Level-1A processing involves computing the ortho-geolocation of each instrument pixel as well as radiometric calibration coefficients and appending them to numerical counts, without applying them. It is divided into five main steps:
- Source packet processing unpacks and validates the source packet data and auxiliary data, converting it to engineering units for the higher processing levels that generate the products and geolocate the data.
- Infra-red (IR) channel calibration calculates the calibration offset and slope that describe the linear relationship between pixel count and radiance for the thermal IR channels, including fire channels. The parameters are determined from the black body pixel counts and the black body temperatures, and the process makes use of look-up tables for the conversion of temperature to radiance.
- Visible/NIR/SWIR channel calibration occurs once per orbit and calculates calibration parameters for all visible, NIR and SWIR channels on the basis of VISCAL data contained within the ISPs acquired when sunlight illuminated the VISCAL diffuser.
- Time calibration is the derivation of the scan time from the time stamps of each ISP and the computation of the time stamp of each pixel (i.e. the time associated to each scan position) in UTC format.
- Geolocationcomputes the ortho-geolocation (i.e. longitude and geodetic latitude, both corrected for the Digital Elevation Model (DEM) and altitude with respect to the reference ellipsoid WGS84) and corresponding (x,y) quasi-cartesian coordinates of the centre of each instrument pixel for each view and four grids defined in SLSTR Level-1B products:
- 1 km grid for TIR and fire channel
- 1 km F1 channel grid
- 0.5 km grid stripe A for visible/NIR and SWIR channels
- 0.5 km grid stripe B for SWIR channels only.