Microwave remote sensing uses both passive and active sensors operating within the wavelength range of 1mm to 1m. Passive sensors such as microwave radiometers record naturally emitted energy, while active sensors like synthetic aperture radar (SAR) generate their own electromagnetic signals. SAR is an example of side-looking radar that uses signal processing to synthesize a very long antenna and improve azimuth resolution. Radar imagery exhibits characteristics like penetration of vegetation and clouds, day/night imaging, and sensitivity to surface properties. However, it also shows distortions from terrain relief and speckle noise from signal interference.

1. MICROWAVE REMOTE SENSING
WAVELENGTH RANGE-1mm to 1m
Presented By:
ROHIT KUMAR
CUJ/I/2013/IGIO/026
5th Semester

2. • Passive and Active Microwave Sensors
• Passive Passive remote sensing systems record electromagnetic energy that is
reflected or emitted from the surface of the Earth
• Sensors Microwave radiometers
• Active Active remote sensors create their own electromagnetic energy
• Sensors Altimeters
• Side-looking real aperture radar
• Scatterometer (SCAT)
• Synthetic Aperture Radar (SAR)

3. Altimeters
 Nadir-looking active microwave
sensors
Measurement of height above
ground

4. Microwave Radiometers
• Typically measure the brightness temperature at vertical and
horizontal polarisation at different frequencies
• Signal is very low, so long integration times are chosen to
improve signal to noise ration (SNR)
• Resolution ~ 10-50 km

5. CryoSat
 Launch in spring 2010
 CryoSat will measures the thickness of
the polar ice sheets and floating sea
ice with a radar altimeter called SIRAL
(Synthetic Aperture Radar
Interferometry Radar Altimeter)

6. Side-Looking Airborne Radar

7. •Scatterometers
• Scatterometers are side-looking real aperture radars
designed to achieve a high radiometric accuracy
• retrieve wind fields over the oceans > several look directions
during one overpass

8. Synthetic Aperture Radar (SAR)
To improve the azimuth resolution, a very long
antenna is synthesized electronically. Many pulses
are sent towards the object.
Due to the motion of the platform the frequency of the
echoes is Doppler shifted.

9. Characteristics of radar remote
sensing
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)
 sensitivity to man made objects
 sensitivity to target structure (use of polarimetry)
 subsurface penetration

10. Characteristics of radar remote sensing
Inconvenients
 Complex interactions (difficulty in understanding, complex processing)
 Speckle effects (difficulty in visual interpretation)
 topograhic effects
 effect of surface roughness

11. Penetration Depth
The penetration of microwaves into vegetation, soil and snow generally increases with wavelength
Response of a pine forest in X-, C- and L-band

12. RADAR Wavelengths and Frequencies
used in Active Microwave Remote
Sensing Investigations
Band Designations
(common wavelengths Wavelength Frequency
shown in parentheses) in cm in GHz
_______________________________________________
Frequency band
Ka
K
Ku
X
C
S
L
P
Wavelength (cm)
0.8-1.1
1.1-1.7
1.7-2.4
2.4-3.8
3.8-7.5
7.5-15
15 -30
30 -100

13. Primary Advantages of RADAR
Remote Sensing of the
Environment
• Active microwave energy penetrates clouds and can be an
all-weather remote sensing system.
• Synoptic views of large areas, for mapping at 1:25,000 to
1:400,000; cloud-shrouded countries may be imaged.
• Coverage can be obtained at user-specified times, even at
night.
• Permits imaging at shallow look angles, resulting in different
perspectives that cannot always be obtained using aerial
photography.
• Senses in wavelengths outside the visible and infrared regions
of the electromagnetic spectrum, providing information on
surface roughness, dielectric properties, and moisture
content.

14. Secondary Advantages of RADAR
Remote Sensing of the Environment
 May penetrate vegetation, sand, and surface layers of snow.
• Has its own illumination, and the angle of illumination can be controlled.
• Enables resolution to be independent of distance to the object, with the
size of a resolution cell being as small as 1 x 1 m.
• Images can be produced from different types of polarized energy (HH,
HV, VV, VH).
• May operate simultaneously in several wavelengths (frequencies) and
thus has multi-frequency potential.
• Can measure ocean wave properties, even from orbital altitudes.
• Can produce overlapping images suitable for stereoscopic viewing and
radargrammetry.
• Supports interferometric operation using two antennas for 3-D mapping,
and analysis of incident-angle signatures of objects.

15. Radar
Nomenclature
 nadir
 azimuth flight direction
 look direction
 range (near and far)
 depression angle (γ)
 incidence angle (θ)
 altitude aboveground-
level, H
 polarization
pulse of microwave energy
Altitude above ground

16. The aircraft travels in a straight
line that is
called the azimuth flight direction
direction.
 The terrain illuminated nearest
the aircraft in the line of sight is
called the near-range.
 The farthest point of terrain
illuminated by the pulse
of energy is called the far-range.

