead to errors in the radiometric measurements, especially when the incident light is fully polarized. To eliminate the errors, an on-orbit polarization correction method is applied to OMSL. The key to this method is to calculate the polarization correction factor by combining the on-orbit polarization measurement and the pre-flight polarization calibration. In this paper, the on-orbit polarization correction method is introduced in detail. Furthermore, the first results of the on-orbit correction are analyzed and compared with simulation of the SCIATRAN.

Inferring global surface HCHO concentrations from multisource hyperspectral satellites and their application to HCHO-related global cancer burden estimation

Formaldehyde (HCHO) is a toxic and hazardous air pollutant that widely exists in atmosphere. Insufficient spatial and temporal coverage of surface HCHO measurements is limiting studies on surface HCHO-related air quality management and health risk assessment. This study develops a method to derive global ground-level HCHO concentrations from satellite-based tropospheric HCHO columns using TM5-simulated surface-to-column conversion factor with coarse spatial resolution. The method improves the factor more representative in finer grids by constraining TM5-simulated vertical profile shapes with satellite HCHO columns. The surface HCHO concentrations derived by the Ozone Mapping and Profiler Suite (OMPS) show good correlation with in situ HCHO measurements (R = 0.59) from the U.S. Environmental Protection Agency surface network. We investigated how surface HCHO relates to urbanization and population aggregation over seven regions with high HCHO pollution. The results show urban HCHO increases as a power function with population size in China, India, and West Asia. HCHO concentrations in rural aeras also present strong log–log relationship with population aggregation in China, India, the United States, and Europe. Moreover, OMPS-derived ground-level HCHO concentrations were used to estimate global cancer burden caused by long-term outdoor HCHO exposure. The results show that up to 418188 more people worldwide will develop this cancer during the human life cycle. The global cancer burden is mainly from the South-East Asia region (33.11 %) and the Western Pacific region (22.95 %). This cancer occurrence in India and China is ranked 1st and 2nd in the world due to the large population size and serious HCHO pollution. Besides, global surface HCHO concentrations and cancer burden derived from the Environmental Trace Gases Monitoring Instrument which is China’s first hyperspectral space-based spectrometer are found similar patterns with that from OMPS. Our results provide new insight into the impact of population urbanization on HCHO pollution and global outdoor HCHO-caused health risks.

Optical Design of a Novel Wide-Field-of-View Space-Based Spectrometer for Climate Monitoring.

We report on a near-infrared imaging spectrometer for sensing the three most prominent greenhouse gases in the atmosphere (water vapor, carbon dioxide and methane). The optical design of the spectrometer involves freeform optics, which enables achieving exceptional performance and allows progressing well beyond the state-of-the-art in terms of compactness, field-of-view, and spatial resolution. The spectrometer is intended to be launched on a small satellite orbiting at 700 km and observing the Earth with a wide field-of-view of 120° and a spatial resolution of 2.6 km at nadir. The satellite will ultimately allow for improved climate change monitoring.

A fibre-based 2D-slit homogenizer concept for high-precision space-based spectrometer missions

The measurement accuracy of recent and future space-based imaging spectrometers with a high spectral and spatial resolution suffer from the inhomogeneity of the radiances of the observed Earth scene. The Instrument Spectral Response Function (ISRF) is distorted due to the inhomogeneous illumination from scene heterogeneity. This gives rise to a pseudo-random error on the measured spectra. In order to assess the spectral stability of the spectrograph, stringent requirements are typically defined on the ISRF such as shape knowledge and the stability of the centroid position of the spectral sample. The high level of spectral accuracy is particularly crucial for missions quantifying small variations in the total column of well-mixed trace gases like hbox {CO}_{2}. In the framework of the hbox {CO}_{2} Monitoring Mission (CO2M) industrial feasibility study (Phase A/B1 study), we investigated a new slit design called 2D-Slit Homogenizer (2DSH). This new concept aims to reduce the Earth scene contrast entering the instrument. The 2DSH is based on optical fibre waveguides assembled in a bundle, which scramble the light in across-track (ACT) and along-track (ALT) direction. A single fibre core dimension in ALT defines the spectral extent of the slit and the dimension in ACT represents the spatial sample of the instrument. The full swath is given by the total size of the adjoined fibres in ACT direction. In this work, we provide experimental measurement data on the stability of representative rectangular core shaped fibre as well as a preliminary pre-development of a 2DSH fibre bundle. In our study, the slit concept has demonstrated significant performance gains in the stability of the ISRF for several extreme high-contrast Earth scenes, achieving a shape stability of <0.5{%} and a centroid stability of <0.25 text {pm} (NIR). Given this unprecedented ISRF stabilization, we conclude that the 2DSH concept efficiently desensitizes the instrument for radiometric and spectral errors with respect to the heterogeneity of the Earth scene radiance.

