Microwave Remote Sensing

3.
Microwave Remote Sensing

3.1 Introduction
Microwave sensing encompasses both active and passive
forms of remote sensing. As described in Chapter 2, the
microwave portion of the spectrum covers the range from
approximately 1cm to 1m in wavelength. Because of their long
wavelengths, compared to the visible and infrared,
microwaves have special properties that are important for
remote sensing. Longer wavelength microwave radiation
can penetrate through cloud cover, haze, dust, and all but
the heaviest rainfall as the longer wavelengths are not
susceptible to atmospheric scattering which affects shorter
optical wavelengths. This property allows detection of
microwave energy under almost all weather and
environmental conditions so that data can be collected at any time.
Passive microwave sensing is similar in concept to thermal remote sensing. All objects emit
microwave energy of some magnitude, but the amounts are generally very small. A passive
microwave sensor detects the naturally emitted microwave energy within its field of view. This
emitted energy is related to the temperature and moisture properties of the emitting object or
surface. Passive microwave sensors are typically radiometers or scanners and operate in
much the same manner as systems discussed previously except that an antenna is used to
detect and record the microwave energy.
Page 92 Section 3.1 Microvaves Introduction
Canada Centre for Remote Sensing
The microwave energy recorded by a passive sensor can be emitted by the atmosphere (1),
reflected from the surface (2), emitted from the surface (3), or transmitted from the subsurface
(4). Because the wavelengths are so long, the energy available is quite small compared to
optical wavelengths. Thus, the fields of view must be large to detect enough energy to record
a signal. Most passive microwave sensors are therefore characterized by low spatial
resolution.
Applications of passive microwave remote sensing include meteorology, hydrology, and
oceanography. By looking "at", or "through" the atmosphere, depending on the wavelength,
meteorologists can use passive microwaves to measure atmospheric profiles and to
determine water and ozone content in the atmosphere. Hydrologists use passive microwaves
to measure soil moisture since microwave emission is influenced by moisture content.
Oceanographic applications include mapping sea ice, currents, and surface winds as well as
detection of pollutants, such as oil slicks.
Active microwave sensors provide their own
source of microwave radiation to illuminate the
target. Active microwave sensors are generally
divided into two distinct categories: imaging and
non-imaging. The most common form of imaging
active microwave sensors is RADAR. RADAR is
an acronym for RAdio Detection And Ranging,
which essentially characterizes the function and
operation of a radar sensor. The sensor transmits
a microwave (radio) signal towards the target and
detects the backscattered portion of the signal.
The strength of the backscattered signal is measured to discriminate between different targets
and the time delay between the transmitted and reflected signals determines the distance (or
range) to the target.
Non-imaging microwave sensors include altimeters and scatterometers. In most cases
these are profiling devices which take measurements in one linear dimension, as opposed to
the two-dimensional representation of imaging sensors. Radar altimeters transmit short
microwave pulses and measure the round trip time delay to targets to determine their distance
from the sensor. Generally altimeters look straight down at nadir below the platform and thus
measure height or elevation (if the altitude of the platform is accurately known). Radar
altimetry is used on aircraft for altitude determination and on aircraft and satellites for
topographic mapping and sea surface height estimation. Scatterometers are also generally
non-imaging sensors and are used to make precise quantitative measurements of the amount
of energy backscattered from targets. The amount of energy backscattered is dependent on
the surface properties (roughness) and the angle at which the microwave energy strikes the
target. Scatterometry measurements over ocean surfaces can be used to estimate wind
speeds based on the sea surface roughness. Ground-based scatterometers are used
extensively to accurately measure the backscatter from various targets in order to
characterize different materials and surface types. This is analogous to the concept of spectral
reflectance curves in the optical spectrum.
For the remainder of this chapter we focus solely on imaging radars. As with passive
microwave sensing, a major advantage of radar is the capability of the radiation to penetrate
through cloud cover and most weather conditions. Because radar is an active sensor, it can
also be used to image the surface at any time, day or night. These are the two primary
advantages of radar: all-weather and day or night imaging. It is also important to understand
that, because of the fundamentally different way in which an active radar operates compared
to the passive sensors we described in Chapter 2, a radar image is quite different from and
has special properties unlike images acquired in the visible and infrared portions of the
spectrum. Because of these differences, radar and optical data can be complementary to one
another as they offer different perspectives of the Earth's surface providing different
information content. We will examine some of these fundamental properties and differences in
more detail in the following sections.
Before we delve into the peculiarities of radar, let's first look briefly at the origins and history of
imaging radar, with particular emphasis on the Canadian experience in radar remote sensing.
The first demonstration of the transmission of radio microwaves and reflection from various
objects was achieved by Hertz in 1886. Shortly after the turn of the century, the first
rudimentary radar was developed for ship detection. In the 1920s and 1930s, experimental
ground-based pulsed radars were developed for detecting objects at a distance. The first
imaging radars used during World War II had rotating sweep displays which were used for
detection and positioning of aircrafts and ships. After World War II, side-looking airborne radar
(SLAR) was developed for military terrain reconnaissance and surveillance where a strip of
the ground parallel to and offset to the side of the aircraft was imaged during flight. In the
1950s, advances in SLAR and the development of higher resolution synthetic aperture radar
(SAR) were developed for military purposes. In the 1960s these radars were declassified and
began to be used for civilian mapping applications. Since this time the development of several
airborne and spaceborne radar systems for mapping and monitoring applications use has
flourished.
Canada initially became involved in radar remote sensing in the mid-1970s. It was recognized
that radar may be particularly well-suited for surveillance of our vast northern expanse, which
is often cloud-covered and shrouded in darkness during the Arctic winter, as well as for
monitoring and mapping our natural resources. Canada's SURSAT (Surveillance Satellite)
project from 1977 to 1979 led to our participation in the (U.S.) SEASAT radar satellite, the first
operational civilian radar satellite. The Convair-580 airborne radar program, carried out by the
Canada Centre for Remote Sensing following the SURSAT program, in conjunction with radar
research programs of other agencies such as NASA and the European Space Agency (ESA),
led to the conclusion that spaceborne remote sensing was feasible. In 1987, the Radar Data
Development Program (RDDP), was initiated by the Canadian government with the objective
of "operationalizing the use of radar data by Canadians". Over the 1980s and early 1990s,
several research and commercial airborne radar systems have collected vast amounts of
imagery throughout the world demonstrating the utility of radar data for a variety of
applications. With the launch of ESA's ERS-1 in 1991, spaceborne radar research intensified,
and was followed by the major launches of Japan's J-ERS satellite in 1992, ERS-2 in 1995,
and Canada's advanced RADARSAT satellite, also in 1995.
3.2 Radar Basics
As noted in the previous section, a radar is essentially a
ranging or distance measuring device. It consists
fundamentally of a transmitter, a receiver, an antenna, and an
electronics system to process and record the data. The
transmitter generates successive short bursts (or pulses of
microwave (A) at regular intervals which are focused by the
antenna into a beam (B). The radar beam illuminates the
surface obliquely at a right angle to the motion of the platform.
The antenna receives a portion of the transmitted energy
reflected (or backscattered) from various objects within the
illuminated beam (C). By measuring the time delay between
the transmission of a pulse and the reception of the
backscattered "echo" from different targets, their distance
from the radar and thus their location can be determined. As the sensor platform moves forward, recording and
processing of the backscattered signals builds up a two-dimensional image of the surface.

