Doppler Radar

The basics of radars is that a beam of energy, called radio waves, is emitted from an antenna. As they strike objects in the atmosphere, the energy is scattered in all directions with some of the energy reflected directly back to the radar.

The larger the object, the greater the amount of energy that is returned to the radar. That provides us with the ability to "see" rain drops in the atmosphere. In addition, the time it takes for the beam of energy to be transmitted and returned to the radar also provides is with the distance to that object.

Analog radars use continuous radio wave transmission and simultaneous listening.

Early National Weather Service radars were transmitting energy and listening for returned energy simultaneously. This is why old images from old radars showed a sweeping line. With each sweep of the radar an object its position would update.

With the advent of Doppler radar, instead of a continuous transmission of energy it emits extremely short bursts of radio waves, called pulses. It then "listens" for and returned energy.

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 (or change) in phase between a transmitted pulse and a received echo, the target's movement directly toward or away from the radar can be calculated, called radial velocity. A positive phase shift implies motion toward the radar and a negative shift suggests motion away from the radar.