Sensor Microwave/Imager (SSM/I), have provided the longest and most regular time series of global sea ice data at a resolution of typically 25 × 25 km (Johannessen et al. 2001). Moreover, the microwave radiation emitted by atmospheric oxygen and water vapor were used for vertical soundings, especially in the upper atmosphere.

In 1972, ERTS 1 (Earth Resources Technology Satellite), later renamed Landsat 1, provided new land-based applications for optical sensors. Landsat carried a return beam videocon (RBV) and a multispectral scanner (MSS) that imaged Earth from an altitude of 900 km with green, red, and two infrared spectral bands at 80-m resolution. Since 1972, Landsats have provided the longest, continuous global record of land cover and its historical changes in existence. Landsat is the premier technology supporting the new geographical field of land-cover science, part of Earth system science.

In the 1970s, laser technology was first employed in combination with satellites (Laser Geodynamics Satellites [LAGEOS] 1 and 2) that were designed for maximum reflectivity to allow for study of Earth’s geoid and the movements of tectonic plates. Interestingly, this type of satellite does not contain any instrumentation, and for that reason the first such satellite launched in 1976 is still operational today.1 Spaceborne synthetic aperture radar enables observation of sea ice with much better accuracy than visible and passive microwave methods, as proven by Seasat, the European Remote Sensing Satellite (ERS 1), Canada’s RADARSAT, and Europe’s Envisat (Figure 2.10) (Johannessen et al. 2001). Although Seasat was able to collect data only for 105 days, it pioneered the exploitation of radar technology and the microwave range to measure ocean topography and winds. Its success led to important follow-on missions such as TOPEX/Poseidon and QuikScat.

FIGURE 2.9 Artist’s drawing of the general design of the Nimbus series of satellites. SOURCE: C. R. Madrid, ed. (1978). The Nimbus 7 Users’ Guide, Goddard Space Flight Center, NASA.

CONCLUSION

In the first two decades of the space age, six nations designed and launched Earth-orbiting satellites for scientific purposes: the Soviet Union (1957), the United States (1958), France (1965), Japan (1970), China (1970), and the United Kingdom (1971). The European Space Agency, a consortium of 17 member nations, was founded in 1974. International cooperation has been an important aspect of the satellite legacy: the Global Weather Experiment (1979) demonstrated what was possible in observation, modeling, understanding, and prediction. Today, such experiments are conducted every

1

The projected lifetime of the LAGEOS satellites is more than 200,000 years.



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