Pulse Radar

Weather radars send pulses of energy. There are two advantages to using pulses of energy rather than a continuous beam. The first advantage is that the same antenna may be used for both transmission and reception. This reduces the cost and complexity of the system, since elevation and azimuth of the target may be determined by merely pointing the antenna. The second advantage is the reduction in the power requirements of the system. If the transmitted power of a radar is 250,000 watts, imagine the energy sage to operate it continuously. The same radar in pulsed mode would require far less energy. Assuming radar pulses lasting 2 microseconds, transmitted 250 times per second, the average power required is just 125 watts, or about 1/10 that of an electric toaster.

Canadian radars are supposed to transmit radar pulses with a peak power of 250,000 watts and a pulse length of 2.0 microseconds; however some radars are operating with a peak power of 160,000 watts and a pulse length of 0.8 microseconds in order to extend the life of the pulse-generating magnetron.

Pulse Length

Weather radars broadcast a brief intense pulse of energy followed by a relatively long listening period during which the very weak signal reflected from targets is received and processed. The actual pulse transmission time for conventional weather radars is four microseconds or less, and the period between pulses is of the order of 1 millisecond.

Using "pulsed" radar allows many measurements to be obtained for each bin. In addition, averaging reduces measurement errors with noise. Random events will not occur in every pulse, but real signals will thus reducing the impact of random echoes.

Definition of a pulse length

The radar returns from precipitation are essentially varying rapidly in time and space. So how long should we illuminate a target with the radar beam to get a realistic estimate of reflectivity?

Using a rapidly pulsed radar to get several samples of reflectivity from each target reduces errors with respect to fluctuations in time and space and reduces the effects of background noise as random events will not occur in every pulse, but real signals will. The radar transmits a stream or "beam" of energy in discrete pulses which propagate away from the radar antenna at approximately the speed of light (~3x108m/s). The volume of each pulse of energy will determine how many targets are illuminated. This directly determines how much energy (power) is returned to the radar. The beamwidth of the radar antenna and the length of time the radar transmits determine the shape and volume of each radar pulse.