Because outer space is such a harsh environment, the performance of all satellite sensors degrades over time. Historically, once an instrument was launched into space, it was eroded by the elements in ways that could not be accurately predicted so that, over time, errors and uncertainties were introduced into the collected data. There is additional concern that jostling an instrument during launch and deployment can affect performance. In anticipation of these problems, EOS sensors will have unprecedented onboard calibration systems enabling engineers on the ground to characterize their performance throughout the lifetime of each satellite's mission and correct for errors introduced into the data by system degradation.

To achieve consistent and accurate measurements that can be used to detect climatic and environmental change, the signals recorded by the detectors in each instrument must be translated into Earth spectral reflectance and temperature units, or the units of reflected and emitted radiance by the Earth and its atmosphere. This translation—or calibration of the instruments—must not only be accurate and consistent among all of the Terra instruments' detectors, but it must also be consistent with the detectors of all of the EOS instruments that will follow over the next 18 years.

Hence, for EOS, calibration is the set of operations or processes that are used to determine the relationship between satellite instrument output values (i.e., digital counts) and corresponding known values of a standard, which are expressed in Systeme Internationale (SI) units. Calibration of the EOS instruments requires that the ongoing performance of each be carefully characterized. Characterization is the set of operations or processes used to quantitatively understand the operation of an instrument and its response as a function of the gamut of operating and viewing conditions experienced by the instrument on orbit. In summary, this effort—unparalleled in other space missions—consists of the following elements:

• pre-flight instrument calibration and characterization;
• pre-flight intercomparison of the performance of the instruments;
• installation of calibration devices on the instruments for on-orbit calibration and characterization;
• on-orbit maneuvers of the Terra platform for additional calibration and characterization;
• validation of the calibration by comparing satellite observations to simultaneous aircraft observations, and to targets on the Earth's surface and in the atmosphere that have known, stable, or measured physical properties;
• analysis of the multiple calibration information, resolution of conflicting information, and formulation of the calibration used in the data analysis; and
• intercomparison with future EOS platforms.