While we have characterized electromagnetic radiation in the visible and infrared portions of the spectrum primarily
by wavelength, microwave portions of the spectrum are often referenced according to both wavelength and
frequency. The microwave region of the spectrum is quite large, relative to the visible and infrared, and there are
several wavelength ranges or bands commonly used which given code letters during World War II, and remain to this
day.
Ka, K, and Ku bands: very short wavelengths used in early airborne radar systems but uncommon today.
X-band: used extensively on airborne systems for military reconnaissance and terrain mapping.
C-band: common on many airborne research systems (CCRS Convair-580 and NASA AirSAR) and
spaceborne systems (including
ERS-1 and 2 and RADARSAT).
Page 96 Section 3.2 Radar Basics
S-band: used on board the Russian ALMAZ satellite.
L-band: used onboard American SEASAT and Japanese JERS-1 satellites and NASA airborne system.
P-band: longest radar wavelengths, used on NASA experimental airborne research system.

Two radar images of the same agricultural fields
Here are two radar images of the same agricultural fields, each image having been collected using a different radar
band. The one on the top was acquired by a C-band radar and the one below was acquired by an L-band radar. You
can clearly see that there are significant differences between the way the various fields and crops appear in each of
the two images. This is due to the different ways in which the radar energy interacts with the fields and crops
depending on the radar wavelength. We will learn more about this in later sections.
When discussing microwave energy, the polarization
of the radiation is also important. Polarization refers to
the orientation of the electric field (recall the definition
of electromagnetic radiation from Chapter 1). Most
radars are designed to transmit microwave radiation
either horizontally polarized (H) or vertically polarized
(V). Similarly, the antenna receives either the
horizontally or vertically polarized backscattered
energy, and some radars can receive both. These two
polarization states are designated by the letters H for
horizontal, and V, for vertical. Thus, there can be four
combinations of both transmit and receive polarizations
as follows:
HH - for horizontal transmit and horizontal receive,
VV - for vertical transmit and vertical receive,
HV - for horizontal transmit and vertical receive, and
VH - for vertical transmit and horizontal receive.
The first two polarization combinations are referred to as like-polarized because the transmit and receive
polarizations are the same. The last two combinations are referred to as cross-polarized because the transmit and
receive polarizations are opposite of one another. These C-band images of agricultural fields demonstrate the
variations in radar response due to changes in polarization. The bottom two images are like-polarized (HH and VV,
respectively), and the upper right image is cross-polarized (HV). The upper left image is the result of displaying each
of the three different polarizations together, one through each of the primary colours (red, green, and blue). Similar to
variations in wavelength, depending on the transmit and receive polarizations, the radiation will interact with and be
backscattered differently from the surface. Both wavelength and polarization affect how a radar "sees" the surface.
Therefore, radar imagery collected using different polarization and wavelength combinations may provide different
and complementary information about the targets on the surface.

C-band images
3.3 Viewing Geometry and Spatial Resolution
The imaging geometry of a radar system is
different from the framing and scanning
systems commonly employed for optical
remote sensing described in Chapter 2. Similar
to optical systems, the platform travels forward
in the flight direction (A) with the nadir (B)
directly beneath the platform. The microwave
beam is transmitted obliquely at right angles to
the direction of flight illuminating a swath (C)
which is offset from nadir. Range (D) refers to
the across-track dimension perpendicular to the
flight direction, while azimuth (E) refers to the
along-track dimension parallel to the flight
direction. This side-looking viewing geometry is
typical of imaging radar systems (airborne or spaceborne).

Near range
The portion of the image swath closest to the nadir track of the radar platform is called the
near range (A) while the portion of the swath farthest from the nadir is called the far range
(B).
Page 99 Section 3.3 Viewing Geometry and Spatial Resolution

Incidence angle
The incidence angle is the angle between the radar beam and ground surface (A) which
increases, moving across the swath from near to far range. The look angle (B) is the angle at
which the radar "looks" at the surface. In the near range, the viewing geometry may be
referred to as being steep, relative to the far range, where the viewing geometry is shallow. At
all ranges the radar antenna measures the radial line of sight distance between the radar and
each target on the surface. This is the slant range distance (C). The ground range
distance (D) is the true horizontal distance along the ground corresponding to each point
measured in slant range.
Unlike optical systems, a radar's spatial resolution
is a function of the specific properties of the
microwave radiation and geometrical effects. If a
Real Aperture Radar (RAR) is used for image
formation (as in Side-Looking Airborne Radar) a
single transmit pulse and the backscattered signal
are used to form the image. In this case, the
resolution is dependent on the effective length of
the pulse in the slant range direction and on the
width of the illumination in the azimuth direction.
The range or across-track resolution is
dependent on the length of the pulse (P). Two
distinct targets on the surface will be resolved in the range dimension if their separation is
greater than half the pulse length. For example, targets 1 and 2 will not be separable while
targets 3 and 4 will. Slant range resolution remains constant, independent of range. However,
when projected into ground range coordinates, the resolution in ground range will be
dependent of the incidence angle. Thus, for fixed slant range resolution, the ground range
resolution will decrease with increasing range.

The azimuth or along-track resolution is determined by the angular width of the radiated
microwave beam and the slant range distance. This beamwidth (A) is a measure of the width
of the illumination pattern. As the radar illumination propagates to increasing distance from the
sensor, the azimuth resolution increases (becomes coarser). In this illustration, targets 1 and
2 in the near range would be separable, but targets 3 and 4 at further range would not. The
radar beamwidth is inversely proportional to the antenna length (also referred to as the
aperture) which means that a longer antenna (or aperture) will produce a narrower beam and
finer resolution.
Finer range resolution can be achieved by using a shorter pulse length, which can be done
within certain engineering design restrictions. Finer azimuth resolution can be achieved by
increasing the antenna length. However, the actual length of the antenna is limited by what
can be carried on an airborne or spaceborne platform. For airborne radars, antennas are
usually limited to one to two metres; for satellites they can be 10 to 15 metres in length. To
overcome this size limitation, the forward motion of the platform and special recording and
processing of the backscattered echoes are used to simulate a very long antenna and thus
increase azimuth resolution.

This figure illustrates how this is achieved. As a target (A) first enters the radar beam (1), the
backscattered echoes from each transmitted pulse begin to be recorded. As the platform
continues to move forward, all echoes from the target for each pulse are recorded during the
entire time that the target is within the beam. The point at which the target leaves the view of
the radar beam (2) some time later, determines the length of the simulated or synthesized
antenna (B). Targets at far range, where the beam is widest will be illuminated for a longer
period of time than objects at near range. The expanding beamwidth, combined with the
increased time a target is within the beam as ground range increases, balance each other,
such that the resolution remains constant across the entire swath. This method of achieving
uniform, fine azimuth resolution across the entire imaging swath is called synthetic aperture
radar, or SAR. Most airborne and spaceborne radars employ this type of radar.
3.4 Radar Image Distortions
As with all remote sensing systems, the viewing
geometry of a radar results in certain geometric
distortions on the resultant imagery. However, there
are key differences for radar imagery which are due
to the side-looking viewing geometry, and the fact
that the radar is fundamentally a distance
measuring device (i.e. measuring range). Slant-
range scale distortion occurs because the radar is
measuring the distance to features in slant-range
rather than the true horizontal distance along the
ground. This results in a varying image scale,
moving from near to far range. Although targets A1
and B1 are the same size on the ground, their apparent dimensions in slant range (A2 and
B2) are different. This causes targets in the near range to appear compressed relative to the
far range. Using trigonometry, ground-range distance can be calculated from the slant-range
distance and platform altitude to convert to the proper ground-range format.

