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

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Previous Page: Pulse Repetition Frequency

Maximum Unambiguous Range (Rmax) and PRF

The primary reason for sending out discrete pulses of E-M energy at given time intervals is that this allows for target ranging. The listening period is the time from when the radar finishes transmitting the first pulse to when it begins transmitting the second pulse. This period allows the first pulse to travel a certain round trip distance. This distance when divided by 2 yields the maximum unambiguous range (Rmax) of a radar. Rmax can be expressed mathematically as

 
Equation 6.  Rmax (Maximum Unambiguous Range).  Click for long description.
 

    where c = speed of light (~ 3 x 108 ms-1)
    and greek symbol tau = listening period (s).

Since the pulse duration is very small (greek symbol musec) when compared to the PRT (msec), Rmax can be approximated by substituting the PRF or PRT for greek symbol ta. As a result, Equation (6) can take on two forms
 
Equation 7.  Rmax.  Click for Long Description.
 
or
 
Equation 8.  Rmax.  Click for long description.
 
    where PRF = pulse repetition frequency (s-1)
    and PRT = pulse repetition time (s) = 1/PRF

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