Synthetic Aperture Radar (SAR) is an active remote sensing technology used in satellites to produce high-resolution images regardless of weather or light conditions. SAR works by emitting microwave pulses and analyzing the echo returns, similar to how bats use echolocation. There are three main types of radar scattering - specular, diffuse, and double-bounce - which appear differently in SAR images and provide information about surface characteristics. Key applications of SAR include search and rescue operations, topographic mapping, and monitoring of events like oil spills.

1. Synthetic Aperture Radar (SAR)
Basics

2. • Synthetic Aperture Radar (SAR) is an emerging
technology in remote sensing.
• In fact, Sentinel-1 is equipped with this active
type of sensor. Likewise, Radarsat and
TerraSAR use synthetic aperture radar.
• The main advantage behind this technology is
how it can synthetically produce higher
resolution images in any weather condition
and even at night.

3. SAR’s Bat-Like Capabilities
• It’s said that synthetic aperture radar is similar to
how bats use echolocation to navigate in a cave.
• When bats fly in a cave, it uses sound to navigate.
Generally, they create sound waves from 50 to
120 dB. When this sound bounces off a wall and
returns to the bat, it understands distance based
on the echo.
• In general, the same principles apply to SAR. The
satellite sends microwave pulses to Earth. The
pulse returns back to the satellite and the sensor
makes an image from the returned echoes.

4. • Generally, synthetic aperture radar is side-
looking. This means they don’t look completely
down at Nadir, but at an angle.
• As mentioned earlier, microwave radar can see at
night and through clouds and smoke. At any time
of the day or in any type of weather condition,
SAR works.
• Actually, longer wavelengths can penetrate
clouds better and even the ground. For example,
L-band (~24 cm) radar has longer wavelengths
than C-band (~6 cm) and X-band (~3cm).

5. Types of Radar Scattering
• Side-looking radar interacts with different
types of terrain.
• The 3 main types of scattering mechanisms
are:
• Specular
• Diffuse
• Double-bounce

6. Smooth Surface
• Smooth Surface reflection comes
from flat terrains like roads or
water. For this type of scattering,
very little energy of the
transmitted pulse returns to the
sensor (similar to a mirror).
• In this example, pixels will
appear black typically with
values less than -20dB.

7. Rough Surface
• Rough surface scattering such as
plowed farm fields and
vegetation. Scattering goes in all
directions diffusely.
• For example, typical pixel values
will be greater than -20dB and
in grey.

8. Double-bounce
• Often, Double bounce occurs
off structures and man-made
objects. The reflected pulse
hits one surface after the other
and back to the sensor.
• For example, typical pixel
values will appear white with
values greater than -10dB.

9. A Little Deeper in the Technology
• Polarization refers to the orientation of the
radar wave from the SAR antenna.
• Both electric and magnetic lines of force are at
right angles to each other, but it’s the electric
field that determines the direction of
polarization of the wave.
• Synthetic Aperture Radar uses an antenna
that can transmit in either the horizontal (H)
or vertical (V) polarization.

10. • When the electromagnetic wave
scatters from a target, the polarisation
state of an electromagnetic wave can
change.
• When the sensor receives the
returning wave, it measures the
degree of change in polarisation from
the target. For example, it can either
be H or V polarization or both
simultaneously.

11. • For single-polarization, these are typical
transmit and receive pairs.
• HH – for horizontal transmit and horizontal
receive
• VV – for vertical transmit and vertical receive
• HV – for horizontal transmit and vertical
receive
• VH – for vertical transmit and horizontal
receive

12. • Finally, it’s synthetic aperture because it can
create higher resolution images.
• Because it receives backscatter along the length
of the synthetic aperture radar, it can
synthetically generate a higher resolution image
for a point target on Earth.
• The entire length of the synthetic aperture radar
has the backscatter information for the point
target. When all the backscatter information is
merged, it’s like a “synthetic aperture”.

14. SAR Image Interpretation
• Now that you know the basics of synthetic
aperture radar, let’s look at a SAR image with
these types of scattering. In this Radarsat-2
example, the image clearly shows all three
types of backscatter.

16. Specular Reflection
• In this scene, there is a river that flows in the
east-west direction. As shown in the schematic
above, very little energy reflects back to the radar
sensor. In this case, the pixel is dark with a low
dB.
• This can also be seen in the southeast portion
with the road/airport paved surface. Again, this is
a specular reflection off a smooth surface.
•

17. Double-Bounce Scattering
• On the other hand, the bright white in the center
of the image can be interpreted as an urban
feature. The radar is receiving double-bounce
backscatter, meaning the transmitted pulses are
returning to the sensor.
• It’s unclear at this scale what this object is but it’s
due to double-bounce returns. Because of its
values greater than -10dB, pixels will appear as a
bright white.
•

18. Diffuse Scattering
• Finally, the majority of the radar image is
rough surface scattering. You have a bit of
specular and double-bounce scattering.
• This may be from annual cropland, vegetation,
grasses, or other features. It is diffuse
scattering because there’s not a high or low
amount of backscatter in the image.

19. Synthetic Aperture Radar Use Cases
• In search and rescue missions, weather
conditions are often poor. In the case of forest
fires, smoke could completely block visibility.
Because microwave SAR is not affected by
these types of conditions, rescuers use it to
find man-made objects on the ground.
Specifically, it looks for double-bounce
scattering where the crash site occurred. Or
even where flooding occurs, if there is a
specular reflection (dark pixels).

20. • Scientists use synthetic aperture radar
to estimate surface elevation with the Space
Shuttle Radar Topography Mission inSAR. This
satellite used interferometry (InSAR)
generating one of the most accurate elevation
models of the whole globe.

21. • SAR is about understanding surface
characteristics based on backscatter. That’s
why we use SAR during oil spills and to
understand ocean waves. During an oil spill,
oil floats on water suppressing waves.