Earth Science Remote Sensing
Overview | Active Sensors | Passive Sensors
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Photo: The MISR instrument, aboard NASA's Terra satellite, is
sensitive to four different wavelengths--red, green, blue, and one
channel in the near-infrared. MISR "sees" the Earth simultaneously at
nine different angles, so it is particularly well designed for measuring
how much sunlight aerosols reflect back to space. (Image courtesy
Shigeru Suzuki and Eric M. De Jong, NASA JPL) |
Remote sensing can be simply defined as the studying of qualities of
an object without coming into contact with it. Within NASA Earth Science, remote
sensing means studying the Earth system from above: from space (in
orbit) or closer in, from our atmosphere. Without this method, studying
the Earth as a whole system would not be possible. Using remote sensing
we measure global ice cover changes, carbon deposits, we track
hurricanes, and forest fires, and many more forcings and events on the
Earth. And we do all this from space!
There are two kinds of remote sensing. Passive sensors detect natural
energy (radiation) that is emitted or reflected by the object or scene
being observed. Reflected sunlight is the most common source of
radiation measured by passive sensors. Examples of passive remote
sensors include the eye, optical telescopes, and radiometers. Active
sensors, on the other hand, provide their own source of energy to
illuminate the objects they observe. An active sensor emits radiation in
the direction of the target to be investigated. The sensor then detects
and measures the radiation that is reflected or backscattered from the
target. RADAR is a widely known form of active remote sensing. In RADAR,
the instrument emits a radio wave and senses the returned energy that
was reflected from the target. Since the speed of light is known and the
time delay between emission and return is measured, the distance to the
target can be determined. Altimeters and Lidar are other examples.
With remote sensing we can study dangerous or difficult to reach
regions. A hazardous area might be a wildfire site, or a hurricane.
Using remote sensing we can better understand water patterns of the
whole Amazon region. We can take measurements from polar regions and
oceans' depths. Taking measurements remotely also ensures that we won't
disturb the object being studied.
Photo: The Atmospheric Infrared Sounder (AIRS) instrument
aboard NASA's Aqua satellite senses temperature using infrared
wavelengths. This image shows temperature of the Earth's surface or
clouds covering it for the month of April 2003.
The instruments aboard orbiting satellites transmit data such as
temperature, electromagnetic spectrum, energy output, heat, and light
from the Earth's surface and atmosphere. Satellite data, in conjunction
with analysis of Earth system data from other sources, provides
scientific teams with enough information to predict many of the Earth's
processes.
This information is utilized for the purposes of protecting the
environment, land use, improving natural resources management, enhancing
economic security and national safety to name a few.
For more information on remote sensor types...
+ Active Sensors
+ Passive Sensors
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