Transcript Modelling the Earth's Evolution

Module 8: The Evolving Earth
Cassini probe at Titan
Activity 2:
Remote Sensing
TOPEX/Poseidon satellite
Summary
In this Activity we will investigate:
(a) what is meant by remote sensing; and
(b) remote sensing using different wavelengths
and applications of remote sensing.
In this Unit we study many images of surface details on
our Earth and also on other planets. Most of these
cannot be obtained using telescopes on, or orbiting,
Earth. For example, existing technology does not allow
us to see this high resolution detail on Jupiter’s natural
satellite Io from Earth:
Colour (left) and infrared
images of the volcano
Pele and a lava ring
around it on Io, taken
using the Galileo probe
spectrometer.
We have not sent people to take photographs as they fly
past Venus or Mars or Saturn.
And if we wanted to know (for example) the number of pine
trees in Tasmania, we would find it easier to calculate the
number of trees shown in an aerial photograph than
sending someone to count trees one by one.
Such images and data can be obtained by means of
REMOTE SENSING techniques applied from aeroplanes,
artificial satellites and probes to the planets.
(a) What is Remote Sensing?
REMOTE SENSING is described as a process by
means of which information is obtained about an object
which is separated from a sensor.
Strictly speaking, an Earth based telescope, or even the
human eye, can be defined as remote sensing
instruments. In the context of this Activity however,
remote sensing is mainly concerned with the derivation
of data about the surface or atmosphere of a planet
using an elevated platform - such as a satellite.
TOPEX-POSEIDON SATELLITE
• Development of Remote Sensing
The first successful attempts to take aerial photographs
were made by a Frenchman, Felicien Tournachon, known
as Nadar, in 1858. Using a hot air balloon Nadar took
photographs of Petit-Clamart, a suburb of Paris.
The first aerial reconnaissance photos are
believed to have been taken from Union
tethered balloons during the American Civil
War. In the following years, American
J. Gairmann designed a camera for aerial
photography.
Late in the nineteenth century and early in the
twentieth, cameras were mounted on manned
and unmanned balloons, kites, rockets and in Munich - even strapped to homing pigeons!
Near Rome, in 1908, a photograph was taken from an
aeroplane for the first time by the pioneer of powered flight
Wilbur Wright. This lead to the development of cameras
and even aircraft designed specifically for remote sensing.
The 1957 launch of Sputnik-1, the first artificial satellite, was
to revolutionize remote sensing. Sending a camera in orbit
means positioning it in an unrivalled vantage point. In 1963
astronaut Gordon Cooper, in orbit on a Mercury capsule,
reported seeing roads and buildings clearly. Photographs
were taken by astronauts in subsequent manned missions.
In 1972, Landsat 1 was launched by NASA for the United States
Geological Survey. Travelling in North-South orbits at an altitude
of around 917km, Landsat 1 was used for carrying out land and
ocean surveys.
It was followed by other Landsat
satellites, right up to Landsat 7 in use
today. Other Earth-observing satellite
systems followed the first Landsats.
Some of these are dedicated to
specific tasks, such as the Seastar
satellite designed for monitoring water
quality, in particular chlorophyll levels.
Landsat 4
And already in 1959, the Russian probe Luna 3 was
taking photographs of the far side of the Moon. All
probes to our moon and the planets carry remote
sensing and data recording and transmission
equipment, providing us with incredible images of the
planets.
GALILEO
VOYAGER
CASSINI
• Remote Sensing Techniques
Most remote sensing techniques use electromagnetic
radiation to obtain information about the target area. *
Remote sensing methods can be subdivided into
two categories: ACTIVE and PASSIVE.
Both categories are used in systems mounted on
aircraft, artificial terrestrial satellites and probes to
the planets.
* Sonar is a form of remote sensing used in and under water to
detect submerged objects by using reflected sound waves rather
than electromagnetic radiation.
In ACTIVE remote sensing, electromagnetic radiation is
emitted by the remote sensing equipment and directed
at the target object. It is reflected by the target and
detected by the sensor part of the instrument. The
reflection provides information about the target.
Sensor
reflected
radiation
Emitter
emitted
radiation
Target
The main type of
active sensor
used is radar.
This is described
further on in this
Activity.
Many remote sensing techniques use electromagnetic
radiation emitted or reflected by the target object in order
to obtain information on the target area. This group of
techniques is called PASSIVE remote sensing.
