On a few recent road trips, I got to really test out a handy feature in my new car: radar sensing. Many newer cars are available with these features whether they be collision avoidance radar, backup traffic monitoring radar, cruise control radar or blind spot radar. It’s incredible to me that this relatively sophisticated technology is becoming available in even cheaper cars. It made me think that it would be interesting to discuss what radar is, how it works, and the different applications. Many of the fundamentals I’ve discussed in other articles, specifically my article about electromagnetic radiation, will be good to know about before we get into this. So if you haven’t read that article, give it a read first and then come back.
Ok, are you refreshed? Good! Radar is one of those words like laser, modem or scuba. Radar was originally an acronym for RAdio Direction And Ranging. The original name should give a hint about how it works. Electromagnetic radiation in the microwave frequency range doesn’t totally penetrate solid objects. This means that when the microwaves hit something they bounce off of it. Depending on the nature of the material involved, some microwaves might be absorbed by the object. In a radar system, there is a transmitter and a receiver. The transmitter sends out microwaves and the receiver receives the reflections back to interpret them. Some properties, like simply knowing where an object is, can be determined by how long the transmitted waves take to return to the receiver. The strength and makeup of the received waves can also help infer the material of the object as well as its size. If the radar is set up to detect frequency shifts, it can use that to determine rotation or relative motion of the object. This kind of radar would be called doppler radar (after the Doppler effect) and is often used for detecting rain motion in meteorology.
Many of the radars that are added onto cars are able to determine relative motion. This means that they can see an object and not only know it’s position in that moment, but also it’s relative velocity (relative to your car). For example, the blind spot radar on my car doesn’t trigger if I pass a car, but it does trigger when a car passes me. The radar is able to distinguish between these two motions. Forward sensing radars are able to detect possible collisions in the same way. This is also how police track your speed by radar. You can notice this effect with sound. When a police siren is approaching you, it sounds higher pitched, and when it passes you, it sounds lower pitched. The reason for this is the Doppler effect.
When a wave (sound or electromagnetic) is emitted from an object in motion, stationary receiving points along the object’s path will see the waves either compressed or elongated depending on whether the moving object is approaching or departing. When the waves compress, this places more peaks and troughs in the same area. More waves in the same size area or shrinking area (shrinking because the distance between the source and you is shrinking) means higher frequency. When the waves are electromagnetic, this shift in frequency can be determined with high precision to determine how fast the object is moving.
Because radar is an electromagnetic wave in the microwave spectrum it can be “jammed” relatively easily. By receiving the microwaves, modifying them, and then retransmitting them back to the source, the radar system may not be able to detect the object properly. Radar is also easily detected because it’s an electromagnetic wave. For these reasons, police often also use LIDAR. Which is the same system except it uses visible or invisible light (“LIDAR” is actually a portmanteau of light and radar) to do the same thing. Remember that light is an electromagnetic wave itself, so all the same principles apply. LIDAR can also be jammed, but as it’s a newer technology than radar, it’s less common. LIDAR also depends on lasers and other light focusing technologies and so wasn’t developed until the 1960s.
You can also use ultrasonic sound to accomplish all of the same exact tasks, but sound is too easily affected by the environment. It’s also horribly slow compared to electromagnetic waves that travel at the speed of light. An ultrasonic rangefinder and speed detector would have to be calibrated every single time it was used. Changes in air density, pressure and temperature would all affect the sound waves. For this reason, we typically only use radar and LIDAR.
Obviously there are hundreds of uses beyond cars and speed tracking when it comes to radar and LIDAR. We use LIDAR for surveying, aerial mapping, and precise distance tracking of celestial objects like the moon. If all of this was interesting, next week I’m going to talk a little bit about lasers. I’ve always been fascinated by lasers and will never forget my parents getting me a little pocket one when I was just 10. So tune in next week!