Remote sensing is "the process of detecting and monitoring the physical
characteristics of an area by measuring its reflected and emitted radiation at
a distance from the targeted area"
While remote sensing is commonly used as a synonym for satellite data,
the concept of remote sensing can also be applied to aerial photography
drones. The remote part of remote sensing means that you are gathering
data from a distance.
The first images of the earth from space were captured
in 1947 from a camera placed in a sub-orbital German V-2 rocket
repurposed by the US after WW II.
Corona Spy Satellite Image of Mys Shmidta Airfield, 1960
(National Reconnaissance Office 1960)
The first truly functional civilian satellite remote sensing system
was the Television Infrared Observation Satellite (TIROS) series,
the first of which launched on 1 April 1960. This inaugurated
the use of satellites for weather observation and forecasting.
TIROS Weather Image, 1960
(NASA 1960) Applications of Remote Sensing
Satellite data and imagery has a wide variety of uses
in the natural sciences in addition to its military and
As an introduction to the wide variety of (perhaps unexpected)
uses for remotely sensed data, skim this list of
100 Earth Shattering Remote Sensing Applications and Uses.
100 Earth Shattering Remote Sensing Applications and Uses
Satellites travel in an
around the earth in a way that the centrifugal force of
the circling of the satellite around the planet
counterbalances the pull of gravity so that the satellite stays aloft.
Orbits can have a number of different characteristics, which involve
different types of movement relative to the earth:
Altitude: How high the orbit is above the surface of the earth.
Low Earth Orbit is used by the International Space Station.
Medium Earth Orbit is used by the GPS constellation of satellites
(12,552 mile altitude) and the other GNSS satellite systems
Inclination: The angle of the orbit relative to the Equator.
Many remote sensing satellites use near-polar orbits
that circle nearly over the poles (inclination of almost 90°)
permitting systematic coverage of the entire surface of the earth
Eccentricity: Whether the orbit is circular or forms an ellipse
SynchronicityThe timing of the orbit relative to the rotation
of the earth or the position of the sun.
in a band 22,236 miles above the Equator, the speed needed to orbit is the same as the rotation of the earth,
so the satellite effectively stays in the same place in the sky
and the satellite can constantly observe
or communicate with a fixed region of the earth. With a
sun-synchronous near-polar orbit, half the orbit follows
the motion of the sun and the satellite can systematically
cover land as it is lit by the sun
The type of orbit determines the
temporal resolution of
a satellite, or how often and long the satellite senses any particular
location on the surface of the earth. Some satellite systems orbit
to the same location daily, while others that are designed to observe
the entire earth may take days or weeks to return to the same location. Rasters
Satellites almost always capture data as
rasters, which are
regular grids of rectangular pixels.
Pixels in a Remotely-Sensed Image
Spatial resolution is the amount
of area on the surface of the earth covered by one pixel. Spatial resolution
is usually measured by the distance in meters between the center of each pixel
in the raster.
Dallas at Medium Spatial Resolution (MODIS)
Dallas at High Spatial Resolution (Landsat)
Because there are technical and cost limitations to resolution
that defines how much detail a satellite sensor can capture at
any one time, satellite data capture follows a narrow path
swath along the ground. The width of this swath
varies by different satellite systems based on their purpose.
Satellites can scan these swaths in two different ways:
Along-track scanning captures the width of the
swath all at one time, similar to using a pushbroom to
sweep a floor. This requires a complex and precise optical
system that places technical limits the width of the swath
Cross-track scanning constantly scans back and
forth across the swath, similar to using a whisk broom
to sweep a floor side to side. This is commonly done
with a rotating mirror that permits a wide swath to be
covered by a simpler optical system than an along-track
scan. However, this also requires complex correction of
distortions introduced by the scanning motion.
Types of Swath
Remote sensing takes advantage of the emission and reflection of electromagnetic
radiation by objects on the surface of the earth to capture
what is where on
the surface of the earth.
