## 2 Post-Launch Calibration and Validation (Cal/Val)  
In a generic manner, a post-launch Cal/Val program for an Earth Observation (EO) mission is typically composed of a set of different complementary activities bringing elements that need to be combined in order to produce consolidated and confident validation results to encompass the full scope of observations of a sensor. Note that the use of the word 'comparison' represents the situation where the nature of the products being compared is similar and of a similar uncertainty. 'Validation' is used where the link to the satellite product is more tenuous and/or the associated uncertainty of the non-satellite measurement is larger and cannot be considered similar, as in the later bullets. 'Calibration' is a special case of comparison where the uncertainty of the reference and the comparison process are sufficiently small that this allows the satellite sensor's measurement to be assessed against it and potentially adjusted as part of the comparison process. Generically, the different components of a validation programme are the following:  
* Comparison against tailored and accurate Fiducial Reference Measurements (FRMs) that provide few points but low uncertainty/high confidence;
* Comparisons against general in situ data that can provide more data over a wider set of conditions, but are generally individually less accurate;
* Comparison against other sources, including: inter-satellite comparison, satellites own Level 3 (including visual interpretation) (i.e., geo-referenced product of merged data derived from the sensor itself, and/or other sensors, e.g., seasonal trends, latitude variations) climatologies, etc.;
* Comparison against models: re-analysis data, data assimilation, integrated model analyses, forward modelling, etc.;
* Validation using monitoring tools: statistics, trends, systematic quality control, etc.;
* Comparison/calibration between operational satellites (e.g., using GSICS best practices).  
All of these components are important and often necessary, but the first component (comparison to FRM) is of particular importance because it gives a reference properly characterised and traceable to a standard on which the validation results can be anchored and an uncertainty assessed. If of a sufficiently low uncertainty, the FRM can also be used to provide calibration information that can be used to update the sensor performance coefficients during operation.  
As indicated above, satellite post-launch Cal/Val has traditionally relied on a wide range of data sets collated by different teams and based on, for example, in situ and aircraft campaigns, (semi)-permanently deployed instrumentation, comparisons with other satellite sensors, opportunistic measurements (e.g., from commercial shipping and aviation) and modelled natural phenomena (e.g., Rayleigh scattering of the atmosphere over dark ocean surfaces).  
To date, these different datasets have had varying levels of adherence to the critical QA4 EO principles, and while all provide information, it is hard for satellite operators to distinguish high-quality data optimised for satellites from those that need more careful treatment. Consistency between datasets for comparison with a single satellite sensor is generally not sufficient to ensure the quality of all comparisons, for all sensors and all conditions. The creation and adherence to the FRM label is designed to address this issue.