August 23, 2018

Instrument

The instrument on board MicroCarb is an infrared passive spectrometer operating in four wavelengths using an echelle grating (dispersive element) to achieve spectral dispersion.

CNES has tasked Airbus Defence & Space with developing and qualifying the instrument.

The instrument measure atmospheric spectra for the following species:

  • Oxygen (O2 at 0.76 and 1.27 µm) to retrive the surface pressure and then normalize the computed CO2 column concentration
  • Carbon dioxide (CO2) in two bands: a first band around 1.6 µm, a second band around 2 µm,
  • Methane (CH4) about 1.67 µm.

The entrance of the spectrometer is a narrow slit perpendicular to the track of the satellite that scans the ground during the detector integration time.

bpc_microcarb-principe-instrument.png

The echelle grating performs spectral dispersion and optical narrow band filters select the useful bands. The light beams are then directed towards a detector which is then used for both dimensions:

  • a spectral dimension, used to collect the spectrum of each band
  • a spatial dimension, which is the image of the ground as seen by the slit for each of the bands

bpc_microcarb-mesure-instrument.png

The pixels illuminated along the spatial dimension (width of the slit) can be added for every element of spectral resolution in order to improve the signal-to-noise ratio.

The instrument includes an imager designed to detect clouds whose presence would cause erroneous measurements by the spectrometer.

TypePassive infrared spectrometer
PrincipleEchelle grating
Wavelengths764-768 nm1,602-1,619 nm2,037-2,065 nm1,660-1,672 nm
Spectral resolution> 25,000> 25,000> 25,000> 25,000
Signal to noise ratio 600600280600
DetectorHgCdTe 1k x 1k matrix
ImagerWavelength: 0.625 µm. Ground resolution: 200 m
Mass60 kg
Power consumption50 W

Instrument characteristics

The choice of a high spectral resolution guarantees a robust instrument that is less sensitive to defects such as optical distortion, non-linear response, stray light, and so on.

Note: the spectral resolution is a key feature of a spectrometer and is defined by R=/ where is the considered wavelength and the minimal difference between two wavelengths detectable by the spectrometer.

The instrument is then made up of:

  • For the optical part
    • a mechanism to point the line of sight either towards the direction of interest or towards the internal calibration systems or a solar diffuser (for purposes of in-flight calibration)
    • an entrance telescope common to the spectrometer and to the imager
    • an entrance slit to the spectrometer
    • an echelle grating
    • optical spectral filters
    • focal detectors (spectrometer and imager)
    • an ultra-stable optical bench supporting these various elements

  • For the electronics part
    • Video electronics associated with the infrared and visible detectors
    • electronics for instrument management and power supply

  • a passive cryogenic cooling system, using a radiator directed to cold space and protected by a baffle from solar and Earth radiation

  • A structure, elements of thermal protection and thermal active control, harness

This instrument concept studied by CNES was checked and sized using an optical bread board.

bpc_microcarb-maquette-optique-instrument.png

Optical bread board of MicroCarb instrument