Christian Buil

IASI instrument: technical overview and measured performances

The Infrared Atmospheric Sounding Interferometer (IASI) is a key payload element of the METOP series of European meteorological polar-orbit satellites. IASI will provide very accurate data about the atmosphere, land and oceans for application to weather predictions and climate studies. The IASI measurement technique is based on passive IR remote sensing using an accurately calibrated Fourier Transform Spectrometer operating in the 3.7 - 15.5 μm spectral range and an associated infrared imager operating in the 10.3-12.5 μm spectral range. The optical configuration of the sounder is based on a Michelson interferometer. Interferograms are processed by the on-board digital processing subsystem which performs the inverse Fourier Transform and the radiometric calibration. The integrated infrared imager allows the co registration of the IASI sounder with AVHRR imager on-board METOP. The first model (proto-flight) of IASI has successfully completed a verification program conducted at ALCATEL SPACE premises in Cannes. This paper provides a brief overview of the IASI mission, instrument architecture and key performances results. A companion paper1 by Alcatel provides more information on instrument design and development.

A new space instrumental concept for the measurement of CO2 concentration in the atmosphere

Measuring the concentration of greenhouse gases from space is a topical challenge. They are measured via a precise analysis of the signature of chemical gaseous species (CO2, CH4, CO, etc.) in the spectrum of the Earths atmosphere. Two types of spectrometer are commonly used. The first is based on the interference between two radiation waves. The Infrared Atmospheric Sounding Interferometer (IASI) aboard the METOP satellite is a good example of a fullyoperational instrument of this kind. The second is based on the use of dispersive optical components. These instruments must have high radiometric and spectral resolutions in narrow spectral bands to be able to discriminate absorption lines from various atmospheric chemical species and to quantify their concentration. This is the case, for example, of the instrument aboard NASAs Orbiting Carbon Observatory (OCO). Our analysis led us to define a new instrument concept, based on a dispersive grating spectrometer, offering similar performance in a more compact and therefore less expensive instrument. After describing this instrument, which uses a specific grating component, a preliminary assessment of performances will be presented, including the theoretical calculations and formulae. A mock-up version of this specific grating demonstrated the feasibility of this concept and its capabilities. This preliminary design is encouraging and shows that such a spectrometer may be compatible with a microsatellite bus. Some prospects for improvement are also considered.

Static Fourier transform spectroscopy breadboards for atmospheric chemistry and climate

New types of sounders dedicated to selected species could be used on small satellites to monitor atmospheric chemistry with simpler instruments. A new kind of Fourier transform spectrometer has been patented by CNES a few years ago. Based on a static configuration, two projects are being studied at CNES with laboratory breadboards. One is dedicated to CO2 concentration monitoring in near infrared. The other one works in thermal infrared to study CO and O3 atmospheric profiles. MoLI breadboard, with a new highly integrated interferometric core, will be used for a long time measurement of CO2 concentration. MOPI is another breadboard under development to transpose this concept in thermal infrared during the SIFTI phase A study. These new generation spectrometers consist in a Michelson interferometer with staircase mirrors assembled by molecular adhesion. They are adapted to narrow spectra sounding from space and could lead to totally static and highly stabilized instruments.

Interferometric Spectro-Imager System (ISIS)

This paper concerns an original design for a spectro-imager, in which spectral analysis is performed by interferometry, with the necessary path difference being obtained as a direct result of the apparent displacement of the source as seen from the satellite while orbiting. Descriptions will be given of the bread-board model used to determine the feasibility of the project, and of the transportable spectro-imager (SIT) currently being produced at Aerospatiale Cannes (France).

Instrumental Aspects of IASI

This paper gives the status of the IASI instrument after more than two years in orbit. It details aspects like stability of the instrument, decontaminations and anomalies caused by radiative-environment.

Static Infrared Fourier Transform Interferometer (SIFTI):Benefits of Phase Modulation Processing

SIFTI (Static Infrared Fourier Transform Interferometer) is a high resolution spectrometer, part of the TRAQ payload. This paper presents a description of this instrument and gives a preliminary radiometric performance in presence of phase modulation.

A new space instrumental concept based on dispersive components for the measurement of CO2 concentration in the atmosphere

Measuring the concentration of greenhouse gases from space is a current challenge. This measurement is achieved via a precise analysis of the signature of chemical gaseous species (CO2, CH4, CO, etc.) in the spectrum of the reflected sunlight. First at all, two families of spectrometers have been studied for the MicroCarb mission. The first family is based on the phenomena of interference between two radiation waves (Michelson Interferometer). The second family is based on the use of dispersive optical components. The second family has been selected for the forthcoming studies in the MicroCarb project. These instruments must have high radiometric and spectral resolutions, in narrow spectral bands, in order to discriminate between absorption lines from various atmospheric chemical species, and to quantify their concentration. This is the case, for example, for the instrument onboard the OCO-2 satellite (NASA/JPL). Our analysis has led us to define a new instrumental concept, based on a dispersive grating spectrometer, with the aim of providing the same accuracy level as the OCO-2, but with a more compact design for accommodation on the Myriade Evolution microsatellite class. This compact design approach will allow us to offer a moderate-cost solution to fulfil mission objectives. Two other studies based on dispersive grating are in progress by CNES prime contractors (ASTRIUM and THALES ALENIA SPACE). A summary of the main specifications of this design will be described, in particular the approach with the so-called “merit function”. After a description of such a space instrument, which uses a specific grating component, a preliminary assessment of performances will be presented, including the theoretical calculations and formula. A breadboard implementation of this specific grating has allowed us to show the practicality of this concept and its capabilities. Some results of this breadboard will be described. In addition, an instrument simulator is being developed to validate the performances of this concept. A grating component prototype has been built, and the specifications, together with the expected performances, will be described, in particular the polarisation ratio. Some elements about detectors will be also given regarding their suitability for the mission. This preliminary design is encouraging and shows that such a spectrometer may be compatible with a microsatellite platform (low mass, low power and compact design). Some prospects of improvements will also be considered.

Probing the hermean exosphere by ultraviolet spectroscopy (PHEBUS): optical simulation of an ultraviolet spectrometer

PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) consists of an ultraviolet spectrometer for the MPO (Mercury Planetary Orbiter) of the Bepi-Colombo Mission. The goal of this instrument is to detect emission lines of Mercury exosphere in the bandwidth from 55 to 315 nm by recording full spectra. This instrument is made of an entrance mobile baffle, which is necessary to scan vertically the Mercury atmosphere, an off-axis mirror entrance, a slit, two gratings and two detectors. A few different designs, simulated by optical software, are analysed in this paper. They provide essential results as the instrument spectral resolution. Besides a radiometric model is established to observe the spectra we would obtain on the detectors.

From the Concept to the Definition of the SIFTI Instrument: Static Infrared Fourier Transform Interferometer

SIFTI, a static interferometer using a pair of crossed staircase fixed mirrors, will provide high quality TIR spectra of O3and CO. At phase A mid-term, we review main technical choices, preliminary budgets and performances.