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DLOC Principles of Meteorological Doppler Radar:
WSR-88D Fundamentals

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The Radar Beam

Propagation of Electromagnetic (E-M) Energy and Pulse Volume

The WSR-88D radar transmits a stream or "beam" of energy in discrete pulses which propagate away from the radar antenna at approximately the speed of light (~3 X 108ms-1). The volume of each pulse of energy will determine how many targets are illuminated. This directly determines how much energy (power) is returned to the radar. The shape of the radar antenna, the wavelength, greek symbol lambda , of the energy transmitted, and the length of time the radar transmits determine the shape and volume of each radar pulse.

The WSR-88D transmits a narrow, conical-shaped beam of pulses with each pulse resembling a truncated cone. The radar pulse volume is illustrated in Figure 1 . The angular width of the radar beam is defined as that region of transmitted energy that is bounded by one-half (-3 dB) the maximum power. The maximum power lies along the beam centerline and decreases outward.  

 

Figure 1.  Illustration of Radar Pulse Volume.  Click for a larger graphic

Figure 1: Beamwidth (greek symbol theta) as defined by half-power points, P/2, which are 3 dB less than along the beam centerline.

 

 

Figure 2.  Radar Pulse Volume Increasing with increasing range.  Click for Larger View.

Figure 2: Radar "beam" geometry and pulse volumes. Pulse 1 has greater volume than pulse 2 even though both pulses have the pulse length, H.

These "half-power" points for the WSR-88D result in an angular width of less than 1°. However, the actual physical width increases with increasing range (the physical length remains constant) such that the pulse volume increases with increasing range (Figure 2). Since the amount of transmitted power is fixed, a radar pulse's power density decreases with increasing range. Pulsed transmission also allows for obtaining target range information.

A Note on Beamwidth

Beamwidth of the WSR-88D varies between 0.87° and 0.96° when the radar antenna is stationary. However, during operations the antenna rotates, which smears the beam. The combination of the physical beamwidth and this smearing is called the effective beamwidth. Impacts of effective beamwidth are presented later on in DLOC Topic 3.

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