This conversion comparison shows a radar image in slant-range display (top) where the
fields and the road in the near range on the left side of the image are compressed, and the
same image converted to ground-range display (bottom) with the features in their proper
geometric shape.
Similar to the distortions encountered when using cameras and scanners, radar images are
also subject to geometric distortions due to relief displacement. As with scanner imagery,
this displacement is one-dimensional and occurs perpendicular to the flight path. However,
the displacement is reversed with targets being displaced towards, instead of away from the
sensor. Radar foreshortening and layover are two consequences which result from relief
displacement.
Page 102 Section 3.4 Radar Image Distortions

When the radar beam reaches the base of a tall feature tilted towards the radar (e.g. a
mountain) before it reaches the top foreshortening will occur. Again, because the radar
measures distance in slant-range, the slope (A to B) will appear compressed and the length of
the slope will be represented incorrectly (A' to B'). Depending on the angle of the hillside or
mountain slope in relation to the incidence angle of the radar beam, the severity of
foreshortening will vary. Maximum foreshortening occurs when the radar beam is
perpendicular to the slope such that the slope, the base, and the top are imaged
simultaneously (C to D). The length of the slope will be reduced to an effective length of zero
in slant range (C'D'). The figure below shows a radar image of steep mountainous terrain
with severe foreshortening effects. The foreshortened slopes appear as bright features on the
image.


Layover occurs when the radar beam reaches the
top of a tall feature (B) before it reaches the base
(A). The return signal from the top of the feature
will be received before the signal from the bottom.
As a result, the top of the feature is displaced
towards the radar from its true position on the
ground, and "lays over" the base of the feature (B'
to A'). Layover effects on a radar image look very
similar to effects due to foreshortening. As with
foreshortening, layover is most severe for small
incidence angles, at the near range of a swath, and
in mountainous terrain.

Both foreshortening and layover result in radar shadow. Radar shadow occurs when the
radar beam is not able to illuminate the ground surface. Shadows occur in the down range
dimension (i.e. towards the far range), behind vertical features or slopes with steep sides.
Since the radar beam does not illuminate the surface, shadowed regions will appear dark on
an image as no energy is available to be backscattered. As incidence angle increases from
near to far range, so will shadow effects as the radar beam looks more and more obliquely at
the surface. This image illustrates radar shadow effects on the right side of the hillsides
which are being illuminated from the left.

Red surfaces are completely in shadow. Black areas in image are shadowed and contain no
information.

Radar shadow effects

3.5 Target Interaction and Image Appearance
The brightness of features in a radar image is dependent on the portion of the transmitted
energy that is returned back to the radar from targets on the surface. The magnitude or
intensity of this backscattered energy is dependent on how the radar energy interacts with the
surface, which is a function of several variables or parameters. These parameters include the
particular characteristics of the radar system (frequency, polarization, viewing geometry, etc.)
as well as the characteristics of the surface (landcover type, topography, relief, etc.). Because
many of these characteristics are interrelated, it is impossible to separate out each of their
individual contributions to the appearance of features in a radar image. Changes in the
various parameters may have an impact on and affect the response of other parameters,
which together will affect the amount of backscatter. Thus, the brightness of features in an
image is usually a combination of several of these variables. However, for the purposes of our
discussion, we can group these characteristics into three areas which fundamentally control
radar energy/target interactions. They are:
Surface roughness of the target
Radar viewing and surface geometry relationship
Moisture content and electrical properties of the target

The surface roughness of a feature controls how the microwave energy interacts with that
surface or target and is generally the dominant factor in determining the tones seen on a radar
image. Surface roughness refers to the average height variations in the surface cover from a
plane surface, and is measured on the order of centimetres. Whether a surface appears rough
or smooth to a radar depends on the wavelength and incidence angle.
Simply put, a surface is considered "smooth" if the
height variations are much smaller than the radar
wavelength. When the surface height variations begin
to approach the size of the wavelength, then the
surface will appear "rough". Thus, a given surface will
appear rougher as the wavelength becomes shorter
and smoother as the wavelength becomes longer. A
smooth surface (A) causes specular reflection of the
incident energy (generally away from the sensor) and
thus only a small amount of energy is returned to the
radar. This results in smooth surfaces appearing as
Page 106 Section 3.5 Target Interaction and Image Appearance
darker toned areas on an image. A rough surface (B) will scatter the energy approximately
equally in all directions (i.e. diffusely) and a significant portion of the energy will be
backscattered to the radar. Thus, rough surfaces will appear lighter in tone on an image.
Incidence angle, in combination with wavelength, also plays a role in the apparent roughness
of a surface. For a given surface and wavelength, the surface will appear smoother as the
incidence angle increases. Thus, as we move farther across the swath, from near to far range,
less energy would be returned to the sensor and the image would become increasingly darker
in tone.
We have already discussed incidence or look angle in relation
to viewing geometry and how changes in this angle affect the
signal returned to the radar. However, in relation to surface
geometry, and its effect on target interaction and image
appearance, the local incidence angle is a more appropriate
and relevant concept. The local incidence angle is the angle
between the radar beam and a line perpendicular to the slope
at the point of incidence (A). Thus, local incidence angle takes
into account the local slope of the terrain in relation to the radar beam. With flat terrain, the
local incidence angle is the same as the look angle (B) of the radar. For terrain with any type
of relief, this is not the case. Generally, slopes facing towards the radar will have small local
incidence angles, causing relatively strong backscattering to the sensor, which results in a
bright-toned appearance in an image.
As the concept of local incidence angle demonstrates, the relationship between viewing
geometry and the geometry of the surface features plays an important role in how the radar
energy interacts with targets and their corresponding brightness on an image. Variations in
viewing geometry will accentuate and enhance topography and relief in different ways, such
that varying degrees of foreshortening, layover, and shadow (section 3.4) may occur
depending on surface slope, orientation, and shape.

The look direction or aspect angle of the radar describes the orientation of the transmitted
radar beam relative to the direction or alignment of linear features on the surface. The look
direction can significantly influence the appearance of features on a radar image, particularly
when ground features are organized in a linear structure (such as agricultural crops or
mountain ranges). If the look direction is close to perpendicular to the orientation of the
feature (A), then a large portion of the incident energy will be reflected back to the sensor and
the feature will appear as a brighter tone. If the look direction is more oblique in relation to the
feature orientation (B), then less energy will be returned to the radar and the feature will
appear darker in tone. Look direction is important for enhancing the contrast between features
in an image. It is particularly important to have the proper look direction in mountainous
regions in order to minimize effects such as layover and shadowing. By acquiring imagery
from different look directions, it may be possible to enhance identification of features with
different orientations relative to the radar.
Features which have two (or more) surfaces (usually
smooth) at right angles to one another, may cause
corner reflection to occur if the 'corner' faces the
general direction of the radar antenna. The orientation of
the surfaces at right angles causes most of the radar
energy to be reflected directly back to the antenna due
to the double bounce (or more) reflection. Corner
reflectors with complex angular shapes are common in
urban environments (e.g. buildings and streets, bridges,
other man-made structures). Naturally occurring corner
reflectors may include severely folded rock and cliff
faces or upright vegetation standing in water. In all cases, corner reflectors show up as very
bright targets in an image, such as the buildings and other man-made structures in this radar
image of a city.

The presence (or absence) of moisture affects the electrical properties of an object or
medium. Changes in the electrical properties influence the absorption, transmission, and
reflection of microwave energy. Thus, the moisture content will influence how targets and
surfaces reflect energy from a radar and how they will appear on an image. Generally,
reflectivity (and image brightness) increases with increased moisture content. For example,
surfaces such as soil and vegetation cover will appear brighter when they are wet than when
they are dry.
When a target is moist or wet, scattering from the topmost portion (surface scattering) is the
dominant process taking place. The type of reflection (ranging from specular to diffuse) and
the magnitude will depend on how rough the material appears to the radar. If the target is very
dry and the surface appears smooth to the radar, the radar energy may be able to penetrate
below the surface, whether that surface is discontinuous (e.g. forest canopy with leaves and
branches), or a homogeneous surface (e.g. soil, sand, or ice). For a given surface, longer
wavelengths are able to penetrate further than shorter wavelengths.

