## 5 Current Status and Limitations of Data Interoperability with Landsat and Sentinel
### Data Interoperability for the International Community  
The international community, through the Committee on Earth Observation Satellites (CEOS), is expressing an expectation of data interoperability as a general principle, beyond specific application areas such as those discussed above. Interoperability is seen as a key requirement to deliver benefit from operational remote-sensing satellite systems such as Landsat and Sentinel 2, and as a logical 'next step' to build on the success of free and open data [65].  
CEOS (www.ceos.org) seeks to optimize the benefits of space-based Earth observation through high level cooperation in areas such as mission planning and provision of compatible data products and policies. As applications become more operational, data suppliers will need to ensure:  
* Continuity so that down-stream products can be developed with confidence that the data streams will continue, with smooth transitions as old instruments are retired and replaced with newer versions;
* Interoperability between data streams to allow reliable and high-quality products, as demonstrated in the NASA HLS Project; and
* That data are fit for use by specialists in agriculture, security, civil engineering, disaster management and so on, who may not be remote sensing scientists. These users will need data that are'ready to use'.  
Figure 15: Impact of differences in the red channels expected due to differences in spectral bands.  
CEOS is therefore establishing a framework for analysis-ready data for land applications, defined as: _CEOS Analysis Ready Data for Land (CARD4L) are satellite data that have been processed to a minimum set of requirements and organized into a form that allows immediate analysis with a minimum of additional user effort, and, interoperability both through time and with other datasets_.  
CEOS analysis ready data places an expectation that satellite data will not only be calibrated at sensor level, but will also be processed to provide a quantitative land surface measurement. At time of writing, land surface reflectance and land surface temperature have been identified by the CEOS community as analysis ready data products for optical land observing satellites. Although measurements taken with differing instruments (e.g., OLI, MSI) will not be identical, they will be fundamentally comparable and, therefore, a critical step toward enduring interoperability.  
Several surface reflectance products are already being produced. Australia ([PERSON] et al., 2016), Canada, China, the European Space Agency ([PERSON] et al., 2016), France, the United Kingdom and the USA ([PERSON] et al., 2016) are actively developing or have developed surface reflectance products from Landsat and Sentinel-2 platforms. In the private sector, Planet released a surface reflectance specification in 2017 ([PERSON] et al., 2016). In addition, [PERSON] et al. ([PERSON] et al., 2017) indicated the development of surface reflectance products for land cover climate change, analysis ready data has been developed in Switzerland ([PERSON] et al., 2016) and [PERSON]'s efforts were summarized previously.  
However, these efforts lack coordination and technical consistency; for example, in approaches to corrections for the atmosphere, the bidirectional reflectance distribution function and terrain. CEOS is developing a framework for analysis ready data that can guide the efforts of individual agencies toward a consistent overall approach.  
To meet the rising expectations of the international community for interoperable data, including preparing data for exploitation in new architectures such as data-cubes ([PERSON] et al., 2016) calibrated data from Landsat and Sentinel 2 will need to be further processed to produce land surface measurements of surface reflectance, validated against in situ measurements. The methods and protocols to achieve this are a significant challenge for the calibration and validation community.