## 1 Introduction
The maximum FAPAR across the eight days is selected to represent the final FAPAR value for this period in MOD15 [35], which means FAPAR does not vary during a given eight-day period. VI-driven models estimate GPP based on the empirical relationship between VIs and FAPAR [36]. A number of prevalent VI-driven models include the temperature and greenness (TG) model [18], the greenness and radiation (GR) model [17], and the vegetation index (VI) model [37]. The Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) are the most widely used VIs in VI-based models [38]. In addition to the NDVI and EVI, numerous studies have utilized vegetation red-edge reflectance VIs to improve GPP estimation [39; 40]. The Medium Resolution Imaging Spectrometer (MERIS) Terrestrial Chlorophyll Index (MTCI), originally derived from the MERIS onboard the Envisat satellite of the European Space Agency (ESA), is one of the red-edge VIs [41]. The global MTCI product, with a spatial resolution of 1 km, was available either weekly or monthly [42]. The MTCI has demonstrated its suitability to evaluate GPP across different biomes such as grasslands [43], deciduous forest, and croplands [44]. [PERSON] and [PERSON] [44] concluded that the MTCIperformed better than the MODIS EVI products for estimating GPP across a diversity of ecosystems and climatic conditions. The MTCI product was discontinued in 2012, but the successful launch of the Ocean and Land Colour Instrument (OLCI) onboard the Sentinel-3 satellite provides the continuity of the MERIS. As a legacy of the MERIS, the Sentinel-3 OLCI has recognized the pivotal deficiencies of MERIS and has undergone many enhancements [45]. The double satellite system of the OLCI instruments (i.e., Sentinel-3A and 3B) enables a revisit period less than two days, and 21 spectral bands exist with wavelengths ranging from optical to near-infrared in OLCI [46]. There are mainly two OLCI products levels, i.e., level-1B and level-2 (land and water), with a spatial resolution of approximately 300 m or 1.2 km. Specifically, the OLCI level-2 land products include the FAPAR and OTCI products, named the OLCI Global Vegetation Index (OGVI) and the OLCI Terrestrial Chlorophyll Index (OTCI), respectively. OGVI and OTCI are the continuity of the MERIS FAPAR (MERIS Global Vegetation Index (MGVI)) and the MTCI datasets, respectively [47]. The apparent difference of the OLCI products compared with MODIS and MERIS is the data composition algorithm. OGVI and OTCI used the actual value for the day while MOD15 and MTCI were composited using the arithmetic mean and maximum value across eight days, respectively [48]. The fine temporal and spectral resolution of OLCI products opened opportunities for GPP estimation. Nevertheless, to the best of our knowledge, only [PERSON] et al. [49] focused on GPP estimation using the OLCI FAPAR product so far. The main objective of their study was to investigate the relationships between a series of satellite FAPAR products (including OLCI FAPAR) and solar-induced chlorophyll fluorescence (SIF) products. Only the relationship between APAR and flux-tower based GPP was established, and neither LUE nor VI-driven models were used. The main objective of this research is to assess the performance of the Sentinel-3 OLCI FAPAR and OTCI products in estimating GPP across different biome types. Thus, this study aims to (1) evaluate the GPP obtained from the OLCI FAPAR and three OTCI-driven models, (2) compare these obtained values of GPP with those derived from EC measurements, and (3) compare the performance between the OLCI FAPAR and MODIS FAPAR products.