IASI

The project

Developed by CNES (Centre National d’Etudes Spatiales) in partnership with Eumetsat (EUropean organisation for the exploitation of METeorological SATellites), the IASI (Infrared Atmospheric Sounding Interferometer) instrument was designed for operational meteorology and for monitoring atmospheric chemistry and climate.
IASI is a thermal infrared Fourier transform spectrometer that provides radiance spectra and derived atmospheric variables to feed operational weather forecasting centers, the Copernicus Services and the scientific community.

Three identical instruments have been developed. They are carried on board Eumetsat’s METOP satellites. This series of IASI instruments provides homogeneous long term data sets. The continuity of the IASI program has been secured with the IASI-New generation (IASI-NG) instruments scheduled to be launched on the Metop Second Generation satellites (METOP-SG) in 2024 and onwards.

From a polar orbit at an altitude of about 800 km and with a swath of more than 2000 km, the IASI instruments observe the entire Earth’s atmosphere twice a day operating in the Short Wave InfraRed (SWIR) and Thermal InfraRed (TIR) domains. They provide useful observations for meteorology, atmospheric composition monitoring and climate studies.

An important contribution to weather forecasting

IASI provides temperature and humidity profiles, operationally and in near real time, which are integrated into the numerical weather prediction (NWP) models of the meteorological agencies and which contribute significantly to the improvement of these models. For example, it makes it possible to refine prediction at daily scale, to extend the forecast period, to better locate rainfall areas, to better anticipate the evolution of storms… IASI’s impact on the quality of the weather prediction is the greatest of all the satellite instruments used in NWP models.

A unique tool for monitoring atmospheric composition

IASI also makes a strong contribution for the monitoring of atmospheric composition on a global scale. The high radiometric quality of the measurements makes it possible to determine the vertical composition of the main greenhouse gases, such as carbon dioxide (CO₂) and methane (CH₄), as well as the totzal column a lot of trace gases, including ozone (O₃), carbon monoxide (CO) and sulfur dioxide (SO₂). IASI is also used to monitor gas plumes and aerosols allowing monitoring of the evolution of large fires and of certain pollution peaks. Other important applications include monitoring the evolution of the ozone hole or measuring the impact of intensive agriculture through the inversion of ammonia (NH₃) emissions.

Valuable data for the study of climate change

The quality and stability of IASI’s measurements and its long operating life (already 15 years and many more to come…) make it a valuable instrument for building up long time series of several essential climate variables (ECV) such as temperature, humidity, cloud cover, aerosols, greenhouse gases, continental surface properties…

Moreover, its unprecedented performance for a space instrument and its exceptional spectral and radiometric stability make it the reference instrument for the intercalibration of infrared Earth observation instruments within the international framework of GSICS.

Satellite

The 3 satellites METOP-A, B and C are identical. They constitute the first series of Eumetsat polar orbiting satellites and were launched in 2006, 2012 and 2018 respectively.

Main characteristics of the satellite:

• Mass at launch: 4085 kg
• Overall size:     6.2 m x 3.4 m x 3.4 m (launch configuration)

17.6 m x 6.7 m x 5.4 m (on-orbit configuration)

• Power: 3890 W (end of life)
• 3 axis stabilization

Since October 2012, at least two satellites have been operating simultaneously. To optimise the use of the observations, the satellites fly in tandem, shifted by half an orbit: in phase opposition on the same orbit. From June 2019 to October 2021, during which time 3 satellites were operating, METOP B and C were considered operational and METOP A was positioned shifted by a quarter of an orbit behind METOP B.

Main METOP orbit parameters:

• Near-circular sun-synchronous polar orbit
• Mean altitude: 817 km
• Inclination: 98.7°
• Local solar time at equator (descending node): 9h30
• Cycle: 29 days (412 orbits)
• Products

The METOP payload consists of a dozen instruments, eight of which, apart from IASI, are used in meteorology.

