Most remote sensing instruments (sensors) are designed to measure photons. The fundamental principle underlying sensor operation centers on what happens in a critical component - the detector. This is the concept of the photoelectric effect (for which Albert Einstein, who first explained it in detail, won his Nobel Prize). This, simply stated, says that there will be an emission of negative particles (electrons) when a negatively charged plate of some appropriate light-sensitive material is subjected to a beam of photons. The electrons can then be made to flow from the plate, collected, and counted as a signal. A key point: The magnitude of the electric current produced (number of photoelectrons per unit time) is directly proportional to the light intensity. Thus, changes in the electric current can be used to measure changes in the photons (numbers; intensity) that strike the plate (detector) during a given time interval. The kinetic energy of the released photoelectrons varies with frequency (or wavelength) of the impinging radiation. But, different materials undergo photoelectric effect release of electrons over different wavelength intervals; each has a threshold wavelength at which the phenomenon begins and a longer wavelength at which it ceases. Meteorological satellite sensors can be broadly classified as two types : passive and active (Fig. 2). Passive sensors do not use their own source of electromagnetic illumination, and depend upon the radiation emitted or reflected from the object of interest. On the other hand, active instruments use their own source of electromagnetic radiation which they use to illuminate the target, and in most cases use the properties of reflected radiation ( e.g. intensity, polarization, and time delay etc.) to deduce the information about the target. These sensors can be further subdivided into the following categories and subcategories :
Equally important is the functional classification of these sensors. Meteorological satellite sensors may be deployed to obtain one and/or more of the following characteristics of different objects of the land-ocean-atmosphere system :
(a) Spatial Information : The examples are the extent and temperature of sea surface, clouds, vegetation, soil moisture, etc. The main objective here is to obtain the required information over a 2-dimensional plane. The best suited sensors for this class are imaging radiometers operating in visible, infrared or microwave frequencies. Active sensors like Synthetic Aperture Radar (SAR) are also put to effective use for the imaging applications.
(b) Spectral Information : For certain applications, the spectral details of an electromagnetic signal are of crucial importance. A particular object of interest, for example an atmospheric layer, or, the ocean surface, interacts differently with different wavelengths of electromagnetic(EM) spectra. In most cases, this may be due to the chemical composition of the object.
Absorption, emission, or reflection of an EM radiation from an object is a function of the wavelength of EM radiation, and the temperature of the object. Thus, the spectral information can provide details of chemical composition, and/or the temperature of the object. Meteorological satellite sensors use this information for sounding applications, where the vertical structure of temperature, humidity, and in some cases, the atmospheric gases is retrieved. An example of this sensor is High Resolution IR Sounder (HIRS), and Advance Microwave Sounding Unit (AMSU) onboard NOAA series of satellites. Future satellites will carry more advanced sensors like imaging spectrometers. Geostationary Imaging Fourier Transform Spectrometer (GIFTS) is a fine example of this new-generation sensor. GIFTS, when operational, is expected to provide the vertical profiles of temperature, humidity, and winds at several atmospheric layers in vertical.
(c) Intensity Information : The intensity of EM radiation can provide several clues about the object of interest. In most cases, the satellite sensors measure the intensity of the radiation reflected from the object to know the dielectric properties and the roughness of the object. By the use of suitable algorithms these parameters can be translated to the properties of geophysical parameters like soil moisture, ocean surface roughness, ocean surface wind speed, and wind direction, etc. The sensors that use this information are radar, scatterometer, and polarimeters.
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