Radar Categorization Based on Antenna Size | RAR (Real Aperture Radar) | SAR (Synthetic Aperture Radar)
Radar is categorized into two main groups on the basis of antenna size:
·
Real Aperture Radar (RAR)
· Synthetic Aperture Radar (SAR)
RAR (Real Aperture Radar)
RAR are
the non-coherent radars that are controlled by the physical length of the
antenna. It is active radar that transmits high-frequency
radar waves from the antenna to the particular piece of terrain that one wants
to obtain the image. This transmitted radar waves is of the narrow-angle beam in the range direction at
right angles to the flight direction called as the azimuth direction and
receives the backscattering from the targets which will be transformed to a
radar image from the received signals, as shown in Figure 1.. The image resolution
is defined by the length of the antenna.
It is impractical to design a large antenna in order to get a high-resolution image because in that case, an antenna has to be at least many times of the length of wavelength in order to minimize the bandwidth of
transmitted signal. If in case, such antenna is made then it is very problematic
to mount such antenna, which requires 1
km diameter length of the antenna to get
25 meters resolution with L band (=25 cm) and 100 km distance from a target, a
RAR hence has a practical constraint for refining the azimuth resolution.
SAR (Synthetic Aperture Radar)
In SAR imaging, an antenna transmits
the microwave pulses towards the earth surface. This high-frequency microwave energy scattered back to the spacecraft is
measured as shown in Figure 1. The SAR makes use of the radar principle
to form an image by utilizing the time
delay of the backscattered signals. SAR is the coherent radar that is attached
to the satellites and aircrafts that
captures the high-resolution images of the wide surface area of the earth. SAR overshadows
photographic and other optical imaging abilities as it has the advanced capability
of taking images in varying weather conditions, day and night due to a different wavelength of camera sensors. SAR works over the
sensors of the wavelength of 1 cm to 1 m,
whereas optical sensors use wavelengths near that of visible light or 1 micron. Due to this difference, SAR
has the capability to see through clouds and storms, whereas optical sensors
are unable to do so. On comparing with RAR, SAR increases the size of the antenna
or aperture synthetically to raise the azimuth resolution.
In SAR, the data processing of received signals and phases from moving
targets with a small antenna is a complicated
process.
When
radar waves hit the earth surface, the proportion of energy scattered back to
the sensor as shown in Figure 1 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 land cover (soil, vegetation or man-made objects).
·
Microwave frequency, polarisation and
incident angle.
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.
Note: If you are taking help from the above work and also incorporating the above content in your research work, Kindly make sure you are citing the below papers.
- https://www.sciencedirect.com/science/article/pii/S2215098617314003
- https://www.sciencedirect.com/science/article/pii/S1319157817301106
- https://www.sciencedirect.com/science/article/pii/S1319157817303865
- https://link.springer.com/article/10.1007/s11831-021-09548-z
Reviewed by IPR
on
January 31, 2022
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