Studying NOAA limb-correction algorithm by simultaneous NOAA-16 and -18 satellite measurements

The images from cross-scanning sensors on polar-orbiting satellites display strong dependence on the scanning angle because of the temperature gradient of the atmosphere and the change in the optical path length between the Earth and the satellite. The dependence on the scanning angle is called limb effect and the effect is often stronger than the real variation of the signatures from scenes. Using a limb adjustment algorithm, all measurements are corrected to nadir-looking-like so that the temperature gradients can be restored from the images. The objective of this paper is to test NOAA limb correction algorithm via real satellite measurements. The newly launched NOAA-18, together with NOAA-16, provides the best opportunity for the first time for such a test. The collocated measurement pairs from NOAA-16 and NOAA-18 contain the data for which both satellites have the same scanning angle and various scanning angles, in particular off-nadir observations from NOAA-16 and nadir observations from NOAA-18. The former data can be used for inter-calibration of the two satellites. The latter can be used for testing the limb adjustment algorithm using pure satellite measurements. NOAA-16 orbit drift, the difference of orbit periods between NOAA-16 and -18, and the performance of the limb correction algorithm will be presented.