Saturday, March 22, 2014


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.




Figure A
Radars create an electromagnetic(EM) pulse that is focused by an antenna transmitted through the atmosphere.

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
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
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.

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
2.The "elevation angle," the angle of the radar beam with respect to the ground.

3.The distance (D) from radar to target.     
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. 

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".

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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