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

PRF (s-1)
PRT (msec)
PRF (s-1)
PRT (msec)
250
4.0
1000
1.0
500
2.0
1250
0.80
750
1.3
1500
0.67

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

WSR-88D Data Collection Operations

Modes

The WSR-88D has two operational modes, Precipitation -- Mode A and Clear Air -- Mode B. The two modes employ six Volume Coverage Patterns (VCPs): VCPs 11, 12, 21 and 121 in Precipitation mode, and VCPs 31 and 32 in Clear Air mode. It should be noted that the term "Clear Air" does not imply "no-precipitation" mode. Even in Clear Air mode, the WSR-88D can detect and accumulate precipitation. At times, using one of the radar's Clear Air VCPs will improve detection and analysis of light, stratiform precipitation (e.g., snow) due to the increased sensitivity that results from the slower antenna rotation rate.

VCP Numbering Scheme

The VCP numbers come from one of four groups. Each group name implies an intended use, but there is some overlap in applications for each VCP.

1. Convection Group -- Two digits starting with number 1
    VCPs 11 and 12 are in this group

2. Shallow Precipitation Group -- Two digits starting with number 2
    VCP 21 is in this group

3. Clear Air Group -- Two digits starting with number 3
    VCPs 31 and 32 are in this group

4. Multiple PRF Dealiasing Algorithm (MPDA) Group -- Three digits starting with number 1, followed by the 2 digit number of VCP with similar elevation angles
    VCP 121 (MPDA version of VCP 21)

Antenna Rotation Rates

The antenna rotates at different speeds depending on the VCP. VCPs 31 and 32 have the slowest antenna speeds, ranging from 71 to 89 seconds for one rotation. In Clear Air mode, the antenna rotates slowly to encounter as many reflectors as possible which increases the statistical accuracy of the data. VCPs 12 and 121 have the fastest antenna speeds, ranging from 12 to 22 seconds for one rotation.

Figure 16. Illustration of VCP 11. Click for a larger version and more information.
Figure 1. VCP 11

Figure 17. Illustration of VCP 12. Click for a larger version and more information.
Figure 2. VCP 12

Figure 18. Illustration of VCP 21. Click for a larger version and more information.
Figure 3. VCP 21

Figure 19. Illustration of VCP 31. Click for a larger version and more information
Figure 4. VCP 31

Figure 20. Illustration of VCP 32. Click for a larger version and more information
Figure 20. VCP 32

Figure 21. Illustration of VCP 121. Click for a larger version and more information
Figure 5. VCP 121