# Validation of Regional-Scale Remote Sensing Products in China: From Site to Network
## 2 Validation Strategy
As feedback, simultaneous satellite, airborne and ground-based remote sensing experiments (e.g., HiWATER) occurring at some key experimental areas (KEAs) can greatly contribute to inspect the validity and effectiveness of Component 1 by primarily validating relatively higher resolution (on the order of tens of meters) RSPs at site/local to watershed scale. Progress with respect to this aspect will be given in Section 4. (3) However, readily available RSPs are usually presented at lower spatial resolution (of the order of kilometers to tens of kilometers) and with larger spatial coverage or even on a global scale basis. To validate this kind of products requires ground measurements to be collected across different climate zones, which depends on joint site-based validation efforts rather than merely over limited spatial extents/reference sites (e.g., within a river basin), as considered in Component 2. Such activities would make it possible to facilitate the establishment of a national validation network, as we accordingly proposed in Component 3. Figure 1: The overall validation strategy. WSN stands for wireless sensor network. In this context, at the prototype stage of this validation network, several anchor sites located at different climate zones and landscapes with sound observational facilities are initially selected. Synergetic multi-platform intensive observations or comprehensive remote sensing experiments are performed at these sites to discern and further rectify the weaknesses of the developed validation guidelines and specifications. Then, the amended Component 1 can be inspected by well-studied validation exercises again. Such a reiterative procedure is conducive to progressively improving and maturing the guidelines to make them more robust and standardised, with wider applicability and firm validity at the national scale. This provides possibilities to coordinate more validation test fields in the future, extending operational validation efforts to meet continental or global range application requirements. Details of this validation network can be found in Section 5.1. (4) Technically, the above three components are integrated and realised by Component 4, i.e., an operational validation system. As a service portal, the functionalities and algorithms embedded in this system are designed to use multi-scale in situ and remote sensing data to validating quantitative RSPs at the regional or global scales. As shown in Figure 1, this system includes three parts related to data sets, accuracy evaluation and scale effect analysis. This setup facilitates the implementation of desired functions ranging from data host to product accuracy assessment. Being compliant with Component 1, scale effect can also be investigated in this process. The introduction of this system can be found in Section 5.2. In conclusion, with commitment of continuous data collection over nationally representative sites, the conceived overall VS and involved work packages are expected to help us gain additional insight into validation issues and especially address the immediate scientific question of how to acquire representative ground truth in remote sensing validation (including modelling) over heterogeneous landscapes.