What is the main advantage of Doppler radar over conventional weather radar?

Edit-Slight Expansions and simplificationsIn General, the Advantages of Doppler RADAR compared to typical "Pulse" RADAR lies in its capability to differentiate between steady objects and moving objects, Doppler RADAR may also provide "velocity search" which later can be used as a basis for target recognition like the second answerer point (man walking and cars) Doppler effect RadarIn a general type of radar, the signal is transmitted/received in short pulses or bursts. The time width (W) for which the signal is sent/received is very short. And the time between two successive transmitted pulses is called pulse repetition time (PRT). Thus, reciprocal of PRT gives its frequency i.e. PRF = 1/PRT. Similarly, the ratio of time width (W) to PRT is called duty cycle (D).

The Radar that works on Doppler effect : it works on the principle of Doppler Effect. This effect is applicable to sound waves and electro-magnetic waves, which travel through some medium or through free space. The Doppler Effect states that when there is relative motion between the source of very high frequency signals and the observer, then frequency shift occurs.

In CW radar, a constant amplitude continuous microwave sine wave is transmitted. The echo is also a constant amplitude microwave sine wave of SAME frequency, but of lower amplitude, due to loss of energy during traveling of the wave.

While detecting the target like aeroplane, ship, missile etc. the echo undergoes change in its frequency. This change of frequency between the transmitted signal and received echo is used to determine the relative speed and direction of motion of the target. For this following formula is used -

V = 1.1 x fd x l

The above is true, but is also worth noting that Doppler radar offer's the forth degree of freedom in radar tracking as Range dot, or Doppler/Range-Rate. With more degrees of freedom then just Azimuth, Elevation & Range, tracking filtering can offer better results in say a Kalman Filter Tracking solution. Doppler profiles can be useful for distinction of distributions the difference between a man walking and a car driving etc (with man-portable radar like BLightER, or MSTAR). Doppler information can be used for SAR and ISAR imagery when the range Doppler matrix is collected at a high fidelity and interpolated correctly for a formation. It also should be noted that Doppler measurement is made via a Fourier transform rather then an statistical process, the statistical process relay own correlation of signal and de-correlation of noise where as the Fourier process is a reordering process and can provide a better signal to noise.