The Simultaneous Nadir Overpass (SNO) Technique to Estimate Inter-Calibration Biases Between Similar Instruments Flying on Different Low-Earth-Orbit (LEO) Satellites
Estimating post-launch measurement biases between similar instruments flying onboard different low-earth-orbiting (LEO) satellites can be difficult. One reason for this difficulty is that often these instruments are not looking at the same Earth scene at the same time. On the other hand, the fact that all LEO satellites revolve around the Earth at slightly different periods causes them to occasionally view the same subsatellite location at nearly the same time. Ideally, identical radiometers flown on different satellites that simultaneously view the same exact Earth target should produce redundant observations. Any deviation from these results would be attributable to relative calibration differences between the “identical” radiometers. Taking advantage of this concept, the Simultaneous Nadir Overpass (SNO) method was developed to estimate and track relative calibration-related measurement biases between radiometers flown on-board different LEO satellites. For a given pair of LEO satellites, the essence of the SNO method is to minimize Earth- scene-related differences found between instrument measurements by utilizing subsatellite observations close to satellite orbital intersections that have a relatively small time difference (~ 30 seconds).