## 2 Global Open Earth Observations Satellite Missions for Monitoring Land Surface Phenology  
### Advanced Very-High Resolution Radiometer (AVHRR)  
AVHRR is a space-borne sensor that presents continuous, 40-year-long harmonized time series combined from images from 14 different platforms [24]. They have been carried out by the National Oceanic and Atmospheric Administration (NOAA) family of polar orbiting platforms (POES) and European MetOp satellites. Sensors provide scanning in five bands: red, near-infrared, medium-wave infrared, and two long-wave infrared (thermal) channels. The combination of the spectral bands enables the transformation and calculation of indices such as the normalized difference vegetation index (NDVI), which well quantifies the health and density of vegetation [25]. NDVI uses the relationship between the visible red part of the spectrum that is absorbed by vegetation and the near infrared that is reflected from the vegetation surface, in the range between 0 and 1. From the AVHRR spectral bands, one of the longest NDVI global time series was created in the framework of the Global Inventory Monitoring and Modeling System (GIMMS) [26].  
The analysis of the AVHRR data showed a global trend of changes in vegetation dynamics going back several decades; the timing of both maturity and senescence within a year advanced in the Northern Hemisphere and the timing of SOS delayed in the Southern Hemisphere [27; 28; 29; 30]. The increase in the growing length is the most evident in the spring-time due to the early disappearance of snow [31]. However, the warming is most significant in Eurasia, but in the United States it is smaller than in most of the world, which shows a fragmented pattern of LSP change [32]. In addition to global dynamics, the AVHRR enabled the analysis and detection of specific changes in vegetation dynamics at the continental or regional level, as well as the breakdown of effects by individual rough land cover categories that can be distinguished on a low spatial resolution sensor. Comparison of the relationship between extreme temperature, precipitation, and LSP in Central Asia [30] showed that, under extreme climate conditions, the start of the season (SOS) of the LSP was gradually delayed, the end of the season (EOS) advanced, and the length of the growing season (LOS) shortened. A similar study performed over Eurasia [33] confirmed that the area of vegetation greenness limited by low temperatures has substantially shrunk, while the area of vegetation greenness limited by precipitation deficit increases over the period 1982-2015. In Europe [34], the northeast areas showed a trend to an earlier and longer growing season while the Mediterranean displayed a growing season shift towards later dates. The correlation between change in LSP and climatic records is more evident in the semi-natural areas, while managed systems have more heterogeneous change patterns. The variations in spring phenology correlate well with anomalies in winter temperature and the winter North Atlantic Oscillation (NAO) [35]. The studies in the eastern Baltic region [36; 37] confirmed the trend of the earlier onset of the growing season and the prolongation of the growing season by 6-7 weeks. The continental phenomentics study with the NDVI dataset produced by Global Inventory Modelling and Mapping Studies (GIMMS) during a period of 1982-2015 over Africa [38] revealed overall trends of later start of growing season, later peak of season, and later EOS. NDVI changes track the longitudinal gradient, with a prolongation of the growing season in northern Africa and a reduction in southern Africa. Drought severity has an impact on the shortening of the growing season, in particular on the earlier termination of vegetation activity, which was confirmed from the study in China [39].