| Passive sensors do not illuminate the area with EM energy such as active systems, they rely on emitted microwave radiation. Longwave radiation has the ability to penetrate cloud cover, and can be captured using passive sensors. Passive Microwave Radiometer (PMR) systems are almost all-weather and observe a wide range of surface emissivity but have a very low spatial resolution (10km+). (Barrett, 1999) A global rainfall map for July 1992 based on DMSP-SSM/1 passive microwave imagery. (Barrett et al., 1999) |