## 3 Data Collections  
### Satellite Measurements  
#### 3.1.1 Background  
Observations of land surfaces from spaceborne platforms started in earnest in the early 1970s with the launch of the Earth Resources Technology Satellite (ERTS-1), subsequently renamed Landsat 1. In turn, the availability of multispectral measurements stimulated the development of methods to explore the significance and implications of those observations in practical applications. [PERSON] et al. [16] and [17]  
Figure 1: Panel (a): Location of the study site in the Lowveld of South Africa with overlapping footprints of Multi-angle Imaging SpectroRadiometer (MISR) Block 110 for Path 168 (dotted rectangle) and 169 (full rectangle). Panel (b): Detailed map of the environment around Skukuza in the Kruger National Park. Source: South African National Biodiversity Institute (SANBI), acquired from [[https://www.sanbi.org/link/bgs-biodiversity-gis/](https://www.sanbi.org/link/bgs-biodiversity-gis/)]([https://www.sanbi.org/link/bgs-biodiversity-gis/](https://www.sanbi.org/link/bgs-biodiversity-gis/)). suggested that the ratio subsequently known as the Normalized Difference Vegetation Index (NDVI) was adequate to monitor \"the vernal advancement and retrogradation (Green wave effect) of natural vegetation\", i.e., to monitor plant phenology as we understand it today. These events mark the start of a sustained effort to monitor the biosphere on the basis of space observations [18]. Over the past 50 years, significant progress has been achieved, and thousands of publications have been published [19], to characterize the state and evolution of plant canopies using observations of the reflectance of the planet in the solar spectral range. A full review of the literature dealing with the documentation of plant phenology on the basis of remote sensing measurements is outside the scope of this paper, but it is worth noting that progress has been achieved on multiple fronts:  
1. Major advances in understanding the processes of radiation transfer in complex environments resulted in the design and implementation of much improved algorithms. These aimed to effectively account for processes unrelated to vegetation (such as atmospheric clouds and aerosols, anisotropic effects, contributions to the measured signals due to variations in soil moisture, etc.) and to deliver reliable and accurate products to describe plant processes. The Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) is a case in point, since it has been identified as one of the _Essential Climate Variables_ (ECV) by the Global Climate Observing System (GCOS) [20]. These developments, in turn, led to a more quantitative representation of plant phenology (in [21; 22; 23]). 2. The technology of measuring reflected radiation on spaceborne platforms has improved drastically, and is currently able to deliver up to hundreds of spectral bands at spatial resolutions of the order of tens of meters or less, sometimes on a daily basis, or to acquire data from multiple directions or in multiple polarizations. Data fusion with observations obtained with different instruments (e.g., LiDARs: see [24; 25; 26]) or in entirely different spectral regions (e.g., microwaves: see [27; 28; 29]) have also been explored, though much remains to be done in order to provide a holistic description of the environment, or of plant phenology in particular. 3. Progress in the field of information technology delivered computing, networking and data archiving resources and capabilities orders of magnitude faster and more performant than even a few decades ago, and permitted the investigation of complex issues. Although the drive to acquire more and better observations continues unabated, the major bottleneck today concerns the actual analysis of those data and the extraction of useful information from the immense archives that have already been accumulated.