RADAR is the acronym of Radio Detection and Ranging. Doppler radar was named after the Austrian Physicist J. Christian Doppler who articulated for the first time in 1842, why the whistle of an approaching train sounds higher than the whistle as the train moves away. This phenomenon is known as Doppler Effect or Doppler shift.
It is the difference between the emitted frequency of a wave and the observed frequency. The variation in frequency is dependent on the direction of the source of the sound wave is moving directly toward or away from the observer.
Doppler Effect
Doppler Effect is the apparent ‘shift’ in the frequency of the radio wave, caused due to the movement of a wave source. This frequency shifts upwards while the wave source is approaching and it moves down, while the wave source is retreating. The Doppler Effect, when analysed, explains the perception of change in the pitch of the sound.
There are four ways to produce Doppler effects, which includes:
- Coherent Pulsed (CP)
- Continuous Wave (CW)
- Pulse-Doppler
- Frequency Modulation
The CW radar used earlier sends continuous radio energy and is then received from any reflecting object. It provides only velocity output.
For the Pulse-Doppler radar, the returned signal is used to determine the velocity of the target object by combining with CW radars. The velocity is called Range-Rate. Later, the Frequency Modulated and CW were clubbed together (FM-CW) that sweeps the transmitted frequency for encoding and for determining the range. It has the limitation of processing only one target, normally.
Original Applicability
During the World War II, it was discovered that the weather was making echoes and was covering the potential enemy target. But, techniques were adopted to filter them. After the war, the radars were used for detecting precipitation, and the weather radar was evolved.
It has been used by national weather services, television newscast and research departments of universities. Raw images were used for making short-term weather forecast, about the future status of rain, hailstorm, snowfall and another weather phenomenon.
Doppler Radar
It combines the features of continuous-wave radars and pulse radars. It is a specialized radar that makes use of Doppler Effect for producing velocity data about distant objects. This is done by the bouncing of microwave signal of the desired target and then analysing, in what way, the motion of the object has changed the frequency of the returned signal.
This gives highly accurate measurements of the radial component of the velocity of the targeted object, relative to the radar. These radars are used in aviation, radiology, sounding satellites and meteorology.
Relationship with Weather Forecast
For weather forecasting, Doppler radar is used for measuring the direction, velocity and speed of the weather-related objects like drops of precipitation. The Doppler Effect is used to for determining if the movement in the atmosphere is horizontally away from or toward the radar.
Radar Operation for Weather
A Transmitter and a Receiver are the two major components of weather radar. The transmitter unit emits pulses of radio waves, known as microwaves, outward in a circular motion. Precipitation of the atmosphere scatters the microwaves and sends some energy back to the transmitter and is detected by the receiver of the radar.
The Next Generation Radar (NEXRAD) obtains information related to precipitation and wind, by the returned energy. When a burst of energy emitted by the radar strikes objects like raindrops, hail, snowflakes or bugs, the energy gets scattered directly in all directions and comes back toward the radar.
However, if the target which is heading at 90 degrees about the antennae beam, it cannot be detected. A Zero Doppler target heads tangentially to the radar antennae beam. If the target has no range-rate or velocity reflects a frequency near the transmitter. So it cannot be detected.
The weather radar, also known as Weather Surveillance Radar (WSR) can locate precipitation, compute its motion and can make an estimation of rain droplets, intensity of precipitation, direction and intensity of the wind, hail, snow, the structure of the storm and their potential for causing severe weather. Because of its strong relationship with the weather forecast, Doppler radar plays a major role in predicting the weather-threatening events. 😊