Section 1. GENERAL PRINCIPLES OF REMOTE SENSING
1.1 USE OF ELECTROMAGNETIC RADIATION FOR REMOTE SENSING
Aerial photography was the earliest form of remote sensing other
than the telescope. For a long time, this technique relied on the
portion of electromagnetic radiation used by our eyes (the visible
spectrum).
Early aerial photography was usually obtained on black and
white film which responded to light over a broad range of visible light.
Later, it was learned that by placing a filter in front of the lens
which would pass only a particular color of light, a black and white
record could be made of the objects reflecting light in that range. For
instance, an aerial photograph of a developed area with a red-passing
filter would show bare ground and many man-made surfaces which reflect a
significant amount of red light. Hence, this photograph would be useful
for identifying man-made features. This technique is used by the Landsat
series of satellites today.
Later, as color photography became available, color film was used
in aerial photography. Again, filters could be used to enhance particular
features.
V-I1.1.1 Near Infrared Aerial Photography. During the second World
War there was a need to detect camouflaged objects. Although a great
deal of aerial photography was obtained, it was often difficult to
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detect objects which had been painted green or had been covered with cut
tree branches. Some experimental film was developed which responded to
to light in the near infrared portion of the spectrum, light just a
little more red than the red light detected by the human eye. One of
the anticipated uses for this film involved the monitoring of healthy
vegetation whose chlorophyll reflects the near infrared extremely well.
This film was simply a black and white film with extended sensitivity
which would record the near infrared if the visible light was filtered
out. Later, a color film was developed which responded to the near
infrared as well as visible colors (except blue).
This was called color
infrared film.
1.1.2 Growth of Remote Sensing. Encouraged by these results,
efforts were made to utilize other electromagnetic wavelengths such as
heat infrared, microwave, and radar for remote sensing purposes. Here
the topic becomes complex because the radiation does not behave exactly
as light does and it is not quite as simple to understand as the near
infrared.
1.1.3 Imaging Satellite Systems. Another important factor in the
development of remote sensing, particularly for ice surveillance, was
the development of satellite systems which routinely return images to
earth. The first of these systems operated in the visible portion of
the spectrum because existing television technology was most easily
applied there. Quickly, however, systems were developed to make use of
other portions of the electromagnetic spectrum. At this time, satellite
remote sensing systems based on radar were being developed.
1.2 ACTIVE VS. PASSIVE SYSTEMS
1.2.1 Source of Electromagnetic Radiation. One of the most important
distinctions among remote sensing systems involves the source of the
radiation used. The easiest example to use is that of a camera. When
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a camera is used utilizing sunlight or even ambient light in a room, it
is said to be a passive system. On the other hand, when it utilizes
flash bulbs, it is an active system. That is, an active system provides
CHAPTER V V-2its own radiation. Ordinary radar is an active system, while imaging
near infrared systems are passive systems. Passive systems are used
when there is sufficient illumination of the object of interest to allow
detection. Active systems are used when there is insufficient radiation
and it must be provided. A second reason for using passive systems is
in situations where the radiation given off is not used for imaging
alone, but also quantitatively describes properties of the object.
Thermal infrared is an example here. The radiation measured is related
to the temperature of the object.
1.2.2 Transmission through the Atmosphere. In all these systems
it is necessary for the radiation to pass through the atmosphere (once
for passive systems, twice for active systems).
Therefore, it is
sometimes necessary to keep in mind the interaction between the atmosphere
and the radiation. Perhaps the best example of this is the scattering
of blue light by the atmosphere. Blue light is actually scattered out
of the beam from the sun. It is then scattered toward us from all
directions. If blue light were not scattered, the sun would look white
instead of red and the sky would be transparent. (We would see stars in
the daytime; shadows would be very pronounced.) Ultraviolet light is
somewhat “bluer” than blue light and it is scattered even more in the
atmosphere than blue light. Furthermore, ultraviolet light will expose
photographic film. On a bright day this scattered ultraviolet light
will fog a photograph of distant objects. In order to avoid this, we
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use a filter which passes visible light but not ultraviolet light
(called a UV filter).
The utilization of almost every remote sensing
system used requires some consideration of the transmission and scattering
properties of the atmosphere for a particular wavelength. These problems
will be discussed where appropriate.
1.2.3 Interaction with the Earth’s Surface. A major aspect of
interpretation of remotely sensed data is the nature of the interaction
of radiation with the earth’s surface. Each kind of surface material
has its own signature. For instance, a water surface absorbs the near
infrared and reflects a fair amount of green light. Snow reflects both.
While it is possible for the observer to catalogue these signatures,
occasionally he will encounter an object whose signature is puzzling.
In those cases it may be necessary to play “detective” and consider the
CHAPTER V
V- 3aspects of the surface which may be producing the signature observed.
For instance, the unusual occurrence of a rainstorm upon snow-covered
sea ice may create an area with unusual absorption in the near infrared.
It is not likely that this signature would be listed in any reference
manual.
The nature of the interaction of radiation with the earth’s surface
can be quite different for active and passive systems. Passive systems
depend on illumination from a natural source, usually the sun or radiation
emitted from the object. In this case, the angle of illumination is
different from the “look” angle. However, usually there is sufficient
illumination that there are few total shadows. Actually, we are quite
used to this situation since we experience it daily. Most active systems
depend on radiation emitted and reflected directly back to the source.
This can create effects we do not experience on a daily basis. Consider
how things look to you when using a flashlight on a dark night; shadows
are troublesome. Yet, this is how the earth looks on airborne imaging
radar.
