ASAR Frequently Asked Questions
This chapter questions
Question 4.1 : What does synthetic aperture mean?
In general the larger the antenna, the more unique
information you can obtain about a particular viewed
object. With more information, you can create a
better image of that object (improved resolution).
It's prohibitively expensive to place very large
radar antennas in space, however, so researchers
found another way to obtain fine resolution: they use
the spacecraft's motion and advanced signal
processing techniques to simulate a larger antenna.
A SAR antenna transmits radar pulses very rapidly. In
fact, the SAR is generally able to transmit several
hundred pulses while its parent spacecraft passes
over a particular object. Many backscattered radar
responses are therefore obtained for that object. After
intensive signal processing, all of those responses can
be manipulated such that the resulting image looks like
the data were obtained from a big, stationary
antenna. The synthetic aperture in this case, therefore,
is the distance traveled by the spacecraft while the
radar antenna collected information about the
object. Please see the associated graphic [below]
The ERS-1 satellite's SAR sends out around 1700
pulses a second, collects about a thousand backscattered
responses from a single object while passing
overhead, and the resulting processed image has a
resolution near 30 metres. The spacecraft travels around
4 kilometres while an object is "within
sight" of the radar, implying that ERS-1's 10
metre x 1 metre radar antenna synthesizes a 4
kilometre-long stationary antenna!
Question 4.2 : Why
is radar often used in remote sensing?
|Figure 4.1 Synthetic Aperture
(Why are radio waves, visible light, and infrared
radiation the most common forms of electromagnetic
radiation sensed by Earth observing satellites?)
The wavelengths of electromagnetic radiation most
commonly used for remotely sensing Earth are: the
spectrum of visible light, a wide spectrum of radio
wavelengths, and several infrared wavelengths. A partial
explanation of why these wavelengths are preferable is
outlined below. When radar (which employs radio
waves) is selected from these possible choices, the
decision is usually based upon radar's
independence of solar illumination and weather
conditions. See the questions: How does radar
"see" at night? and How does radar
"see" through clouds? for more details.
There is a good reason why our eyes sense the
electromagnetic radiation (light) they do: visible light
represents a significant portion of the
electromagnetic radiation which can pass through
Earth's atmosphere and ionosphere. A wide spectrum
of radio waves, which radars employ, and some
infrared radiation can also pass through to the
There are many reasons why other wavelengths/frequencies
of electromagnetic radiation don't make it through
Earth's atmosphere. For example many people
know that ozone in Earth's upper atmosphere helps
protect us from ultraviolet radiation. This occurs
because the structure of the ozone molecule is
particularly sensitive to ultraviolet frequencies; it
has a natural resonance near those frequencies.
Think of a person swinging; he swings back and forth
with a particular frequency. If you push him with the
same frequency (every time he comes back) or at a
related periodic frequency (say, every other time he
comes back), he will keep going higher and higher.
Incoming ultraviolet radiation likewise keeps
"pushing" the ozone moleule at the
structure's resonant frequency - in a sense
like pushing the swinging person higher. Soon the ozone
molecule breaks into an oxygen atom and an O2 molecule
which go on to other adventures. The incoming
ultraviolet radiation's energy was used to break
apart the ozone molecule, a process by which we say the
radiation was "absorbed." Many molecules in
Earth's atmosphere have various kinds of resonances
which absorb other frequencies of electromagnetic
radiation. The visible spectrum, a wide spectrum of
radio frequencies, and some infrared frequencies
don't match well with those resonances, however,
and thus are not much affected by absorption. These
frequencies of electromagnetic radiation are
therefore most commonly used for remote sensing
purposes. As a side note - some radar wavelengths
reflect off the ionosphere and therefore cannot be used for
remote sensing purposes. You may know a HAM radio
operator who utilizes this phenomenon to talk to a companion
halfway around the world. In this case radio waves are
transmitted up to the ionosphere and reflected back down
to Earth elsewhere, rather than passing through the
ionosphere. This happens because the radio waves cause
electrons in the ionosphere to oscillate, and the
oscillating electrons in turn radiate electromagnetic waves.
Question 4.3 : How does radar "see" at night?
SAR instruments transmit radar signals and then measure
how strongly those signals are scattered back. An
analogy with photography can be made: when it's
dark, a camera's flash sends out light and then the
film records objects that the flash illuminates. In both
cases the SAR and the camera are not dependent upon the
sun because they provide their own illumination.
Question 4.4 : How does radar "see"
Light does often make it through clouds, but that light
has just been scattered all over the place, making it
nearly impossible to tell how the light was oriented
before it entered the cloud. This is why we can't
see objects through clouds. The difference with radar is
how much less it's distorted while passing through
The reason why clouds scatter visible light while leaving
radar undistorted is a matter of relative scale.
