November 19, 2023

Infrared Imager Radiometer

The Imaging Infrared Radiometer instrument (IIR) is the French contribution to the CALIPSO science payload.
It was developed by SODERN with CNES as prime contractor.
The LMD dynamic meteorology laboratory designed and developed the black body of the instrument and performed ground calibration.

The IIR instrument is a three-channel imaging radiometer in the thermal infrared at 8.65 µm, 10.6 µm and 12.05 µm.
IIR images will provide context for lidar measurements by night and enable co-registration with the MODIS multispectral radiometer on the Aqua satellite.
IIR measurements, combined with the lidar information, will furthermore provide the ability to determine the size of ice particles in semi-transparent clouds.

The number and choice of spectral bands are optimized to improve these measurements. While the traditional pair of 11-12-µm channels is sensitive to small particles, the 8.5-12-µm channel pair is more sensitive to large particles. Better discrimination of the absorption and scattering effects of complex-shape crystals is expected, as well as the ability to differentiate spheres and hexagonal shapes.

The design of the IIR instrument is based on an adaptation of a single-channel infrared camera (ISM) developed for the IASI instrument on the METOP satellites. It uses a non-cooled microbolometer detector.

IIR Features

The IIR is composed of:

  • A camera (ISM), consisting of an objective (aperture 0.75) optimized in the thermal infrared, a microbolometer detector array, specific electronics, a passive cooler and mechanical parts.
  • A filter wheel enabling three spectral filters to be positioned alternately in front of the camera.
  • A black body to calibrate the camera.
  • A pointing mirror to sequentially select between the Earth view, the black body and cold space (second calibration source).

Main technical characteristics

These include the spectral responses of the bolometer and objective/mirror assembly.

  • Spectral bands: three bands selected by three spectral filters

Centre: 8.65 µm Bandwidth: 0.9 µm
Centre: 10.6 µm Bandwidth: 0.6 µm
Centre: 12.05 µm Bandwidth: 1 µm

  • Instantaneous field of view of 64 km x 64 km on the ground
  • Pixel size of 1 km x 1 km
  • Radiometric performance

NedT < 0.5 K @ 210 K
Absolute calibration accuracy < 1 K

  • Mass: 24 kg
  • Volume: 490 x 550 x 320 mm
  • Consumption: 27 W
  • Data rate: 50 kbps


The IIR was built by SODERN in Limeil-Brévannes, France, under contract to CNES. Among the components, the microbolometrer detector array is a Boeing U3000 commercial detector.

On-ground calibration

Measuring infrared brightness temperatures with the accuracy needed for scientific applications requires on-board calibration.
This is achieved by inserting uniform cold and warm scenes with stable and accurately known temperature between Earth images, like calibrating a household thermometer. The cold scene is obtained by pointing at deep space (temperature: 3 K) while the warm scene is a black body.
is the black body an annular device 9 cm in diameter painted black and designed by LMD to have properties approaching an ideal black body. Its emissivity is estimated higher than 0.986.

Complete radiometric characterization of the instrument was performed before launch by LMD in a vacuum chamber.

Level 1 processing

Raw IIR images are operationally processed by NASA using a processing sequence specified by CNES.

This level 1 processing performs:

  • joining and projection of images (in the three channels) on the same geographic grid centred on the lidar spot position 
  • in-flight radiometric calibration using the calibration images interleaved with the Earth views.

Projected and calibrated radiances are available for users in an IIR level 1B product covering half-orbits.

Level 2 processing

The level 2 processing goal is to provide the emissivity of clouds and the size of particles in semi-transparent ice clouds.
Two products are generated, one along the lidar track and the other across the swath (64 km) of the IIR.
The processing algorithms were specified by the Institut Pierre Simon Laplace (IPSL) and run by NASA.

The conventional split-window technique was adapted to get the most out of altitude measurements and scene classification by the lidar.
Cloud emissivity calculations use the cloud altitude as input, converted into temperature via the meteorological profile, which reduces a major error contribution in the method.
The depolarization measured by the lidar also provides information on particle shape, enabling their size to be more finely determined.
Another improvement is in the variety of particle models used in precomputed tables and in the care taken in calculating the optical properties of particles.

Simulations with airborne data show a significant improvement in the determination of particle size.

Outside the lidar track, an algorithm extends size determination to neighbouring pixels for which the lidar measurement is sufficiently representative.
The homogeneity of the scene is estimated from the IIR image itself and from the corresponding visible image, acquired by day by the WFC camera.

The IIR Technical Expertise Centre

The IIR Technical Expertise Centre at CNES performs a dual role:

  • an instrumental role: technical in-flight monitoring of the instrument
  • an image quality role: in-flight commissioning and in-orbit monitoring of the IIR’s geometric and radiometric quality, i.e. the quality of the IIR level 1 product.

The IIR Technical Expertise Centre relies upon a computing facility named TEC, which provides operators and experts with the working environment required to conduct their investigations. TEC receives IIR housekeeping data from the Mission Operations Control Center (MOCC), and raw data (level 0), level 1 and calibration products from the Atmospheric Science Data Center (ASDC) or the supercooled liquid cloud fraction (SCF).

TEC was developed by Cap Gemini with CNES as prime contractor. It is located at the Toulouse Space Centre and  operated by CNES teams.