WHAT IS RADAR?
The word "radar" is
derived from the words RAdio Detection And Ranging. It refers to the
technique of using radio waves to detect the presence of objects in the
atmosphere. It was designed before World War II to detect the presence of
aircraft.
WORK FUNCTION OF RADAR
1.TARGET DETECTION
Figure A
Radars create an
electromagnetic(EM) pulse that is focused by an antenna transmitted through
the atmosphere.
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Figure B
Objects in the path of the
transmitted pulse, called
"targets" or "echoes," scatter most of the energy, but
some will be reflected back toward the radar
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Figure C
The receiving antenna (normally also the
transmitting antenna) gathers back-scattered radiation and feeds it to a
"receiver." An EM pulse encountering a target is scattered in
all directions. The larger the target, the stronger the scattered signal
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Figure D
Also, the more targets, the
stronger the return signal, that is, the targets combine to produce a
stronger signal. The radar measures the returned signal, called
"reflectivity." Reflectivity magnitude is related to the
number and size of the targets encountered.
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2.TARGET
LOCATION
The radar needs 3 pieces of
information to determine the location of a target.
1.The "azimuth angle," the angle of the radar
beam with respect to north
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2.The "elevation angle," the angle of the radar
beam with respect to the ground.
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3.The distance (D) from radar to target.
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Distance is determined by measuring the time it takes for
the EM pulse to make a round trip from the radar to the target and back using
the relation:
distance = time x velocity
The pulse travels at the speed of light (c). Since the
pulse travels to and from the target, the total distance is 2D. If t is the
time it takes, then 2D = cxt or D = (cxt)/2.
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3.TARGET VELOCITY
Doppler radars, like NEXRAD, can measure "radial velocity," the component of target velocity moving toward or away from the radar.
at "time interval 1"
(T1), an EM pulse transmitted by the radar is intercepted by a target at
distance "D1".
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At "time interval 2"
(T2), another pulse returns a target distance "D2." Doppler radars
measure the change in "D" from T1 to T2. These changes, the radar's
wavelength, and the time interval between T1 and T2, are used to compute
target velocity.
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