Radar's longer wavelengths in effect average
the properties of air with the properties and shapes of
many individual water droplets, making the cloud look
homogeneous - i.e. like moist air. Visible light has
short enough wavelengths to respond to all the
individual boundaries between air and water
droplets. At each boundary the light is reflected to a
new direction, and by the time it escapes the cloud,
information on the light's original direction
is hopelessly lost. The radar signals, on the other
hand, are only affected while entering and exiting the
cloud. Because they don't suffer multiple bounces,
the radar waves are relatively undistorted by clouds.
Question 4.5 : How
is radar data different from what I would see? Why
isn't there any colour?
(See the previous question for an explanation of why you
don't see clouds in a typical radar image.)
Question 4.6 : Our eyes perceive what is called
visible electromagnetic radiation, or
electromagnetic radiation with wavelengths between 0.4
and 0.7 microns. Even though we can't see other
wavelengths of electromagnetic radiation, they
certainly affect us. Ultraviolet radiation, for example,
can burn our skin or hurt our eyes, and X-Rays can
inform us if we have broken a bone or have developed
cavities in our teeth!
When we think about all the information light provides us
about our world, and about how other electromagnetic
waves impact our lives, it seems only natural that
people would want to detect and "visualize"
many other kinds of electromagnetic radiation. The Earth
Observing System does just that; many satellites'
instruments "see" certain electromagnetic
waves and relay that data to a "brain"
(computer), where the information is then converted into
an image for humans to interpret. Each wavelength
indicates something different about the imaged
object, just as you might associate the wavelength
corresponding to bright green light with young plants.
Visible light contains a range of wavelengths, but with
radar we often measure one very specific wavelength.
Just think of how differently things would look if
you could only see yellow. Your eyes would only detect
how brightly an object scattered yellow, so the
reflection's intensity, not the colour, is what
would give you new and useful information. Similarly,
radar antennas are often made to detect how brightly
objects reflect one particular wavelength. Since there
are no other "colours" (wavelengths) to mix
in, we really only care about the backscatter's
intensity and therefore often use greyscale in our
visualizations of this data.
Question 4.7 : What's the smallest object
you can see in a SAR image?
In ASF's full-resolution SAR images, you can
distinguish objects as small as about 30 metres wide.
Some of the smaller items that we've spotted
have been ships and their wakes. When the SAR happens to
be aligned at a certain angle, long thin objects such as
roads or even the Alaskan oil pipeline can also be seen.
Question 4.8 : What's the difference between
resolution and pixel spacing?
Pixel spacing represents how much area each pixel covers,
while resolution indicates the smallest object you could
pick out in an image. Each pixel represents one
solid colour, so of course you can't see anything
within it. When you place other pixels around it,
though, you might notice a few pixels are rather
different in colour than surrounding pixels and conclude
that you have identified a distinct object.
ASF's full-resolution ERS-1 SAR images have 12.5 m
pixel spacing and about 30 m resolution. This means that
each pixel represents a 12.5 x 12.5 m area on the
ground, and you can discern individual objects which are
around 30 m wide or larger.
Question 4.9 : How is this
SAR data used?
SAR's ability to pass relatively unaffected through
clouds, illuminate the Earth's surface with its own
signals, and precisely measure distances makes it
especially useful for the following applications:
- Sea ice monitoring
- Surface deformation detection
- Glacier monitoring
- Crop production forecasting
- Forest cover mapping
- Ocean wave spectra
- Urban planning
- Coastal surveillance (erosion)
- Monitoring disasters such as forest fires, floods,
volcanic eruptions, and oil spills
Some of the larger current research
projects include: mapping the Antarctic continent;
mapping the Amazon rainforest; using
interferometric analysis for predicting or analyzing
earthquakes and volcanic activity; and generating
"Arctic Snapshots" of the Arctic ice
Question 4.10 : What's the difference between
slant range and ground range?
The time it takes for a transmitted signal to travel to
an object and back tells you how far away the object is.
If you transmit a signal and receive two separate
"echoes," you can use the time difference
between when you record the first and second responses
to determine the distance between the two sensed objects
(dependent on where you stand). In this way the
spaceborne SAR measures how far objects are from the
spacecraft and the distance between the two objects,
along the direction the spacecraft is looking. These
distances are said to be recorded in slant range,
since they are measured in a direction which is at an
angle/slant to the ground.
Often researchers don't really care about distances
from the spacecraft; they want to know about distances
on the ground. Perhaps they need "real"
(ground) distances to determine how much land was used
for farming or what percentage of the sea was covered
with ice, but the spacecraft samples the returning radar
signals at specific time intervals which correspond to
discrete distances from the spacecraft. That means
that the data are originally in slant range. Given
various parameters, the data can be processed such that
each data value covers the same amount of area
(distance) on the ground. We then say that the data are
in ground range.