Objects reflect, absorb, and emit energy in a unique way, and at all times.
This energy is called
electromagnetic radiation and is emitted in waves that are
able to transmit energy from one place to another. These waves originate from
billions of vibrating electrons, atoms, and molecules, which emit and absorb
Different types of electromagnetic radiation are distinguished by
the speed of their vibration. As these waves travel through space
at 300,000 kilometers per second (186,000 miles/sec) the distance
between the vibrating pulses is called the
and is usually measured in meters or nanometers (one billionth of a meter).
The speed of the vibration is called frequency
and is usually measured in Hertz (1 Hz = one vibration per second)
The higher the temperature of an object, the faster its
electrons vibrate and the shorter its peak wavelength of emitted radiation.
Conversely, the lower the temperature of an object, the slower its electrons
vibrate, and the longer its peak wavelength of emitted radiation.
The fundamental unit of electromagnetic phenomena is the
photon, the smallest
possible amount of electromagnetic energy of a particular wavelength. Photons
are units of energy rather than matter, so they have no mass.
The energy of a photon determines the frequency (and wavelength) of
light that is associated with it. The greater the energy of the photon, the
greater the frequency and vice versa.
Electromagnetic radiation is a part of our lives in many ways.
Different frequencies of electromagnetic radiation have different
characteristics. These characteristics make different frequencies of electromagnetic
radiation useful for different types of remote sensing.
Radio waves (100,000 km - 1,000 nanometers) are used for communication through radio/TV broadcast,
cellphone signals, GPS signals, signals sent through communications satellites, etc. Different frequencies
of radio waves pass through or are reflected from different types of materials.
Infrared radiation (1,000 - 700 nanometers at a frequency just below the red visible light we can see)
is associated with heat and can often be used in situations where there is
little visible light (night-vision). Photosynthetic plants reflect infrared
radiation (to avoid overheating), so infrared radiation can be used in conjunction
with visible light to detect vegetation
Visible light (380 - 700 nanometers) is a form of electromagnetic radiation that we can see
and which we can create with light bulbs. It is useful for capturing the way we see the world from above.
However, visible light does not travel through objects or thick gases like clouds.
X-rays (0.01 nm - 10 nm) pass through soft human tissue and are used for medical diagnosis
The Electromagnetic Spectrum
(Lawrence Berkeley National Laboratory 1996)
Electromagnetic radiation is different from
associated with radioactive materials like uranium and nuclear power plants.
Particle radiation results from subatomic particles being thrown off by
nuclear reactions. Particle radiation is often associated with electromagnetic
radiation, but the primary health concern with any kind of radiation is
ionization, which occurs when radiation pushes electrons out of atoms and leaves them as
ions with a positive charge. With living cells, this ionization damages the
cell DNA and can lead to cell death or mutations and cancer. Bands
While early satellites captured only panchromatic (grayscale) visible
light, contemporary satellites often have sensors that capture
different ranges or
bands of electromagnetic radiation.
The number of different bands that can be handled by the satellite
sensors is called the
spectral resolution. The appropriate
spectral resolution depends on the purpose of the satellite.
For space imagery we are usually most interested in the red (430-480 THz), green (540-580 THz),
and blue (610-670 THz) bands that the three different types of
in our eye retinas can detect as visible light colors.
Other bands are useful for analyzing a variety of phenomena. For example,
biogeographers commonly use
a combination of red and near-infrared bands called
normalized difference vegetation index (NDVI)
to determine levels of vegetation in a particular area.
NDVI is based on a characteristic that photosynthetic
green plants tend to reflect infrared light
to avoid overheating. They also reflect green light,
which is why they appear green to our eyes. However, they absorb
red light to power the process of photosynthesis.
This phenomena can be used with the Landsat near infrared
band (band 5) and the red band (band 4) to calculate
an index that is highest in areas with large amounts
of vegetation, and lower in areas of low vegetation.
Normalized Difference Vegetation Index
The range of the index is negative one to positive one.