If the radar energy does manage to penetrate through the topmost surface, then volume
scattering may occur. Volume scattering is the scattering of radar energy within a volume or
medium, and usually consists of multiple bounces and reflections from different components
within the volume. For example, in a forest, scattering may come from the leaf canopy at the
tops of the trees, the leaves and branches further below, and the tree trunks and soil at the
ground level. Volume scattering may serve to decrease or increase image brightness,
depending on how much of the energy is scattered out of the volume and back to the radar.
3.6 Radar Image Properties

All radar images appear with some degree of what we call radar speckle. Speckle appears as
a grainy "salt and pepper" texture in an image. This is caused by random constructive and
destructive interference from the multiple scattering returns that will occur within each
resolution cell. As an example, an homogeneous target, such as a large grass-covered field,
without the effects of speckle would generally result in light-toned pixel values on an image
(A). However, reflections from the individual blades of grass within each resolution cell results
in some image pixels being brighter and some being darker than the average tone (B), such
that the field appears speckled.

Speckle is essentially a form of noise which degrades the quality of an image and may make
interpretation (visual or digital) more difficult. Thus, it is generally desirable to reduce speckle
prior to interpretation and analysis. Speckle reduction can be achieved in two ways:
multi-look processing, or
spatial filtering.

Page 110 Section 3.6 Radar Image Properties

Multi-look processing refers to the division of the radar beam (A)
into several (in this example, five) narrower sub-beams (1 to 5).
Each sub-beam provides an independent "look" at the
illuminated scene, as the name suggests. Each of these "looks"
will also be subject to speckle, but by summing and averaging
them together to form the final output image, the amount of
speckle will be reduced.
While multi-looking is
usually done during data
acquisition, speckle reduction by spatial filtering is
performed on the output image in a digital (i.e. computer)
image analysis environment. Speckle reduction filtering
consists of moving a small window of a few pixels in
dimension (e.g. 3x3 or 5x5) over each pixel in the image,
applying a mathematical calculation using the pixel
values under that window (e.g. calculating the average), and replacing the central pixel with
the new value. The window is moved along in both the row and column dimensions one pixel
at a time, until the entire image has been covered. By calculating the average of a small
window around each pixel, a smoothing effect is achieved and the visual appearance of the
speckle is reduced.

Speckle reduction using an averaging filter
This graphic shows a radar image before (top) and after (bottom) speckle reduction using an
averaging filter. The median (or middle) value of all the pixels underneath the moving window
is also often used to reduce speckle. Other more complex filtering calculations can be
performed to reduce speckle while minimizing the amount of smoothing taking place.
Both multi-look processing and spatial filtering reduce speckle at the expense of resolution,
since they both essentially smooth the image. Therefore, the amount of speckle reduction
desired must be balanced with the particular application the image is being used for, and the
amount of detail required. If fine detail and high resolution is required then little or no multi-
looking/spatial filtering should be done. If broad-scale interpretation and mapping is the
application, then speckle reduction techniques may be more appropriate and acceptable.
Another property peculiar to radar images is slant-range distortion, which was discussed in
some detail in section 3.4. Features in the near-range are compressed relative to features in
the far range due to the slant-range scale variability. For most applications, it is desirable to
have the radar image presented in a format which corrects for this distortion, to enable true
distance measurements between features. This requires the slant-range image to be
converted to 'ground range' display. This can be done by the radar processor prior to creating
an image or after data acquisition by applying a transformation to the slant range image. In
most cases, this conversion will only be an estimate of the geometry of the ground features
due to the complications introduced by variations in terrain relief and topography.
A radar antenna transmits more power in the mid-range portion of the illuminated swath than
at the near and far ranges. This effect is known as antenna pattern and results in stronger
returns from the center portion of the swath than at the edges. Combined with this antenna
pattern effect is the fact that the energy returned to the radar decreases dramatically as the
range distance increases. Thus, for a given surface, the strength of the returned signal
becomes smaller and smaller moving farther across the swath. These effects combine to
produce an image which varies in intensity (tone) in the range direction across the image. A
process known as antenna pattern correction may be applied to produce a uniform average
brightness across the imaged swath, to better facilitate visual interpretation.

The range of brightness levels a remote sensing system can differentiate is related to
radiometric resolution (section 2.5) and is referred to as the dynamic range. While optical
sensors, such as those carried by satellites such as Landsat and SPOT, typically produce 256
intensity levels, radar systems can differentiate intensity levels up to around 100,000 levels!
Since the human eye can only discriminate about 40 intensity levels at one time, this is too
much information for visual interpretation. Even a typical computer would have difficulty
dealing with this range of information. Therefore, most radars record and process the original
data as 16 bits (65,536 levels of intensity), which are then further scaled down to 8 bits (256
levels) for visual interpretation and/or digital computer analysis.
Calibration is a process which ensures that the radar system and the signals that it measures
are as consistent and as accurate as possible. Prior to analysis, most radar images will
require relative calibration. Relative calibration corrects for known variations in radar
antenna and systems response and ensures that uniform, repeatable measurements can be
made over time. This allows relative comparisons between the response of features within a
single image, and between separate images to be made with confidence. However, if we wish
to make accurate quantitative measurements representing the actual energy or power
returned from various features or targets for comparative purposes, then absolute
calibration is necessary.
Absolute calibration, a much more involved process than relative calibration, attempts to
relate the magnitude of the recorded signal strength to the actual amount of energy
backscattered from each resolution cell. To achieve this, detailed measurements of the radar
system properties are required as well as quantitative measurements of the scattering
properties of specific targets. The latter are often obtained using ground-based
scatterometers, as described in section 3.1. Also, devices called transponders may be
placed on the ground prior to data acquisition to calibrate an image. These devices receive
the incoming radar signal, amplify it, and transmit a return signal of known strength back to
the radar. By knowing the actual strength of this return signal in the image, the responses
from other features can be referenced to it.
3.7 Advanced Radar Applications
In addition to standard acquisition and use of radar data, there are three specific applications
worth mentioning.

The first is stereo radar which is similar in concept to stereo mapping using aerial
photography (described in section 2.7). Stereo radar image pairs are acquired covering the
same area, but with different look/incidence angles (A), or opposite look directions (B). Unlike
aerial photos where the displacement is radially outward from the nadir point directly below
the camera, radar images show displacement only in the range direction. Stereo pairs taken
from opposite look directions (i.e. one looking north and the other south) may show significant
contrast and may be difficult to interpret visually or digitally. In mountainous terrain, this will be
even more pronounced as shadowing on opposite sides of features will eliminate the stereo
effect. Same side stereo imaging (A) has been used operationally for years to assist in
interpretation for forestry and geology and also to generate topographic maps. The estimation
of distance measurements and terrain height for topographic mapping from stereo radar data
is called radargrammetry, and is analogous to photogrammetry carried out for similar
purposes with aerial photographs.

