Predicting Simultaneous Nadir Overpasses among Polar-Orbiting Meteorological Satellites for the Intersatellite Calibration of Radiometers

A method for accurately predicting simultaneous nadir overpasses (SNOs) among different sun-synchronous polar-orbiting meteorological satellites is presented for intersatellite radiometer calibration. At each SNO, the radiometers on the two satellites view the earth and its atmosphere at nadir within a few seconds of each other, providing an ideal scenario for the intercalibration of radiometers. The basic mechanism and frequency of occurrences of such events are analyzed. Prediction using the Simplified General Perturbations No. 4 (SGP4), an orbital perturbation model, is presented, and examples of SNOs among the NOAA-16, NOAA-17, Terra, and Aqua satellites are provided. Intersatellite calibration using this approach has the potential for achieving the calibration consistency and traceability required for long-term climate studies.