## 4 Current Status and Identified Needs
### Level 2 Bio-Geophysical Land Products
#### 4.3.6 Field Campaigns and Advanced Technological Solutions  
In addition to the operational ground-based networks, the validation of satellite-based land products is largely based on ad-hoc field campaigns. While operational networks typically rely on well-consolidated technology and standardised measurement procedures, to ensure robustness and consistency across the sites, new measurement solutions and practices can be tested during ad-hoc field campaigns, for in-depth validation of satellite EO data over specific ecosystems.  
One very popular example of such advanced solutions are drones, also called Unmanned Aerial Vehicles (UAV), which are a flexible and cost-effective solution for field surveys, notably if compared to airborne campaigns. UAV-based measurement systems are being tested in various land (and water) remote sensing studies since they allow for enhanced spatial and multi-angular sampling [166] over the region of interest. UAV-based techniques also permit the surveillance of remote locations difficult to access, such as wetland, coastlines and dense tropical forest. The UAV flight plan, both the altitude and the acquisition paths, can be defined pre-launch and progressively optimised in successive flights to attain the best spatial sampling over the considered ecosystem. UAV-based hyperspectral sensors are nowadays considered extremely valuable to complement in-situ and airborne spectrometry for the validation of satellite SR products and for ecological monitoring, as demonstrated in recent activities over the Canadian Mer Bleue peatland site [167; 168]. The relevance of these measurement systems is also demonstrated by the recent SRIX4 VEG initiative [154], which opens a call for a field inter-comparison campaign of UAV-based hyperspectral sensors, to contribute towards developing community-agreed best practices for satellite SR product validation.  
Similar to UAV-based systems, new low-cost automated devices are becoming a very interesting alternative to enhance temporal sampling over the region of interest. Such sensors are usually based on inexpensive commercial off-the shelf (COTS) technologies, such as miniaturised sensors controlled with small single-board computers, which can be easily deployed and replicated at different locations. The most widely used sensors are the automated digital hemispherical photography (DHP), which allows tracking the phenological cycle of the canopy at very high temporal interval [169; 170]. The use of these devices in the land domain has significantly grown in the past decade for complementing the in-situ traditional field measurements.  
Another technique receiving an increasing interest in the land community, is active sensing, using LiDAR systems, both terrestrial-based (TLS) or UAV-based systems (UAV-LS). Attaching a LiDAR scanner to a moving platform, such as a terrestrial pointing system or UAV-based platform, allows 3D mapping of large and multi-layered canopy structures, notably over forest sites [171]. The derived point cloud measurements can be used to retrieve land bio-geophysical parameters, such as LAI [172], Leaf Area Density [173] or detailed tree volume for Above-Ground Biomass (AGB) estimation [174]. In this latter study, the combination of UAV-based platform and LiDAR systems, has shown to be extremely effective for overcoming the limitations of TLS time-demanding fieldwork, while providing comparable accuracy.  
To conclude, there is an ever-increasing number of new advanced solutions for land validation purposes. However, the best practices are not consolidated yet and the estimated accuracy of such methods, both for Cal/Val and for biophysical products estimate still needs to be properly characterised and benchmarked against traditional methods.