Pre-launch calibration of AVHRR reflected-solar channels

Abstract

Demand for quantitative (rather than qualitative) measurements of reflected-solar radiance has grown during the period of availability of AVHRR data, but even today the most important applications involve image products. The principal purpose of pre-launch radiance calibration of Channels 1 and 2 (centered at approximately 630 and 850 nm) has therefore been to ensure that the channel gains are correctly set to make best use of the dynamic range of the instrument rather than to calibrate the radiance response characteristics of the instrument with high accuracy. This emphasis was encouraged by the technical difficulty of accurate calibration in this part of the spectrum, as well as by the absence of in-orbit calibration for Channels 1 and 2. This paper briefly reviews pre-launch calibration methodology for the AVHRR at the manufacturer's facility, and the absolute accuracy of the process, which is adequate only for the purpose of setting the gain of the instrument. A 1989 proposal by the National Institute for Standards and Technology (NIST), formerly known as the National Bureau of Standards (NBS), to improve the accuracy has not yet been funded. The importance of improving the accuracy is expected to increase in the future as new applications for quantitative measurements emerge in support of research into the mechanisms of global change.