Technical Description

Image processing

PROBA-V raw observations and the describing platform and instrument auxiliary data are downlinked to Data Reception Stations in Fairbanks (USA, AL) and  Kiruna (Sweden) as Transfer Frame Files (TFFs) once the satellite passes over these locations. In the Data Ingestion Facility (DIF), Polar Motion Files are used to convert these raw observations into Level 1A data, which are then stored in the Long Term Data Archive (LTDA). Polar Motion Files describe the Earth’s rotational axis movement with respect to the axis’ average location over the year 1900.

Subsequently, a geolocation step is performed to determine the latitude and longitude for each Level 1A data pixel. The satellite position and velocity are interpolated per scan line using an orbital propagation model. The geolocation accuracy is refined using the geometric Instrument Calibration Parameters (ICP) file. A geometric processing model additionally calculates the viewing and solar zenith angles (VZA and SZA, respectively), which are required for further processing. The output of the geometric processing is Level 1B data (see the flow chart below). The user is referred to Sterckx et al. (2014) for further details on the geometric processing model.

The radiometric processing converts the digital number count at a certain spectral band (DN) into physical Top of Atmosphere (TOA) reflectance values. First, the DN number is corrected for non-linearities, dark currents, and inter-pixel non-uniformities. Second, these numbers are converted to at-sensor radiance L, using the band-specific calibration coefficients derived from the radiometric ICP file. Finally, the TOA radiance L is converted into TOA reflectance, yielding Level 1C data. These data, as well as the radiometric and geometric calibration files, are available at the Product Distribution Facility.

Level 1C TOA reflectances are mapped to a standard geographic lat/lon grid (WGS84) and snow/ice and cloud/cloud shadow detection algorithms are applied, resulting in Level 2A data. Alternatively, an atmospheric correction is applied to account for absorption by atmospheric constituents, which results in Top of Canopy (TOC) reflectances (Level 2B data).

The final step comprises the compositing of the daily Level 2 data into either TOA (from Level 2A) or TOC (from Level 2B) multi-day synthesis files. See the flow chart below for the available synthesis data at 100 m, 300 m, and 1 km resolution.

PROBA-V process chain