Simulation of a RADAR speed gun; illustrates the measurement of the phase shift of radio waves reflected off a moving object.
To try this
simulation:
Use the slider to control the speed of the car. Observe the reflecting radio waves and their phase shift as they bounce back to the radar gun.
What's Going On?
RADAR Guns work on the principle of Doppler shift. The gun transmits a microwave beam at a very high frequency. When the beam hits an object, the beam is reflected back to the gun. If the object is standing still, the reflected beam has the same frequency as the transmitted beam.
If the object is moving towards the gun, the reflected beam has a higher frequency. The electronics in the gun compares the two beams and measures the difference between the two. This difference is called a phase shift. When something is approaching its phase shift is positive. If the object is moving away from the gun, the phase shift will be negative. In either case, the bigger the shift, the faster the object is moving. The circuit in the gun calculates the speed from the phase shift.
There's More!
Doppler Radar is also being used at airports to detect wind shear - a condition of air turbulence that is dangerous to airplanes when they are close to the ground. When wind shear occurs there are quickly moving currents of air of varying densities close to the runways. The Doppler radar is directed to the areas where shear is suspected. The moving currents of air reflect the radar and create phase shift depending upon the direction of the air movement. By knowing the rate of the phase shift, airport controllers can determine if the wind shear is occurring at dangerous levels and pilots can be warned.
