Absolute sensor calibration is the basis on which measured radiance from multiple sensors can be
compared with each other or with time. Despite the increasing sophistication and reliability of on-board
satellite sensor calibration systems, vicarious (or ground-look) calibration methods remain an important
component of calibration validation. These methods, particularly the surface reflectance method, typically
involve intensive measurements, at the time of satellite over-flight, of the surface and atmospheric
properties (such as, surface reflectance and temperature, atmospheric pressure, water vapor and temperature
profiles, and aerosol optical properties) that are ideally uniform, stable, and well defined. Such conditions
are found, for example, in the desert southwest of the United States or in the atmosphere above high
altitude lakes. A key measurement that has been often neglected and one that is becoming increasingly
important due to rapid industrialization of developing countries, is the single scattering albedo of aerosols--a measure of the fraction of light that is scattered from the total amount extinguished from the direct beam;
typical values range from close to 1 to 0.8 or lower for highly absorbing aerosols. It is obtained from a
measurement of aerosol absorption for which many techniques are available. The error in the measurement
of absorptance disproportionately impacts the error in the scattered radiance as seen by the remote sensor.
Here we describe the sensitivity of sensor calibration to aerosol absorption and illustrate why it should be
an important measurement in the field calibration campaigns, including those planned for the next
generation satellites of the US National Polar-Orbiting Operational Environmental Satellite System.