How do speed guns work?

timeToday’s world is a dynamic world. People always want to get somewhere. The sky is full of satellites, the sky is full of airplanes and rockets, the ocean is full of ships and submarines, and the land is full of cars, bicycles and trains. Humans have developed laws, rules, technology, and subsequently entire industries to ensure that all these vehicles move smoothly without harming humans or each other. A small but important part of this photo is the speed gun.

What is a speed gun?

A speed gun is a device that measures the speed of a moving object without making contact with the object. To achieve this, the device reflects electromagnetic radiation of specific frequencies off objects, captures the reflections and uses the Doppler effect to infer the object’s speed. Speed ​​guns are electronic and use complex circuits to emit the radiation used for measurement.

These devices are widely used by law enforcement officials to monitor traffic speeds, coaches to measure athlete performance, and other industries that require accurate movement tracking.

What is the Doppler effect?

The Doppler effect, named after Austrian physicist Christian Doppler, relies on the simple concept of relative velocity. Suppose a man is sitting in the middle of a field and whistling. Sound waves are centered on the whistle and spread outward in concentric circles, spreading evenly. A woman standing at the edge of a field regularly receives the waves as their crests reach her. (Because the speed of sound waves in air is 343 m/s, the human ear cannot hear the gap.)

Every wave has a frequency and a wavelength. The higher the frequency, the higher the pitch and vice versa.

Now, imagine that the whistler is moving around the field on an off-road vehicle. If the SUV were to drive toward the woman, waves would build up in front of the vehicle. In other words, from a woman’s perspective, the wave acquires the speed of a carriage in addition to the speed of a sound wave. Therefore, the waves will reach the woman more frequently and she will perceive a higher pitch. (For the same reason, the sound will be lower pitched in the direction behind the off-road vehicle.)

That’s why when a train comes into a station, people on the platform hear a higher-pitched horn than when the train leaves. This effect is the Doppler effect.

Speed ​​guns were originally developed for military use during World War II and use radio waves rather than sound waves to exert their effect. Speed ​​guns have a radio transmitter and a receiver. The transmitter emits radio waves that a person holding a speed gun can point at an object. The receiver collects waves reflected from objects back in the direction of the speed gun.

If an object approaches a speed gun, the frequency of the returning wave will be slightly higher than the frequency of the emitted wave. A simple computer in the gun can deduce the object’s velocity from this difference.

How are speed and effect linked?

All electromagnetic waves have a fixed speed – equal to the speed of light in that medium. In a vacuum, this value is expressed as c: 299,792,458 m/s. Any change in frequency detected by a speed gun directly corresponds to the Doppler shift caused by the motion of the object. This principle is powerful because it allows the speed gun to operate accurately over a wide range of distances and speeds, independent of air resistance.

A speed gun can calculate the speed of a moving object by multiplying the difference (between the receiving frequency and the transmitting frequency) by c and dividing it by the transmitting frequency times 2.

This relationship shows that the difference is proportional to the speed of the object: the faster the object moves, the more pronounced the difference. In other words, the only condition is that the object should move much slower than the speed of light – which is the case in most, if not all, practical applications of speed guns.

Are there any downsides to speed guns?

Today, the technology to emit radio waves is everywhere. The principle is simple: When an antenna is excited by alternating current at radio wave frequencies, it emits radio waves. Radio wave frequencies range from 30 Hz to 300 billion Hz.

For a long time, the equipment that created the waves was cumbersome. That changed when scientists invented the transistor in the 1940s. Electronic circuits built using transistors greatly simplified the process of generating radio waves and allowed transmitters to become smaller.

However, radio waves have inherent disadvantages that transmitters cannot fully adapt to. For example, radio waves diverge as they travel through the air. If the antenna is 5 cm long, the waves it emits will diverge 22° to both sides, resulting in an overall 44° wide beam.

Such a beam could hit more than one moving vehicle and produce inaccurate speed readings.

Continuous wave radar (which emits radio waves and continuously tracks their reflections) may also produce readings from multiple vehicles.

Engineers developed systems to compensate for these errors, but the final setup was more complex and expensive.

For these reasons, lidar speed guns have replaced radar speed guns. The name is an abbreviation for “Light Detection and Ranging”. LIDAR uses lasers instead of radio waves; the gun’s operation is otherwise similar. The divergence of the laser is very low, thus providing better aiming.

Amartya Srinivasan is a student of Class XI at PS High School, Mylapore, Chennai. Vasudevan Mukunth is deputy science editor of The Hindu.