Other meteorological instruments on board METOP:

• AMSU (2 instruments), Advanced Microwave Sounding Unit – a microwave temperature sounder with 15 channels to yield temperature sounding over completely overcast conditions and to help cloud detection for the companion infrared sounding instruments – NOAA instruments
• MHS, Microwave Humidity Sounder –  a five microwave channels instrument sensitive to liquid water in clouds which also provides a qualitative estimate of the rainfall rate – Eumetsat instrument
• HIRS/4, High Resolution Infrared Radiation Sounder –  a sounder with 19 channels in the infrared and 1 channel in the visible spectral range which mainly provides temperature and humidity profiles and ozone content – NOAA instrument
• AVHRR/3, Advanced Very High Resolution Radiometer –  a multispectral imager with 6 channels in the visible and infrared spectral ranges. The focus is on cloud parameters (distribution, top height, emissivity, etc.), sea surface temperatures, sea ice, snow and vegetation cover – NOAA instrument
• GOME-2, Global Ozone Monitoring Experiment-2 – a UV/VIS spectrometer for atmospheric ozone and other trace gaz concentration –ESA/Eumetsat instrument
• ASCAT, Advanced Scatterometer – a scatterometer for surface stress and surface wind measurements over ocean – ESA instrument
• GRAS, GNSS Receiver for Atmospheric Sounding – a GNSS receiver for radio occultation measurements of temperature et humidity profiles in limb – ESA/Eumetsat instrument

IASI instrument

The IASI instrument is designed to measure atmospheric spectra in the infrared. It consists of a Fourier Transform Spectrometer (FTS) based on a Michelson interferometer (the flat mirrors are replaced with retroreflecting corner-cube mirrors) operating in the 3.6-15.5 µm spectral range (2760-645 cm^-1), coupled to an integrated imaging system, permitting the characterization of cloudiness inside the field of view of the FTS.

It comprises an onboard management unit (sensor module) which acquires raw interferograms and a digital processing subsystem which performs the inverse Fourier transform and the radiometric calibration. The emitted infrared radiation is splitted into 8461 channels.

To achieve global coverage, IASI observes the Earth up to an angle of 48.3º on both sides of the satellite track. Every 8s (48 km at nadir), it scans the full swath width of about 2050 km in the cross-track direction. During the scan, the mirror moves to 30 measurement positions. Each instantaneous field of view (IFOV) of 3.33 º x 3.33º or about 50 km x 50 km at nadir is composed of 2 x 2 pixels, each corresponds to a circular field-of-view of 0.83º (about 12 km diameter footprint on the ground at nadir). A calibration (cold-space viewing and updating of calibration coefficients) is performed after each scan cycle.

Main instrument characterisitcs:

Products:

The processing of IASI level 1 products, and of some levels 2, is implemented by Eumetsat as part of EPS (Eumetsat Polar System). The data are processed as soon as they are received at the station (less than 2 hours after acquisition), transferred immediately to the weather forecasting centres and made more widely available thanks to the Eumetcast broadcast system.

The AERIS ESPRI data center collects the Eumetsat level 1C (atmospheric radiance spectra) and level 2 (temperature, humidity and cloud content) products through an antenna dedicated to the reception of data transmitted by the Eumetcast satellite link. These products are archived at AERIS but, according to an agreement with Eumetsat, are subject to restricted access for the French scientific community.

From the received level 1 data, AERIS generates level 2 and level 3 products of atmospheric compounds such as carbon monoxide (CO), ozone (O₃), methane (CH₄), sulfur dioxide (SO₂), ammonia (NH₃), dust, formic acid (HCOOH). An original inversion processing using the synergy of IASI and GOME2 instruments is also carried out to provide vertical profiles of O₃.
Moreovere, in the framework of the GEWEX (Global Energy and Water Exchanges) programme, AERIS distributes cloud characterisation products obtained from IASI data.
All these products are freely available on the AERIS/IASI portal.

Products available in open access at AERIS

• Vertical profile (level 2) and total column-integrated content of CO (level 2 and level 3)
• Total column-integrated content of HCOOH (level 2 and level 3)
• Total column-integrated content of NH₃ (level 2 and level 3) – near real time and reanalysis
• Vertical profile (level 2) and total column-integrated content of O₃ (level 2 and level 3)
• Total column-integrated content of SO₂ and plume altitude (level 2)
• Mid-tropospheric column of CH₄ (level 2)
• Mid-tropospheric column of CO₂ (level 2)
• Dust Aerosol Optical Depth and mean layer altitude (level 2)
• Clouds temperature, pressure, type, emissivity and height (level 2)
• IASI + GOME2 : vertical profile of O₃ (level 2)

Level 3 products are monthly average on a 1°x1° grid.

Contacts

Point of contact

Cathy Boonne (Institut Pierre Simon Laplace)

cathy.boonne@ipsl.jussieu.fr Data manager

Cathy Clerbaux (Laboratoire Atmosphères, Milieux, Observations Spatiales)

Cathy.Clerbaux@latmos.ipsl.fr Scientific coordinator