The Doppler effect is the change in observed frequency caused by relative motion between a source and a listener. If the source moves toward the receiver, the frequency appears higher; if it moves away, it appears lower. In sound systems this is heard as a pitch shift, and in radio or radar it shows up as a measurable frequency offset.
In cyber security, the Doppler effect matters anywhere signals are used for sensing, tracking, or authentication. Wireless tools can use frequency shifts to detect movement, estimate speed, or distinguish genuine reflections from static clutter. Attackers may try to exploit motion-related ambiguity to confuse RF localization, evade tracking, or distort sensor readings. Defenders use the same physics in radar, sonar, and other monitoring systems to identify moving devices, drones, or vehicles and to separate real targets from background interference.