## 2 Materials and Methods
### SIF Data  
OCO-2 belongs to a sun-synchronous orbit satellite that was launched on 21 September 2014 and has an overpass time at \(\sim\)1:30 pm. The OCO-2 spectrometer measures spectra in the O\({}_{2}\)-A band with far-red SIF retrieved at 757 nm and 771 nm based on the filling in Fraunhofer lines [25; 42; 43]. OCO-2 data were produced by the OCO-2 project at the Jet Propulsion Laboratory, California Institute of Technology, USA. The sun-sensor acquires data in the nadir, glint, and target modes, with a revisit period of approximate 16 days, and its ground-pixel size is 1.3 \(\times\) 2.25 km\({}^{2}\), which directly facilitates local studies. The latest version of the B8100r OCO-2 dataset was used in this study. A conversion factor was provided in the dataset, which was used to convert the instantaneous SIF to a daily average SIF [36]. The OCO-2 SIF retrievals at 757 nm and SIF 771 nm are denoted SIF\({}_{757}\) and SIF\({}_{771}\) in this study, respectively. OCO-2 SIF retrievals are noisy as single observations, even though OCO-2 has a small footprint. The OCO-2 SIF retrievals for cloudy days were removed according to the solar radiation measured by the flux tower in this study [44; 45].  
GOME-2 on MetOp-A platform belongs to a sun-synchronous orbit satellite which was first launched on 19 October 2006 with an overpass time at \(\sim\)9:30 am. GOME-2 measures spectra for top-of-atmosphere radiance between 240 and 790 nm with a spectral resolution between 0.2 and 0.5 nm, and it has a signal-to-noise ratio of \(\sim\)1000 and a relatively large footprint (approximately 40 km \(\times\) 80 km at nadir for MetOp-A before 15 July 2013 and 40 km \(\times\) 40 km since 15 July 2013) [46]. SIF was retrieved near the wavelength of 740 nm by using a principal component analysis algorithm [47]. The GOME-2 SIF retrievals were quality filtered and controlled (e.g., heavy cloud contamination and data with relatively low signal-to-noise ratios were removed) and binned in 0.5\({}^{\circ}\) latitude-longitude grid cells [45; 48]. Version 27 (V27) level 2 retrievals are the results of new postprocessing based on Version 26 (V26), and these retrievals provide additional fields (daily-averaged SIF and quality flag, i.e., users can apply quality flags to filter out bad data) and have adjusted bias corrections and calibrations. Only good quality data (quality flag = 2, details are given in Section 2 in the supplementary material) were used in this study. As a new proxy for GPP, RSIF was developed using a machine learning approach based on the inputs-MODIS reflectance to predict GOME-2 SIF data, and detailed parameters on RSIF can be found in a previous study [38].  
To remain spatially consistent with GOME-2 SIF retrievals, a 0.5\({}^{\circ}\) grid cell based on the coordinates of the flux towers was used to extract the OCO-2 SIF data for the period from September 2014 to December 2017 and to match the GOME-2 SIF at the Daman and A'rou sites. The OCO-2 SIF data were calculated using the averaged values of the available pixels around the towers over a squareof 0.5\({}^{\circ}\) by 0.5\({}^{\circ}\). GOME-2 SIF values were extracted within a 0.5\({}^{\circ}\) grid cell based on the coordinates of the flux towers. The daily-averaged GOME-2 SIF data were noisy as single measurements; thus, they were averaged over a 16-d period to investigate the GPP-SIF relationship. Due to the different platforms, wavelengths and overpass times between OCO-2 and GOME-2, we fitted the GPP-SIF relationship separately. The daily OCO-2 SIF and a 16-d averaged GOME-2 SIF data were used to ensure a reasonable comparison between OCO-2 and GOME-2.