Since 1960, NOAA has operated a fleet of Polar-orbiting Environmental Satellites called POES, complimented by the higher altitude (36,000km) Geostationary Operational Environmental Satellites (GOES). The stationary GOES satellites give a constant view from two points in space, while the polar-orbiting NOAA-N Prime circles the Earth at a lower altitude (860km) once every 102 minutes. These two systems provide continuous data about the global atmosphere.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 188.8.131.52.0