Question 4.11 : What
does geocoded mean?
|Figure 4.2 Slant Range vs Ground Range
A standard ASF SAR image has gone through a lot of
processing to look "normal." One step in this
process involves manipulating the data such that
each pixel represents a specific distance on the ground.
The latitude and longitude coordinates of each
image's centre and corner pixels are also
known. Sometimes, however, it's convenient to map
the data onto a standard grid - such as a mercator
projection. Then the pixels in each row would be evenly
spaced in terms of longitude, and the entire row would
be located at a specific latitude. The data would
then be termed geocoded. It is often much easier to
compare/overlay geocoded SAR data with non-SAR data sets.
Question 4.12 : What does terrain correction mean?
If you have information about a region's topography,
like a digital elevation model (DEM), you can make the
slant to ground range conversion more sophisticated.
In effect this terrain correction can compensate for
foreshortening by spreading data representing the
mountain's facing side into more pixels and
compacting returns from the back face into fewer pixels.
It's nearly impossible, though, to reliably
extract the separate returns from data values
representing the facing slope. Sometimes people try to
compensate for shadowing as well. Knowing the
mountain's slopes, they can approximate how the
strength of backscattered signals were affected by the
changed incidence angle and adjust results accordingly.
These procedures, though inexact, can greatly improve
SAR image analysis.
Question 4.13 : What is a look (e.g. 4-look data)?
What is speckle?
As the spacecraft moves along in its orbit, the radar
antenna transmits pulses very rapidly. It can therefore
obtain many backscattered radar responses from a
particular object while passing overhead. In fact the
ERS-1 SAR records about 1,000 responses for a single
object. The SAR processor could use all of these
responses to obtain the object's radar
cross-section (i.e. how brightly the object
backscattered the incoming radar), but the result often
contains quite a bit of speckle.
Speckle, generally considered to be noise, is due in part
to the SAR's fine resolution and its signals'
coherency. Speckle can be caused by an object that
behaves as a very strong reflector at a particular
alignment between itself and the spacecraft, or by a
coherent sum of all the various responses within a grid
cell which happen to randomly sum (as vectors with
magnitude and phase) to a large resultant magnitude
at a given phase.
To reduce speckle, the data are sometimes processed in
sections which are later combined. With ERS-1's
1,000 samples per object, we might wish to use an
object's first 250 responses to determine its radar
cross-section. If we then processed the next 250
responses to get another estimate, and so on, we
would end up with four estimates of the object's
radar cross-section. Combining these four estimates,
or looks, together would reduce the amount of speckle.
When an image has been processed as "4-looks":
the first 250 (or so) samples of each viewed object were
processed to make one image; the next 250 samples
for each object were processed to make a second image;
the third and fourth images were created with the next
chunks of data; and the four images (looks) were
combined to create the final result.
The more looks that are used to process an image, the
less speckle there is. (The Complex-Format SAR Data
Example [given below] demonstrates this.) It must be
taken into account that information deemed important is
also lost in this process, however, and that
resolution is reduced. Several research groups are
developing/improving algorithms to reduce speckle while
saving as much accurate information as possible.
Question 4.14 : What do you mean by "Complex
|Figure 4.3 Coherent Summing of Radar Backscatter
Used here the term "complex" refers to complex
numbers, or complex-format data. You might be used to
hearing of complex numbers with their
"real" and "imaginary" components,
also known as cosine and sine components. For example a
wave might be described in complex format by:
A*(cos(wt) + i*sin(wt)), where 'w' represents
the wave's frequency and 'A' its
amplitude. [see below] The cosine value would describe
the wave's real component, sine the imaginary
component, and the two would combine as vectors to
provide the wave's overall phase (inverse tangent
of sin/cos) and amplitude.
|Figure 4.4 Representing Waves in Complex Format
Both the cosine and sine components of backscattered SAR
signals are measured and digitized on-board the satellite.
|Figure 4.5 Obtaining the Input Signal's In-Phase and Quadrature Components
The two resulting data streams are then transmitted to a
ground station for further processing. People sometimes
call these the 'I' (representing In-Phase,
or the cosine or real component) and 'Q'
(representing Quadrature, the 90 degrees shifted, sine
or imaginary component) data streams. For standard
processing these two data values are combined to obtain
the composite signal intensity (sqrt[I^2 + Q^2]).
Sometimes, though, it's desired to process the two
data streams separately - usually to maintain the
signals' phase information. Then ASF
distributes the individually-processed I and Q data
values for each pixel location, calling this product
"Complex SAR Data." A more detailed
example/tutorial of complex SAR data is also available.