Long-term climate data records often consist of observations made by multiple sensors. It is, therefore, extremely
important to have instrument overlap, to be able to track instrument stability, to quantify measurement uncertainties, and
to establish an absolute measurement scale traceable to the International System of Units (SI). The Moderate Resolution
Imaging Spectroradiometer (MODIS) is a key instrument for both the Terra and Aqua missions, which were launched in
December 1999 and May 2002, respectively. It has 20 reflective solar bands (RSB) with wavelengths from 0.41 to
2.2μm and observes the Earth at three nadir spatial resolutions: 0.25km, 0.5km, and 1km. MODIS RSB on-orbit
calibration is reflectance based with reference to the bi-directional reflectance factor (BRF) of its on-board solar diffuser
(SD). The SD BRF characterization was made pre-launch by the instrument vendor using reference samples traceable
directly to the National Institute of Standards and Technology (NIST). On-orbit SD reflectance degradation is tracked by
an on-board solar diffuser stability monitor (SDSM). This paper provides details of this calibration chain, from pre-launch
to on-orbit operation, and associated uncertainty assessments. Using MODIS as an example, this paper also
discusses challenges and key design requirements for future missions developed for accurate climate studies.
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