## 4 Current Status and Identified Needs
### Level 2 Bio-Geophysical Land Products
#### 4.3.4 Existing Land Ground-Based Networks  
There is a large number of existing ground-based networks, which could be potentially used for satellite land products validation. We list here those that are currently active and widely used in the land domain:  
* The AERONET network [52] is providing globally-spread, continuous and readily accessible Cal/Val data of aerosol optical, microphysical and radiative properties. Despite the fact that AERONET is focused on atmospheric measurements, it has been largely used in the land community, both as ancillary data as well as for indirect validation of SR products, as proposed within ACIX [118]. The major strength of AERONET is its spatiotemporal and geographical coverage and the consistency across the whole network, allowing to perform unbiased, comprehensive and statistically robust validation of satellite-based EO data at global scale. * The National Science Foundation's National Ecological Observatory Network (NEON) [156; 157] provides open data from 81 field sites located in different ecosystems across the United States (US). Data collection methods are standardised across the sites and include automated measurements, observational field sampling and airborne remote sensing surveys. The NEON data catalogue includes over 175 data products, providing key information about plants, soil, freshwater and the atmosphere. The validation of vegetation variables, i.e., FAPAR, LAI, fraction of vegetation cover (fCover), within GBOV [53] (c.f. Section 4.3.5) mostly relies on NEON data, with clear limitation in terms of global coverage, since NEON observations are only representative of US biomes. * The Surface Radiation Budget Network (SURFRAD) [158; 159] was established with the primary objective to support climate research with accurate, continuous, long-term measurements of the surface radiation budget over US. Independent measures of upwelling and downwelling, solar and infrared radiation are the primary SURFRAD products. Observations from SURFRAD have been used for evaluating satellite-based estimates of surface radiation, as in GBOV [53], and for validating hydrologic, weather prediction and climate models. * The Integrated Carbon Observation System (ICOS) [160] is a pan-European network aiming at quantifying and understanding the greenhouse gas balance of Europe and neighbouring regions. ICOS infrastructure includes more than 100 measurement stations in twelve European countries. These stations measure greenhouse gas concentrations in the atmosphere and fluxes over the terrestrial and marine ecosystems [161]. ICOS measurements are used within GBOV [53] to validate surface reflectance and albedo products. Inclusion of ICOS data for fAPAR, LAI and fCover, as well as LST, is currently under investigation within GBOV, although some adaptation of the measurement protocols will be required [3]. * FLUXNET [162] is a global network of micrometeorological tower sites, providing information for validating remote sensing products for net primary productivity (NPP), evaporation, and energy absorption. The network includes more than 800 active and historic flux measurement sites, dispersed across most of the world's climate space and representative biomes [163]. At each FLUXNET tower site, the eddy covariance method is applied to quantify water, carbon and energy fluxes between the biosphere and atmosphere. FLUXNET data are used to complement the GBOV [53] database for validation of surface reflectance and albedo variables. * The Terrestrial Ecosystem Research Network (TERN) [164] is Australia's infrastructure for long-term ecosystem observations. The TERN database includes a large ensemble of validation data gathered during field campaigns (>1000 sites) [165]. In addition, as part of the TERN network, 10 sites were identified, which were intensively characterised (super sites) and are suitable for multi-instrumental land product validation and algorithm development.