Basic Concepts, Explanation, and Application. Fundamental Remote Sensing; Advantage/ disadvantages, Imaging/non Imaging sensors, RAR and SAR, SAR Geometry, Resolutions in the microwave, Geometric Distortions in SAR, Polarization in SAR, Target Interaction, SAR Interferometry

1. MICROWAVE REMOTE SENSING
CONCEPTS, EXPLANATION AND
APPLICATION
-BY
SOUMIK CHAKRABORTY
(M.TECH GIS)
NU, NEEMRANA

2. TABLE OF CONTENTS:
• Fundamentals of Remote Sensing and Introduction of Microwave
• Advantage & Disadvantages of Microwave Sensor
• Active & Passive Microwave Remote sensing
• Microwave Sensors & Equipments
• Imaging & Non Imaging Sensors
• Radar Geometry
• SAR (Synthetic Aperture Radar)
• SAR PROPERTIES (Azimuthal & Range resolution)
• SAR Geometric Distortions
• SAR POLARIMETRY
• Modes of Data Acquisition
• Target Interaction & Types
• SAR Application (INTERFEROMETRY)

3. FUNDAMENTALS OF REMOTE SENSING:
• Remote sensing is a set of multidisciplinary techniques and methodologies
that aim at obtaining information about the environment through “remote”
measurements.
• Introduction of basic remote sensing concept begins with the understanding of
electromagnetic spectrum in detail.
• Electromagnetic radiation consists of an electrical field(e) which varies in
magnitude in a direction perpendicular to the direction in which the radiation
is traveling, and a magnetic field (m) oriented at right angles to the electrical
field. Both these fields travel at the speed of light (c)

4. • Fig.1:- explaining electromagnetic spectrum with microwave’s location within it.
• Fig.02:- Classification of microwave region in electro magnetic spectrum based on
wavelength and frequency and arrangement in increasing order of wavelength.

5. ADVANTAGES & DISADVANTAGES OF
MICROWAVE SENSOR
• Advantages compared to optical remote sensing
• All weather capability (small sensitivity of clouds, light rain)
• Day and night operation (independence of sun illumination)
• No effects of atmospheric constituents (multi-temporal
analysis)
• Sensitivity to dielectric properties (water content, biomass, ice)
• Sensitivity to surface roughness (ocean wind speed)
• Accurate measurements of distance (interferometry)
• Sensitivity to man-made objects
• Sensitivity to target structure (use of polarimetry)
• Subsurface penetration
• DISADVANTAGES
• Complex interactions (difficulty in understanding,
complex processing)
• Speckle effects (difficulty in visual interpretation)
• Topographic effectseffect of surface roughness

6. MICROWAVE SENSORS & EQUIPMENTS:
• A microwave radiometer is a passive device which records the natural microwave
emission from the earth. It can be used to measure the total water content of the
atmosphere within its field of view.
• A radar altimeter sends out pulses of microwave signals and record the signal
scattered back from the earth surface. The height of the surface can be measured
from the time delay of the return signals.
• A wind scatterometer can be used to measure wind speed and direction over the
ocean surface. Sends out pulses of microwaves along several directions and records
the magnitude of the signals backscattered from the ocean surface.

7. • Microwave
radiometer
• Scatterometer
• Radar Altimeter

8. ACTIVE & PASSIVE MICROWAVE REMOTE
SENSING:
• Active systems are characterized by the
presence of their own source
(transmitter) that “lights up” the
observed scene and, therefore, can be
used both at night and day,
independently of the presence of sun.
The sensor transmits a (radio) signal in
the microwave bandwidth and records
the part that is backscattered by the
target towards the sensor itself. The
power of the backscattered signal allows
to discriminate between different targets
within the scene, while the time between
the sent and the received signal is used
to measure the distance of the target.
• Passive systems collect the
radiation that is naturally emitted
by the observed surface. In fact,
objects emit energy at the
microwave frequencies, although
sometimes in an extremely small
amount. These systems are
generally characterized by
relatively low spatial resolutions.

9. • Description of passive and active microwave remote sensing respectively.

10. IMAGING & NON- IMAGING SENSORS
• The most common form of imaging
active microwave sensors is RADAR.
RADAR is an acronym for radio detection
and ranging. The sensor transmits a
microwave (radio) signal towards the
target and detects the backscattered
portion of the signal. By measuring the
time difference between the transmission
of pulse and reception of the
backscattered echo from different
targets, their distance from radar and
thus location is determined.
• 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
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.

