How is radar used to determind the speed of object?

Answer 1

Radar (RAdio Detection And Ranging) transmits high-frequency, short-wavelength radio waves at the moving object and receives waves reflected from the object. If the object is not moving, the waves bounce back at the same frequency they were transmitted at, and you can determine the distance by how long they take to return.

If the object is moving towards the detector, the frequency is higher because each cycle travels a slightly smaller distance, hence returns faster (the Doppler effect). If the object is moving away, the frequency is slower for the same reason. The difference is, at low speeds, proportional to the speed of the object. If the object approaches the speed of light, relativity messes up this simple relationship.

So, if the object moves towards you at 100 wavelengths of the radar wave per second, the received frequency will be 100Hz (or cycles per second) faster than the transmitted frequency. Since you are generating the radar wave at a known frequency, you know the wavelength also.

Continuous wave/doppler radar has a problem if the object is travelling obliquely to the receiver; the shift is not as pronounced as it would be on an object traveling directly toward or away from the receiver. This is where pulse radar comes into it's own. Bang out a narrow pulse then 'listen' for a return echo. The time it takes to return gives the distance. Because the antenna rotation speed is constant, if the object moves obliquely then the angular difference between returns can be used to determine the speed.

Answer 2

Radar uses the Doppler effect to determine the speed of an object by measuring the shift in the frequency of the radar signal reflected off the object. As the object moves towards or away from the radar, the frequency of the reflected signal changes in a predictable manner that allows the radar system to calculate the object's speed accurately.