A New Version of the SOLAR-ISS Spectrum Covering the 165 – 3000 nm Spectral Region

The accurate measurement of the solar spectrum at the top of the atmosphere and its variability are fundamental inputs for solar physics (Sun modeling), terrestrial atmospheric photochemistry, and Earth’s climate (climate’s modeling). These inputs were the prime objective set in 1996 for the SOLAR International Space Station (ISS). The SOLAR package represents a set of three solar instruments measuring the total and spectral absolute irradiance from 16 nm to 3088 nm. SOLAR was launched with the European Columbus space laboratory in February 2008 aboard the NASA Space Shuttle Atlantis. SOLAR on the ISS tracked the Sun until it was decommissioned in February 2017. The SOLar SPECtrum (SOLSPEC) instrument of the SOLAR payload allowed the measurement of solar spectra in the 165 – 3000 nm wavelength range for almost a decade. Until the end of its mission, SOLAR/SOLSPEC was pushed to its limits to test how it was affected by space environmental effects (external thermal factors) and to better calibrate the space-based spectrometer. To that end, a new solar reference spectrum (SOLAR-ISS – V1.1) representative of the 2008 solar minimum was obtained from the measurements made by the SOLAR/SOLSPEC instrument and its calibrations. The main purpose of this article is to improve the SOLAR-ISS reference spectrum (between 165 and 180 nm in the far ultraviolet, between 216.9 and 226.8 nm in the middle ultraviolet, and between 2400 and 3000 nm in the near-infrared). SOLAR-ISS has a resolution better than 0.1 nm between 165 and 1000 nm, and 1 nm in the 1000 – 3000 nm wavelength range. Finally, a first comparison is made between the new SOLAR-ISS spectrum (V2.0) and the Total and Spectral solar Irradiance Sensor (TSIS-1) spectrum obtained from its first observations from the ISS. Indeed, the launch of TSIS in December 2017 provides a new light on the absolute determination of the solar spectrum and especially in the infrared region of the spectrum.

Effects of spectral sampling rate and range of CO2 absorption bands on XCO2 retrieval from TanSat hyperspectral spectrometer

The spectral sampling rate and range of CO2 absorption bands are critical for the optimal design of hyperspectral instrument for CO2 observation satellite. Undersampling of spectra in space-based spectrometer significantly contaminates signals measured in the CO2 1.61 μm-band. The CO2 dry-air column (XCO2) error due to spectral undersampling can be up to ~1 ppm, which is the target precision of the Chinese Carbon Satellite (TanSat) for a single sounding. Undersampling error depends on surface albedo, solar zenith angle, and scattering properties in the atmosphere. The spectral sampling rate is recommended to be greater than 2.0 pixels per full width at half maximum to avoid undersampling. Reduction of spectral resolution and the use of narrower spectral regions can improve spectral sampling with little changes in CO2 retrieval sensitivity without losing much information. The full-band approach provides direct constraints on the wavelength-dependent surface albedo and particle scattering from the measurements. To keep a broader band, we recommend reduction of the spectral resolution by a factor of two.

Speckle effect on diffusers in space-based spectrometers

The use of diffusers in space-based spectrometers leads to the creation of speckles, in the entrance slit. Due to these speckles, unwanted modulations in the measured spectral data are observed. We explain the origin of these unwanted modulations, which we refer to as speckle-induced spectral features or, in short, spectral features. A single value is introduced that can be used to indicate the effect of the speckle-induced spectral features, the spectral feature amplitude (SFA). Measurement and modeling of the SFA parameter on aluminum diffusers are presented.

Improvement of the O 2A band spectroscopic database for satellite-based cloud detection

Detection of cloud parameters from space-based spectrometers can employ the vibrational bands of O 2 in the b 1∑ g + ← X 3∑ g − spin-forbidden electronic transition manifold, particularly the Δ v = 0 A band. The European Space Agency's Global Ozone Monitoring Experiment (GOME), on board the European Remote Sensing 2 satellite, uses the A band in the Initial Cloud Fitting Algorithm (ICFA). The work reported here consists of making substantial improvements in the line-by-line spectral database for the A band, testing whether an additional correction to the line shape function is necessary in order to model correctly the atmospheric transmission in this band, and calculating prototype cloud and ground template spectra for comparison with satellite measurements.