Encyclopedia of Environmental Science and Engineering, Volume I and II

1062 REMOTE SENSING

Airborne and satellite MSS systems have become widely

used in many environmental science and resource management

applications. Examples of different types of MSS system

include the following:

• The U.S. Landsat satellite series includes two MSS
  systems. The original Landsat MSS (on board sys-
  tems launched between 1972 and 1978) includes
  four spectral bands in the visible and near-infrared
  portions of the spectrum, with a spatial resolution
  of 80 m. The Landsat Thematic Mapper (TM)
  instrument (since 1982) includes six bands in the
  visible, near-, and mid-infrared regions, with a
  spatial resolution of 30 m (and a thermal infrared
  band with a resolution of 120 m). Both instruments
  operate in an across-track configuration with a
  swath width of 185 km, and a current orbital repeat
  cycle of 16 days.


• The HRV instrument on the French SPOT satel-
  lite series can collect data in either a single wide
  “panchromatic” band, with 10m resolution, or in
  three narrower bands in the visible (green and
  red) and near-infrared, with 20 m resolution. The
  orbital repeat cycle is 26 days, but the sensor’s
  ability to be rotated (via ground command) up to
  27° left or right allows more frequent imaging
  of a given location on the Earth’s surface. The
  HRV is an along-track scanner, with a swath
  width of 60 to 80 km depending on the viewing
  angle. Two identical HRVs are included on each
  SPOT satellite.

Imaging Radar Systems

Whereas the previous types of remote sensing systems oper-

ate in the visible and infrared portions of the electromagnetic

spectrum, imaging radar systems operate in the microwave

portion of the spectrum, with wavelengths from approxi-

mately 1 cm to 1 m. At these wavelengths, radar is unaf-

fected by clouds or haze (shorter wavelength systems are

used for meteorological remote sensing). In addition, radar

systems are active sensors, transmitting their own radiation

rather than passively measuring reflected solar or emitted

radiation; thus, they can be operated at any time of day or

night. Imaging radar systems are sensitive to the geometric

structure and dielectric properties of objects, with the pri-

mary determinant of an object’s dielectric properties being

its liquid water content. Current satellite radar systems

include the European ERS-series and the Canadian Radarsat,

which each have a single 5-cm wavelength band, and the

Japanese JERS-1 system with a 23-cm band. Several air-

borne radar systems have been developed, such as the NASA/

JPL AIRSAR, which operates at multiple wavelengths. 5,6,7

Photographic cameras, video cameras, and multispectral

scanners can be operated in a vertical configuration to mini-

mize the geometric distortion of the image, or at an oblique

angle to provide a side view of the landscape. Imaging radar

systems are not operated vertically, but in a side-looking

configuration with a broad range of possible look angles.

ENVIRONMENTAL APPLICATIONS OF

REMOTE SENSING

Remote sensing has been used for a wide variety of applica-

tions in the environmental sciences. Among the earliest uses

of remote sensing was geologic mapping, including the dis-

crimination of rock and mineral types, lineament mapping,

and identifying landforms and geologic structures. Today,

many types of remotely-sensed data are used for geologi-

cal applications at a variety of spatial scales, ranging from

high-resolution aerial photography, to thermal-scanner

images, to lower-resolution Landsat images covering large

areas.

Agricultural applications of remote sensing are also

common. Aerial photography and other remotely-sensed data

are widely used as a base for soil mapping, while multispec-

tral and thermal images are used for soil moisture mapping.

Imaging radar systems, with their sensitivity to moisture-

related dielectric surface properties, can also be used to mea-

sure soil moisture. Multispectral visible and infrared data are

used for crop classification and assessment, including moni-

toring the health and productivity of crops, with the goal of

predicting yields and identifying areas of crop damage.

In forestry, aerial photographs are used to delineate

timber stands and to estimate tree heights, stocking densities,

crown diameters, and other variables relating to timber

volume. Color infrared photography and multispectral imag-

ery can be used to map forest types and to identify areas of

stress due to pest infestations, air pollution, and other causes.

Aerial and satellite imagery can be used to map the effects of

wildfires, windthrow, and other phenomena in forested

regions. Wildlife habitat can be assessed using remote sens-

ing at a variety of scales. High-resolution aerial photography

can also be used to assist in wildlife censuses in non-forested

areas such as rangeland.

Many aquatic and hydrological applications make use of

remote sensing. Water pollution can be monitored using aerial

photography or MSS systems, and imaging radar can be used

to detect oil slicks. Thermal imagery is used to study currents

and circulation patterns in lakes and oceans. Both optical and

radar data are used to monitor flooding, including flooding

beneath a forest canopy in the case of radar. Wetlands delin-

eation and characterization can both be assisted by remote

sensing. Radar systems are used to measure ocean waves, and

both radar and optical images have been used to detect sea

and lake ice.

Remote sensing is often used to assist in site selection

and infrastructure location, urban and regional planning, and

civil engineering applications. Aerial photographs are often

acquired with a significant overlap between adjacent photos,

allowing heights to be measured using the stereoscopic

effect. This process is extensively used for topographic map-

ping and for creation of geometrically-correct orthorectified

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