Calibration of the Solar Channels of the NOAA-9 AVHRR Using High Altitude Aircraft Measurements

Abstract

A method for calibrating satellite radiometers is investigated. A calibrated spectral radiometer carried aboard a U2 aircraft at an altitude of 60 000 ft was aligned with White Sands. New Mexico along the same view vector as the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA-9 spacecraft at the time of the spacecraft's overpass on 26 August 1985. Both sets of data have been transformed into best estimates of the radiance at satellite altitude inside the footprint of the aircraft radiometer, allowing an estimate of radiance calibration changes in the AVHRR to be made. It is assumed that both instrument systems are linear, that the spectral response function of AVHRR has not changed from its prelaunch value, and that the zero radiance responses of both instruments are accurately known. Extrapolation of the radiances measured from the aircraft to those expected at satellite altitude is achieved by modeling the experimental conditions at White Sands and calculating the ratio of radiances at the two altitudes through the LOWTRAN VI computer program. Results from data taken within 2 minutes either side of the satellite overpass indicate a 98.9% correlation between the two sets of data, and a change in gain relative to the prelaunch calibration of +2 ± 5% for channel 1 and ?2 ± 5% for channel 2 of the NOAA-9 AVHRR. Analysis of other coincident data for the NOAA-9 AVHRR and the aircraft spectral radiometer, including a large dataset from October and November 1986, is now in progress and will establish the day-to-day repeatability of results using this method.