BOX 2.1

The Origin of Earth Remote Sensing

Earth remote sensing techniques have developed over many years, evolving out of astronomy and accelerating as satellite technology became more robust.

  • Before 1932: Use of optical astronomy (initial passive spectral observations of stellar and planetary surface and atmospheric temperatures and compositions, demonstrating basic methods).

  • 1932: The first radio astronomy observations by pioneer radio astronomer Karl Jansky, revealing cosmic radiation.

  • 1940-1945: Wartime studies of centimeter- and millimeter-wave atmospheric absorption spectra and passive radiation; the development of sensitive radiometry.

  • 1968: Launch of the first passive microwave radiometer on the Soviet Cosmos-243 satellite—it observed sea surface temperature, land temperature, snow/ice cover, water vapor, and liquid water using four unscanned window channels at 3.5-37 GHz (unfortunately short-lived, operating only for weeks).

  • 1972, 1975: The first long-lived satellites to image window-channel parameters (humidity over ocean, sea ice, ocean roughness and wind, snow cover, precipitation, land temperature, etc.) and atmospheric temperature profiles: Nimbus-E Microwave Spectrometer (NEMS; two window channels and three opaque channels) and Electrical Scanning Microwave Radiometer (ESMR) imaging at 19.36 GHz launched on the NASA Nimbus-5 satellite in 1972, and the Scanning Microwave Radiometer (a wide-swath imaging version of NEMS) and the dual-polarized ESMR imaging at 37 GHz launched on Nimbus-6 in 1975.

  • 1978: The first operational weather satellites to incorporate imaging passive microwave spectrometers for temperature sounding (microwave sounding unit with four opaque-band channels at 50-58 GHz on TIROS-N and NOAA-6 and -7).

  • 1987: The first operational satellites to monitor surface parameters and atmospheric water (Special Sensor Microwave/Imager with seven window channels at four frequencies, 19.35-85.5 GHz, first launched by the Defense Meteorological Satellite Program).

  • Post-1987: Launch of continually improved research (NASA) and operational (National Oceanic and Atmospheric Administration and Department of Defense) passive microwave instrument types.

ocean internal waves). The full global coverage provided by satellites enables scientists to monitor Earth’s environment far more accurately and completely than had been possible using traditional means such as weather stations and balloon sounders. Satellite data have also greatly improved the accuracy of weather forecasts and enabled sensitive, large-scale climate studies revealing, for example, the effects of ozone-modifying trace gases. Figure 2.2 presents a typical image of the abundance of water vapor over the oceans as observed by combining observations in multiple frequencies obtained by the Advanced Microwave Scanning Radiometer-Earth (AMSR-E) imaging passive microwave spectrometer.



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