Electromagnetic radiation in the microwave
wavelength region is used in remote sensing to provide useful information about
the Earth's atmosphere, land and ocean.
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.
it sends out pulses of microwaves along several directions and
records the magnitude of the signals backscattered from the ocean
surface. The magnitude of the backscattered signal is related
to the ocean surface roughness, which in turns is dependent on
the sea surface wind condition, and hence the wind speed and direction
can be derived.
orne platforms to generate high resolution images of the earth surface
using microwave energy.
Synthetic Aperture Radar (SAR)
In synthetic aperture radar (SAR) imaging, 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 utilising the time delay of the backscattered signals.
|A radar pulse is transmitted from the antenna to the ground||The radar pulse is scattered by the ground targets back to the antenna.|
In real aperture radar imaging, the ground resolution
is limited by the size of the microwave beam sent out from the antenna. Finer
details on the ground can be resolved by using a narrower beam. The beam width
is inversely proportional to the size of the antenna, i.e. the longer the antenna,
the narrower the beam.
||The microwave beam sent out by the antenna illuminates an area on the ground
(known as the antenna's "footprint"). In radar imaging, the recorded signal
strength depends on the microwave energy backscattered from the ground
targets inside this footprint. Increasing the length of the antenna will
decrease the width of the footprint.
It is not feasible for a spacecraft to carry a very long antenna which is
required for high resolution imaging of the earth surface. To overcome this
limitation, SAR capitalises on the motion of the space craft to emulate a large
antenna (about 4 km for the ERS SAR) from the small antenna (10 m on the ERS
satellite) it actually carries on board.
Imaging geometry for a typical strip-mapping synthetic aperture radar
imaging system. The antenna's footprint sweeps out a strip parallel to
the direction of the satellite's ground track.
Interaction between Microwaves and Earth's Surface
When microwaves strike a surface, the proportion
of energy scattered back to the sensor depends on many factors:
- Physical factors such as the dielectric constant of the surface
materials which also depends strongly on the moisture content;
- Geometric factors such as surface roughness, slopes, orientation
of the objects relative to the radar beam direction;
- The types of landcover (soil, vegetation or man-made objects).
- Microwave frequency, polarisation and incident angle.
Click here to read more about microwave frequency, polarisation and incident angle in SAR imaging.
Due to the cloud penetrating property of microwave, SAR is able to acquire "cloud-free"
images in all weather. This is especially useful in the tropical regions which
are frequently under cloud covers throughout the year. Being an active remote
sensing device, it is also capable of night-time operation.
Infrared Remote Sensing
Interpreting SAR Images
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