Atmospheric sounding
Such measurements are performed in a variety of ways including remote sensing and in situ observations.Remote sensing soundings generally use passive infrared and microwave radiometers: Sensors that measure atmospheric constituents directly, such as thermometers, barometers, and humidity sensors, can be sent aloft on balloons, rockets or dropsondes.The AMSU instruments on three NOAA and two EUMETSAT satellites, for instance, can sample the entire globe at better than one degree resolution in less than a day.We can distinguish between two broad classes of sensor: active, such as radar, that have their own source, and passive that only detect what is already there.There can be a variety of sources for a passive instrument, including scattered radiation, light emitted directly from the sun, moon or stars—both more appropriate in the visual or ultra-violet range—as well light emitted from warm objects, which is more appropriate in the microwave and infrared.The measurement vector is typically counts, radiances or brightness temperatures from a radiometer or similar detector but could include any other quantity germane to the problem.Satellite and many other remote sensing instruments do not measure the relevant physical properties, that is the state, but rather the amount of radiation emitted in a particular direction, at a particular frequency.It is usually easy to go from the state space to the measurement space—for instance with Beer's law or radiative transfer—but not the other way around, therefore we need some method of inverting