## 2 Framework of the VRPC  
Figure 1 shows the framework of the VRPC, which includes four basic components. The first element is the design and construction of the observation stations and sites. The stations will be selected according to several principles. The existing bases and resources for remote sensing product validation need to be considered first. The sites involved in the stations need to be adequately typical and representative, while also being relatively heterogeneous over a sufficient area. The installation of observation systems must fully consider the spatial distribution characteristics of the underlying surface. Thus, prior knowledge of the key parameters of the sites' land surface need to be acquired and the installation strategy needs to be optimized based on prior knowledge [12]. The validation stations can be divided into two main types. The first type consists of one core site or several core sites, which are set up especially for validation of remote sensing products. These sites normally have special observation techniques such as unmanned aerial vehicles, ground-based remote-sensing platforms, and wireless sensor networks (WSN). Simultaneous satellite-airborne-ground experiments, ground-based remote sensing control experiments, and intensive ground experiments can be launched periodically. The theories and methodologies for the scale transformation and validation are studied based on these thematic experiments. The second type of validation station is based on other existing observation networks, such as the Chinese Ecosystem Research Network (CERN), the Chinese Terrestrial Ecosystem Flux Research Network (ChinaFLUX), and the High-Cold Region Observation and Research Network for Land Surface Processes and Environment (HORN). The stations for these networks are set up for special scientific aims. Based on their existing observation base and systems, some improvements can be processed for the demands of remote sensing validation. Then, these sites can perform some continuous and fixed observations. The obtained data are normally used to validate remote sensing products directly based on theories and methodologies. Some validation standards and guidelines for remote sensing products are being studied and formulated for a special project. These standards and guidelines can be used to construct the sites and to establish collaborations with other networks' sites. The results of the applications and the theory or methodology studies can be used to improve the standards and guidelines. Some operation and management mechanisms also need to be formulated for the VRPC. These mechanisms include _in situ_ observation guidelines, data processing guidelines, data quality control guidelines, data archiving guidelines, and data sharing regulations, and they need to be discussed by administrative and technological staff of the core sites. The above four basic components constitute the VPRC. The selection of the stations and core sites is the basis of the observations, and the pixel-scale observation sites need to be constructed, which represents the largest difference between normal stations and the VPRC's stations. The validation standards and guidelines are the theoretical basis. Scale is the key problem in the validation strategies for different remote sensing retrieval variables, and a great deal of research is required on this topic. The management mechanisms are the operating basis for the collaborative observation network in China, which ensures that the data are consistent and shared. The core sites of the special remote sensing stations and other stations involved in existing observation networks are the platform for the _in situ_ measurements of several atmospheric and land surface parameters. The prototype design above needs to be practiced on these platforms. Additionally, the design can be improved according to the application practices. The settings of the core sites are involved in the validation strategies. Thus, the settings affect the validation strategies. Figure 1: Framework of the VRPC.