The MetOp-SG A satellite(s) will carry the Sentinel-5 instrument as the Sentinel-5 mission as part of the EU's Copernicus Programme. The development of the Sentinel-5 instrument is done by Airbus Defence & Space, Ottobrunn, under contract with ESA. Many European industries have contributed to the subsystems.

The main characteristics of Sentinel-5 instrument are:

• Type: passive grating imaging spectrometer
• Configuration: Push broom staring (non-scanning) in nadir viewing
• Swath width: 2 670 km
• Spatial sampling: 50x50 km²(UV1), 7.5x7.5 km² (all other channels),
• Spectral: 5 spectrometers (1 in UV1, 1 in UV2VIS, 1 in NIR, 2 in SWIR)
• Radiometric accuracy (absolute): 3%, 6%(SWIR) of the measured earth spectral reflectance.
• Overall mass: 290 Kg.
• Dimensions (x.y.z): 1.145 x 1.032 x 1.026 m³
• Design lifetime: 7.5 years
• Power Demand: 300 W
• Generated data volume: 139 Gbits per full orbit.

Observation is performed over the complete sunlit (with respect to the sub-satellite point) part of the orbit while the instrument calibrations are done during the dark part of the orbit. The expected instrument operation is shown schematically in Figure 6. The five spectral chains of Sentinel-5 are shown in Figure 7 along with several key associated instrument performance requirements.

Figure 6: Sentinel-5 Instrument Operations Scenario

The Sentinel-5 Instrument comprises five spectral chains subdivided behind two co-aligned telescopes: one telescope for UV1 and SWIR-1 & -3 and one telescope for UV2VIS and NIR spectral channels. In nominal observation mode the telescopes provide free access to the earth field-of-view. Included in each of the telescopes optical path is a polarisation scrambler to reduce the instrument's sensitivity to the polarisation status of the observed scene. After each of the telescopes the converging light path is split into the spectral bands by means of dichroic mirrors and is imaged on slits for the spectral channels. Each of the five spectral bands is analysed in a dispersive spectrometer optics after which the spectrum is detected together with spatial across track information on a detector array. The spectrometer slits incorporate a new design to provide signal mixing in the along-track direction, providing signal spatial scrambling within each spatial pixel and thus reducing the instrument's sensitivity to inhomogeneous scenes.

Figure 7: Sentinel-5 Spectral Bands

For in-orbit calibration purposes the Sentinel-5 instrument features a Calibration Subsystem, CAS, one per telescope, which injects different calibration signals either via an actuated injection mirror for internal calibration sources or a diffusing element for sun calibration measurements. Located right before the first optical element of the telescopes this allows a calibration of the instruments full optical path. For absolute radiometric calibration the CAS provides a "sun via diffuser" signal. Other signals used for calibration include internal sources such as a broadband white-light-source, WLS, and spectral line sources, SLS, used for relative spectral calibration and the characterisation of the spectral resolution. Another on-board calibration signal is from LEDs located in each of the spectrometers and is used for diagnostic flat field illumination of the detectors. Figure 8 provides an overview of the Sentinel-5 instrument architecture as currently foreseen. Figure 9 provides an illustration of the instrument as currently envisaged.

Figure 8: Sentinel-5 Instrument Architecture

Figure 9: Sentinel-5 Instrument