## 4 Current Status and Identified Needs
### Level 2 Land Surface Reflectances
#### 4.2.3 HYPERNETS Project  
The HYPERNETS project [136,137] aims at developing a global automated network of ground-based hyperspectral radiometers, measuring water and land bidirectional surface reflectance for supporting the validation of satellite-based SR products. The radiometers will be equipped with an embedded calibration device and a pointing system, allowing it to cover a wide range of viewing and azimuth angles for the full characterisation of the surface BRDF.  
The HYPERNETS network will ensure accurate SR validation data over land and water sites, supporting the needs of any space-borne optical mission and filling a well-recognised data gap in the current Cal/Val ground-based infrastructure. In the water domain, the AERONET-OC network [138] has been providing long-term continuous data for validation of water leaving reflectance. However, AERONET-OC multi-spectral measurements were specifically tailored to the needs of ocean colour narrow-band sensors, such as MERIS, OLCI or SeaWIFS, and they are not well suited for validating broad-band high-resolution sensors, such as S2 or L8. For these sensors, the common approach consists of accounting for the differences in spectral band definition using band adjustment procedures [139,140], although this introduces uncertainties in the validation analysis. Hyper-spectral measurements from HYPERNETS will allow users to overcome these limitations, by ensuring suitable validation data for any spectral bands in the solar spectrum domain and by filling the Cal/Val needs of future hyperspectral missions, such as ESA's CHIME mission.  
At the time of writing this paper, preparatory activities are on-going for setting up the future HYPERNETS network; this includes the deployment of first autonomous hyperspectral radiometers in coastal and inland waters and initial exploitation for validation of aquatic reflectances derived from high-resolution satellite data [141]. The project will continue in the coming years with evolution of the instrumental set-up, using more advanced hyperspectral systems with additional SWIR channels for land observations and with the progressive deployment in a number of land and water sites. The location of the land sites was defined to cover a wide range of surface types, aerosol loading and ecoclimatic areas for challenging the performances of the AC algorithms in realistic conditions.  
\begin{table}
\begin{tabular}{c c} \hline
**ID** & **Status and Needs** & **Recommendations** \\ \hline \hline IDN-07 & The lack of ground-based surface reflectance measurements over land is currently limiting our capability to validate SR products and assess the quality of AC algorithms. Such measurements should ideally be: continuous (increase match-up), hyper-spectral (multi-mission) and multi-angular (for BRDF characterisation). & Space Agencies, in coordination with CEOS-LPV, should support on-going efforts (e.g., HYPERNETS) working towards setting up a globally representative network of ground-based measurements for supporting the validation of satellite-derived land SR products. \\ \hline \hline \end{tabular}
\end{table}
Table 3: Identified needs and derived recommendations for assessing quality of land SR products.