Products - GEO-LEO Intercalibration

The premise of space-based inter-calibration is that two sensors should obtain identical measurements over the same location, at the same time, under the same viewing geometry, and within the same spectral region. In reality, such conditions rarely exist, but the differences can be minimized and the residuals can be accounted for. An essential task of GSICS, therefore, is to minimize and account for those differences.

In the case that one sensor is on a geostationary (GEO) satellite and another is on a low earth orbiting (LEO) satellite, the LEO regularly flies under the GEO within the GEO's field of regard. Unlike LEO, GEO's scan pattern is complicated, sometimes even unpredictable, so knowing the GEO position does not guarantee knowledge of where the GEO is looking. On the other hand, GEO usually scans its domain fairly quickly, increasing the chance that GEO and LEO look at the same place at approximately the same time. Also, thanks to the huge difference between the GEO and LEO orbit altitude, a LEO could find viewing geometry similar to that of GEO in about every corner of GEO's domain. All things considered, there are typically a lot more collocations between GEO-LEO than between LEO-LEO or GEO-GEO, except that GEO-LEO collocations have to be searched for empirically (instead of being predicted by orbit parameters). Such large sample size is a statistical advantage for GEO- LEO inter-calibration.

In fact, it is important to collect off-nadir collocations not only to increase the sample size. Since all GEOs nominally stay above a fixed location on the Equator and most (though not all) LEOs are on a sun- synchronous orbit, those LEOs which pass the nadir of a GEO at the same local time and afford comparisons for nadir view only. For this reason, off-nadir collocations are critical to evaluate GEO performance at different local time and view angle.