Top of Atmosphere
Traceability is defined by the Committee of Earth Observation Satellites (CEOS) as:Property of a measurement result relating the result to a stated metrological reference through an unbroken chain of calibrations of a measuring system or comparisons, each contributing to the stated measurement uncertainty.
Traceability involves both an unbroken chain to that reference – a clear link of “A was calibrated against B, which was calibrated against C and so on to the reference” and the documentary evidence that each step was performed in a reliable way, with clear uncertainty analysis in the form of an uncertainty budget for each step which includes the previous step as input as well as the uncertainties introduced by the current step. Ideally this documentation is reviewed through peer review or formal audit.
Note that there are other common uses of the term “traceability” including that it is possible to “trace” the origin of all the input data sets and that there are appropriate algorithmic documents (e.g. ATBDs) and that software is formally checked. These are all important aspects of a quality system. Metrological traceability includes all this, and also the unbroken chain of calibration and uncertainty analysis.
The GUM distinguishes Type A and Type B methods for evaluating uncertainty. A Type A method uses statistical analysis of repeated observations. Usually this is used to estimate the uncertainty associated with random effects. It is possible to use Type A methods to estimate the uncertainty associated with effects that are systematic for the measurement of interest but consciously randomised for the purposes of uncertainty evaluation (e.g. by realigning an instrument that would normally not be realigned, or varying a temperature that would normally be constant). In Earth Observation Type A methods are generally used to estimate noise statistics – a random effect process.
The GUM describes Type B methods for evaluating uncertainty as using “other methods”. This may include prior knowledge (e.g. from a calibration certificate or the behaviour of similar instruments), it may include performing theoretical modelling.