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

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Previous Page: The Radar Beam

Power Density

The power density determines how much energy targets intercept and reflect or "backscatter" toward the radar antenna. Two pulses at different ranges have been transmitted by the radar in Figure 2 . The pulse on the right is further from the radar, and thus has a larger volume and lower power density than the second transmitted pulse on the left.

The actual length of the pulses shown in Figure 2 is determined by the pulse duration or length of time the radar transmits energy. Pulse duration for the WSR-88D when operating in short pulse mode (VCP 11, 12, 21, 121, 211, 212, 221, or 32) is 1.57 X 10 -6 s and in long pulse mode (VCP 31) is 4.7 X 10 -6 s. Also, since the main beam describes a cone, a cross section perpendicular to the beam centerline is a circle. Another way of describing the power density is power per unit area of a cross section. This is illustrated in Figure 3 where the cross-sectional area of pulse 1 is 4 times larger than the area of pulse 2. Since the transmitted energy is the same for both pulse samples, then pulse 2 has a power density 4 times greater than the power density of pulse 1.

Figure 3.  Cross section view of radar beam.  Click for larger version.

Figure 3: Example of a pulse volume and power density difference viewed from either end of the pulse sample.

Next Page: Backscattered Energy