## 1 Introduction  
The land surface albedo, which is defined as the ratio of the radiant flux reflected from the Earth's land surface to the incident flux, is a key forcing parameter controlling the planetary radiative energy budget and the partitioning of the radiative energy between the atmosphere and surface. The surface albedo varies in space and time as a result of both natural processes (e.g., solar illumination, snowfall, and vegetation growth) and human activities (e.g., the clearing and replanting of forests, the sowing and harvesting of crops, and the burning and grazing of rangelands) and is a sensitive indicator of environmental vulnerability [1]. Consequently, a long-term record of the surface albedo for the global landmass is required by climate, biogeochemical, hydrological, and weather forecast, models at a range of spatial (from a few meters to 30 km) and temporal (from daily to monthly) scales, and it is therefore, mandatory to quantitatively, and efficiency assess the uncertainties in SA measurement using these products. The European's Union funded C3S [2] provides key indicators on climate change drivers by means of combining the observations of the climate system with the latest science to develop authoritative, quality-assured information about the past, current and future states of the climate. The C3S portfolio includes consistent estimates of multiple Essential Climate Variables (ECVs), including the surface albedo, which is primarily based on satellite data and provided as a series of gridded CDRs. In the frame of C3S, SA products have been derived from the data collected by NOAA/AVHRR (1981-2005) and SPOT/VGT (1998-2014). The Project for Onboard Autonomy satellite (PROBA-V) satellite [3], which was launched on May 2013 for seven years, was designed to bridge the gap in space-borne vegetation observation between SPOT/VGT (March 1998-May 2014) and the Sentinel-3 satellites. PROBA-V is comparable to the VGT sensors on SPOT platforms. In C3S, the PROBA-V SA v1.0 products are generated with the aim to extend the CDRs based on AVHRR and VGT observations over time. The algorithms for these C3S SA products were designed by Meteo-France based on previous research conducted within the Satellite Application Facility for Land Surface Analysis (LSA SAF) program of EUMETSAT [4; 5]. Meteo-France developed the SA algorithms, Refs. [6; 7; 8; 9] based on MSG/SEVIRI and MetOp/AVHRR within the EUMETSAT LSA SAF program, and were later adapted to these other sensors in C3S and Copernicus Global Land Service (CGLS) [10]. These products are generated by the processing line infrastructure implemented by VITO and openly distributed through the C3S Climate Data Store (CDS) [11]. Within the C3S CDS, a framework for the Evaluation and Quality Control (EQC) of climate data products derived from satellite and in situ observations to be catalogued was developed [12]. Validation, or quality assessment, is one of the main components defined in this EQC framework, and it is defined as the process of independently assessing the quality of the data products derived from the system outputs [13]. In terms of satellite-derived land products, validation is the procedure to assess their accuracy and quantify their uncertainties via analytical comparisons with reference data. Validation also constitutes the means of guaranteeing the compliance of products with user requirements, and helps end-users understand to what extent the product is suitable for their specific applications [14]. Based on previous validation works, mainly based on NASA products [15; 16; 17; 18; 19], but also on EUMETSAT [7], the CEOS WGCV LPV sub-group [20] has defined the global albedo product validation good practices [21].