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.
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| 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.
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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.
All-Weather 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.
Microwave Frequency
The ability of microwave to penetrate clouds, precipitation, or land surface cover depends on its frequency. Generally, the penetration power increases for longer wavelength (lower frequency).The SAR backscattered intensity generally increases with the surface roughness. However, "roughness" is a relative quantity. Whether a surface is considered rough or not depends on the length scale of the measuring instrument. If a meter-rule is used to measure surface roughness, then any surface fluctuation of the order of 1 cm or less will be considered smooth. On the other hand, if a surface is examined under a microscope, then a fluctuation of the order of a fraction of a millimiter is considered very rough. In SAR imaging, the reference length scale for surface roughness is the wavelength of the microwave. If the surface fluctuation is less than the microwave wavelength, then the surface is considered smooth. For example, little radiation is backscattered from a surface with a fluctuation of the order of 5 cm if a L-band (15 to 30 cm wavelength) SAR is used and the surface will appear dark. However, the same surface will appear bright due to increased backscattering in a X-band (2.4 to 3.8 cm wavelength) SAR image.
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The land surface appears smooth to a long wavelength radar. Little radiation is backscattered from the surface. |
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The same land surface appears rough to a short wavelength radar. The surface appears bright in the radar image due to increased backscattering from the surface. |
Both the ERS and RADARSAT SARs use the C band microwave while the JERS SAR uses the L band. The C band is useful for imaging ocean and ice features. However, it also finds numerous land applications. The L band has a longer wavelength and is more penetrating than the C band. Hence, it is more useful in forest and vegetation study as it is able to penetrate deeper into the vegetation canopy.
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The short wavelength radar interacts mainly with the top layer of the forest canopy while the longer wavelength radar is able to penetrate deeper into the canopy to undergo multiple scattering between the canopy, trunks and soil. |
Microwave Polarisation in Synthetic Aperture Radar
The microwave polarisation refers to the orientation of the electric field vector of the transmitted beam with respect to the horizontal direction. If the electric field vector oscillates along a direction parallel to the horizontal direction, the beam is said to be "H" polarised. On the other hand, if the electric field vector oscillates along a direction perpendicular to the horizontal direction, the beam is "V" polarised.
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Microwave Polarisation: If the electric field vector oscillates along the horizontal direction, the wave is H polarised. If the electric field vector oscillates perpendicular to the horizontal direction, the wave is V polarised. |
Incident Angles
The incident angle refers to the angle between the incident radar beam and the direction perpendicular to the ground surface. The interaction between microwaves and the surface depends on the incident angle of the radar pulse on the surface. ERS SAR has a constant incident angle of 23o at the scene centre. RADARSAT is the first spaceborne SAR that is equipped with multiple beam modes enabling microwave imaging at different incident angles and resolutions.The incident angle of 23o for the ERS SAR is optimal for detecting ocean waves and other ocean surface features. A larger incident angle may be more suitable for other applications. For example, a large incident angle will increase the contrast between the forested and clearcut areas.
Acquisition of SAR images of an area using two different incident angles will also enable the construction of a stereo image for the area.
SAR Images
Synthetic Aperture Radar(SAR) images can be obtained from satellites such as ERS, JERS and RADARSAT. Since radar interacts with the ground features in ways different from the optical radiation, special care has to be taken when interpreting radar images.An example of a ERS SAR image is shown below together with a SPOT multispectral natural colour composite image of the same area for comparison.
ERS SAR image (pixel size=12.5 m)
SPOT Multispectral image in Natural Colour
(pixel size=20 m)
The urban area on the left appears bright in the SAR image while the vegetated areas on the right have intermediate tone. The clearings and water (sea and river) appear dark in the image. These features will be explained in the following sections. The SAR image was acquired in September 1995 while the SPOT image was acquired in February 1994. Additional clearings can be seen in the SAR image.
