As the radar antenna turns, it emits extremely short bursts of radio waves, called pulses. Each pulse lasts about 0.00000157 seconds (1.57x10-6), with a 0.00099843-second (998.43x10-6) "listening period" in between. The transmitted radio waves move through the atmosphere at about the speed of light.
By recording the direction in which the antenna was pointed, the direction of the target is known as well. Generally, the better the target is at reflecting radio waves (i.e., more raindrops, larger hailstones, etc.), the stronger the reflected radio waves, or echo, will be.
This information is observed within the approximately 0.001-second listening period with the process repeated up to 1,300 times per second. By keeping track of the time it takes the radio waves to leave the antenna, hit the target, and return to the antenna, the radar can calculate the distance to the target.
The WSR-88D's pulses have an average transmitted power of about 450,000 watts. By comparison, a typical home microwave oven will generate about 1000 watts of energy. However, because of the very short period the radar is actually transmitting, when the time of all pulses each hour are totaled (the time the radar is actually transmitting), the radar is "on" for a little over 7 seconds each hour. The remaining 59 minutes and 53 seconds are spent listening for any returned signals.
Take it to the MAX! Beam me up!
The Doppler Advantage
By their design, Doppler radar systems can provide information regarding the movement of targets as well their position. When the WSR-88D transmits a pulse of radio waves, the system keeps track of the phase (shape, position, and form) of the transmitted radio waves.
By measuring the shift in phase between a transmitted pulse and a received echo, the target's radial velocity (the movement of the target directly toward or away from the radar) can be calculated. A positive phase shift implies motion toward the radar and a negative shift suggests motion away from the radar.
The phase shift effect is similar to the "Doppler shift" observed with sound waves. With the "Doppler shift", the sound pitch of an object moving toward your location is higher due to compression of sound waves. As an object moves away from your location, sound waves are stretched resulting in a lower frequency. You have probably heard this effect from an emergency vehicle or train. As the vehicle or train passes your location, the siren or whistle's pitch lowers as the object passes by.
For the Doppler radar, atmospheric objects moving inbound (toward the radar) produce a positive shift in frequency of the radar signal. Objects moving away from the radar (outbound) produce a negative shift in frequency. It is this change in frequency that allows us to "see" motion in the atmosphere. The larger the phase shift, the greater the target's radial velocity.
Scanning the horizon
The WSR-88D employs scanning strategies in which the antenna automatically raises to higher and higher preset angles, or elevation slices, as it rotates. These elevation slices comprise a volume coverage pattern or VCP. Once the radar sweeps through all elevation slices a volume scan is complete. In precipitation mode, the WSR-88D completes a volume scan every 4-6 minutes depending upon which VCP is in effect, providing an updated 3-dimensional look at the atmosphere around the radar site.
Take it to the MAX! Volume Coverage Patterns: Turn it up!