Radargrammetry is one method of estimating terrain height using radar. Another, more
advanced method is called interferometry. Interferometry relies on being able to measure a
property of electromagnetic waves called phase. Suppose we have two waves with the exact
Page 114 Section 3.7 Advanced Radar Applications
same wavelength and frequency traveling along in space, but the starting point of one is offset
slightly from the other. The offset between matching points on these two waves (A) is called
the phase difference. Interferometric systems use two
antennas, separated in the range dimension by a small
distance, both recording the returns from each resolution
cell. The two antennas can be on the same platform (as
with some airborne SARs), or the data can be acquired from
two different passes with the same sensor, such has been
done with both airborne and satellite radars. By measuring
the exact phase
difference between the
two returns (A), the
path length difference can be calculated to an accuracy
that is on the order of the wavelength (i.e centimetres).
Knowing the position of the antennas with respect to the
Earth's surface, the position of the resolution cell,
including its elevation, can be determined. The phase
difference between adjacent resolution cells, is
illustrated in this interferogram, where colours
represents the variations in height. The information contained in an interferogram can be used
to derive topographic information and produce three-dimensional imagery of terrain height.

The concept of radar polarimetry was already alluded to in our discussion of radar
fundamentals in section 3.2. As its name implies, polarimetry involves discriminating between
the polarizations that a radar system is able to transmit and receive. Most radars transmit
microwave radiation in either horizontal (H) or vertical (V) polarization, and similarly, receive
the backscattered signal at only one of these polarizations. Multi-polarization radars are able
to transmit either H or V polarization and receive both the like- and cross-polarized returns
(e.g. HH and HV or VV and VH, where the first letter stands for the polarization transmitted
and the second letter the polarization received). Polarimetric radars are able to transmit and
receive both horizontal and vertical polarizations. Thus, they are able to receive and process
all four combinations of these polarizations: HH, HV, VH, and VV. Each of these "polarization
channels" have varying sensitivities to different surface characteristics and properties. Thus,
the availability of multi-polarization data helps to improve the identification of, and the
discrimination between features. In addition to recording the magnitude (i.e. the strength) of
the returned signal for each polarization, most polarimetric radars are also able to record the
phase information of the returned signals. This can be used to further characterize the
polarimetric "signature" of different surface features.
3.8 Radar Polarimetry
Introduction to Polarization
When discussing microwave energy propagation and scattering, the polarization of the
radiation is an important property. For a plane electromagnetic (EM) wave, polarization refers
to the locus of the electric field vector in the plane perpendicular to the direction of propagation.
While the length of the vector represents the amplitude of the wave, and the rotation rate of
the vector represents the frequency of the wave, polarization refers to the orientation and
shape of the pattern traced by the tip of the vector.
The waveform of the electric field strength (voltage) of an EM wave can be predictable (the
wave is polarized) or random (the wave is unpolarized), or a combination of both. In the latter
case, the degree of polarization describes the ratio of polarized power to total power of the
wave. An example of a fully polarized wave would be a monochromatic sine wave, with a
single, constant frequency and stable amplitude.

Examples of horizontal (black) and vertical (red) polarizations of a plane electromagnetic wave

Many radars are designed to transmit microwave radiation that is either horizontally polarized
(H) or vertically polarized (V). A transmitted wave of either polarization can generate a
backscattered wave with a variety of polarizations. It is the analysis of these transmit and
receive polarization combinations that constitutes the science of radar polarimetry.
Any polarization on either transmission or reception can be synthesized by using H and V
components with a well-defined relationship between them. For this reason, systems that
transmit and receive both of these linear polarizations are commonly used. With these radars,
there can be four combinations of transmit and receive polarizations:
HH - for horizontal transmit and horizontal receive
VV - for vertical transmit and vertical receive
Page 117 Section 3.8 Radar Polarimetry
HV - for horizontal transmit and vertical receive, and
VH - for vertical transmit and horizontal receive.

The first two polarization combinations are referred to as "like-polarized" because the transmit
and receive polarizations are the same. The last two combinations are referred to as "cross-
polarized" because the transmit and receive polarizations are orthogonal to one another.
Radar systems can have one, two or all four of these transmit/receive polarization
combinations. Examples include the following types of radar systems:

Note that "quadrature polarization" and "fully polarimetric" can be used as synonyms for
"polarimetric". The relative phase between channels is measured in a polarimetric radar, and
is a very important component of the measurement. In the other radar types, relative phase
may or may not be measured. The alternating polarization mode has been introduced on
ENVISAT - relative phase is measured but the important HH-VV phase is not meaningful
because of the time lapse between the measurements.
These C-band images of agricultural fields demonstrate the dependence of the radar response
on polarization. The top two images are like-polarized (HH on left, VV on right), and the lower
left image is cross-polarized (HV). The lower right image is the result of displaying these three
images as a colour composite (in this case, HH - red, VV - green, and HV - blue).
Both wavelength and polarization affect how a radar system "sees" the elements in the scene.
Therefore, radar imagery collected using different polarization and wavelength combinations
may provide different and complementary information. Furthermore, when three polarizations
are combined in a colour composite, the information is presented in a way that an image
interpreter can infer more information of the surface characteristics.

single polarized - HH or VV (or possibly HV or VH)
dual polarized - HH and HV, VV and VH, or HH and VV
alternating polarization - HH and HV, alternating with VV and VH
polarimetric - HH, VV, HV, and VH

Illustration of how different polarizations (HH, VV, HV & colour composite) bring out different
features in an agricultural scene

Polarimetric Information
The primary description of how a radar target or surface feature scatters EM energy is given by
the scattering matrix. From the scattering matrix, other forms of polarimetric information can be
derived, such as synthesized images and polarization signatures.
Polarization Synthesis
A polarimetric radar can be used to determine the target response or scattering matrix using
two orthogonal polarizations, typically linear H and linear V on each of transmit and receive. If a
scattering matrix is known, the response of the target to any combination of incident and
received polarizations can be computed. This is referred to as polarization synthesis, and
illustrates the power and flexibility of a fully polarimetric radar.
Through polarization synthesis, an image can be created to improve the detectability of
selected features. An example is the detection of ships in ocean images. To find the best
transmit-receive polarization combination to use, the polarization signature of a typical ship and
that of the ocean is calculated for a number of polarizations. Then the ratio of the ship to ocean
backscatter is computed for each polarization. The transmit-receive polarization combination
that maximises the ratio of backscatter strength is then used to improve the detectability of
ships. This procedure is called "polarimetric contrast enhancement" or the use of a
"polarimetric matched filter".
Polarization Signatures
Because the incident and scattered waves can take on so many different polarizations, and the
scattering matrix consists of four complex numbers, it is helpful to simplify the interpretation of
the scattering behaviour using three-dimensional plots. The "polarization signature" of the
target provides a convenient way of visualising a target's scattering properties. The signatures
are also called "polarization response plots".
An incident electromagnetic wave can be selected to have an electric field with ellipticity
between -45 and +45, and an orientation between 0 and 180. These variables are used as
the x- and y-axes of a 3-D plot portraying the polarization signature. For each of these possible
incident polarizations, the strength of the backscatter can be computed for the same
polarization on transmit and receive (the co-polarized signature) and for orthogonal
polarizations on transmit and receive (the cross-polarized signature). The strength is displayed
on the z-axis of the signatures.

