Active Microwave Remote
Sensing
Lecture 8
Oct 22, 2007
Recap: passive and active RS
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Passive: uses natural energy, either reflected
sunlight (solar energy) or emitted thermal or
microwave radiation.
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Active: sensor creates its own energy
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Transmitted toward Earth or other targets
Interacts with atmosphere and/or surface
Reflects back toward sensor (backscatter)
Advantages: all weathers and all times
Widely used active RS systems
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RADAR: RAdio Detection And Ranging
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LIDAR: LIght Detection And Ranging
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Long-wavelength microwaves (1 – 100 cm)
Short-wavelength laser light (UV, visible, near IR)
SONAR: SOund Navigation And Ranging: (very long wave, low Hz)
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Sound can not travel through vacuum
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Earth and water absorb acoustic energy far less than EMR energy
Seismic survey use small explosions, record the reflected sound
Medical imaging using ultrasound
Sound waves through a water column.
Sound waves are extremely slow (300 m/s in air, 1,530 m/s in sea-water)
Bathymetric sonar (measure water depths and changes in bottom topography )
Imaging sonar or sidescan imaging sonar (imaging the bottom topography and
bottom roughness)
Types of radar
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Nonimaging radar
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Traffic police use handheld Doppler radar system
determine the speed by measuring frequency shift
between transmitted and return microwave signal
Plan position indicator (PPI) radars use a rotating antenna
to detect targets over a circular area, such as NEXRDA
Satellite-based radar altimeters (low spatial resolution but
high vertical resolution)
Imaging radar
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Usually high spatial resolution,
Consists of a transmitter, a receiver, one or more antennas,
GPS, computers
Microwaves
Band Designations
(common wavelengths
Wavelength () Frequency ()
shown in parentheses)
in cm
in GHz
_______________________________________________
Ka (0.86 cm)
0.75 - 1.18
40.0 to 26.5
K
Ku
X (3.0 and 3.2 cm)
C (7.5, 6.0 cm)
S (8.0, 9.6, 12.6 cm)
L (23.5, 24.0, 25.0 cm)
P (68.0 cm)
1.18 - 1.67
1.67 - 2.4
2.4 - 3.8
3.8 - 7.5
7.5 - 15.0
15.0 - 30.0
30.0 - 100
26.5 to 18.0
18.0 to 12.5
12.5 - 8.0
8.0 - 4.0
4.0 - 2.0
2.0 - 1.0
1.0 - 0.3
Two imaging radar systems
In World War II, ground based radar was used to detect incoming planes and ships
(non-imaging radar).
Imaging RADAR was not developed until the 1950s (after World War II). Since
then, side-looking airborne radar (SLAR) has been used to get detailed
images of enemy sites along the edge of the flight field. SLAR is usually a
real aperture radar. The longer the antenna (but there is limitation), the better
the spatial resolution
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Real aperture radar (RAR)
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Aperture means antenna
A fixed length (for example: 1 - 15m)
Synthetic aperture radar (SAR)
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1m (11m) antenna can be synthesized electronically into a 600m (15 km)
synthetic length.
Most (air-, space-borne) radar systems now use SAR.
1. RAR: Principle of SLAR
waveform
Radar Nomenclature and Geometry
Azimuth flight direction
Flightline groundtrack
Near range
Far range
Radar Nomenclature
• nadir
• azimuth (or flight) direction
• look (or range) direction
• range (near, middle, and far)
• depression angle ()
• incidence angle ()

