1.1.2 Scientific Background
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Figure 1.2 ERS-1 image, May 17, 1992 Mt. Redoubt, Southern Alaska (Copyright ESA, 1992) |
1.1.2.1 Radar Imaging
Radar is the commonly used acronym for Radio
Detection and Ranging. Radio waves are that
part of the electromagnetic spectrum that
have wavelengths considerably longer
than visible light, as shown in figure1.3 below.
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Figure 1.3 The Electromagnetic Spectrum |
Imaging radar is an active illumination
system, in contrast to passive optical
imaging systems that require the Sun's
illumination. An antenna, mounted on an
aircraft or spacecraft, transmits a radar
signal in a side-looking direction
towards the earth's surface. The
reflected signal, known as the echo, is backscattered from the
surface and received a fraction of a second
later at the same antenna, as shown in
figure1.4 below. The
brightness, or amplitude (A), of this
received echo is measured and recorded and
the data are then used to construct an
image. For coherent Radar systems such as
Synthetic Aperture Radar (SAR), the
phase of the received echo is also measured
and used to construct the image.
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Figure 1.4 Echoes received back by antenna |
Radar uses a single frequency for
illumination; therefore there is no color
associated with raw Radar imagery (unlike
optical imagery which is illuminated by
all the various colours from the ambient
visible light.) However, Radar provides at
least two significant benefits from its
not being dependent on natural light: the
ability to image through clouds, and the
ability to image at night. The wavelength of
the microwaves used in Radar are longer than
those of visible light, and are less
responsive to the boundaries between air and
the water droplets within the clouds.
The result is that, for Radar, the clouds
appear homogeneous with only slight
distortions occurring when the waves
enter and leave the clouds.
1.1.2.1.1 Polarisation
Irrespective of wavelength, radar signals
can be transmitted and/or received in
different modes of polarisation. Being
electromagnetic waves, microwaves are
transverse; that is the vibrations
are perpendicular to the direction of
wave propagation (unlike sound waves
which are longitudinal and vibrate in
the same direction as the wave
propagation.) For Radar, the waves are
typically polarised in a plane, either
horizontal or vertical; although it is
also possible for the waves to not be
restricted to a plane (they could be
elliptically or circularly polarised).
1.1.2.1.2 Backscatter
Radar images
are composed of many picture
elements referred to as pixels. Each
pixel in the radar image
represents an estimate of the
radar backscatter for that
area on the ground. Darker
areas in the image represent low
backscatter, while brighter
areas represent high backscatter. Bright
features mean that a large fraction of
the radar energy was
reflected back to the radar, while dark
features imply that very
little energy was reflected. Backscatter
for a target area at a
particular wavelength will vary for
a variety of conditions, such as the
physical size of the
scatterers in the target area, the
target's electrical
properties and the moisture content,
with wetter objects
appearing bright, and drier targets
appearing dark. (The
exception to this is a smooth body of
water, which will act as a
flat surface and reflect incoming pulses
away from a target. These bodies
will appear dark ). The
wavelength and polarisation of the Radar
pulses, and the observation
angles will also affect backscatter
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Figure 1.5 Backscatter from various surfaces types |
A useful rule-of-thumb in analysing
radar images is that
the higher or brighter the
backscatter on the image, the
rougher the surface being
imaged. Flat surfaces
that reflect little or no radio or
microwave energy back
towards the radar will always appear
dark in radar
images. Vegetation is usually
moderately rough on the
scale of most radar wavelengths and
appears as grey or
light grey in a radar image.
Surfaces inclined
towards the radar will have a
stronger backscatter than
surfaces which slope away from the
radar and will tend
to appear brighter in a radar
image. Some areas
not illuminated by the radar, like
the back slope of
mountains, are in shadow, and will
appear dark. When
city streets or buildings are lined
up in such a way
that the incoming radar pulses are
able to bounce off
the streets and then bounce
again off the
buildings (called a double-bounce)
and directly back
towards the radar they appear very
bright (white) in
radar images. Roads and freeways are
flat surfaces and so
appear dark. Buildings which do not
line up so that the
radar pulses are reflected
straight back will
appear light grey, like very rough surfaces.
1.1.2.2 Real Aperture Radar (RAR)
Before considering the properties of a
Synthetic Aperture Radar system, we will
consider a Real Aperture System. Aperture
means the opening used to collect the
reflected energy that is used to form an
image. In the case of radar imaging this is
the antenna. For RAR systems, only the
amplitude (and not the phase) of each echo
return is measured and processed.
