from orbital altitudes. Systematic orbital observations of the Earth began in 1960 with the launch of Tiros I, the first meteorological satellite, using a low‐resolution imaging system. Each Tiros spacecraft carried a narrow‐angle TV, five‐channel scanning radiometer, and a bolometer.
In 1961, orbital color photography was acquired by an automatic camera in the unmanned MA‐4 Mercury spacecraft. This was followed by photography acquired during the Mercury, Gemini, Apollo, and Skylab missions. On Apollo 9, the first multispectral images were acquired to assess their use for Earth resources observation. This was followed by the launch in 1972 of the first Earth Resources Technology Satellite (ERTS‐1, later renamed Landsat‐1), which was one of the major milestones in the field of Earth remote sensing. ERTS‐1 was followed by the series of Landsat missions.
Figure 1.15 Sea surface height over two trenches in the Caribbean acquired with the Seasat altimeter.
Source: Townsend (1980). © 1980, IEEE.
Figure 1.16 Shaded relief display of the topography of California measured by Shuttle Radar Topography Mission using an interferometric SAR.
Figure 1.17 Subsurface layering in the ice cover and bedrock profile acquired with an airborne electromagnetic sounder over a part of the Antarctic ice sheet.
Earth orbital spacecraft were also used to acquire remote sensing data other than regular photography. To name just a few, the Nimbus spacecraft carry passive microwave radiometers, infrared spectrometers, and infrared radiometers. The Synchronous Meteorological Satellite (SMS) carried visible and IR spin‐scan cameras. Skylab (1972) carried a radiometer and a radar scatterometer. Seasat (1978) carried an imaging radar, a scatterometer, and an altimeter.
In the 1980s and 1990s, the Space Shuttle provided an additional platform for remote sensing. A number of shuttle flights carried imaging radar systems. In particular, the Shuttle Radar Topography Mission, flown on the Space Shuttle in 2000, allowed global mapping of the Earth’s topography.
Remote sensing activity was also expanding dramatically using planetary spacecraft. Images were acquired of the surfaces of the Moon, Mercury, Venus, Mars, the Jovian and Saturnian satellites, Pluto, numerous Asteroids and comets, and of the atmospheres of Venus, Jupiter, Saturn Uranus, and Neptune. Other types of remote sensors, such as radar altimeters, sounders, gamma‐ray detectors, infrared radiometers, and spectrometers were used on a number of planetary missions.
The use of orbiting spacecraft is becoming a necessity in a number of geophysical disciplines because they allow the acquisition of global and synoptic coverage with a relatively short repetitive period. These features are essential for observing dynamic atmospheric, oceanic, and biologic phenomena. The global coverage capability is also essential in a number of geologic applications where large‐scale structures are being investigated. In addition, planetary rovers are using remote sensing instruments to conduct close‐up analysis of planetary surfaces. Over the last decade, with the advances in detectors, light optics, microwave technology, antennas, materials, spacecraft technology and data systems, there has been a great expansion in the development, deployment, and utilization of remote sensors. These will be discussed throughout this textbook.
1.4 Transmission Through the Earth and Planetary Atmospheres
The presence of an atmosphere puts limitations on the spectral regions that can be used to observe the underlying surface. This is a result of wave interactions with atmospheric and ionospheric constituents leading to absorption or scattering in specific spectral regions (Figure 1.19).
Figure 1.18 Comparison of temperature profiles acquired with a microwave sounder (NEMS) and radiosonde. The spatial and temporal differences, Δs and Δt, between the two measurements are indicated.
Source: Waters et al. (1975). © 1975, American Meteorological Society.
Figure 1.19 Generalized absorption spectrum of the Earth’s atmosphere at zenith. The curve shows the total atmospheric transmission.
At radio frequencies below 10 MHz, the Earth’s ionosphere blocks any transmission to or from the surface. In the rest of the radio frequency region, up to the low microwave (10 GHz), the atmosphere is effectively transparent. In the rest of the microwave region, there are a number of strong absorption bands, mainly associated with water vapor and oxygen.
In the submillimeter and far‐infrared region, the atmosphere is almost completely opaque, and the surface is invisible. This opacity is due mainly to the presence of absorption spectral bands associated with the atmospheric constituents. This makes the spectral region most appropriate for atmospheric remote sensing.
The opacity of the atmosphere in the visible and near infrared is high in selected bands where the high absorption coefficients are due to a variety of electronic and vibrational processes mainly related to the water vapor and carbon dioxide molecules. In the ultraviolet, the opacity is mainly due to the ozone layer in the upper atmosphere.
The presence of clouds leads to additional opacity due to absorption and scattering by cloud drops. This limits the observation capabilities in the visible, infrared, and submillimeter regions. In the microwave and radio frequency regions, clouds are basically transparent.
In the case of the other planets, more extreme conditions are encountered. In the case of Mercury, the Moon, asteroids, comets, and Pluto, no significant atmosphere exists, and the whole electromagnetic spectrum can be used for surface observation. In the case of Venus and Titan, the continuous and complete cloud, or haze, coverage limits surface observation to the longer wavelength regions, particularly radio frequency and microwave bands. In the case of Mars, the tenuous atmosphere is essentially transparent across the spectrum even though a number of absorption bands are present. In the case of the giant planets, the upper atmosphere is essentially all that can be observed and studied remotely with some deeper access from emitted microwave radiation.
References and Further Reading
1 Bracalante,
