GCE Kannur

1. Microwave Sensing Systems
Uses sensors that operate in the microwave
portion of the EM Spectrum
Microwave Spectrum includes wavelengths within
the approximate range of 1 mm to 1m.
Scanners and radiometers can operate outside
the visible and photographic wavelength regions.
Note: scale is logarithmic.

3. ●
Electromagnetic Spectrum

Visible (VIS) : 0.4 µm to 0.7 µm
(400 to 700
nanometers)

Infrared (IR): 0.7 µm to 14 µm

Microwave : 1 mm to 1 m
In the visible, features are observed by virtue of
reflected solar energy. By contrast, in the infrared
portion, sensing of emitted energy predominates.
The dividing line between sensing reflected and
emitted radiation is the wavelength of ~3 µm.

6. Radar is an acronym for Radio Detection and
Ranging. Radar is an active sensor systems. It
generates its own illumination as an outgoing
signal that interacts with the target such that some
of the signal is returned as backscatter that is
picked up by the same antenna that emitted the
radar beam.
Radar operates in part of the microwave region of
the electromagnetic spectrum, specifically in the
frequency interval from 40,000 to 300 megahertz
(MHz). The latter frequency extends into the higher
frequencies of the broadcast-radio region.
Radar

8. Water vapor, dust, smoke, airborne pollutants and other small
particles close in magnitude to visible and infrared (VIR)
wavelengths cause interference in the path between target
and sensor. VIR wavelengths can be dispersed or blocked
before they reach the sensors. Since microwaves are longer,
they are not as affected by these types of small particle
matter. Therefore, sensors transmitting and/or receiving
microwaves are able to 'see' through haze, cloud, light rain,
snow, smoke, and pollution. As a result, radar images can
yield valuable information that is not available in VIR images.
Atmospheric Penetration

17. categories of Remote
sensing radars
Real aperture and
Synthetic Aperture Radars (SAR).
Side-Looking Airborne Radar (SLAR)
Radar altimeter
Microwave scatterometer

18. Real aperture radars transmit and receive microwave
signals with a fixed length antenna. They are limited in
their ability to produce resolutions fine enough for
most remote sensing applications, simply because it is
difficult to transport a very long antenna.
To solve this problem synthetic aperture radars (SAR)
were developed. SARs have physically shorter
antennas, which simulate or synthesize very long
antennas. This is accomplished through modified data
recording and signal processing techniques.

19. Side-Looking Airborne Radar (SLAR)
A radar pulse is transmitted off-nadir by an antenna fixed
below an aircraft to image large ground areas adjacent to the
flight line. The echoes are processed to produce an
amplitude/time video signal which is then recorded as an
image line, with brighter pixels indicating higher energy
returns. The oblique look angle used to acquire radar
imagery results in characteristic geometric distortions such as
radar shadows and layover effects.

20. The propagation of one radar pulse is shown using solid lines to indicate
the wavefront locations at time intervals 1 to 12. The reflected waves or
echoes are shown by dashed lines beginning at time 7 for the house and 9
for the tree. These return signals reach the antennae at times 13 and 17
respectively.

21. The antennae response graph shows a strong echo for the house at time 13 and
a weaker echo for the tree at time 17. The strength of the echo depends on the
way an object reflects radio waves.

22. Pixel size in SLAR imagery is determined by the time duration
of the pulse and the beamwidth of the SLAR antennae.

23. Radar altimeter
This is a non-imaging radar which detects the EM
backscattering of a surface from a narrow pulse
with near normal incidence. By directing the pulse
onto the Earth's surface from a nadir position
(that is, directly overhead) the distance being
measured is the altitude of the scanning platform
above the surface.

24. Radar altimeter
Over oceans, radar altimeters are used to
determine significant wave height, wind speed
and mesoscale topography. Data from this non-
imaging device can also be plotted in image
format, such as to map sea surface topography,
if the required data volume is available. Other
applications include mapping surface
topography and type of ice masses and sea/ice
boundaries.

26. Microwave scatterometer
This device measures the microwave scattering or reflective
properties of surfaces. It is also non-imaging, and is
specifically designed to measure backscattering. The surface
is scanned in two or more directions, usually by multiple
sensors.
The primary application of microwave scatterometers is to
measure wind vectors (that is, speed and direction) over the
ocean surface. Although radar scatterometers are non-
imaging, their data may be used to construct global wind
maps when collected over a sufficiently large area.

27. Scatterometer
A scatterometer is a
microwave radar sensor
used to measure the
reflection or scattering
effect produced while
scanning the surface of the
earth from an aircraft or a
satellite.

30. Two Generic Types of Sensors
• A "passive" system generally consists of an array of
small sensors or detectors which record the amount
of electro-magnetic radiation reflected and/or
emitted from the Earth's surface.
• An "active" system propagates its own electro-
magnetic radiation and measures the intensity of the
return signal.

31. Examples of Passive Sensors
• Advanced Very High Resolution Radiometer
(AVHRR) Sea Surface Temperature
• Sea-viewing Wide Field-of-View Sensor
(SeaWiFS) Ocean Color

32. Why Sea-Surface-Temperature?
• Sea surface temperature (SST) is a key
parameter for energy transfer between the
oceans and the atmosphere and AVHRR-
derived SST can deliver important data to
meteorological and climatological models.
• AVHRR imagery is also useful for meteorological
applications (for example, providing detailed
imagery of cloud systems), and land applications
such as vegetation mapping and monitoring.

33. Advantages of passive microwave remoteAdvantages of passive microwave remote
sensingsensing
• Sees through clouds at lower frequencies
• Long heritage, various instruments since 1978
• Emissivity sensitive to state of surface, particularly
moisture
– Soil moisture
– Snow-water equivalent
– (water is ~80× as absorptive as ice at these frequencies,
whereas in visible through infrared, water and ice have
similar absorption coefficients)
• But, because of small amount of energy emitted, pixel
size must be large

34. Microwave Remote Sensing: Principles
and Applications.
• Advantages
– Day/night coverage.
– All weather except
during periods of
heavy rain.
– Complementary
information to that in
optical and IR regions.
• Disadvantages
– Data are difficult to
interpret.
– Coarse resolution
except for SAR.

35. Infared Satellite Imagery
• These images were obtained with an AVHRR sensor
(Advanced Very High Resolution Radiometer) carried
on a NOAA Polar Orbiting Environmental Satellite
(POES).
• Measures the amount of thermal infared radiation
given off by the surface of the ocean.
• The amount of thermal infared radiation given off by
an object is related to its temperature (dying embers
give off less radiation than a hot fire and a person
gives off even less).
• Thus by measuring the amount of radiation given off
by the ocean we can calculate its temperature.

36. Infared Satellite Imagery
• These images were obtained with an AVHRR sensor
(Advanced Very High Resolution Radiometer) carried
on a NOAA Polar Orbiting Environmental Satellite
(POES).
• Measures the amount of thermal infared radiation
given off by the surface of the ocean.
• The amount of thermal infared radiation given off by
an object is related to its temperature (dying embers
give off less radiation than a hot fire and a person
gives off even less).
• Thus by measuring the amount of radiation given off
by the ocean we can calculate its temperature.