In the diagram below, radiation emitted by the sun is
partially reflected by the object being observed; it is
then detected by the sensor.
Sun
emitted
light
sensor
One example of
use is the detection of
reflected near infrared radiation
light
reflected by vegetation
by means of infraredsensitive film.
Click here to find out more about the electromagnetic spectrum
Different types of terrain in the target area act on the
electromagnetic radiation hitting them in different ways.
For example, a green object will absorb blue and red
light, and reflect green.
Sensor
Blue: Absorbed
The sensor will in
this case detect just
the reflected green
Green: Reflected light.
Red: Absorbed
Green Target Object
A leaf on a plant absorbs blue and red wavelengths of
light but reflects green and near infrared. Our eyes cannot
detect the latter, so we see leaves as being green.
But a sensor which can detect radiation at near infrared
wavelengths (around 1 micrometre wavelength) will ‘see’
the leaf in infrared. This would normally be indicated by
the colour red in a screen or printed image, to permit us
to view the picture in wavelengths which are visible to us.
Of course, some other colour must then be used for
detected red wavelengths. This can make remotely-sensed
images including data in non-visible wavelengths appear
confusing until one gets used to them.
Colour and near
infrared image of
fields in Malta. The
red area to the right is
a vineyard. The
yellow areas all
around are fields with
bare soil, whilst the
orange or pale red
areas are poorlyirrigated vegetation,
which emit a lower
intensity of near
infrared. Near infrared
images are a good
indicator of the health
of plants.
© Euro-Mediterranean Centre on Insular Coastal Dynamics, Malta.
Reproduced with permission.
In some instances, the electromagnetic radiation detected
by a sensor is emitted directly by the target object.
In the diagram below, heat generated by a power plant
is emitted in the form of thermal infrared radiation from
chimney stacks and detected by a thermal infrared
sensor.
In many cases, the
electromagnetic
radiation detected by
a sensor may be a
combination of both
reflected and emitted
radiation.
The spectrum or signature of a target object as recorded
by the sensor or sensors being used gives an indication of
the object’s properties, such as its colour, temperature or
composition.
Each different type of
surface reflects or emits a
particular spectrum of
electromagnetic radiation.
By analysing this
spectrum, the nature of the
target areas observed can
be determined.
Spectrum of one point (or pixel)
as measured by the NASA
AVIRIS spectrometer
Certain parts of the electromagnetic spectrum are absorbed
by a planet’s atmosphere. Remote sensing cannot be
carried out using these wavelengths, as the radiation cannot
travel through the atmosphere to reach the sensor without
being absorbed.
The regions in the spectrum which are not blocked by the
atmosphere are called atmospheric windows.
Fog and clouds, smoke or smog, or ash from volcano
eruptions are all conditions which may prevent remote sensing
even within atmospheric windows. The particles making up
these features reflect and scatter or absorb certain
wavelengths of electromagnetic radiation. A near infrared
sensor cannot ‘see’ through clouds on Earth - or Venus - and
a sensor operating in a different wavelength has to be used.
(b) Remote Sensing at Different Wavelengths
The wavebands most frequently used in remote sensing the
surface of a planet are:
• ultraviolet
electromechanical scanner or CCD
• visible
film, electromechanical scanner or CCD
• near infrared
film, electromechanical scanner or CCD
• thermal infrared electromechanical scanner
• microwave
radiometer or radar
The devices most commonly used to sense each band of
radiation are indicated to the right.
All these devices can be used either on aircraft or satellites,
and in most cases on both.
• Ultraviolet to near-infrared wavelengths (1)
Aerial photography is still a frequently used technique.
Dedicated high speed cameras are installed in aeroplanes
which fly at predetermined altitudes and flight lines in order
to obtain a sequence of images.
This method is not used on satellites and probes, as
retrieving the film would pose a problem.
Photographic film is widely used because of the high resolution
which can be obtained. This is made possible by the fine grain
of the film. The fast image registration speed permits remote
sensing at high aircraft speed and a wide range of altitudes.
Infrared-sensitive film, developed for military purposes, has
proved particularly useful for agricultural and forestry surveys.
As the aeroplane moves along its flight-path the operator takes
a series of pictures of the surface below, capturing images in a
sequence or SWATHE mapping out a long, narrow area.
S
W
A
T
H
E
Image
Image
Image
Image
Image
Image
1
2
3
4
5
6
After each flight-line the aeroplane is turned around to map
another swathe adjacent to the first. By laying out swathes of
images alongside each other, a MOSAIC is formed, mapping
the area of interest.