Polarization signatures of a large conducting sphere.
P = Power, O = Orientation (degrees), E = Ellipticity (degrees)
This figure shows the polarization signatures of the most simple of all targets - a large
conducting sphere or a trihedral corner reflector. The wave is backscattered with the same
polarization, except for a change of sign of the ellipticity (or in the case of linear polarization, a
Co-polarized signature Cross-polarized signature

change of the phase angle between Eh and Ev of 180o). The sign changes once for every
reflection - the sphere represents a single reflection, and the trihedral gives three reflections, so
each behaves as an "odd-bounce" reflector.
For more complicated targets, the polarization signature takes on different shapes. Two
interesting signatures come from a dihedral corner reflector and Bragg scattering from the sea
surface. In the case of the dihedral reflector, the co-pol signature has a double peak,
characteristic of "even-bounce" reflectors. In the case of Bragg scattering, the response is
similar to the single-bounce sphere, except that the backscatter of the vertical polarization is
higher than that of the horizontal polarization.
Data Calibration
One critical requirement of polarimetric radar systems is the need for calibration. This is
because much of the information lies in the ratios of amplitudes and the differences in phase
angle between the four transmit-receive polarization combinations. If the calibration is not
sufficiently accurate, the scattering mechanisms will be misinterpreted and the advantages of
using polarization will not be realised.
Calibration is achieved by a combination of radar system design and data analysis. Imagine the
response to a trihedral corner reflector. Its ideal response is only obtained if the four channels
of the radar system all have the same gain, system-dependent phase differences between
channels are absent, and there is no energy leakage from one channel to another.
In terms of the radar system design, the channel gains and phases should be as carefully
matched as possible. In the case of the phase balance, this means that the signal path lengths
should be effectively the same in all channels. Calibration signals are often built into the design
to help verify these channel balances.
In terms of data analysis, channel balances, cross-talk and noise effects can be measured and
corrected by analysing the received data. In addition to analysing the response of internal
calibration signals, the signals from known targets such as corner reflectors, active
transponders, and uniform clutter can be used to calibrate some of the parameters.

Polarimetric Applications
Synthetic Aperture Radar polarimetry has been limited to a number of experimental airborne
SAR systems and the SIR-C (shuttle) mission. With these data, researchers have studied a
number of applications, and have shown that the interpretation of a number of features in a
scene is facilitated when the radar is operated in polarimetric mode. The launch of
RADARSAT-2 will make polarimetric data available on an operational basis, and uses of such
data will become more routine and more sophisticated.
Some applications in which polarimetric SAR has already proved useful include:
Agriculture: for crop type identification, crop condition monitoring, soil moisture
measurement, and soil tillage and crop residue identification;
Forestry: for clearcuts and linear features mapping, biomass estimation, species
identification and fire scar mapping;
Geology: for geological mapping;
Hydrology: for monitoring wetlands and snow cover;
Oceanography: for sea ice identification, coastal windfield measurement, and wave slope
measurement;
Shipping: for ship detection and classification;
Coastal Zone: for shoreline detection, substrate mapping, slick detection and general
vegetation mapping.

3.9 Airborne versus Spaceborne Radars
Like other remote sensing systems, an imaging radar sensor may be carried on either an
airborne or spaceborne platform. Depending on the use of the prospective imagery, there are
trade-offs between the two types of platforms. Regardless of the platform used, a significant
advantage of using a Synthetic Aperture Radar (SAR) is that the spatial resolution is
independent of platform altitude. Thus, fine resolution can be achieved from both airborne and
spaceborne platforms.

Although spatial resolution is independent of altitude, viewing geometry and swath coverage
can be greatly affected by altitude variations. At aircraft operating altitudes, an airborne radar
must image over a wide range of incidence angles, perhaps as much as 60 or 70 degrees, in
order to achieve relatively wide swaths (let's say 50 to 70 km). As we have learned in the
preceding sections, incidence angle (or look angle) has a significant effect on the backscatter
from surface features and on their appearance on an image. Image characteristics such as
foreshortening, layover, and shadowing will be subject to wide variations, across a large
incidence angle range. Spaceborne radars are able to avoid some of these imaging geometry
problems since they operate at altitudes up to one hundred times higher than airborne radars.
At altitudes of several hundred kilometres, spaceborne radars can image comparable swath
widths, but over a much narrower range of incidence angles, typically ranging from five to 15
degrees. This provides for more uniform illumination and reduces undesirable imaging
variations across the swath due to viewing geometry.
Page 123 Section 3.9 Airborne versus Spaceborne Radars

Although airborne radar systems may be more susceptible to imaging geometry problems,
they are flexible in their capability to collect data from different look angles and look directions.
By optimizing the geometry for the particular terrain being imaged, or by acquiring imagery
from more than one look direction, some of these effects may be reduced. Additionally, an
airborne radar is able to collect data anywhere and at any time (as long as weather and flying
conditions are acceptable!). A spaceborne radar does not have this degree of flexibility, as its
viewing geometry and data acquisition schedule is controlled by the pattern of its orbit.
However, satellite radars do have the advantage of being able to collect imagery more quickly
over a larger area than an airborne radar, and provide consistent viewing geometry. The
frequency of coverage may not be as often as that possible with an airborne platform, but
depending on the orbit parameters, the viewing geometry flexibility, and the geographic area
of interest, a spaceborne radar may have a revisit period as short as one day.
As with any aircraft, an airborne radar will be susceptible to variations in velocity and other
motions of the aircraft as well as to environmental (weather) conditions. In order to avoid
image artifacts or geometric positioning errors due to random variations in the motion of the
aircraft, the radar system must use sophisticated navigation/positioning equipment and
advanced image processing to compensate for these variations. Generally, this will be able to
correct for all but the most severe variations in motion, such as significant air turbulence.
Spaceborne radars are not affected by motion of this type. Indeed, the geometry of their orbits
is usually very stable and their positions can be accurately calculated. However, geometric
correction of imagery from spaceborne platforms must take into account other factors, such as
the rotation and curvature of the Earth, to achieve proper geometric positioning of features on
the surface.
Page 124 Section 3.9 Airborne versus Spaceborne Radars
3.10 Airborne and Spaceborne Radar Systems
In order to more clearly illustrate the differences between airborne and spaceborne radars, we
will briefly outline a few of the representative systems of each type, starting with airborne
systems.

The Convair-580 C/X SAR system developed and operated by the Canada Centre for
Remote Sensing was a workhorse for experimental research into advanced SAR applications
in Canada and around the world, particularly in preparation for satellite-borne SARs. The
system was transferred to Environment Canada in 1996 for use in oil spill research and other
environmental applications. This system operates at two radar bands, C- (5.66 cm) and X-
(3.24 cm). Cross-polarization data can be recorded simultaneously for both the C- and X-band
channels, and the C-band system can be operated as a fully polarimetric radar. Imagery can
be acquired at three different imaging geometries (nadir, narrow and wide swath modes) over
a wide range of incidence angles (five degrees to almost 90 degrees). In addition to being a
fully calibratable system for quantitative measurements, the system has a second antenna
mounted on the aircraft fuselage to allow the C-band system to be operated as an
interferometric radar.
The Sea Ice and Terrain Assessment (STAR)
systems operated by Intera Technologies Limited of
Calgary, Alberta, Canada, (later Intermap
Technologies ) were among the first SAR systems
used commercially around the world. Both STAR-1
and STAR-2 operate at X-band (3.2 cm) with HH
polarization in two different resolution modes. The
swath coverage varies from 19 to 50 km, and the
resolution from 5 to 18 m. They were primarily
designed for monitoring sea ice (one of the key
applications for radar, in Canada) and for terrain
analysis. Radar's all-weather, day or night imaging capabilities are well-suited to monitoring
ice in Canada's northern and coastal waters. STAR-1 was also the first SAR system to use
on-board data processing and to offer real-time downlinking of data to surface stations.

