Pulse Repetition Frequency
Pulse Repetition Frequency (PRF) is the number of pulses transmitted per second by the radar. The reciprocal of this is called the Pulse Repetition Time (PRT), which is the elapsed time from the beginning of one pulse to the beginning of the next pulse. PRF is important since it determines the maximum target range (Rmax, discussed in next sections)* that can be accurately determined by the WSR-88D.
The PRF used by the WSR-88D steers a middle course between these two goals, and is around 326 pulses s-1. The typical pulse length for the WSR-88D is 1.6 microseconds. Traveling at the speed of light, this pulse will encounter a target at the outer end of the maxium target range (often referred to as the Maximum Unambiguous Range) in 499 microseconds.
Once a pulse is emitted, the radar then "waits" (called "Listening Time") about 1000 microseconds. This allows the backscattered radiation from a target at that range to return to the radar. This Listening Time can cause some confusion, as will be discussed in the section on Range Folding.
The Doppler Dilemma
It turns out that a low PRF is desirable for the most accurate esimate of target range and power returned (reflectivity. However, to measure the Doppler shift exhibited by the back scattered radation, it turns out that a high PRF is desirable for the most accurate estimate of target velocity (extremely important in detecting rotation in thunderstorms, for example).
The "juggling" of these competing goals is sometimes called "The Doppler Dilemma" (discussed more completely next time).
*and maximum Doppler velocity (Vmax)
Table 5: PRTs for Hypothetical PRFs
WSR-88D Sampling Techniques
By employing different PRFs, the WSR-88D uses different techniques to gather reflectivity and velocity information. These techniques, known as waveforms, are used in different combinations in each VCP. For this