SATELLITE IMAGERY
Charles Braxmeier
Department of Computer Science and Engineering
University of Minnesota
*** NOTE TO T.A.: I WAS AWAY ON BUSINESS TRIP LAST WEEK AND
FOUND OUT ABOUT THE CHANGE IN LENGTH ON MONDAY 10/23 --- WILL
HAVE FULL ROUGH DRAFT ON GROUP WEBSITE BY THURSDAY 10/26.
HERE IS WHAT I HAVE SO FAR. ***
SYNONYMS
None
DEFINITION
Satellite imagery is a technique useful in many different areas of work. Earth-orbiting
artificial satellites take photographs of a specific area over the Earth’s surface, and collect
data of the image area. The data is then transmitted to receivers that reconstruct the
image.
The data collected by these satellites are of two varieties: radiation emissions or light
particles. The wavelengths of the radiation collected by the satellite drive the type of
image that can be produced.
There are a few main types of satellite imagery, notably:
 Visible imagery: An image based on the visible light reflected off of the Earth
and its atmosphere. (Figure 1)
 Infrared (IR) imagery: An image based on radiation emitted from the Earth and
its atmosphere within IR wavelength ranges. (Figure 2)
o Note, there is also enhanced infrared, which just colors the image based on
the amount of radiation collected. In meteorological terms, this roughly
equates to the temperature of the object in the image.
 Water vapor imagery: An image based on radiation emitted by water vapor off of
the Earth and its atmosphere. (Figure 3)
The satellites that orbit around the earth for imagery purposes are “geo-stationery”, which
means that they orbit in such a manner that they are always stationary with respect to a
point on the ground of the Earth.
The resolution of the satellite images can vary depending on the imaging equipment on a
given satellite, and the satellite’s altitude relative to the Earth. If a satellite is further
away from the object of interest, the resolution will not be as great as the same satellite
half the distance to the object. The sample images below have a resolution of 8km,
which, in modern imagery, is not very granular. Most major cities have resolutions as
low as 10 meters, while rural areas have resolutions around 500 meters.
Figure 1: Infrared Satellite image
Figure 2. Visible Satellite image. It appears to be night-time in the Western US.
Figure 3. Water Vapor Satellite image.
HISTORICAL BACKGROUND
Images taken from outer space are not a new concept to the scientific community.
Cameras were mounted onto rockets as early as 1890, but the technology did not take off
until near World War II, when rocket technology development crested.
Meteorological satellites were invented in the early 1960s. Images of the Earth via
satellite were transmitted for the first time on April 1, 1960 by the meteorological
satellite TIROS-1. More currently, the main meteorological satellites being used today
are GOES. GOES (Geostationary Operational Environmental Satellites) operate in geostationary orbit above the Earth. These satellites were designed to provide a better
understanding of the weather patterns at a given location over a period of time.
The United States has launched many satellites to take non-meterological images of the
Earth, notably through the Landsat program. The Landsat program was created in 1972,
and has sent seven image-collecting satellites into orbit, with the most recent being sent
in 1999. The first Landsat satellite was sent into orbit on July 23, 1972. Of the seven
Landsat satellites to be launched into orbit, only two are still functioning (Landsat 5 and
Landsat 7). Another notable satellite in US history is the KH-11. The KH-11 provided
the first real-time satellite images on January 20, 1977.
A notable image in history is the Blue Marble photograph. Originally, a project was
developed to map the visible color of the earth with a 1-km resolution. The famous Blue
Marble photograph is shown below in Figure 4. The image was taken by the crew of the
Apollo 17 mission.
Figure 4: “Blue Marble” photograph
SCIENTIFIC FUNDAMENTALS
A broader term for acquiring data about an object in which the object and the
measurement device are not intimate is called “remote sensing.” Following this
definition, many more types of observation besides satellite images are considered remote
sensing (for example, an ultrasound on a fetus). However, in the context of this article,
we’ll focus on the satellite imagery.
A satellite, in fact, does not simply take a picture of the area of the Earth it is targeting
(think of the megapixels that would be involved!). Instead, a satellite sensor will measure
the intensity of the radiation reflected from the Earth at given wavelength ranges. Some
of the ranges include the visible light wavelengths, infrared, and near-infrared. These
wavelength ranges, or “bands”, filter only the information needed for the type of data
being collected. By collecting the information from the sensors, the satellites are
providing the data necessary to produce an image.
Taking the data collected by the satellites instruments, the satellite will register how
intensely the radiation was being reflected. This intensity is given a numerical
assignment for each radiation wavelength band that was being measured. These numbers
provide the basis for how the image is eventually put together.
As an example, let’s assume we would like a visible image from a satellite hovering
above the eastern United States (for reference, this is the GOES-8 satellite). At the time
of data capture, the satellite’s sensors will record numerical entries for three particular
wavelength bands:
 0.45 µm to 0.5 µm (blue visible light band)
 0.5 µm to 0.6 µm (green visible light band)
 0.6 µm to 0.7 µm (red visible light band).
Once a number is registered for each band, the numbers make up a classical RGB color
combination that can be interpreted by many image processors.
The color assignments (either true color or grey-scale images) can be assigned from
radiation detection in different ways; however the simplest example is that of a visible
satellite image, given above.
After all the data is captured for a given image, it is transmitted to receivers back at Earth
for collection, storage, and processing.
The scientific processes behind capturing the satellite-based data is relatively simplistic,
however its uses in the real-world are many, and all quite diverse.
KEY APPLICATIONS
Environmental: Agriculture, Forestry
In agriculture, satellite imagery can be used in a variety of ways. Recall, in rural areas,
the resolution of the most detailed satellite images is roughly 500 meters, which is
significantly less detailed than major cities’ images. This constitutes lower-quality
images in many non-urban areas, making use in agriculture – which is fundamentally
based in rural areas – rather difficult.
Weather: Meteorology & Atmospheric Studies
In its most famous sense, satellite imagery is heavily used in meteorology.
Demographic: Land Use Planning
Governmental: Intelligence, Reconnaissance, War
FUTURE DIRECTIONS
Expanded Space Exploration
Radar-type products done via Satellite
Addressing Privacy Concerns
CROSS REFERENCES
Remote Sensing
Image Classification
GPS
RECOMMENDED READING
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