11. IMAGING SYSTEMS: RAR (REAL APERTURE
RADAR)
• Aperture means the opening used to
collect the reflected energy that is
used to form an image. In the case of
radar imaging this is the antenna. For
RAR systems, only the amplitude of
each echo return is measured and
processed. The spatial resolution of
RAR is primarily determined by the
size of the antenna used. The larger
the antenna, the better the spatial
resolution. Other determining factors
include the pulse duration (Ԏ) and the
antenna beamwidth.

12. RADAR GEOMETRY
• The incidence angle is the angle
between the radar pulse of EMR and
a line perpendicular to the earth’s
surface where it makes contact.
When the terrain is flat, the
incidence angle is the complement (
90 - γ) of the depression angle (γ).
• If the terrain is sloped, there is no
relationship between depression
angle and incident angle. The
incidence angle best describes the
relationship between the radar beam
and surface slope.

14. SAR (SYNTHETIC APERTURE RADAR)
• The most commonly used microwave
imaging sensor is the synthetic aperture
radar (SAR) that is a radar system
capable of providing high-resolution
microwave images.
• Microwave pulses are transmitted by an
antenna towards the earth surface. The
microwave energy scattered back to the
spacecraft is measured.
• The SAR makes use of the radar
principle to form an image by utilizing
the time delay of the backscattered
signals.

15. SAR PROPERTIES:
• Determined specifically by amplitude & phase information. Amplitude is
determined by:- physical characteristics of surface features- (surface
roughness, geometric structure, orientation)
• Electrical characteristics of surface features- (dielectric constant, moisture
content, conductivity) radar frequency of the sensor
• Phase- is the fraction of one complete sine wave cycle (a single SAR
wavelength). Phase of the SAR image is determined primarily by the distance
between the satellite antenna and the ground targets.

16. SAR GEOMETRY:
• C = speed of light, L = antenna
length, R = distance antenna-object,
λ = wavelength
• The beam width (β) is a function of the
wavelength λ and the antenna aperture d,
where d = 2l (l = length of radar antenna).
• Beam width will be narrow if large antenna
dimension increases. There are three major
types of antenna: horn antenna, parabolic
antenna and array antenna.

17. AZIMUTHAL RESOLUTION:
• The ability of the sensor to distinguish
between two closely spaced objects in
direction parallel to the motion vector of the
sensor.
• Azimuthal resolution is given by the product of
effective horizontal beam width and slant
range distance to target. Horizontal beam
width is given by this following formula where
L = antenna length, λ = wavelength.
• So the azimuthal resolution is given by as
follows where r = slant range of distance to
target.

18. RANGE RESOLUTION:
• Ability to distinguish between two closely
spaced elements perpendicular to motion
vector of sensor having echoes received at
separate timing. Since range resolution is
heavily dependent upon pulse width.
• The range resolution of a pulsed radar
system is limited by the bandwidth of the
transmitted pulse.
• However, the shorter the pulse, the lower
the transmitted energy and the poorer the
radiometric resolution.
• To preserve the radiometric resolution,
SAR systems generate a long pulse with
a linear frequency modulation or
Compressed High Intensity Radar Pulse
(CHIRP). After the received signal has
been compressed, the range resolution
is optimized without loss of radiometric
resolution.

19. SAR GEOMETRIC DISTORTIONS:
FORESHOTENING:
• 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. The slope (A to B)
will appear compressed and the length of the
slope will be represented incorrectly (A' to B').
All terrain that has a slope inclined toward
the radar will appear compressed or
foreshortened relative to slopes inclined away
from the radar. The foreshortened slopes
appear as bright features on the image.

20. • Foreshortening Image example

21. LAYOVER:
• 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 is just an extreme case of
Foreshortening. Layover displacement is
greatest at short range, where the look
angle is smaller

22. 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. Both
foreshortening and
layover result in radar
shadow. This results in
dark pixel values.

23. SAR POLARIMETRY:
• Un-polarized energy vibrates in all
possible directions perpendicular to
the direction of travel.
• Radar antennas send and receive
polarized energy. This means that
the pulse of energy is filtered so
that its electrical wave vibrations
are only in a single plane that is
perpendicular to the direction of
travel.
• The pulse of electromagnetic
energy sent out by the antenna may
be vertically or horizontally
polarized.

