InfraRed Sounder

The IRS acquires a number of spectral soundings simultaneously over a dwell using a two dimensional detector array. It uses a ‘step-and-stare’ mechanism to probe the Earth´s atmosphere.

The dwell coverage is stepped in an east/west direction to form a line of dwell spectral soundings, before moving northward to form the next line, covering the local area coverage (LAC) within the repeat cycle duration (‘step-and-stare’ principle of probing the atmosphere).

Four separate LAC zones are defined and scanned sequentially (Figure 2). One LAC is acquired within 15 minutes, consisting of overlapping dwells following a step and stare scan pattern. Each dwell consists of 160x160 pixels (spectral soundings), with 4km spatial sampling distance at nadir. Europe (LAC4) is observed every 30 minutes.

The spectral soundings are transmitted to the ground as interferograms and transformed to spectral channels as part of the ground processing, before dissemination to the end users as Level 1 datasets.

The IRS is based on an imaging Fourier-interferometer with a hyperspectral resolution of 0.625cm^-1 wave-number, taking measurements in two bands, the Long-Wave Infrared (LWIR) and the Mid-Wave Infrared (MWIR).

Summary of technical specifications

• Two spectral bands: MWIR: 1600 to 2250cm^-1 (4.44–6.25µm) and LWIR: 680 to 1210cm^-1 (8.26–14.70µm).
• Full disc coverage in 60 min, Europe region (Local Area Coverage) 30 min.
• Spatial resolution of 4km x 4km at nadir.
• Radiometric measurement ranges between 180K and 313K (equivalent black-body temperature.)
• Spectral radiometric noise (excluding spectral calibration) at 280K black body: between 170 and 900mK depending on the considered wave-number inside the band of interest.

The IRS includes the ozone band within LWIR and the carbon monoxide band within MWIR. This will allow measurement within the free troposphere, leading to information on enhanced levels of pollution in the boundary layer below.