Page 125 Section 3.10 Airborne and Spaceborne Radar Systems

The United States National Aeronautics
and Space Administration (NASA) has
been at the forefront of multi-frequency,
multi-polarization synthetic aperture radar
research for many years. The Jet
Propulsion Laboratory (JPL) in California
has operated various advanced systems
on contract for NASA. The AirSAR
system is a C-, L-, and P-band advanced polarimetric SAR which can collect data for each of
these bands at all possible combinations of horizontal and vertical transmit and receive
polarizations (i.e. HH, HV, VH, and VV). Data from the AirSAR system can be fully calibrated
to allow extraction of quantitative measurements of radar backscatter. Spatial resolution of the
AirSAR system is on the order of 12 metres in both range and azimuth. Incidence angle
ranges from zero degrees at nadir to about 70 degrees at the far range. This capability to
collect multi-frequency, multi-polarization data over such a diverse range of incidence angles
allows a wide variety of specialized research experiments to be carried out.
With the advances and success of airborne imaging radar, satellite radars
were the next logical step to complement the optical satellite sensors in
operation. SEASAT, launched in 1978, was the first civilian remote sensing
satellite to carry a spaceborne SAR sensor. The SAR operated at L-band
(23.5 cm) with HH polarization. The viewing geometry was fixed between
nine and 15 degrees with a swath width of 100 km and a spatial resolution of
25 metres. This steep viewing geometry was designed primarily for
observations of ocean and sea ice, but a great deal of imagery was also collected over land
areas. However, the small incidence angles amplified foreshortening and layover effects over
terrain with high relief, limiting its utility in these areas. Although the satellite was only
operational for three months, it demonstrated the wealth of information (and the large volumes
of data!) possible from a spaceborne radar.
With the success of the short-lived SEASAT mission, and
impetus provided from positive results with several airborne
SARs, the European Space Agency (ESA) launched ERS-1
in July of 1991. ERS-1 carried on-board a radar altimeter, an
infrared radiometer and microwave sounder, and a C-band
(5.66 cm), active microwave instrument. This is a flexible
instrument which can be operated as a scatterometer to
measure reflectivity of the ocean surface, as well as ocean
surface wind speed and direction. It can also operate as a
synthetic aperture radar, collecting imagery over a 100 km
swath over an incidence angle range of 20 to 26 degrees, at
a resolution of approximately 30 metres. Polarization is
vertical transmit and vertical receive (VV) which, combined
with the fairly steep viewing angles, make ERS-1 particularly sensitive to surface roughness.
The revisit period (or repeat cycle) of ERS-1 can be varied by adjusting the orbit, and has
ranged from three to 168 days, depending on the mode of operation. Generally, the repeat
cycle is about 35 days. A second satellite, ERS-2, was launched in April of 1995 and carries
the same active microwave sensor as ERS-1. Designed primarily for ocean monitoring
applications and research, ERS-1 provided the worldwide remote sensing community with the
first wide-spread access to spaceborne SAR data. Imagery from both satellites has been used
in a wide range of applications, over both ocean and land environments. Like SEASAT, the
steep viewing angles limit their utility for some land applications due to geometry effects.
The National Space Development Agency of Japan
(NASDA), launched the JERS-1 satellite in February of
1992. In addition to carrying two optical sensors, JERS-
1 has an L-band (23.5 cm) SAR operating at HH
polarization. The swath width is approximately 75 km
and spatial resolution is approximately 18 metres in
both range and azimuth. The imaging geometry of
JERS-1 is slightly shallower than either SEASAT or the
ERS satellites, with the incidence angle at the middle of
the swath being 35 degrees. Thus, JERS-1 images are
slightly less susceptible to geometry and terrain effects.
The longer L-band wavelength of JERS-1 allows some penetration of the radar energy
through vegetation and other surface types.
Spaceborne SAR remote sensing took a giant leap forward
with the launch of Canada's RADARSAT satellite on Nov.
4, 1995. The RADARSAT project, led by the Canadian
Space Agency (CSA), was built on the development of
remote sensing technologies and applications work carried
out by the Canada Centre for Remote Sensing (CCRS)
since the 1970s. RADARSAT carries an advanced C-band (5.6 cm), HH-polarized SAR with a
steerable radar beam allowing various imaging options over a 500 km range. Imaging
swaths can be varied from 35 to 500 km in width, with resolutions from 10 to 100 metres.
Viewing geometry is also flexible, with incidence angles ranging from less than 20 degrees to
more than 50 degrees. Although the satellite's orbit repeat cycle is 24 days, the flexibility of
the steerable radar beam gives RADARSAT the ability to image regions much more frequently
and to address specific geographic requests for data acquisition. RADARSAT's orbit is
optimized for frequent coverage of mid-latitude to polar regions, and is able to provide daily
images of the entire Arctic region as well as view any part of Canada within three days. Even
at equatorial latitudes, complete coverage can be obtained within six days using the widest
swath of 500 km.

Imaging options over a 500 km range
3. Endnotes

3.11 Endnotes
You have just completed Chapter 3 - Microwave Sensing. You can continue to Chapter 4 -
Image Interpretation and Analysis or first browse the CCRS Web site for other articles related
to microwave remote sensing.
You can get more information about remote sensing radars by checking out an the overview
1

or the more detailed technical specifications
2
of Canada's own microwave satellite:
RADARSAT. You can even see photos
3
of the satellite being built, watch a video of the
launch and see the very first image!
4
As well, learn how microwave remote sensing is used for
various applications such as agriculture
5
, forestry
6
, and geology
7
, and see international
applications
8
of RADARSAT imagery.

Learn about a new way of reducing speckle
9
in a radar image that was developed by
scientists at CCRS in co-operation with scientists in France. As well, learn how a digital
elevation model can be used to correct topographic distortions
10
in radar images or how a
radar image is calibrated using precision transponders
11
and see the mysterious movement of
the transponder
12
when shown in an image!

A training manual
13
has been prepared on how radar data can be used to obtain stereo
images. Interferometry
14
is another fascinating technique studied extensively at CCRS. It has
been tried with both airborne and satellite data and applied to detecting changes in the land
15
,
glacier movement
16
and ocean studies
17
.

Our Remote Sensing Glossary has extensive terminology and explanations of microwave-
related concepts which can be accessed through individual term searches
18
or by selecting
the radar" category.
1
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/radarsat/specs/rsatoview_e.html

2
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/radarsat/specs/radspec_e.html

3
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/radarsat/photos/radpix_e.html

4
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/radarsat/photos/radpix_e.html

5
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/airborne/sarbro/sbagri_e.html

6
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/airborne/sarbro/sbfort_e.html

Page 129 Section 3.11 Endnotes
7
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/airborne/sarbro/sbgeol_e.html

8
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/airborne/sarbro/sbgbsar_e.html

9
http://www.ccrs.nrcan.gc.ca/ccrs/com/rsnewsltr/2303/2303ap1_e.html

10
http://www.ccrs.nrcan.gc.ca/ccrs/com/rsnewsltr/2401/2401ap3_e.html

11
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/radarsat/trans/transpo_e.html

12
http://www.ccrs.nrcan.gc.ca/ccrs/rd/ana/transpond/rpt_e.html

13
http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/stereosc/chap1/chapter1_1_e.html

14
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/airborne/sarbro/sbinter_e.html

15
http://www.ccrs.nrcan.gc.ca/ccrs/com/rsnewsltr/2301/2301rn2_e.html

16
http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/radarsat/images/ant/rant01_e.html

17
http://www.ccrs.nrcan.gc.ca/ccrs/rd/ana/split/insar_e.html

18
http://www.ccrs.nrcan.gc.ca/ccrs/learn/terms/glossary/glossary_e.html

Page 130 Section 3.11 Endnotes
3. Did You Know?

3.1 Did You Know?

'S' band magnetrons are typically used for microwave oven power sources. They operate in
the range of 2-4 GHz. The corresponding wavelengths are 15 cm to 7.5 cm. The screening
mesh used on microwave oven doors is sufficiently fine (much smaller than 7.5 cm) that it
behaves as a continuous, thin, metal sheet, preventing the escape of the radar energy, yet
allowing good visibility of the interior (using visible wavelengths, which are much shorter yet).
Page 131 Section 3 Did you know?
3.2 Did You Know?
"....Just what do those numbers mean?!"

