pulse recurrence rate
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pulse recurrence rate[′pəls ri′kə·rəns ‚rāt]
pulse repetition frequency (PRF)
In radar, the number of pulses that occur each second. Also called the pulse recurrence rate, or PRR. It should not be confused with the transmission frequency, which is determined by the rate at which cycles are repeated within the transmitted pulse. The distance between two pulses in time is called the pulse recurrence period (PRP), and its relationship with PRF can be described as PRP = 1/PRF second. The illustration shows returns from two targets, one closer than the other. The return from the distant target is smaller than that of the closer target because of the spreading out of radar energy as it propagates and the wave attenuation in the atmosphere. If the next pulse were sent before the distant target arrived, the timing circuit would be confused and an ambiguous measurement might take place. Thus, the time between successive pulses is arranged to correspond to the outward and return time for the targets at the maximum possible range of detection. Returns from targets beyond this range will be so small that they usually will not be detected.