The Usefulness of In-Flight Measurements of Space Count to Improve Calibration of the AVHRR Solar Reflectance Bands

The solar reflectance bands (SRB; centered at λ1 = 0.63, λ2 = 0.83, and λ3A = 1.61 μm) of the Advanced Very High Resolution Radiometers (AVHRR) flown on board NOAA satellites are often referred to as noncalibrated in-flight. In contrast, the Earth emission bands (EEBs; centered at λ3B = 3.7, λ4 = 11, and λ5 = 12 μm) are calibrated using two reference points: deep space and the internal calibration targets. In the SRBs, measurements of space count (SC) are also available; however, historically they are not used to specify the calibration offset [zero count (ZC)], which does not even appear in the calibration equation. A regression calibration formulation is used instead, equivalent to setting the ZC to a constant, whose value is specified from prelaunch measurements. The analyses below, supported by a review of the instrument design and a wealth of historical SC information, show that the SC varies in-flight and differs from its prelaunch value. It is therefore suggested that 1) the AVHRR calibration equation in the SRBs be reformulated to explicitly use the ZC, consistently with the EEBs; and 2) the value of ZC be specified from the onboard measurements of SC. The ZC formulation of the calibration equation is physically solid, and it minimizes human-induced calibration errors resulting from the use of a regression formulation with an unconstrained intercept. Specifying the calibration offset improves radiances, most notably at the low end of radiometric scale, and subsequently provides for more accurate vicarious determinations of the calibration slope (gain). These calibration improvements are important for the products derived from the AVHRR low radiances, such as aerosol over ocean, and are particularly critical when generating their long-term climate data records (CDRs).