In Doppler radar, the returned signal is commonly called an echo.
A radar transmitter sends radio waves toward a target. When those waves strike an object, some energy is reflected or scattered back toward the radar receiver. This returning energy is called an echo, much as a sound reflection is called an echo. The radar compares the transmitted signal with the received echo to determine a target’s range, direction, and often speed.
Doppler radar focuses especially on frequency change. If the target moves toward the radar, the reflected waves arrive at a higher frequency; if it moves away, they arrive at a lower frequency. That shift lets the system estimate the target’s radial velocity—the component of motion toward or away from the antenna.
A common mix-up is confusing the echo with the transmitted carrier, beam, or pulse. The carrier is the radio-frequency wave used to convey the signal, the beam describes its directional spread, and a pulse is a brief transmission. The echo is specifically the returned reflection. Weather radar uses echoes from raindrops, while police radar uses echoes from vehicles.