## 1 Introduction  
Sentinel-3A was launched on 16 February 2016 and accompanied by Sentinel-3B on 25 April 2018, with onboard multiple sensing instruments focusing on Earth observation to support Copernicus ocean, land, atmosphere, emergency, security and cryosphere applications. Co-located observations of two of these instruments--the Ocean and Land Colour Instrument (OLCI) and the Sea and Land Surface Temperature Radiometer (SLSTR)--are used as synergistic inputs for the so-called'synergy' (SYN) processing chain [1]. The Sentinel-3 SYN processing chain combines observations in the visible and near-infrared (VNIR) and short-wave infrared (SWIR) channels to provide several higher-level products. One particular series of output products of the SYN processing chain are the SYN VEGETATION (SYN VGT) products, which are designed to provide surface reflectance products similar to those obtained from the VEGETATION1 and VEGETATION2 (VGT) instruments onboard SPOT4 (launched 24 March 1998) and SPOT5 (launched 4 May 2002), respectively, to meet Copernicus--previously known as Global Monitoring for Environment and Security (GMES)--user needs [2]. SPOT5 was, however, decommissioned in early 2015, i.e., before the launch of the first Sentinel-3 satellite. From 2013 to 2020, PROBA-V (Project for On-Board Autonomy--Vegetation), which was launched on 7 May 2013, provided data continuity to the SPOT VGT payloadsand acted as a gap filler to the Sentinel-3 mission [3]. The PROBA-V mission was designed, and products were defined to closely match SPOT VGT products [4], and after the latest reprocessing campaigns, both the SPOT VGT Collection 3 and PROBA-V Collection 2 archives were shown to be highly consistent [5; 6]. Similar to the standard products of SPOT VGT and PROBA-V, Sentinel-3 SYN VGT products provide complete Earth coverage every 1 to 2 days in four spectral bands (BLUE, with a central wavelength around 460 nm; RED, 655 nm; NIR, 835 nm; and SWIR, 1600 nm) (see also Table 1) projected on the same regular 1 km 'Plate carree' grid [1]. Output products include (i) top-of-atmosphere (TOA) reflectance (VGP), (ii) 1-day synthesis surface reflectance and Normalized Difference Vegetation Index (NDVI) (VG1) and (iii) 10-day synthesis surface reflectance and NDVI (V10). In principle, users thus have access to a continuous series of data products from the combined data archives of SPOT VGT (1998-2014), PROBA-V (2013-2020) and Sentinel-3 SYN VGT (from 2018 onwards). There are, however, several aspects related to the satellite/sensor and processing definitions that, to a greater or lesser extent, impact the consistency between these product series, such as differences in the acquisition scheme, sensor design, spectral response, atmospheric correction and sensor calibration. Another crucial aspect is that since the release of the first Sentinel-3 SYN VGT products in October 2018, several important changes in the SYN VGT processing baseline have been implemented, thereby gradually improving the quality of SYN VGT products. However, to date, reprocessing of the SYN VGT archive has not taken place. The archive of SYN VGT products is therefore intrinsically inconsistent, and consequently, different consistency levels with SPOT VGT and PROBA-V will be reached throughout the years. Numerous studies have addressed the challenge of transitioning between consecutive wide swath land monitoring sensors while attempting to provide consistent product archives. These include, for example, the assessment of the uncertainties related to the transition from the Moderate Resolution Imaging Spectroradiometer (MODIS) to Visible Infrared Imaging Radiometer Suite (VIIRS) [10], between different AVHRR sensors [11; 12; 13], from the Advanced Very-High-Resolution Radiometer (AVHRR) to MODIS [14; 15; 16] or from the AVHRR to SPOT VGT [15; 17].