## 2 Remote Sensing Surface Albedo Products
### C3S SA v1.0 Based on SPOT/VGT Data  
C3S SPOT/VGT SA v1.0, which is available from C3S CDS [11], provides a CDR of the global black-sky and white-sky albedo estimates for the period from 1999 to 2014 at a ground sampling distance of 1/112\({}^{\circ}\) and a temporal frequency of 10 days using a compositing window of 20 days. The dates of production are the 10 th, 20 th and 30 th of each month. As for PROBA-V, the albedo quantities are provided for the three broadband domains (visible [0.4-0.7 \(\upmu\)m], NIR [0.7-4 \(\upmu\)m] and total shortwave [0.3-4 \(\upmu\)m]) and in the four spectral channels (see Table 2) of the VEGETATION sensors.  
Figure 1: Global distribution of PROBA-V SA v1.0 black-sky albedo values in the total shortwave (AL-DH-BB) on 23 September 2014.  
The albedo products are based on the data acquired by the VEGETATION-1 and VEGETATION-2 sensors aboard the SPOT-4 and SPOT-5 satellites, respectively.  
The retrieval algorithm [6; 7; 9] was developed based on the previous developments of the SA products [6; 7; 8; 9] based on MSG/SEVIRI and Metop/AVHRR in the framework of the EUMETSAT LSA SAF project [4], and it was later adapted here to these other sensors in CGLS and C3S [43]. The input data were Collection 3 of SPOT/VGT, which corrects the bug previously detected regarding the incorrect calculation of the Sun-Earth distance of Collection 2, includes improved cloud and snow/ice detection, and revises the absolute radiometric calibration for the entire archive [44]. The main differences with the C3S PROBA-V SA v1.0 algorithm are related to the use of a different Bidirectional Reflectance Distribution Function (BRDF) model and temporal composite scheme. PROBA-V SA v1.0 implemented the Ross_Thick kernel for volumetric scattering and the Roujean kernel for geometric scattering [42], whereas SPOT/VGT SA v1.0 implemented the Ross_Thick kernel [42] for volumetric scattering and the Li_Sparse_Reciprocal kernel for geometrical scattering [45]. The synthesis period of PROBA-V SA v1.0 is 30 days using a semi-Gaussian weighting function. The SPOT/VGT SA v1.0 retrieval methodology uses a 20-day temporal composite, meaning that each surface albedo product is built from the valid SPOT/VGT observations corresponding to the 20-day period preceding the calculation date. At the BRDF inversion step, the previous inversion result is used as a priori information. Hence, a recursive temporal composition of the information over a longer time period (approximately three times longer than the production period) can be achieved to guarantee the coherence and spatial completeness of the product. The \"age\" (AGE parameter in the output product) of the clear-sky observations exploited in the recursive inversion scheme is an important piece of information for potential applications, and is therefore, also made available to users. This age corresponds to the mean age with respect to the date of the calculation of the clear observations considered for the albedo calculation. More details are given in the ATBD [36].  
[PERSON] et al. [46] showed that the C3S SPOT/VGT SA v1.0 products were good quality based on most of the criteria evaluated, reaching validated stage 1 in the CEOS LPV hierarchy. However, two main limitations were found: (i) Some temporal noise existed at the short-time scale; and (ii) the product was not able to capture some snow events, showing large uncertainties (>0.2) in those cases. Comparisons with ground measurements (461 samples, 2000-2005 period) from 15 FLUXNET stations showed similar overall uncertainty (RMSD = 0.05) as other satellite products (MCD43A3 C6, GlobAlbedo, and GLASS), but a positive bias (14%) was found.