Speckle Noise
Unlike optical images, radar images are formed by coherent interaction of the transmitted microwave with the targets. Hence, it suffers from the effects of speckle noise which arises from coherent summation of the signals scattered from ground scatterers distributed randomly within each pixel. A radar image appears more noisy than an optical image. The speckle noise is sometimes suppressed by applying a speckle removal filter on the digital image before display and further analysis. |
This image is extracted from the above SAR image, showing the clearing areas between the river and the coastline. The image appears "grainy" due to the presence of speckles. |
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This image shows the effect of applying a speckle removal filter to the SAR image. The vegetated areas and the clearings now appear more homogeneous. |
Backscattered Radar Intensity
A single radar image is usually displayed as a grey scale image, such as the one shown above. The intensity of each pixel represents the proportion of microwave backscattered from that area on the ground which depends on a variety of factors: types, sizes, shapes and orientations of the scatterers in the target area; moisture content of the target area; frequency and polarisation of the radar pulses; as well as the incident angles of the radar beam. The pixel intensity values are often converted to a physical quantity called the backscattering coefficient or normalised radar cross-section measured in decibel (dB) units with values ranging from +5 dB for very bright objects to -40 dB for very dark surfaces.Interpreting SAR Images
Interpreting a radar image is not a straightforward task. It very often requires some familiarity with the ground conditions of the areas imaged. As a useful rule of thumb, the higher the backscattered intensity, the rougher is the surface being imaged.Flat surfaces such as paved roads, runways or calm water normally appear as dark areas in a radar image since most of the incident radar pulses are specularly reflected away.
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Specular Reflection: A smooth surface acts like a mirror for the incident radar pulse. Most of the incident radar energy is reflected away according to the law of specular reflection, i.e. the angle of reflection is equal to the angle of incidence. Very little energy is scattered back to the radar sensor. |
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Diffused Reflection: A rough surface reflects the incident radar pulse in all directions. Part of the radar energy is scattered back to the radar sensor. The amount of energy backscattered depends on the properties of the target on the ground. |
Calm sea surfaces appear dark in SAR images. However, rough sea surfaces may appear bright especially when the incidence angle is small. The presence of oil films smoothen out the sea surface. Under certain conditions when the sea surface is sufficiently rough, oil films can be detected as dark patches against a bright background.
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A ship (bright target near the bottom left corner) is seen discharging oil into the sea in this ERS SAR image. |
Trees and other vegetations are usually moderately rough on the wavelength scale. Hence, they appear as moderately bright features in the image. The tropical rain forests have a characteristic backscatter coefficient of between -6 and -7 dB, which is spatially homogeneous and remains stable in time. For this reason, the tropical rainforests have been used as calibrating targets in performing radiometric calibration of SAR images.
Very bright targets may appear in the image due to the corner-reflector or double-bounce effect where the radar pulse bounces off the horizontal ground (or the sea) towards the target, and then reflected from one vertical surface of the target back to the sensor. Examples of such targets are ships on the sea, high-rise buildings and regular metallic objects such as cargo containers. Built-up areas and many man-made features usually appear as bright patches in a radar image due to the corner reflector effect.
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Corner Reflection: When two smooth surfaces form a right angle facing the radar beam, the beam bounces twice off the surfaces and most of the radar energy is reflected back to the radar sensor. |
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This SAR image shows an area of the sea near a busy port. Many ships can be seen as bright spots in this image due to corner reflection. The sea is calm, and hence the ships can be easily detected against the dark background. |
The brightness of areas covered by bare soil may vary from very dark to very bright depending on its roughness and moisture content. Typically, rough soil appears bright in the image. For similar soil roughness, the surface with a higher moisture content will appear brighter.
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Dry Soil: Some of the incident radar energy is able to penetrate into the soil surface, resulting in less backscattered intensity. |
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Wet Soil: The large difference in electrical properties between water and air results in higher backscattered radar intensity. |
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Flooded Soil: Radar is specularly reflected off the water surface, resulting in low backscattered intensity. The flooded area appears dark in the SAR image. |
Multitemporal SAR images
If more than one radar images of the same area acquired at different time are available, they can be combined to give a multitemporal colour composite image of the area. For example, if three images are available, then one image can be assigned to the Red, the second to the Green and the third to the Blue colour channels for display. This technique is especially useful in detecting landcover changes over the period of image acquisition. The areas where no change in landcover occurs will appear in grey while areas with landcover changes will appear as colourful patches in the image.
Source : http://www.crisp.nus.edu.sg