1.1.2.2.1 Spatial Resolution
The spatial resolution of a Real Aperture
Radar system is determined by, among
other things, the size of the antenna
used. For any given wavelength, the
larger the antenna the better the
spatial resolution. Other determining
factors include the pulse length and the
antenna beamwidth. The pulse
length of the radar signal is determined
by the length of time that the
antenna emits its burst of energy.
Consider an image to be a set of values
A(x,y), where the x coordinate is in the
direction of platform motion and the y
coordinate is in the direction of
illumination. Then the value of y, or range direction ,
and its resolution (range
resolution) is based on the pulse
length, the arrival time of the echo,
and the timing precision of the radar.
The value of x, which is azimuth direction
(also referred to as the along-track
direction) , and its resolution
(azimuth resolution) depends on the
position of the platform that carries
the transmitting antenna and the
beamwidth of the radar.
1.1.2.2.2 Range Resolution
For a Radar system to image separately
two ground features that are close
together in the range direction, it is
necessary for all parts of the two
objects' reflected signals to be
received separately by the antenna. Any
time overlap between the signals from
two objects will cause their images
to be blurred together.
1.1.2.2.3 Azimuth Resolution
As mentioned above, the azimuth
resolution is affected by the beamwidth.
As the antenna beam fans out with
increasing distance from the earth
to the platform carrying the pulse
transmitting source and receiver, the
azimuth resolution deteriorates. The
beamwidth of the antenna is directly
proportional to the wavelength of the
transmitted pulses and inversely
proportional to the length of the antenna.
So, for any given wavelength, antenna
beamwidth can be best controlled by one
of two different means:
- by controlling the physical length
of the antenna, or
- by synthesising an effective length
of the antenna
Those systems where beamwidth is
controlled by the physical antenna length
are referred to as Real Aperture, or
Noncoherent Radars and the natural
resolution of such an orbiting radar
instrument, observing from 1000 km, is
typically 10 km on the ground. While these
systems enjoy relative simplicity of
design and data processing, the resolution
difficulties restrict them to short-range,
low altitude operation and the use of
relatively short wavelengths. These
restrictions limit the area of coverage
obtainable and the short wavelengths
experience more atmospheric dispersion.
1.1.2.3 Synthetic Aperture Radar (SAR)
Synthetic Aperture Radar (SAR) takes
advantage of the Doppler history of the
radar echoes generated by the forward motion
of the spacecraft to synthesise a large
antenna, enabling high azimuthal resolution
in the resulting image despite a
physically small antenna, as shown is figure1.6 . As the radar
moves, a pulse is transmitted at each
position. The return echoes pass through the
receiver and are recorded in an echo store.
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Figure 1.6 Constructing a synthetic antenna |
SAR is a coherent, active,
microwave imaging method that improves
natural radar resolution by focusing the
image through a process known as synthetic
aperture processing. This typically requires
a complex integrated array of onboard
navigational and control systems, with
location accuracy provided by both Doppler
and inertial navigation equipment. For a
C band instrument, (such as ERS-1, ERS-2 or
ASAR) 1000 km from its target, the area on
the ground covered by a single
transmitted EM pulse, known as the radar
footprint, is on the order of 5 km long in
the along-track (azimuth) direction. In SAR,
the satellite must not cover more than
half of the azimuth antenna length between
the emission of successive pulses, so as not
to degrade the range resolution 1.1.2.2.2. . For
example, a 10 m antenna should advance only
5 m between pulses, to produce a 5 m
long final elementary resolution cell, or pixel. Therefore, each
5 km long footprint is a collection of
signals, each one of which is a mixture of a
thousand 5 m samples, each of which
contributes to a thousand signals. Focusing
is the reconstruction of the contribution of
each 5 m cell, which results in an
improvement of resolution of
approximately a thousand times of a Real
Aperture Radar. This effectively provides a
synthetic aperture of a 20 km antenna.
In essence, return signals from the centre
portion of the beamwidth are discriminated
by detecting Doppler frequency shifts, which
is a change in wave frequency resulting
from the relative velocities of a
transmitter and a reflector. Within the wide
antenna beam, returns from features in
the area ahead of the platform will have
upshifted, or higher, frequencies resulting
from the Doppler effect. Conversely,
returns from features behind the platform
will have downshifted, or lower,
frequencies. Returns from features near the
centreline of the beamwidth (the
so-called Zero-Doppler line) will experience
no frequency shift.
