Which radio signal feature is crucial for determining speed in radar systems?

The Doppler effect is crucial for determining speed in radar systems because it describes the change in frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave. When radar waves are emitted towards a moving object, the frequency of the reflected waves changes based on the object's speed and its direction of movement relative to the radar device.

If the object is moving toward the radar source, the frequency of the reflected waves is higher than that of the emitted waves, while if it is moving away, the frequency is lower. This shift in frequency is what the radar system measures, allowing it to calculate the speed of the moving object accurately.

Amplitude modulation, waveform analysis, and frequency shift are all important in various contexts of signal processing and communication but do not play as direct a role in the speed measurement aspect as the Doppler effect does. The ability to measure the changes in frequency directly correlates with the motion of the target, making it the key element for speed determination in radar technology.