Here we see the major weather and climate satellites in the US fleet. (There are more than a dozen smaller ones, such as GRACE, Cloudsat, Icesat, etc.) The five currently flying — NASA's Terra, Aqua, and Aura; NOAA's POES; the Air Force's DMSP — represent some 20B$ of national investment. NPOES, the next generation US platform, represents another 12B$. These platforms carry instruments with such labels as MODIS, MISR, AIRS, MLS, AMSU-A, AMSR-E, HIRS, AVHRR, MHS, SBUV, ATMS, CMIS, CrIS, and VIIRS. These sensors have one thing in common: They passively sense the natural electromagnetic emissions or reflected sunlight ("radiance") from the atmosphere and surface to derive environmental information. They are collectively termed radiometers.