Radar Imaging
GISC 6325 / GEOS 5325
Dr. Stuart Murchison
Non-Imaging Radar
• Police radar, which detects the speed
of passing vehicles and displays that
speed on a screen, operates on the
principle of the Doppler effect. The
Doppler effect is the change in
frequency (number per unit time) of
sound or light waves emitted from a
moving source. The Doppler effect
Radar (an acronym for "radio
was explained by Austrian physicist detection and ranging") is a device
Christian Doppler (1803-1853) in that emits and receives radio waves.
1842. According to Doppler, waves The waves bounce off the targeted
bunch up as they approach their vehicle and are received by a
target and spread out as they move recorder. The recorder compares the
difference between the sent and
away from their target. received waves, and translates the
information into miles per hour.
Non-Imaging Radar
To provide a polar-coordinate map-
like display of targets, NRL originated
the radar PLAN-POSITION INDICATOR
(PPI)-the well-known radar scope
with the round face and the
sweeping hand-between 1939 and
1940. The PPI is now universally used
by military and commercial interests
around the world for the display of
radar information for such functions
as air and surface detection,
navigation, air traffic control, air
intercept, and object identification
Imaging Radar - General
• RADAR = RAdio Detection and Ranging
• Uses the microwave region of the
electromagnetic spectrum.
• Wavelengths used in imaging radar range
between 1 mm and 1 m
• Longer wavelengths are used for
communication and navigation.
Microwave Region
Radar Bands
Wavelength Range and Descriptions
• 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
– on many airborne research systems (CCRS Convair-580 and NASA AirSAR) and
spaceborne systems (including ERS-1 and 2 and RADARSAT).
• 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
Radar
Bands
Imaging Radar - Advantages
• Active system (works day or night).
– There is also passive microwave imaging (radiometer)
mode. This senses surface radio-emission, which can be
converted to radiant temperatures.
• Not affected by cloud cover or haze if l > 2 cm. It operates
independent of weather conditions. Water clouds have a
significant effect on radar with wavelength l 4 cm.
• Can penetrate well-sorted dry sand in hyper-arid regions to a
depth of about 2 m.
Imaging Radar - Operation
• Transmitting antenna sends an electro-
magnetic signal.
• Target reflects some energy back to
source.
• Receiving antenna receives reflected
signal
• Signal is processed and image produced.
Terminology
• RAR: Real Aperture Radar
• SAR: Synthetic Aperture Radar
• SLAR: Side-looking airborne radar (could be RAR
or SAR).
• SIR: Shuttle imaging radar (a SAR)
– 3 missions: SIR-A (1981), SIR-B (1984) and SIR-
C (1994)
• INSAR: Interfereometric SAR. Can be satellite or
airborne.
• SRTM: Shuttle Radar Topography Mission (an
INSAR mapping mission)
How Radar Works
Microwave energy pulses (A) are
emitted at regular intervals and focused
by the antenna into a radar beam (B)
directed downwards and to the side.
The radar beam illuminates the surface
obliquely at a right angle to the motion
of the platform. Objects on the ground
reflect the microwave energy
depending on factors such as roughness
and attitude. The antenna receives this
reflected (or backscattered) energy (C).
How Radar Works
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.
Radar Geometry
• In airborne and spaceborne radar imaging 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.
Near Range is the portion of the image swath closest to the nadir track
Far Range is the portion of the swath farthest from the nadir track.
Depression or Grazing Angle is the angle between the horizontal and a radar ray
path.
Slant Range Distance is the radial line of sight distance between the radar and
each target on the surface.
Ground Range Distance is the true horizontal distance along the ground
corresponding to each point measured in slant range.
Incidence Angle is the angle between the radar beam and ground surface
Look Angle is the angle at which the radar "looks" at the surface, or the angle
between vertical and a ray path
Radar Image Geometry - Shadow
Radar Image Geometry - Shadow
Radar Image Geometry - Shadow
Shadow is more of a problem at far
range
Radar Image Geometry - Layover
Radar Image Geometry - Layover
Layover occurs when the radar beam reaches
the top of a tall feature before it reaches the
base. The top of the feature is displaced
towards the radar sensor and is displaced from
its true ground position - it 'lays over' the base.
The visual effect on the image is similar to that
of foreshortening.
Foreshortening
• Even if there is no layover, radar returns from facing steep
slopes will make the terrain look steeper than it is. This is
known as ‘foreshortening’. Features which show layover in
the near range will show foreshortening in the far range.
Foreshortening occurs because radar measure distance in the slant-
range direction such that the slope A-B appears as compressed in the
image (A'B') and slope C-D is severely compressed (C'D')
Radar Image
Geometry -
Shadow &
Foreshortening
Mt. Shasta, California 4,317 meters
(14,161 feet)
Stratovolcano
Target Interaction and Image
Signatures
• Unlike aerial photographs and satellite images which are
passive remote sensing systems, in active systems such as
radar, the brightness or darkness of the image is dependent
on the portion of the transmitted energy that is returned
back to the radar from targets on the surface. Bright areas
are produce by strong radar response and darker areas are
from weak radar responses.
