Operating at microwave wavelengths, these instruments have the advantage of cloud penetration and all-weather capability. Channels within 1 to 40 GHz and 80 to 100 GHz are used to get day/night information on the Earth’s surface. They have the advantage over visible/IR radiometers of being able to probe the dielectric properties of a surface or penetrate certain surfaces, a capability that is especially useful with vegetation, soil, sea ice and snow. Observations by instruments like AMSU-A, with channels between 50 and 60 GHz, are used for deriving atmospheric parameters, especially atmospheric temperature.
Like other microwave instruments, these passive instruments offer accurate spectral information but their spatial resolution is poor. At 90 GHz, their spatial resolution is typically 5 km, and for the lower frequencies it is of order tens of kilometres – poorer than that of their visible or infrared counterparts. As a consequence, they are most used for global analysis rather than regional or local, although some instruments are used to correct measurements from other sensors, rather than for imaging applications. These include the microwave radiometers on the ERS/Envisat and Topex/Poseidon/Jason series satellites, which are used to estimate and correct for atmospheric water vapour content in the column through which altimetric readings are being taken.