Each photo in a swathe overlaps
the previous one; similarly, each
swathe overlaps the previous
swathe. This precaution ensures
that no gaps exist in the final
composite image.
This is important as gusts of wind can easily affect the
direction of motion and orientation of an aeroplane, resulting
in areas being missed out unless sufficient overlapping is
planned for.
On the Internet, a movie of an aerial survey can be found at
http://www.digitalradiance.com/sng/farming.htm
Although expensive, aerial photography is still used for a
wide variety of purposes including:
hydrology
cartography
agricultural surveys
urban studies
geological surveys
forestry surveys
The data collected is often compiled in Geographic
Information Systems (GIS), which combine layers of maps
and images with databases on computer.
The main advantages of using aeroplanes are a higher
operational flexibility (a satellite remote sensing operation
can only be carried out when the satellite overflies, or orbits
above, the target area) and a higher resolution of objects on
the ground, as an aeroplane can fly lower and closer to the
target - although satellites are catching up in this field.
• Ultraviolet to near-infrared wavelengths (2)
Electromechanical scanning devices have sensors with
oscillating or rotating mirrors which reflect an image of a
point in the area below them onto a photoelectric detector.
As the mirror either oscillates or rotates, a side to side
scanning movement is produced which forms a row of data
points. The vehicle carrying the sensor moves along its flight
path, and a second row of points is recorded. The rows make
up a picture as wide as the scan and as long as the flight-line.
The data is registered and recorded, or transmitted,
electronically, and this technique can therefore be used on
satellites and probes.
These instruments are also
direction
of rotation
known as across-track or
whiskbroom scanners.
direction of motion
This method of scanning images is frequently used in
multi-spectral scanners. These devices have a small
battery of photoelectric detectors which record the
intensity of radiation emanated from a point in a number
of wavebands. For example, a multi-spectral scanner
may record the values in ultraviolet, blue, green, red, and
two wavebands of near-infrared simultaneously.
The Landsat family of satellites
have multispectral scanners on
board as their main remote
sensing instruments.
Landsat 7
A more advanced version of the original Landsat
Multispectral Scanner instrument is used on some Landsat
satellites: this is the Thematic Mapper. It is similar in
principle to the multi-spectral scanner, but the selection of
the seven wavebands of the scanner was based on specific
application requirements.
0.42-0.52
0.52-0.60
0.63-0.69
0.76-0.90
1.55-1.75
10.4-12.5
2.08-2.35
blue
green
red
near infrared
mid-infrared
thermal infrared
mid-infrared
coastal water mapping
identification of vegetation
plant species differentiation
determination of plant vigour
soil moisture content
thermal mapping
discrimination of rock types
Landsat 5
Thematic Mapper
image of Miami,
Florida.
Landsat 5
Thematic Mapper
NASA AVIRIS
airborne visible
and near infrared
whiskbroom
scanner.
AVIRIS image in
different wavebands.
• Ultraviolet to near-infrared wavelengths (3)
Charge-Coupled Devices or CCDs are thin wafers of silicon
with a surface divided into an array of square light-sensitive
picture elements.
A CCD can be linear, that is, the picture elements are
arranged in a line. In this case, a line of data is captured
when the array is exposed to light from the target. As the
vehicle carrying the instrument advances along its flightpath
or orbit it collects subsequent lines of data which together
make up an image.
These linear array
scanners are also known
as along-track or
pushbroom scanners.
flightline
In other instruments, the CCD array is rectangular. This layout
‘captures’ a rectangular image. This is similar to a photograph,
but in this case the image is recorded electronically rather than
on film. Images are batched together to form map mosaics in a
way identical to that described earlier on for photographic
images.
This rectangular
array layout is
also used in
digital cameras
and some
telescopes,
including the
Hubble Space
Telescope.
Near infrared image
showing the
Cottonera Lines
defensive works and
a shipyard in Malta.
The image was taken
using a digital
camera with a
rectangular CCD
array mounted in a
light aircraft. The red
areas indicate
reflection of near
infrared radiation by
plants.
© Euro-Mediterranean
Centre on Insular
Coastal Dynamics,
Malta. Reproduced with
permission.
• Picture Perfect
Before using the images obtained by remote sensing, the
data has to be rectified and processed. This is usually done
by means of dedicated computer software (photo images
are often scanned to be processed electronically).