• The response to radar energy by the target is primarily
dependent on three factors:
– Surface roughness of the target
– Radar viewing and surface geometry relationship
– Moisture content and electrical properties of the target
Surface Roughness
• Specular Reflection (A) is caused by a smooth surface where the incident energy is
reflected and not backscattered. This results in smooth surfaces appearing as darker
toned areas on an image.
• Diffuse Reflection (B) is caused by a rough surface which scatters the energy equally in
all directions. A significant portion of the energy will be backscattered to the radar, such
that a rough surface will appear lighter in tone on an image.
• Corner Reflection (C) occurs when the target object reflect most of the energy directly
back to the antenna resulting in a very bright appearance to the object. This occurs
where there are buildings, metallic structures (urban environments) and cliff faces,
folded rock (natural environments).
Radar viewing and surface geometry
relationship
Different vegetation types (e.g., desert, grasslands, forests or frozen tundra) will all have different backscatter
properties. In addition, the basic reflectivity of the soil, called the "dielectric constant" will change depending on
the amount of water that the soil contains. Dry soil has a low dielectric constant, so that little radar energy will
be reflected. Saturated soil will have the opposite effect, and will be a strong reflector. Moist and partially frozen
soils will have intermediate values.
Moisture content and electrical
properties of the target
By affecting the absorption and propagation of electromagnetic
waves, dielectric constant strongly influence the interaction of
electromagnetic radiation with the terrain surface.
Most common materials have dielectric constants 1-100
Dielectric constant is controlled by the amount of moisture content,
hence, the return of radar signal is influenced by the amount of moisture
in the soil and vegetation.
Dielectric
Material
constant
Vacuum 1 (by definition)
Increasing the moisture content reduces the
Air 1.00054
penetration of the radar signal beneath the soil and
Paper 3.5
vegetation canopy.
Pyrex glass 4.7
Water (20°) 80.4
Radar Speckle
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.
Both multi-look processing and spatial
filtering reduce speckle at the expense of
Speckle reduction can be achieved in two ways: resolution, since they both essentially
multi-look processing smooth the image. Therefore, the amount of
speckle reduction desired must be balanced
spatial filtering. with the particular application the image is
being used for, and the amount of detail
required.
Airborne versus Spaceborne Radar
Spaceborne radar does not require
a wide range of incidence angles to
cover a wide swath.
Airborne radar must image over
a wide range of incidence angles
in order to cover a wide swath.
Polarization
The photograph on the right was taken
through polarizing sunglasses and through
the rear window of a car. Light from the sky
is reflected by the windshield of the other
car at an angle, making it mostly horizontally
polarized. The rear window is made of
tempered glass. Stress in the glass, left from
its heat treatment, causes it to alter the
polarization of light passing through it, like a
wave plate. Without this effect, the
sunglasses would block the horizontally
polarized light reflected from the other car's
window. The stress in the rear window,
however, changes some of the horizontally
polarized light into vertically polarized light
that can pass through the glasses. As a
result, the regular pattern of the heat
treatment becomes visible.
Radar Signal Polarization
Polarization of the radar signal
is the orientation of the the
electromagnetic field and is a
factor in the way in which the
radar signal interacts with
ground objects and the
resulting energy reflected back.
Most radar imaging sensors
are designed to transmit
microwave radiation either
horizontally polarized (H) or
vertically polarized (V), and
receive either the horizontally
or vertically polarized
backscattered energy.
Radar Signal Polarization
• Polarizing Radar has four possible 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, (cross-polarized)
– VH - for vertical transmit and horizontal receive
(cross-polarized).
Radar Polarization Example
DEM’s and Radar Interfereometry
DEM’s and Radar Interfereometry
• Radar interferometry
uses synthetic
aperture radar
mapping satellites to
form detailed images
of geological surfaces.
This powerful
technique can reveal
centimeter-sized
changes in the Earth's
crust due to natural
phenomena.
DEM’s and Radar
Interfereometry
• Interferometric synthetic aperture radar,
also abbreviated InSAR or IfSAR, is a radar
technique used in geodesy and remote
sensing. This geodetic method uses two
or more synthetic aperture radar (SAR)
images to generate maps of surface
deformation or digital elevation, using
differences in the phase of the waves
returning to the satellite. The technique
can potentially measure centimeter-scale
changes in deformation over time spans
of days to years. It has applications for
geophysical monitoring of natural
hazards, for example earthquakes,
volcanoes and landslides, and also in
structural engineering, in particular
monitoring of subsidence and structural
stability.
Sandia Labs RTV
Rapid Terrain Visualization
This radar provides the ability to generate highly accurate map
products in real-time, including digital elevation models (DEMs),
orthorectified SAR images, as well as a measure of the data
quality.
Dome of metamorphic rocks in the Sahara desert
(Sudan)
Landsat SIR-C radar
SIR-C Image of
Vesuvius and
Naples, Italy
• Mt. Vesuvius, one of the best known
volcanoes in the world primarily for the
eruption that buried the Roman city of
Pompeii in AD 79, is shown in the
center of this radar image. The central
cone of Vesuvius is the dark purple
feature in the center of the volcano.
