Using Moderate Resolution Imaging Spectrometer (MODIS) to calibrate advanced very high resolution radiometer reflectance channels

A series of 10 advanced very high resolution radiometers (AVHRRs) flown on National Oceanic and Atmospheric Administration (NOAA)'s polar‐orbiting satellites for over 20 years has provided data suitable for many quantitative remote sensing applications. To be useful for geophysical research, each radiometer must be accurately calibrated, which poses problems in the AVHRR reflectance channels because they have no onboard calibration. Previous studies have shown that values of the reflectance channel calibrations, accurately measured during preflight, change abruptly immediately after launch and then change slowly during the satellite's lifetime. The presence of the dual‐gain reflectance channels on the current series of AVHRRs also complicates the application of previous calibration techniques. A technique is presented here for calibrating the AVHRR dual‐gain reflectance channels using Moderate Resolution Imaging Spectrometer (MODIS) data. This method employs selective criteria to reproduce a laboratory type calibration where instrument counts observed by AVHRR are matched to reflectances measured by MODIS on a pixel by pixel basis for coincident and co‐located scenes. Unlike AVHRR, MODIS employs onboard calibration of its reflectance channels. The goal here was to explore the utility of using MODIS to calibrate the new dual‐gain reflectance channels of the AVHRR. The AVHRRs in the NOAA‐KLM series of spacecraft employ a dual‐gain approach to increase the sensitivity to dark scenes. Traditional methods using radiometrically stable targets to calibrate the reflectance channels of AVHRR typically do not provide data for both gain settings. The data from two scenes that met the over‐pass criteria are analyzed. The regression of the MODIS reflectances versus the AVHRR counts for these scenes were able to produce calibration slopes and intercepts in both the low and high gain regions. The reflectance differences using the MODIS‐derived calibration compared to preflight calibration are well within the expected behavior of the AVHRR during its first year in orbit. Comparison with reference ch1 and ch2 reflectance values from NOAA 9 for a Libyan Desert Target were within 5% of those using the MODIS‐derived calibration. While the determination of the absolute accuracy this approach needs further study, it clearly offers the potential for calibration of the AVHRR dual‐gain reflectance channels.