The amplitude and phase of the signals
returned from objects are recorded in the
echo store throughout the time period in
which the objects are within the beam of
the moving antenna. By processing the return
signals according to their Doppler shifts, a
very narrow effective antenna beamwidth
can be achieved, even at far ranges, without
requiring a physically long antenna or a
short operating wavelength.
Because the signals received by a SAR system
are recorded over a long time period, the
system translates the real antenna over a
correspondingly long distance, which becomes
the effective length of the antenna. The azimuth resolution 1.1.2.2.3.
with this synthetic antenna length is
greatly improved, due to the effective
narrowing of the beamwidth. The azimuth
resolution is also essentially independent
of range, because at long range an object is
in the beam longer, meaning that returns
from it are recorded over a longer distance.
1.1.2.4 Image Interpretation
Determining and describing how various
objects reflect radar energy has been mostly
derived from empirical observation. It has
been found that the primary factors
influencing an object's return signal
intensity are their geometric and electrical
characteristics, as well as their
general composition. That is, radar waves
interact differently with soil, vegetation,
water, ice, or man-made objects.
1.1.2.4.1 Geometric Characteristics
One of the most readily apparent features
of radar imagery is its sidelighted
character, which arises through
variations in the relative sensor to
terrain geometry. Shadow areas and radar
backscatter 1.1.2.1.2. are
affected by different surface
properties over a range of incidence angles.
For flat terrain, the local reflection
angle is the same as the incidence
angle; most of the incident energy is
reflected away from the sensor,
resulting in a very low return signal.
Rough surfaces, on the other hand, will
scatter incidence energy in all
directions and return a significant
portion of the incident energy back to
the antenna, as show in the scattering
mechanisms figure below.
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Figure 1.7 Scattering mechanisms |
The shape and orientation of objects must
be considered, as well as their surface
roughness, when evaluating radar
returns. For instance a particularly
bright response will come from a corner
reflector, which will produce a double
bounce, as shown above. It is also worth
noting that some features, such as corn
fields, might appear rough when seen in
both the visible and microwave portion
of the spectrum, whereas other
surfaces, such as roads, may appear
rough in the visible range but look
smooth in the microwave spectrum. In
general, SAR images will manifest
many more smooth, or specular, surfaces
than those produced with optical sensors.
Since one factor affecting backscatter is
the polarisation used, as
was discussed
previously 1.1.2.1.2. , those SAR systems that
can transmit pulses in either
horizontal (H) or vertical (V)
polarisation and receive in either H or
V, such as the ASAR sensor, can better
utilise this property for image interpretation.
In addition, SARs measure the phase of
the incoming pulse and can therefore
measure the phase difference in the
return of the HH and VV signals.
This difference can be thought of as a
difference in the round-trip times of HH
and VV signals and is frequently the
result of the structural
characteristics of the scatterers, or
targets. The phase information in the
products from these SARs can used to
derive an estimate of the
correlation coefficient for the HH and
VV returns, which can be considered as a
measure of how alike the HH and VV
scatterers are.
As an aside, the phase information
derived by a SAR may also be exploited
in Interferometric SAR (InSAR). InSAR
requires the phase differences of at
least two complex-valued SAR images,
acquired from different orbit positions
and/or at different times. These phase
differences can be compared after proper
image registration and produce a new
kind of image called an interferogram.
This subject is discussed in greater
detail in the section entitled "Interferometry 1.1.5.4. ".
1.1.2.4.2 Electrical Characteristics
The electrical characteristics of terrain
features interact with their geometric
characteristics to determine the
intensity of radar returns. One
measure of an object's electrical
character is the 'complex
dielectric constant', which is a
parameter that indicates the
reflectivity and conductivity of various
materials. As reflectivity and
conductivity increases, so does the
value of this constant.
In the microwave region
of the spectrum, most natural materials
have a dielectric constant in the
range of 3 to 8 when dry, whereas water
has a dielectric constant of
approximately 80. This means that the
presence of moisture in either soil
or vegetation will result in
significantly greater reflectivity.
Other examples of sources of high
reflectivity are metal bridges,
silos, and railroad tracks.
