Question

1. In your own words, explain how Doppler radar works. Describe the properties of electromagnetic waves and interactions that make Doppler radar possible

Answer 1

Doppler radar is a device that makes use of the doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the original signal has been altered by the objects that reflected it. Variations in the frequency of the signal give direct and highly accurate measurements of a target's velocity relative to the radar source and the direction of the microwave beam. Doppler radars are used in aviation, sounding satellites, police speed guns, and radiology. The specific term "Doppler Radar", due in part to its extremely common use by television meteorologists in on-air weather reporting, has erroneously become popularly synonymous with the type of radar used in meteorology. Most modern weather radars use the pulse-doppler technique to examine the motion of precipitation, but it is only a part of the processing of their data. So, while these radars use a highly specialized form of doppler radar, the term is much broader in its meaning and its applications.

The Doppler effect, named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the waves. It is commonly heard when a vehicle sounding a siren approaches, passes and recedes from an observer. The received frequency is increased (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is decreased during the recession. Consider the following analogy: A pitcher throws one ball every second in a person's direction (a frequency of 1 ball per second). Assume that the balls travel at a constant velocity. If the pitcher is stationary, the man will catch one ball every second. However, if the pitcher is jogging towards the man, he will catch balls more frequently because the balls will be less spaced out (the frequency increases). The inverse is true if the pitcher is moving away from the man; he will catch balls less frequently due to the pitcher's backward motion (the frequency decreases). If the pitcher shoots microwaves instead of throwing balls, the analogy demonstrates that the frequency of the microwaves is affected by the movement of the pitcher; as he runs towards the man, the frequency of the microwaves the man receives increases, and as he runs away from the man the frequency of the microwaves the man receives decreases. Note that, from the point of view of the pitcher, the frequency remains constant (whether he's throwing balls or transmitting microwaves). Since with electromagnetic radiation like microwaves frequency is inversely proportional to wavelength, the wavelength of the waves is also affected. Thus, the relative difference in velocity between a source and an observer is what gives rise to the doppler effect.