What do satellites really measure and how well do they measure it?

Rather than answering this question in the abstract, we chose a representative suite of instruments of interest to ARM. These instruments include several visible-infrared (IR) imagers-the Advanced Very High Resolution Radiometer (AVHRR) and the High Resolution Infrared Sounder (HIRS) on the National Oceanic and Atmospheric Administration's (NOAA) polar orbiters, as well as the current VISSR (Visible and Infrared Spin-Scan Radiometer) Atmospheric Sounder (VAS), and the next generation Imager and Sounder on NOAA's Geostationary Operational Environmental Satellites (GOES). We also include the scanning Earth Radiation Budget Experiment (ERBE) radiometer and its follow-on, the Cloud and Earth Radiation Energy System (CERES), to be launched on the Earth Observing System (EOS). Table S.1 summarizes key features and the major measurement uncertainties for each of these instruments.

In each case, the instrument really measures the TOA radiance () in some spectral band. Therefore, we have paid particular attention to the radiometric accuracy. With the exception of VISSR/VAS, these instruments do well in the infrared, with typical accuracies of 0.6K, corresponding to about 1% of the measured longwave radiance. These accuracies are established and maintained first through a pre-launch calibration with a carefully temperature-controlled blackbody and then by an on-board two-point calibration (deep space view and a temperature- monitored blackbody) in which the calibration sources are viewed through the entire optical train of the instrument. (Some of the AVHRR channels exhibit a small non-linearity, but this is adequately handled by non-linear corrections developed during the pre-launch calibration.) VISSR/VAS is the one major exception to this rule, as the instrument foreoptics are beyond the on-board calibration target and the influence of the foreoptics on the radiance measurements must be determined by model calculations.

The picture is much less encouraging for visible calibration. Of the representative instruments only ERBE/CERES has on-board visible calibration. Furthermore, instruments such as AVHRR (on NOAA-9, -10) have been known to experience significant degradation in their response (up to 7% per year for a total of 20-25%) after launch. One solution to this calibration problem is to use targets on the Earth's surface as the reference radiance sources. In this way an indirect (i.e., not intrinsic to the instrument) calibration can be made. Currently the best indirect calibrations have an absolute accuracy of about 5% as discussed below.