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PROBA-V data improve surface albedo products in Africa

08 May 2023

As ESA’s PROBA-V satellite marks its 10-year launch milestone, data tasked during the mission’s lifetime are still being used to assess changes in land cover.

The Agency funded SPAR@MEP project delivers a consistent long-term data record (LTDR) of aerosol optical properties and surface reflectance from SPOT-VGT and PROBA-V observations. The project has recently released global products at 5 km spatial resolution.

Following the success of the Vegetation (VGT) instrument flown on the French SPOT (Satellite Pour l’Observation de la Terre) satellites, PROBA-V aimed to extend this data set, when it launched on 7 May 2013.

Despite being only the size of a washing machine, for seven years the mission acquired valuable land cover data across the entire planet every day. It used an imager to collect light in the blue, red, near-infrared and mid-infrared wavebands, to distinguish between different land cover types and vegetation growth.

In the framework of the SPAR@MEP project, PROBA-V acquisitions have recently been aggregated at 5km and processed globally with the CISAR algorithm.

CISAR is an inversion algorithm for the simultaneous retrieval of surface reflectance, free from any atmospheric effects and aerosol and cloud single scattering properties. It has been developed by Rayference in Belgium, who lead the project.

The satellite observations are accumulated for 15 days, during which, the surface reflectance is assumed invariant. A new retrieval takes place every 10 days. As this inversion method is based on an Optimal Estimation approach, accurate and reliable characterisation of input uncertainties are fundamental, so as to avoid impacting the retrieval.


Surface Albedo derived from PROBA-V data in 2019

Rayference have produced an animation showing the surface albedo of the Northern part of Africa during 2019, expressed as BiHemispherical Reflectance (BHR). It is possible to observe seasonal expansion and regression of vegetation in the Sahel region, following the rainy season starting in May.

Timeseries of PROBA-V data show stability of surface albedo in Sahara
Timeseries of PROBA-V data show stability of surface albedo in Sahara

Albedo refers to the fraction of light that is reflected off a body or surface. As vegetation grows in the Sahel region, the albedo in the near infrared (NIR) bands increases while the reflectance in the red band decreases.

Conversely, in the Sahara region, the surface albedo shows very high stability, irrespective of atmospheric conditions, owing to the lack of vegetation dynamic in this desert region.

The new global BHR dataset at 5km is available on the Zenodo repository via the following link.