Typical output products (e.g. RADARSAT imagery) have used 8-bit or 16-bit data formats
(digital numbers) for data storage. In order to obtain the original physically meaningful
backscatter values ( sigma nought, beta nought) of calibrated radar products, it is
necessary to reverse the final steps in the SAR processing chain. For RADARSAT imagery,
this must include the squaring of the digital values and the application of a lookup table (which
can have range dependent values). Thus, as you can see, the relationships among the digital
numbers in the imagery are not that simple!
3.4 Did You Know?
"...look to the left, look to the right, stand up, sit down..."
...although a radar's side-looking geometry can
result in several image effects such as
foreshortening, layover, and shadow, this geometry
is exactly what makes radar so useful for terrain
analysis. These effects, if not too severe, actually
enhance the visual appearance of relief and terrain
structure, making radar imagery excellent for
applications such as topographic mapping and
identifying geologic structure.
3.5 Did You Know?
"...rivers in the Sahara desert?...you're crazy!..."

... that an L-band radar (23.5 cm wavelength) imaging from the orbiting space shuttle was
able to discover ancient river channels beneath the Sahara Desert in Northern Africa.
Because of the long wavelength and the extreme dryness of the sand, the radar was able to
penetrate several metres below the desert surface to reveal the old river beds during ancient
times when this area was not so dry.
Page 133 Section 3.Did you know?
3.7 Did You Know?
"...we've picked up an unidentified moving object on the radar, sir..."

... besides being able to determine terrain height using interferometry, it is also possible to
measure the velocity of targets moving towards or away from the radar sensor, using only one
pass over the target. This is done by recording the returns from two antennas mounted on the
platform, separated by a short distance in the along-track or flight direction. The phase
differences between the returns at each antenna are used to derive the speed of motion of
targets in the illuminated scene. Potential applications include determination of sea-ice drift,
ocean currents, and ocean wave parameters.
3.8 Did You Know?
That many other polarizations can be transmitted (or received) if a radar system can transmit
or receive the H and V channels simultaneously. For example, if a radar system transmits an
H and a V signal simultaneously, and the V signal is 90o out of phase with respect to the H
signal, the resulting transmitted wave will have circular polarization.
3. Whiz Quiz and Answers

Page 135 Section 3 Whiz Quiz and Answers
3.2 Answers
1. Much the same as with optical sensors that have different bands or channels of data, multi-
wavelength and multi-frequency radar images can provide complementary information. Radar
data collected at different wavelengths is analogous to the different bands of data in optical
remote sensing. Similarly, the various polarizations may also be considered as different bands
of information. Depending on the wavelength and polarization of the radar energy, it will
interact differently with features on the surface. As with multi-band optical data, we can
combine these different "channels" of data together to produce colour images which may
highlight subtle variations in features as a function of wavelength or polarization.
2. A scatterometer is used to precisely measure the intensity of backscatter reflected from an
object or surface. By accurately characterizing (i.e. measuring) the intensity of energy
reflected from a variety of objects or surface types, these measurements can be used to
generate typical backscatter signatures, similar to the concept of spectral signatures with
optical data. These measurements can be used as references for calibrating imagery from an
imaging radar sensor so that more accurate comparisons can be made of the response
between different features.
3.2 Whiz Quiz
How could we use radar images of different
wavelengths and/or polarizations to extract
more information about a particular scene?
Think back to Chapter 1, the general
characteristics of remote sensing images,
and Chapter 2, interpretation of data from
optical sensors.
Explain how data from a non-imaging
scatterometer could be used to extract more
accurate information from an imaging radar.
3.3 Whiz Quiz

Explain why the use of a synthetic aperture radar (SAR) is the only practical option for radar
remote sensing from space.
3.3 Answer
The high altitudes of spaceborne platforms (i.e. hundreds of kilometres) preclude the use of
real aperture radar (RAR) because the azimuth resolution, which is a function of the range
distance, would be too coarse to be useful. In a spaceborne RAR, the only way to achieve fine
resolution would be to have a very, very narrow beam which would require an extremely long
physical antenna. However, an antenna of several kilometres in length is physically
impossible to build, let alone fly on a spacecraft. Therefore, we need to use synthetic aperture
radar to synthesize a long antenna to achieve fine azimuth resolution.
3.5 Whiz Quiz

If an agricultural area, with crops such as wheat and corn, became flooded, what do you think
these areas might look like on a radar image? Explain the reasons for your answers based on
your knowledge of how radar energy interacts with a target.
3.5 Answer
Generally, image brightness increases with increased moisture content. However, in the case
of flooding, the surface is completely saturated and results in standing water. Areas where the
water has risen above the height of the crops will likely appear dark in tone, as the water acts
as a specular reflector bouncing the energy away from the radar sensor. Flooded areas would
generally be distinguishable by a darker tone from the surrounding agricultural crops which
are not flooded and would scatter more diffusely. However, if the wheat and corn stalks are
not completely submersed, then these areas may actually appear brighter on the image. In
this situation, specular reflections off the water which then bounce and hit the wheat and corn
stalks may act like corner reflectors and return most of the incoming energy back to the radar.
This would result in these areas appearing quite bright on the image. Thus, the degree of
flooding and how much the crops are submersed will impact the appearance of the image.
3.6 Whiz Quiz

Outline the basic steps you might want to perform on a radar image before carrying out any
visual interpretation.
3.6 Answer
Before visually interpreting and analyzing a radar image, there are several procedures which
would be useful to perform, including:
Converting the slant-range image to the ground-range plane display. This will remove
the effects of slant-range scale distortion so that features appear in their proper relative
size across the entire swath and distances on the ground are represented correctly.
Correcting for antenna pattern. This will provide a uniform average brightness of image
tone making visual interpretation and comparison of feature responses at different
ranges easier.
Reducing the effects of speckle to some degree. Unless there is a need for detailed
analysis of very small features (i.e. less than a few pixels in size), speckle reduction will
reduce the "grainy" appearance of the image and make general image interpretation
simpler.
Scaling of the dynamic range in the image to a maximum of 8-bits (256 grey levels).
Because of the limitations of most desktop computer systems, as well as of the human
eye in discriminating brightness levels, any more grey levels would not be useful.
3.8 Whiz Quiz

Can sound waves be polarized?
3.8 Answer
Polarization is a phenomenon which is characteristic
of those waves that vibrate in a direction
perpendicular to their direction of propagation. While
electromagnetic waves vibrate up/down, side to side
and in intermediate directions, sound waves vibrate in
the same direction as their direction of travel, so
cannot be polarized.

3.10 Whiz Quiz

A particular object or feature may not have the same appearance (i.e. backscatter response)
on all radar images, particularly airborne versus spaceborne radars. List some of the factors
which might account for this.
3.10 Answer
The backscatter response, and thus the appearance of an object or feature on a radar image,
is dependent on several things.
Different radar wavelengths or frequencies will result in variations due to their differing
sensitivities to surface roughness, which controls the amount of energy backscattered.
Using different polarizations will also affect how the energy interacts with a target and
the subsequent energy that is reflected back to the radar.
Variations in viewing geometry, including look/incidence angle, the look direction and
orientation of features to the radar, and the local incidence angle at which the radar
energy strikes the surface, play a major role in the amount of energy reflected.
Generally, these differences can be quite significant between airborne and spaceborne
platforms.
Changes in the moisture content of an object or feature will also change the amount of
backscatter.

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