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What is radar?


How does it work?

WHAT IS RADAR?


     The word "radar" is an acronym 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. Radar was designed shortly before World War II. Its primary purpose was to detect the presence of aircraft. Today, radar is used for a wide array of applications, but primarily to detect precipitation and other meteorological events.

     NEXRAD is short for NEXt generation RADar and refers to the nationwide network of Doppler radar sites installed by the National Oceanographic and Atmospheric Administration (NOAA). Its official designation is WSR-88D (WSR88D = Weather Surveillance Radar - 1988 - Doppler). NOAA designed this network specifically to provide comprehensive radar coverage of meteorological events occurring in the United States.
     An extensive network of NEXRAD stations provides almost complete radar coverage of the continental United States, Alaska, and Hawaii. The range of each NEXRAD is 124 nautical miles.


Map of continental US showing location of all NEXRAD stations and the extent of their 124 nautical mile detection range.

HOW DOES IT WORK?

TARGET DETECTION

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

  • Objects in the path of the transmitted EM pulse, called "targets" or "echoes," scatter most of the energy, but some will be reflected back toward the radar (Figure B). 

  • 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 C). 

  • Also, the more targets, the stronger the return signal, that is, the targets combine to produce a stronger signal (Figure D).

  • The radar measures the returned signal, generally called the "reflectivity." 

  • Reflectivity magnitude is related to the number and size of the targets encountered.

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.

  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 (t) * 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 = c*t or D = (c*t)/2. 

TARGET VELOCITY

  • Doppler radars, like NEXRAD, can also measure "radial velocity," the component of target velocity moving toward or away from the radar. 

  • For example, 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.