17.  The range or look direction for any radar image is
the direction of the radar illumination that is at right angles to the
direction the aircraft or spacecraft is traveling.
 Generally, objects that trend (or strike) in a
direction that is orthogonal (perpendicular) to the
range or look direction are enhanced much more
than those objects in the terrain that lie parallel
to the look direction.
Consequently, linear features that appear dark or are
imperceptible in a radar image using one look direction may
appear bright in another radar image with a different look
direction.
Range Direction

18. The incident angle (θ) is the angle between the radar
pulse of EMR and line perpendicular to the Earth’s
surface where it makes contact. When the
terrain is flat, the incident angle (θ) is the
complement (θ = 90 - g) of the depression angle(γ). If the
terrain is sloped, there is no
relationship between depression angle and
incident angle. The incident angle best describes the
relationship between the radar beam and surface slope.
• Many mathematical radar studies assume the terrain surface
is flat (horizontal) therefore, the incident angle is assumed to
be the complement of the depression angle.

19. Unpolarized 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.

20. It is possible to:
• send vertically polarized energy and receive only
vertically polarized energy (designated VV),
• send horizontal and receive horizontally
polarized energy (HH),
• send horizontal and receive vertically polarized
energy (HV), or
• send vertical and receive horizontally polarized
energy (VH).
• HH and VV configurations produce
like-polarized radar imagery.
• HV and VH configurations produce
cross-polarized imagery.

21. RADAR Resolution
To determine the spatial resolution at any point
in a radar image, it is necessary to compute the
resolution in two dimensions: the range and
azimuth resolutions.
Radar is in effect a ranging device that measures the distance
to objects in the terrain by means of sending out and receiving
pulses of active microwave energy.
The range resolution in the across-track
direction is proportional to the length of the
microwave pulse.
The shorter the pulse length, the finer the range
resolution. Pulse length is a function of the speed of
light (c) multiplied by the duration of the transmission
(t).

22. Range Resolution
The range resolution (Rr) at any point between the near
and far-range of the illuminated strip can be computed if
the depression angle (γ) of the sensor at that location
and the pulse length (τ) are known.
It is possible to convert pulse length into distance by
multiplying it times the speed of light (c = 3 x 108 m sec-
1). The resulting
distance is measured in the slant-range previously
discussed. Because we want to know the range resolution
in the ground-range (not the slant-range) it is necessary
to convert slant-range to ground-range by dividing the
slant-range distance by the cosine of the depression angle
(γ). Therefore, the equation for
computing the range resolution is:
____τ x c______
Rr = 2 cos γ

23. To know both the length and width of the
resolution element, we must also
compute the width of the resolution
element in the direction the aircraft or
spacecraft is flying — the azimuth
direction.
Azimuth
Resolution
Azimuth resolution (Ra) is
determined by computing the
width of the terrain strip that is
illuminated by the radar
beam.

24. • Real aperture active microwave radars produce a lobe shaped
beam which is narrower in the near-range and
spreads out in the far-range.
Basically, the angular beam width is directly proportional
to the wavelength of the transmitted pulse of energy, i.e.,
the longer the wavelength, the wider the beam width, and
the shorter the wavelength, the narrower the beam width.
Therefore, in real aperture (brute force) radars a shorter
wavelength pulse will result in improved azimuth
resolution.
Unfortunately, the shorter the wavelength, the poorer the
atmospheric and vegetation penetration capability.

25. Fortunately, the beam width is also inversely
proportional to antenna length (L).
This means that the longer the radar antenna,
the narrower the beam width and the higher
the azimuth resolution. The relationship
between wavelength (λ) and antenna length
(L) is summarized below, which can be used to
compute the azimuth resolution:
Ra = S x λ
L
where S is the slant-range distance to the
point of interest.

26. GEOMETRIC DISTORTIONS
 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.

27. Similar to the distortions encountered when using cameras and scanners,
radar images are also subject to geometric distortions due to relief
displacement.
Radar foreshortening and layover are two consequences which result from
relief displacement.
Foreshortening

28. Foreshortening
 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').

29. 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 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.

30. Shadowing
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.

31. 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.

32. RADAR Image Speckle
Speckle is a grainy salt-and-pepper pattern in
radar imagery present due to the coherent
nature of the radar wave, which causes
random constructive and destructive
interference, and hence random bright and
dark areas in a radar image.
The speckle can be reduced by processing separate portions of an
aperture and recombining these portions so that interference does not
occur. This process, called multiple looks or non-coherent integration,
produces a more pleasing appearance, and in some cases may aid in
interpretation of the image but at a cost of degraded resolution.