24. TYPES OF POSSIBLE POLARIZATION SYSTEMS
• Single Polarimetric has only one linearly polarized channel (HH or HV or VH or
VV)
• Dual Polarimetric refers to combination of two polarizations (HH+HV or HH+VV
or VH+VV or VH+HH).
• Quad-Polarimetric All four polarizations are available (HH+HV+VH+VV)
• Hybrid Polarimetric is circular transmission of EM pulses and receiving them
back linearly (RISAT-1) (RH, RV).

26. SAR MODES OF DATA ACQUISITION:
• SAR data is collected in different
beam modes such as Fine Beam
Mode (Stripmap Mode), ScanSAR
(Medium Resolution, Coarse
Resolution), Extended High and
Spotlight Mode (Sliding Spotlight,
Starring Spotlight).
• This is achieved through beam
Stirring Capacity and Changing
Angle of Incidence / Off-Nadir
Angle. Resolution increases from
Beam mode to ScanSar and
Spotlight mode respectively
whereas Swath area decreases in
that order.
Fig.07 – describing different modes resolution

27. MICROWAVE TARGET INTERACTIONS:
• The ability of microwave to penetrate clouds, precipitation, or land surface cover
depends on its frequency. Generally, the penetration power increases for longer
wavelength (lower frequency).
• The SAR backscattered intensity generally increases with the surface
roughness. However, "roughness" is a relative quantity. The land surface
appears smooth to a long wavelength radar. Little radiation is backscattered
from the surface.
• The same land surface appears rough to a short wavelength radar. The surface
appears bright in the radar image due to increased backscattering from the
surface

28. SPECULAR REFLECTION:
• Occurs in very smooth surfaces, when height of feature is less than wavelength
of electromagnetic radiation.

29. MICROWAVE SCATTERING TYPES:
• Most surfaces are not specular in nature.
These are called diffuse reflectors or
scatterers. The roughness of the surface w.r.t
wavelength governs the scattering pattern.
• The dielectric constant of the medium governs
the strength of the backscatter. If dielectricity
of a substance is greater than other substance
signifying it is more wet; hence will have more
amount of backscatter.

30. WAVELENGTH DEPENDENT INTERACTION
• Both the ERS and RADARSAT SARs use the C band microwave while the JERS SAR
uses the L band. The C band is useful for imaging ocean and ice features.
However, it also finds numerous land applications.
• The short wavelength radar interacts mainly with the top layer of the forest
canopy while the longer wavelength radar is able to penetrate deeper into the
canopy to undergo multiple scattering between the canopy, trunks and soil.
• The L band has a longer wavelength and is more penetrating than the C band.
Hence, it is more useful in forest and vegetation study as it is able to penetrate
deeper into the vegetation canopy.

32. SAR INTERFEROMETRY
• Earthquakes, landslides, volcanic eruptions, or,
more generally, deformation phenomena of the
earth's surface, can be monitored through the use
of Synthetic Aperture Radar (SAR) sensors.
• Interferometric SAR (InSAR) is based upon
utilization of the phase difference between two
complex radar SAR observations of the same area.
• Two perform interferometry SAR images are taken
from slightly different sensor position and then
the phase of the two images are combined after
coregistration.
• Interferogram can be generated where phase is
highly correlated to the terrain topography and
deformation patterns can be mapped. Selection of
Image for Interferometry should have two or More
SLC Images having same Mode, Track, Pass and
most importantly same Polarization.

33. • When images are acquired in different times (temporal baseline), using the
Differential SAR Interferometry (DInSAR) technique, it is possible to measure the
changes of the surface. The SAR is able to revisit the same area at regular
intervals, providing information at very high spatial resolution of the observed
scene.
• In the case of ERS 1 / 2 and Envisat European Space Agency (ESA), active since
1992 has set the time to review every 35 days, while for the new generation
sensors such as the constellation Cosmo Sky-Med, the interval was reduced to 8
days. These measures are shown by a series of colored bands, the so-called
fringes or interferogram.
• The interferometric techniques produce not only the maps of ground deformation
measured along the line of sight of the sensor, but taking advantage of a series
of images (instead of only two) acquired over time, allow us to follow the itself
temporal evolution of deformation.
• For example, the measurement of ground deformation in volcanic areas is
extremely important because these are often precursors of eruptions, or however
indicate an increase of volcanic activity.

34. • Fig.11- Flowchart explaining SAR interferometry calculation and analysis

35. • Fig.12:- showing ultimate result in the form of interferrogram detecting shift in position
indicating volcanic activity.

36. THANK YOU!