• altitude above-ground-level,
H
• polarization

Slant-range vs. Ground-range
geometry
Radar imagery has a different geometry than that produced by most
conventional remote sensor systems, such as cameras, multispectral scanners
or area-array detectors. Therefore, one must be very careful when attempting
to make radargrammetric measurements.
• Uncorrected radar imagery is displayed in what is called slant-range
geometry, i.e., it is based on the actual distance from the radar to each of the
respective features in the scene.
• It is possible to convert the slant-range display into the true ground-range
display on the x-axis so that features in the scene are in their proper
planimetric (x,y) position relative to one another in the final radar image.
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Most radar systems
and data providers
now provide the data
in ground-range
geometry
Range (or across-track) Resolution
Rr 
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t c
2 cos 
t.c called pulse
length. It seems the
short pulse length
will lead fine range
resolution.
However, the shorter
the pulse length, the
less the total amount
of energy that
illuminates the
target.
Pulse duration (t)
= 0.1 x 10 -6 sec
t.c/2
t.c/2
Azimuth (or along-track) Resolution
S 
Ra 
D
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The shorter
wavelength and longer
antenna will improve
azimuth resolution.
The shorter the
wavelength, the poorer
the atmospheric and
vegetation penetration
capability
There is practical
limitation to the
antenna length, while
SAR will solve this
problem.
2. SAR
A major advance in radar remote sensing has been the improvement in azimuth resolution through the
development of synthetic aperture radar (SAR) systems. Great improvement in azimuth resolution
could be realized if a longer antenna were used. Engineers have developed procedures to synthesize a
very long antenna electronically. Like a brute force or real aperture radar, a synthetic aperture radar
also uses a relatively small antenna (e.g., 1 m) that sends out a relatively broad beam perpendicular to
the aircraft. The major difference is that a greater number of additional beams are sent toward the
object. Doppler principles are then used to monitor the returns from all these additional microwave
pulses to synthesize the azimuth resolution to become one very narrow beam.
Azimuth resolution is constant = D/2, it is
independent of the slant range distance,  ,
and the platform altitude. So the same SAR
system in a aircraft and in a spacecraft
should have the same resolution. There is no
other remote sensing system with this
capability.
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
pulses of
microwave energy
9
a.
8
7
6
5
4
object is a
3
constant dist ance
from the flightline
2
time n
1
c.
b.
8
7
time n+1
time n+2
interference signal
radar hologram
9
9
8
9
8
7
7
6.5
time n+4
time n+3
9
8
7
d.
6.5
9
8
7
6.5
7
e.
At time n+3, the shortest distance and area of zero Doppler shift
Speckle noise
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Using SAR, we can get high spatial resolution in the azimuth
dimension (direction). But the coherently recording returned
echoes (SAR) also causes speckle noise.
For one-single channel SAR system, the speckle noise has a
multiplicative nature for the amplitude and an additive nature
for the phase.
For multi-dimensional (or polarimetric) SAR (or PolSAR)
system, speckle noise is even complicated.
There are two ways to remove speckle noise:
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Using several looks, i.e., averaging takes place, usually 4 or 16 looks
(N). But lose resolution: Azimuth resolution = N(D/2)
Modeling the noise, then remove them.
Backscatter
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The portion of the outgoing radar
signal that the target redirects
directly back towards the radar
antenna.
When a radar system transmits a
pulse of energy to the ground
(A), it scatters off the ground in
all directions (C). A portion of
the scattered energy is directed
back toward the radar receiver
(B), and this portion is referred
to as "backscatter".
Amount of backscatter per unit area
http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar_Course_III/parameters_affecting.htm
Fundamental radar equation
t
wrong
Intermediate
h

8 sin 
Penetration ability to forest
Response of A Pine Forest Stand to X-, C- and L-band Microwave Energy
L-band
23.5 cm
a.
C-band
5.8 cm
b.
X-band
3 cm
c.
Polarization
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Unpolarized energy
vibrates in all possible
directions perpendicular to
the direction of travel.
The pulse of
electromagnetic energy is
filtered and sent out by the
antenna may be vertically
or horizontally polarized.
The pulse of energy
received by the antenna
may be vertically or
horizontally polarized
VV, HH – like-polarized
imagery
VH, HV- cross-polarized
imagery
Penetration ability
into subsurface
Penetration ability
to heavy rainfall
SIR-C/X-SAR
Images of a Portion
of Rondonia,
Brazil, Obtained on
April 10, 1994
Radar Shadow
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Shadows in radar images can enhance the geomorphology and texture of the terrain.
Shadows can also obscure the most important features in a radar image, such as the
information behind tall buildings or land use in deep valleys. If certain conditions are
met, any feature protruding above the local datum can cause the incident pulse of
microwave energy to reflect all of its energy on the foreslope of the object and
produce a black shadow for the backslope
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Unlike airphotos, where light may be scattered into the shadow area and then
recorded on film, there is no information within the radar shadow area. It is black.
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Two terrain features (e.g., mountains) with identical heights and fore- and backslopes
may be recorded with entirely different shadows, depending upon where they are in
the across-track. A feature that casts an extensive shadow in the far-range might have
its backslope completely illuminated in the near-range.
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Radar shadows occur only in the cross-track dimension. Therefore, the orientation of
shadows in a radar image provides information about the look direction and the
location of the near- and far-range
Shadows and look direction
Shuttle Imaging Radar (SIR-C) Image of Maui
Major Active Radar Systems
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Seasat, June 1978, 105 days mission, L-HH band, 25 m resolution
SIR-A, Nov. 1981, 2.5 days mission, L-HH band, 40 m resolution
SIR-B, Oct. 1984, 8 days mission, L-HH band, about 25 m resolution
SIR-C, April and Sept. 1994, 10 days each. X-, C-, L- bands multipolarization
(HH, VV, HV, VH), 10-30 m resolution
JERS-1, 1992-1998, L-band, 15-30 m resolution
(Japan)
RADARSAT, Jan. 1995-now, C-HH band, 10, 50, and 100 m
(Canada)
ERS-1, 2, July 1991-now, C-VV band, 20-30 m
(ESA)
ASAR on EnviSat, 2002-now, C band
(ESA)
AIRSAR/TOPSAR, 1998-now, C,L,P bands with full polarization, 10m
NEXRAD, 1988-now, S-band, 1-4 km,
TRMM precipitation radar, 1997, Ku-band, 4km, vertical 250m (USA and
Japan)
Active Radar Systems for Mars
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MARSIS (Mars advanced radar for subsurface and ionosphere sounding)
of Mras Express, 2003, 1.8-5 MHz, up to 5 KM deep (ESA)
http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=34826&fbodylongid=1601
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SHARAD (shallow subsurface radar) of MRO, 2005, 15-25 MHz, up to
1Km deep (ISA-NASA)
http://mars.jpl.nasa.gov/mro/mission/sc_instru_sharad.html