The images have to be rectified. Any errors due to motion of
the vehicle carrying the remote sensing instrumentation are
eliminated, and elements within the image located at different
altitudes are corrected for perspective.
Ground coordinates are added to the image by ‘draping’ it
over a digital elevation (or terrain) model during the
rectification process. These are required for creating
mosaics, for comparing or combining images of the same
areas taken using different wavelengths or at different
times, and for obtaining measurements from the image.
The image may then be processed by applying mathematical
filters included in the computer software used to clarify or
sharpen the image.
FILTER
By combining images taken at different wavelengths - for
example, by choosing three different wavebands from a multispectral scanner - certain features may be enhanced,
permitting the image analyst to clearly identify the details
which interest him. For example, selecting green, red and a
near-infrared band would enable him to identify vegetation
types more clearly.
• Thermal infrared remote sensing
Thermal infrared remote sensing detects thermal radiation in
two wavebands: 3.5 to 5.5 micrometres and 8 to 14
micrometres. Transmittance of thermal radiation in
wavelengths between these two regions are blocked by the
Earth’s atmosphere.
Photoelectric detectors with electromechanical scanner
mechanisms are used to detect thermal radiation. The data
may be output from the sensor as a linear chart or (as is
more often the case) as a false-colour image, each colour
depicting a particular temperature range.
Thermal imagers can be so sensitive as to detect differences
in temperature as small as 0.03K. This sensitivity is called
the thermal resolution of the sensor.
The image to the right shows the
superstructure of a moored tanker.
The red areas are hottest, followed
by orange, yellow, green, light blue,
dark blue, purple and black. The top
surfaces, warmed by the sun, are
hottest
© Euro-Mediterranean Centre on Insular Coastal
Dynamics, Malta. Reproduced with permission.
The Prometheus volcano on Io. The farther
image was taken in the near infrared; the
nearer is a false colour thermal infrared image
taken at wavelengths around 4.5micrometres.
The highest temperature area is coloured
white, followed by yellow, red, green and blue.
• Remote sensing at microwave frequencies
Microwaves have a distinct advantage over the other
wavelengths discussed so far: they can easily penetrate
clouds and fog. Passive microwave radiometers can detect
thermal-related microwave emissions from surface
features. One use is the measurement of sea and ice
surface temperatures on a large (or global) scale; another
is the detection of oil slicks in water.
Active microwave remote sensing, utilising any one of a
variety of RADAR systems, is another important tool. In this
case, the radar emits a microwave beam which is reflected by
the target back to the radar system. A simple form of radar is
used as an altimeter, giving the altitude of an aeroplane or
satellite above a planet’s surface by timing the reflection.
The French TOPEX/Poseidon satellite, operated in
conjunction with NASA, measures sea level beneath its
orbit using radar altimeters. This information is used to
relate changes in ocean currents with atmospheric and
climate patterns. Measurements from a Microwave
Radiometer on the satellite provide estimates of the
water-vapor content in the atmosphere. This is used to
correct errors in altimeter measurements.
These combined measurements allow
scientists to chart the height of the
sea across ocean basins with an
accuracy better than 10 centimeters.
TOPEX/Poseidon image of La Nina
weather conditions in the Pacific
In a Synthetic Aperture Radar (SAR) system, reflected
signals are received from targets at successive antenna
positions as the vehicle carrying the SAR moves along its
path, building up an image which could otherwise be
obtained only by using an antenna several hundred metres
wide. A very high resolution can therefore be obtained, as
the wavelength is small compared to the simulated large
antenna.
Radar systems are deployed on both aircraft and satellites,
and can be used for a wide range of applications including
the monitoring of sea- and ocean-going vessels and oil spills,
the observation of water conditions, geological surveys and
land-use surveys.
One spectacular application of radar (SAR) has been the
Magellan Radar Mapper on the Magellan probe to Venus.
This instrument gave us our
first map of the surface of
Venus. Radar was selected
because of the thick cloud
cover of Venus which
absorbs visible and infrared
radiation but permits the
passage of microwaves.
Three dimensional digital elevation models of the surface of
Venus could be constructed using views of the same
features taken at different angles.
(See also the movies of Magellan mapping Venus and the 3D models of
the surface on the Universe textbook CD.)
In this Activity we have seen how remote sensing can reveal
features of our own planet and help us control our use - and
abuse - of the Earth and its resources-
-and how remote sensing techniques can provide
images and maps of other planets in our solar
system.
