## 2 Global Open Earth Observations Satellite Missions for Monitoring Land Surface Phenology
### Landsat Program  
The Landsat program provides continuous observations since 1972, for the last 50 years, so that it represents one of the longest observations of the Earth's surface [71]. It consisted of nine satellites, from the first, Landsat 1 (23 July 1972 to 6 January 1978), to the last, Landsat 9 (27 September 2021 to present). The main spatial resolution of imagery is 30 m, which includes channels in the visible and near-infrared part of the spectrum and a panchromatic band of 15 m. It also contains thermal bands of long wavelengths with a resolution of 100 m, which enable monitoring of the Earth's surface temperature. Temporal resolution is usually 16 days (Landsat 7). Due to the multidecadal availability of images, Landsat enables global analysis of vegetation trends on a global scale in a relatively high spatial resolution. This is of relevance for the analysis of historical changes in vegetation cover over the areas on Earth that are more exposed to the risk of climate change. Some of the mentioned cases include the analysis of greenness trends over circumpolar Arctic tundra [72], Russian tundra [73], Tibetan plateau [74], snow cover retreats in the Himalayas [75] and Alps [76,77], and the shift of the boreal forest biome [78]. Landsat data are also very applicable for tracking the abrupt changes in the land cover, i.e., change detection [79], long-term land use change analysis [80], and recovery of vegetation after abrupt changes [81, 82, 83]. Due to lower temporal resolution or revisit frequency, unavailability of observations due to the cloud cover, and accidental technical problems (Landsat 7 ETM scan line correct failure), Landsat was not used as extensively for the analysis of the LSP compared with MODIS and AVHRR. In addition, the problem represents the irregular time series of observations with diverse timestamps that are not suitable for widely used applications for the calculation of phenology metrics such as TIMESAT [84]. Irregular time steps and seasonal gaps of available imagery, along with the larger computing load due to the 30 m spatial resolution, affect more demanding data processing requirements for the extraction of characteristic phenology dates from Landsat images. A new tendency in the analysis of LSP at high to medium spatial resolution is to combine Landsat and Sentinel observations (HSL--Harmonized Landsat and Sentinel-2 [85, 86, 87, 88, 89]), which jointly achieves a significantly better temporal frequency of images and partially cancels out the shortcomings associated with the problem of cloud cover. Sentinel-2 represents a constellation of two platforms, Sentinel-2A and Sentinel-2B, which performed active observation of the Earth from June 2015 and March 2017, respectively. The optical sensor performs surface scanning in 13 bands with a spatial resolution of 10-60 m, with a revisit cycle of 10 days (5 for constellation). A significant advantage of the mentioned sensors, compared to MODIS and AVHRR (Table 1), is that due to their high resolution, they enable monitoring of phenology at the level of tree species [90,91] or small [92] or large-scale cropping systems [93,92]. The fusion of the aforementioned sensors will therefore enable much more precise monitoring and determination of the effect of global climate change on vegetation dynamics, which is one of the key advantages of using images from both satellites. Trends in land surface phenology topic-related research papers indexed in the Web of Science Core Collection (WoSCC) by using various global open Earth observation satellite missions are shown in Figure 1.