the application of color infrared and multispectral photography were undertaken under the sponsorship of NASA, leading to the launch of multispectral imagers on the Landsat satellites in the 1970s. A major breakthrough was the advent of electronic detectors particularly in large arrays and across many spectral regions.
Figure 1.8 Spectral signature of some vegetation types.
Source: From Brooks (1972).
At the long wavelength end of the spectrum, active microwave systems have been used since early this century and particularly after World War II to detect and track moving objects such as ships and, later, planes. More recently, active microwave sensors have been developed providing two‐dimensional images that look very similar to regular photography, except the image brightness is a reflection of the scattering properties of the surface in the microwave region. Passive microwave sensors were also developed to provide “photographs” of the microwave emission of natural objects.
The tracking and ranging capabilities of radio systems were known as early as 1889, when Heinrich Hertz showed that solid objects reflected radio waves. In the first quarter of this century, a number of investigations were conducted in the use of radar systems for the detection and tracking of ships and planes and for the study of the ionosphere.
Radar work expanded dramatically during World War II. Today, the diversity of applications for radar is truly startling. It is being used to study ocean surface features, lower and upper atmospheric phenomena, subsurface and surface land structures, and surface cover. Radar sensors exist in many different configurations. These include altimeters to provide topographic measurements, scatterometers to measure surface roughness, and polarimetric and interferometric imagers.
Figure 1.9 Landsat TM images of Death Valley acquired at 0.48 μm (a), 0.56 μm (b), 0.66 μm (c), 0.83 μm (d), 1.65 μm (e), and 11.5 μm (f).
In the mid‐1950s, extensive work took place in the development of real aperture airborne imaging radars. At about the same time, work was ongoing in developing synthetic aperture imaging radars (SAR), which use coherent signals to achieve high‐resolution capability from high‐flying aircraft. These systems became available to the scientific community in the mid‐1960s. Since then, work has continued at a number of institutions to develop the capability of radar sensors to study natural surfaces. This work led to the orbital flight around the Earth of the Seasat SAR (1978) and the Shuttle Imaging Radar (1981, 1984). Since then, several countries have flown orbital SAR systems.
The most recently introduced remote sensing instrument is the laser, which was first developed in 1960. It is mainly being used for atmospheric studies, topographic mapping, and surface studies by fluorescence.
There has been great progress in spaceborne remote sensing over the past three decades. Most of the early remote sensing satellites were developed exclusively by government agencies in a small number of countries. Now, nearly 20 countries are either developing or flying remote sensing satellites. And many of these satellites are developed, launched, and operated by commercial firms. In some cases, these commercial firms have completely replaced government developments, and the original developers in the governments now are simply the customers of the commercial firms.
Figure 1.10 Images of an area near Cuprite, Nevada, acquired with an airborne imaging spectrometer. The image is shown to the left. The spectral curves derived from the image data are compared to the spectral curves measured in the laboratory using samples from the same area.
Source: Courtesy of JPL. See color section.
The capabilities of remote sensing satellites have also dramatically increased over the past three decades. The number of spectral channels available has grown from a few to more than 200 in the case of the Hyperion instrument. Resolutions of a few meters or less are now available from commercial vendors. Synthetic aperture radars are now capable of collecting images on demand in many different modes. Satellites are now acquiring images of other planets in more spectral channels and with better resolutions than what was available for the Earth two decades ago. And as the remote sensing data have become more available, the number of applications has grown. In many cases, the limitation now has shifted from the technology that acquires the data to the techniques and training to optimally exploit the information embedded in the remote sensing data.
Figure 1.11 Sea surface temperature derived from ship observations (a) and from the Seasat Multispectral Microwave Radiometer (b). (c) shows the difference.
Source: From Liu (1983). © 1983, John Wiley & Sons.
Figure 1.12 Backscatter data acquired over the Amazon region (insert). The different curves correspond to different incidence angles. Data were acquired by the Seasat Scatterometer at 14.6 GHz and at VV polarization.
Source: Bracalante et al. (1980). © 1980, IEEE.
Figure 1.13 Profile of Tharsis region (Mars) acquired with Earth‐based radar.
Figure 1.14 Profiles of an unnamed impact basin on Mars using Earth‐based radar. The set of profiles shown correspond to the box overlay on the figure.
1.3 Remote Sensing Space Platforms
Up until 1946, remote sensing data were mainly acquired from airplanes or balloons. In 1946, pictures were taken from V‐2 rockets. The sounding rocket photographs proved invaluable in illustrating