As we move on to explore our knowledge of the other
bodies in our Solar System, bear in mind that the
information we have about these bodies has mostly
been gathered by remote sensing.
Remote Sensing will be discussed in more detail in the
Unit on Studies in Space Exploration.
Image Credits
NASA: TOPEX/Poseidon satellite
http://www.jpl.nasa.gov/pictures/
NASA: Cassini probe at Titan
http://www.jpl.nasa.gov/pictures/
NASA: Mount Pele on Io from Galileo
http://science.nasa.gov/newhome/headlines/ast19nov99_1.htm
NASA: Mars
http://pds.jpl.nasa.gov/planets/welcome.htm
NASA: Landsat 4
http://www.earth.nasa.gov/history/landsat/landsat4.html
NASA: Voyager probe
http://www.jpl.nasa.gov/pictures/
NASA: Galileo probe
http://www.jpl.nasa.gov/pictures/
NASA: Cassini probe
http://www.jpl.nasa.gov/pictures/
Image Credits
Euro-Mediterranean Centre on insular coastal Dynamics Malta:
colour-near infrared image of vineyards
NASA: Landsat 7 satellite
http://landsat.gsfc.nasa.gov/
NASA: Landsat 5 image of Miami
http://landsat.gsfc.nasa.gov/
NASA: Landsat 5 thematic mapper
http://www.earth.nasa.gov/history/landsat/landsat5.html
NASA: AVIRIS spectrum, aircraft and sensor, remotely sensed images
http://makalu.jpl.nasa.gov/html/overview.html
NASA: Hubble Space Telescope
http://www510.gsfc.nasa.gov/512/hst.htm
Euro-Mediterranean Centre on insular coastal Dynamics Malta:
colour-infrared image of Cottonera Lines and shipyard
Euro-Mediterranean Centre on insular coastal Dynamics Malta:
colour image of countryside in Crete
Image Credits
Euro-Mediterranean Centre on insular coastal Dynamics Malta:
thermal image of aft end of moored tanker
NASA: Prometheus volcano on Io by Galileo probe
http://science.nasa.gov/newhome/headlines/ast19nov99_1.htm
NASA: TOPEX/Poseidon image of La Nina
http://www.jpl.nasa.gov/elnino/991129.html
NASA: Magellan probe at Venus
http://www.jpl.nasa.gov/pictures/
Hit the Esc key (escape)
to return to the Module 8 Home Page
The Electromagnetic Spectrum
Visible light is made up of a whole spectrum of colours:
Each of which corresponds to a characteristic
frequency f (or wavelength ) range,
because light, as an electromagnetic wave which
travels at the speed of light c,
obeys the equation
c = f
Optical astronomers observing in visible light
work between wavelengths of about
 = 400 nm (or frequency f = 7.5 x 1014 Hz)
- the blue end of the visible spectrum
And
 = 700 nm (or frequency f = 4.3 x 1014 Hz)
- the red end of the visible spectrum.
Forgotten what 1014 means?
Click here to revise scientific notation.
300 kHz
= 3x105 Hz
1 km
1 nm
= 10-9 m
10-13 m
Radio waves
wavelength
3 GHz
= 3x109 Hz
Microwave &
Infrared
Visible
Ultraviolet
frequency
The visible
spectrum
is a small 10 cm
part of the
whole
electro 10-5 m
magnetic
spectrum.
3x1013 Hz
3x1017 Hz
X rays
Gamma rays
3x1021 Hz
Astronomers try to access as much of
the electromagnetic spectrum as possible
with their telescopes & detectors,
ranging from radio waves to gamma rays.
Click here to return to the Activity!
Scientific notation
In order to save writing heaps of zeroes, scientists use a
system of notation where very large numbers are written
with the number of factors of ten as an exponent.
For instance:
5 000 is written 5 x 103
6 000 000 000 is written 6 x 109
42 700 is written 4.27 x 104
In scientific notation the aim is to present the number as a
number between 1 and 10 multiplied by a power of ten:
e.g. 4.27 x 104
Also, in order to save writing heaps of decimal places,
scientists use a system of notation where very small
numbers are written with the number of factors of ten as
an exponent.
For instance: .007 is written 7 x 10-3
0.00000010436 is written 1.0346 x 10-7
0.000060001 is written 6.0001 x 10-5
In scientific notation the aim is to present the number as
a number between 1 and 10 multiplied by a power of ten:
e.g. 6.0001 x 10-5
Click here to return to the discussion on
the Electromagnetic Spectrum