## 6 Recommendations
### Develop Validation of Level 2 Products  
Perhaps the most ambitious recommendation from the workshop was to develop a process for validation of Level 2 products. The definition of Level 2 products, as least for the purposes of this paper, are surface reflectance and surface temperature products produced from Level 1 at-sensor products through atmospheric compensation. The major driver for this is the observation that the vast majority of scientists who use data from these sensors are interested in what is occurring at the surface and not at the sensor. In fact, every indication for the past several years suggests that surface products will become, or already are, the'standard' product of most users. Within the NASA Earth Observation System (EOS) Program, Terra and Aqua MODIS surface reflectance products have been the standard source for generating higher-level land products at global scales. Due to the large footprint of MODIS pixels it has not been practical to validate surface reflectance using ground-based radiometers or spectrometers. Instead, considerable progress has been made by first comparing radiative transfer models [73], and then validating the most uncertain part of the atmospheric correction--aerosol estimation--using AERONET optical thickness data. This approach has been extended to Landsat surface reflectance products [26], and formed the basis for the recent CEOS Atmospheric Correction Intercomparison Experiment (ACIX), which involved both Landsat and Sentinel-2. However, the 10-30 m resolution of Landsat and Sentinel-2 now admit the possibility of direct, ground-based validation of spectral reflectance itself and use of in situ surface reflectance observations for validation was explicitly highlight by the panel. This will be especially challenging because few of these measurements are made on a regular basis, standard procedures for acquiring the measurements have not been developed, accuracy and precision will have to be determined, and it will likely require global cooperation to obtain a satisfactory representation of the Earth's surfaces. It will be necessary to develop a systematic process for obtaining surface reflectance measurements. Implicit in this will be the need for a standard procedure for the measurement that includes methodology, data format, estimates of accuracy and precision, traceability to the International System of Units (SI traceability) and data management/availability. Fortunately, there are a few activities already in progress that can lend support to the validation of surface reflectance products. The RadCalNet program, being developed in the context of CEOS WGCV IVOS, is a good example ([[http://calvalportal.ceos.org/test-sites/radcalnet-prototyping](http://calvalportal.ceos.org/test-sites/radcalnet-prototyping)]([http://calvalportal.ceos.org/test-sites/radcalnet-prototyping](http://calvalportal.ceos.org/test-sites/radcalnet-prototyping))). The intent of RadCalNet is to provide top of atmosphere radiance/reflectance estimates over stable radiometric calibration sites. However, to do so, each of these sites directly measures surface reflectance and makes those data available to users. Currently in a beta test configuration, it is anticipated that the data will soon be available to the public. Another related activity is the Fiducial Reference Measurements (FRM) program sponsored by the European Space Agency ([[https://earth.esa.int/web/sppa/activities/frm](https://earth.esa.int/web/sppa/activities/frm)]([https://earth.esa.int/web/sppa/activities/frm](https://earth.esa.int/web/sppa/activities/frm))). This program provides a variety of in situ measurements for satellite ocean color, altimetry, and air quality, to name a few. As such, it provides a promising framework that could be adapted for surface reflectance measurements. As a final note, successfully accomplishing this recommendation will clearly require a global effort.