How Radar Works

Published December 31st, 2019

How Radar Works 

By Curtis Sluder 

Basically, radar works by sending out a series of radio waves at a specific frequency then calculating the change in frequency when the waves return. Think of the waves as “wiggling” at a certain speed.  When these waves hit things, they bounce off and are scattered in different directions.  Some of the radio waves that hit an object are reflected back to the device that sent out the radio waves. These waves are the ones the radar unit is looking for.  The waves reflected back will “wiggle” at a slightly different frequency, and the computer processor in the radar device can then determine how fast an object is travelling based on how much faster or slower the “wiggle”, or frequency.  These calculations are possible because of the Doppler Effect. 

The Doppler effect is the change in frequency of radio waves when an object emitting radio waves is moving (or the item they are reflected off is moving.) As the waves are reflected back, the frequency is higher coming from objects getting closer, and lower when bounced back from objects getting further away.  Think of the change in pitch when a fire truck approaches and passes.  The pitch is high as it approaches, accurate when it is beside you, and lower when it passes and moves away.  The reason is, on approach, each sound wave reaches the hearer slightly faster than the last, because the vehicle transmitting the sound is getting closer.  As it moves away, each sound wave reaches the hearer slightly slower, because the transmitting vehicle is getting further away with each wave. Think of the waves as being pressed together when the siren approaches, and spread apart after it passes.

)))))))))))))))))   <– Direction of  travel     )  )  )  )  )  )  )  )  ) 


The computer processor in the radar device in a police car “talks” to the speedometer of the car.  This is how moving radar can be accurate. The computer accounts for the speed of the police car when determining how fast an object is travelling.  

Some Patrol cars are equipped with rear radar.  This means as you speed up and approach a patrol car, it can detect your speed as you approach. This is why its never a good idea to be the lead car in a group of speeders. The lead car will be clocked first.  (Sit back and let them get the ticket) 

Patrolmen and women are trained to visually detect the speed of vehicles as part of their radar training.  They must be able to accurately estimate the speed of vehicles without the use of other devices.  One way this is done is by using a length of a known distance, say the distance across a bridge, and time the vehicles mentally as they cross.  The faster they cross the bridge, the faster they are travelling.  Relying solely on visual estimates is fraught with problems, though.  Size, shape, color, speed, direction and other factors influence the estimate.  For example, think of how slowly an airplane seems to move when you are observing it land from a distance.  

 Mathematical calculations can be performed to determine how many feet per second the vehicle is traveling and convert that number to miles per hour.  For example, at 100 feet per second, a vehicle would be traveling 68.2 miles per hour.  At 150 feet per second, a vehicle is travelling 102.3 mph. At 50 feet per second, a vehicle is only travelling 34 mph.  To determine speed in MPH when you know the FPS, divide the FPS by 1.466.   Example:  90 fps / 1.466 = 61.39 mph 

One method of visually detecting speed used a device in the vehicle to calculate the speed of a target vehicle based on how long it takes it to go across a certain distance.  The officer hit a button when the target vehicle went under a bridge, for example, and then hit the button again at the next bridge.  The distance between the bridges was known and entered into the device, and it then calculates the speed.  This was called a VASCAR speed detection device, and despite the possibility of human error, was relatively reliable. VASCAR stands for “Visual Average Speed Computer And Recorder”. Of note, a 1991 study by the National Highway Traffic Safety Administration found that VASCAR-plus units produced errors of less than 2 mph if used correctly.  Obviously, a radar/laser detector would be useless against VASCAR, and some states and localities have outlawed the use of radar. 

These VASCAR devices can also be tied to the speed of the patrol vehicle, making this an accurate testing device when the patrol car is moving as well. This also enables more accurate measurement of the distance between two points.  

I will do a future article on VASCAR, but for now, lets get back to the radar. Once an officer has made a visual estimate, the radar device is then used to confirm that estimate- In theory.  In reality, sound tells the officer a car is speeding. The radar device emits a tone as it senses speed–  Higher pitch means faster, lower pitch means slower. They may visually estimate the speed as well, but you cannot eliminate the human tendency to recognize the pitch of the tone.  As the officer gains experience, the ear also becomes trained to hear that specific pitch that indicates a speeder. 

After each stop, the police officer should perform a tuning fork test to determine that the radar sensing device is accurate.     This is just what it seems. A tuning fork, which is known to vibrate at a specific speed, is “detected” by the device to determine that it is properly calibrated.  

Are there ways to “beat” these tickets based on Radar?  Yes, but only in rare cases. In North Carolina, the officer must testify convincingly that you were “exceeding the posted speed limit”, or “speeding in excess of 80 mph”. The officer may not have to prove the specific speed. Even the best devices and the most trained officers still are going to be plus or minus 2 or 3 mph in most cases.   This is what creates the ability to negotiate different possible outcomes for a skilled attorney.  Juries are unpredictable, and a Jury trial is expensive, so a known outcome that protects your insurance and driving record is far preferable to the possibility of losing your license when its left to the hands of a Jury or Judge

I hope you have enjoyed this short article and have found it informative.