1.1.2.4.3 Responses of Soil, Vegetation,
Mountains, Water and Ice
1.1.2.4.3.1 Soil
Because the dielectric constant for
water is at least 10 times that for
dry soil, the presence of water in
the top few centimetres of bare,
unvegetated soil can be detected in
radar imagery, becoming particularly
apparent at longer wavelengths.
1.1.2.4.3.2 Vegetation
A vegetation canopy will interact
with radar waves as a group of
volume scatterers, as shown in figure1.7 above. The
canopy is composed of a large number
of discrete plant components,
such as leaves, stems, stalks, limbs
and so on. In addition, the canopy
is underlain by soil that may result
in surface scattering of the
energy that penetrates the
vegetation canopy. In general,
shorter wavelengths, of
approximately 2 to 6 cm, are best
for sensing crop canopies and
tree leaves, because at these
wavelengths volume scattering
predominates and surface scattering
from the underlying soil is
minimised. However, longer
wavelengths, of approximately 10 to
30 cm are best for sensing tree
trunks or limbs.
Another factor influencing the affect
of radar backscatter from vegetation
is the polarisation of
the beam, with like-polarised waves
(HH or VV) penetrating vegetation
more than cross-polarised (HV or
VH) waves. In addition, more energy
is returned from crops having their
rows aligned in the azimuth direction
than from those aligned in the range direction,
especially in the case of
like-polarised beams.
1.1.2.4.3.3 Mountains
When a spaceborne SAR looks down and
to the side toward a steep mountain,
many objects on the mountain's
facing slope may appear to be
located at the same distance from
the spacecraft, as though the
farther out in range an object is,
the higher, or closer, to the
spacecraft the ground is raised by
the mountain to compensate. Since
those many objects are located at
nearly the same distance from the
SAR, their backscattered signals
will return to the spacecraft at
about the same time. The SAR will
conclude that the object located at
that distance backscattered
brightly, mapping all those
responses into one location while in
truth they came from many. This is
called foreshortening, or
layover in the extreme case where
responses from, say, a
mountain's peak are positioned
before surrounding locations. This
is depicted in the figure below
entitled "Geometric Distortions".
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Figure 1.8 Geometric distortions |
The resulting SAR image will show
much of the mountain's bright
facing slope was mapped onto a few
pixels, while the darker
backfacing slope was considered to
cover many more pixels. Therefore it
appears as though the mountains are
lying over. Steeper topography
or a smaller SAR look angle can
worsen foreshortening effects.
1.1.2.4.3.4 Water and Ice
Smooth water surfaces yield no
returns to the antenna, but rough
water surfaces return radar signals
of varying strengths. In addition,
waves moving in the range direction,
that is moving toward or away from
the radar system, are easier to
detect that waves moving in the azimuthal direction.
Radar backscatter from sea ice is
dependent on the dielectric
properties and spatial distribution
of the ice. Such factors as ice age,
surface roughness, internal
geometry, temperature, and snow
cover will also all play a role in
the affect of radar backscatter.
Each of these above topics are
discussed in more detail in the
sections within the handbook
relating to these applications.
1.1.2.5 General Rules for SAR Image Interpretation
In summary, the general rules for SAR Image
Interpretation are as follows:
- Regions of calm water and other smooth
surfaces will appear black, because the
incident radar reflects away from the spacecraft.
- Surface variations near the size of the
radar's wavelength cause strong backscattering.
- A rough surface backscatters more
brightly when it is wet.
- Wind-roughened water can backscatter
brightly when the resulting waves are
close in size to the incident
radar's wavelength.
- Hills and other large-scale surface
variations tend to appear bright on the
side that faces the sensor and dim on
the side that faces away from the
sensor. Mountains show this effect to
the extreme, in part due to increased foreshortening.
- Due to the reflectivity and angular
structure of buildings, bridges, and
other human-made objects, these targets
tend to behave as corner
reflectors and show up as bright spots
in a SAR image.
- A particularly strong response, say from
a corner reflector, can look like a
bright cross in a processed SAR image.
1.1.2.6 References
T.M.
Lillesand, R.W. Kiefer, "Remote Sensing
and Image Interpretation", 2nd ed,
Wiley 1987.
D.
Massonet, K.L.Feigl, "Radar
interferometry and its application to
changes in the earth surface",
Reviews of Geophysics Vol. 36, Number 4,
Nov. 1998.
R.
Bamler, P. Hartl, "Synthetic aperture
radar interferometry", 1998.
ALASKA
Sar Facility, www.asf.alaska.edu
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