This cone is surrounded on the northern
and eastern sides by the old crater rim,
called Mt. Somma. Recent lava flows
are the pale yellow areas on the
southern and western sides of the cone.
It shows an area 100 kilometers by 55
kilometers (62 miles by 34 miles.)
• The top image is a SIR-C image of Nile
photograph taken with color
infrared film from Space
Shuttle Columbia in
Paleochannel, Sudan
November 1995. The radar
image at the bottom is a SIR-
C/X-SAR image. The thick,
white band in the top right of
the radar image is an ancient
channel of the Nile that is
now buried under layers of
sand. This channel cannot be
seen in the photograph and its
existence was not known
before this radar image was
processed. The area to the left
in both images shows how
the Nile is forced to flow
through a chaotic set of
fractures that causes the river
to break up into smaller
channels, suggesting that the
Nile has only recently
established this course. Each
image is about 50 kilometers
by 19 kilometers. Red = Chv;
Green = Lhv; Blue = Lhh
SIR-C/X-SAR image of the
Mississippi River
• This image of the Mississippi River in Mississippi,
Arkansas, and Louisiana shows regions that are
prone to flooding. The image covers an area of
about 23 km by 40 km. Red = Lvv; Green = Lvh;
Blue = Cvv. This site along the Mississippi River
lies north of Vicksburg along the Arkansas-
Louisiana-Mississippi state borders. This region is
characterized by rich farmland. The town in the
extreme upper left is Eudora, Arkansas. The long,
narrow lakes which parallel the river are called
oxbow lakes, named for the U-shaped harness
worn by an ox. Oxbows form when a river changes
course, abandoning old channels in favor of a new
course. As the river changes course, the
surrounding land dries out, leaving these lakes
isolated. Oxbow lakes are common in areas where
rivers flow through generally flat terrain, allowing
the river to easily change course. The green
regions bordering the river are undeveloped
forested areas.
• A damaged oil
tanker off the Nov. 2002 Oil spill
northwest coast of
Spain split in half
on November 19,
in Spain
2002, creating a
series of large oil
slicks. The image
shows the oil slick
with RADARSAT
data. Black areas
indicate the
location of the
slick on November
18. The land is
shown using
Landsat falsecolor
Radarsat Mosaic of USA (190 images)
Radar Imaging of Cities
(San Francisco)
• This SIR-C/X-SAR image of San Francisco,
California shows how the radar distinguishes
between densely populated urban areas and
nearby areas that are relatively unsettled.
Downtown San Francisco is at the center and
the city of Oakland is at the right across the
San Francisco Bay. Some city areas, such as
the South of Market appear bright red due to
the alignment of streets and buildings to the
incoming radar beam. Various bridges in the
area are also visible. All the dark areas on the
image are water. Two major faults are
visible. The San Andreas fault, on the San
Francisco peninsula, is seen in the lower left
of the image. The fault trace is the straight
feature filled with linear reservoirs which
appear dark. The Hayward fault is the
straight feature on the right side between the
urban areas and the hillier terrain to the east.
The image is about 42x58 km.
Archeology of Angor,
Cambodia
• The city houses an ancient complex of more than
60 temples dating to the 9th to 15th centuries.
Today the Angkor complex is hidden beneath a
dense rainforest canopy, making it difficult for
researchers on the ground. The principal complex,
Angkor Wat, is the bright square just left of the
center of the image. It is surrounded by a
reservoir that appears in this image as a thick
black line. The larger bright square above Angkor
Wat is another temple complex called Angkor
Thom. Archeologists studying this image believe
the blue-purple area slightly north of Angkor
Thom may be previously undiscovered structures.
In the lower right is a bright rectangle surrounded
by a dark reservoir, which houses the temple
complex Chau Srei Vibol. Image is 55x85km.
Red=Lhh, Green =Lhv, and Blue =Chv.
C-Band
image of
Dallas
35 km (21 miles)
by 26 km (16
miles)
SIR-C/X-SAR image of Mississippi Delta
• The area shown is approximately 63 km
by 43 km. As the river enters the Gulf of
Mexico, it dumps its load of sediment,
building up the delta front. As one part of
the delta becomes clogged with sediment,
the delta front will migrate in search of
new areas to grow. The area shown on
this image is the currently active delta
front of the Mississippi. Most of the land
in the image consists of mud flats and
marsh lands. There is little human
settlement in this area due to the
instability of the sediments. The main
shipping channel of the Mississippi River
is the broad red stripe running northwest
to southeast down the left side of the
image. The bright spots within the
channel are ships. Red = Lvv; Green =
Cvv; Blue = Xvv.
USSR Landers Indicate Venus has a Rocky
Surface
Venera 9, 1975
Venera 10, 1975
Venera 13, 1982
• 98% of the Venutian
surface was mapped.
• Magellan finally
burned up in the
venusian
atmosphere, in mid-
October, 1994.
• Image at right: Blues
represent the lowest
surfaces followed by
greens, then yellows
and oranges with red
being highest.
• High region, near the
equatorial center, is
Aphrodite Terra.
Beta Regio, near the
central left, is also
elevated.