internationally accepted units of physical measurement, the SI units.2 For optimum accuracy and the long-term stability of an absolute calibration, it is necessary to establish traceability to an SI unit.
However, it is more important to determine to which SI unit the measurement must be traceable and whether traceability to an SI unit is even necessary. This can be done by examining the requirements of the data product algorithm. The question of which SI unit to use is answered by determining which chain of measurements has the lowest accumulated uncertainty. Often it is the shortest measurement chain and sometimes the most convenient one. In the case of a relative measurement, that is, in the measurement of a ratio (for example, reflectance), traceability to an SI unit is meaningless. The accuracy in this case will be a function of all the uncertainties accumulated in determining the ratio of the outgoing to the incoming radiation.
The specific characterization and calibration requirements and the needed accuracy are determined by the needs of the algorithms for each data product—that is, the parameters that must be measured and the accuracy to which they must be known. This list of requirements is usually presented as a list of specifications in the contract to build the sensor. It is obvious that if the accuracy requirements are set too high, needless expense will be incurred. If they are set too low, the algorithm will not produce an acceptable data product. The optimum range should be set by sensitivity analyses of the algorithms of several key data products.
3. Calibration verification is the process that verifies the estimated accuracy of the calibration before launch and the stability of the calibration after launch. Prelaunch calibration verification could take the form of documentation of accumulated uncertainty or it could be determined by comparisons with other, similar, well-calibrated and well-documented systems. The latter method of calibration verification is preferred since one or more sources of
The General Conference on Weights and Measures, Conférence Générale des Poids et Mesures (CGPM), recommended the establishment of a practical system of units of measurement that was adopted by all signatories to the Treaty of the Meter (Convention du Metre). The name Système International d’Unités, abbreviated SI, was given to the system by the 11th session of the CGPM in 1960. In 1970 at the 14th CGPM, the current version of the SI was completed by adding a seventh base unit, the mole. The other six base units are the ampere, kelvin, kilogram, meter, second, and candela. These well-defined units (by internationally agreed upon experimental methods for realization) are by convention considered to be dimensionally independent and are to be used to establish the quantities for the measurement of electricity, temperature, mass, length, time, luminous intensity, and amount of substance. All other quantities are derived from the SI units and are defined by the International Organization for Standardization. At the end of the chain of measurements needed to establish a derived quantity is the standard, or measurement artifact, to which has been assigned a value in one of the units derived from the SI. In principle, similar standards obtained from two different national measurement laboratories should be equivalent within the combined uncertainty accumulated in their respective chain of measurements.