When near infrared is high and red is low, that is
when plants are reflecting infrared and absorbing red,
that's when NDVI is high and closer to one.
1 - 0
----- = 1
1 + 0
When near infrared is low and red is high, such as with
bare ground or water, NDVI is low and closer to negative one.
0 - 1
----- = -1
0 + 1
Ground Control and Downlinks
As with GPS, satellites used for remote sensing
are controlled through a mission operations center (MOC).
All contemporary satellite data is returned to
earth with radio signals through
that then relay that data to the MOC
for processing, storage and communication.
The photo below is of a particularly remote downlink
station in the Arctic used by the Landsat system.
German Remote Sensing Data Center, Neustrelitz
(DLR 2016) Landsat
Of the hundreds of remote-sensing satellites launched in
the past half century, the
has proved especially valuable for civilian use.
Landsat 1 was launched on 23 July 1972 and subsequent satellites
have provided continuous satellite imagery of the Earth.
This is arguably one of the most important scientific enterprises of
our time, and if you work with remote sensing, you will probably
use Landsat data on multiple occasions.
Landsat 7 (launched 1999) and Landsat 8 (launched 2013) are currently operational.
Landsat satellites complete just over 14 orbits a day, covering
the entire earth every 16 days. This gives a
of 16 days, although if there is cloud cover when the satellite passes
over a location, the data may be unusable. Landsat satellites
travel in a near-polar, sun-synchronous orbit.
Landsat data is publicly available as
scenes, or images
that cover an area around 100 miles square. Scenes are
designated by a path number and row number.
Landsat Orbit Coverage
The scene below was acquired from Landsat 8 on 10 November 2015.
It is path 34, row 32, covering North Central Colorado with
Denver in the lower right-hand corner.
Landsat Image of Denver, 10 November 2015 (NASA)
Landsat 8 captures data in 11 different bands, giving it a
that includes the visible light, near-infrared, short wave infrared, and thermal infrared bands
spatial resolution varies from 15 meters with panchromatic data (grayscale) to
30 meters with multispectral data, to 100 meters with thermal data
(USGS 2017). Google Maps / Earth
When using satellite view in Google Maps or Google Earth, you will
notice a copyright at the bottom for
TerraMetrics, a commercial geospatial data company
that Google buys their imagery from. Although this is referred to as
"satellite" view, what you
see at different levels of zoom is "multiple layers of data such as satellite imagery,
aerial photography, synthetic ocean imagery, roadways, location names,
addresses and more, which come from many different data and imagery providers."
Indeed, rather than giving a purely faithful picture of what you would
see from space, this is an artistic representation of the
Earth that is used to effectively communicate
what is where
so that users can interpret that data more clearly and act accordingly. Threats to Satellite Systems
There are around
1,100 active satellites plus another 2,600 or so that have been decommissioned.
This does not include as many as
500,000 pieces of "space junk" the size of a marble or
larger that NASA tracks to help safeguard space operations.
While most space junk will eventually return to earth, the constant addition of
new satellites and launch stages, and the occasional
collision of satellites
(turning two satellites into multiple pieces of junk) means that
the threat to space operations by space junk
is increasing and irreversible.
As the list of space-faring nations grows,
terrestrial conflicts could extend to actions against
spaceborne systems, making military and civilian
geospatial technology highly vulnerable to disruption or
destruction by state and non-state actors.
All satellite systems are expensive to build, launch,
maintain, and renew. As such, they are dependent upon
political and economic support that
is tenuous in the contemporary American political environment.
If this is an area of interest to you, you might
consider browsing the 2001
Report of the Commission to Assess United States National Security
Space Management and Organization. Landsat Raster Visualization in ArcMap
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Landsat Raster Visualization in ArcMap
History of satellite imagery
EarthExplorer satellite data web portal
Electromagnetic radiation and Landsat bands
Creating a Landsat RGB image (Composite Bands tool)
Normalized Difference Vegetation Index (NDVI)
Raster algebra and the Raster Calculator tool