## 6 Recommendations
### Improve Calibration Methodologies  
As expected there were several recommendations from the workshop that fall into the category of improving calibration methodologies. These can be divided into geometric, radiometric, and cross-calibration categories. Under the category of geometric calibration, three recommendations were made. The first was to establish inter-agency agreement to share the LIC Geometric Reference Image (GRI) that has been developed by ESA and is being incorporated into the Sentinel 2 mission. This approach would improve the geometric accuracy of Landsat imagery and also provide greater consistency between the two systems. The second recommendation was for the Sentinel 2 Digital Elevation Model (DEM) to be shared with the Landsat program which would allow for a modest improvement in geometric consistency as well. Lastly, it was recommended to publish both Landsat 8 and Sentinel 2 point spread functions with the goal of allowing the user community to more effectively resample higher resolution imagery to Landsat and Sentinel scales. Radiometric calibration recommendations were also threefold in nature. The first recommendation was to use a common solar irradiance model. This is a shared concern that has received significant attention in the calibration community over the years, but has been limited in resolution. As an example, in the case of Sentinel 2 and Landsat, the Thuillier and ChKur models are used, respectively. Differences in these models are in the 2-3% range in the SWIR, so noticeable improvement in consistency is possible with this recommendation. Use of multiple models has a direct impact on vicarious calibration methods, especially when comparisons are done across sensors. Therefore, because several solar irradiance models exist and it has also been hard to standardize on one, several systems are moving to reflectance-based radiometric calibration which obviates the need for a solar irradiance model. In addition, reflectance-based calibration has an advantage of less uncertainty in the methodology. The second specific topic for radiometric improvement was the use of consistent Earth-Sun distances. This factor affects absolute radiometric calibration up to 3% over the year. Any inaccuracy of its estimate directly impacts the assessment of absolute gain coefficients. It was recommended at the workshop to compare the values of Earth-Sun distances estimated over one year between NASA for OLI calibration and MPC for S2 calibration. A third radiometric calibration recommendation was to use common methods for PICS-based calibration. Pseudo Invariant Calibration Sites are routinely used to monitor the long-term stability of optical satellites using stable sites that are primarily located in the Sahara Desert. But, there has been a lack of standardization for using these sites. For example, different regions are used, as are different models for surface BRDF, spectral responses, and atmospheric conditions. Fortunately, there are efforts underway for standardization (the CEOS Working Group on Calibration and Validation (WGCV) Infrared Visible and Optical Sensors (IVOS) PICS Characterization (PICSCAR) project as one example), but significant effort will be required to reach agreement on standardized methods. A series of recommendations were developed for improving cross-calibration. The first was to generate consistent cross-calibration coefficients within the Landsat and Sentinel 2 series. Sensitivity to this, of course, is most prevalent for Landsat which has been flying sensors since the 1970s. Because of substantial differences in sensor design, as well as incremental improvements to calibration that have occurred over the intervening decades, the consistency among these sensors has not always been optimal. Fortunately, USGS EROS has been addressing this issue and a consistent calibration for all sensors from Landsat 8 OLI back to the Landsat 1 Multispectral Scanner (MSS) will be incorporated in Collection 2 Processing which is due to be released in 2018.