What is police radar and how does it work?
Radar stands for RAdio Detection And Ranging.
It works by transmitting radio waves that bounce off of different targets up ahead and then reflect back to the radar transmitter. The military uses radar to track fighter jets in the sky, weathermen use radar to track clouds in the atmosphere, newer cars use radar to alert you if there’s a vehicle in your blind spot, and police officers use radar to measure the speeds of nearby vehicles.
How does a radar gun calculate your speed?
To measure speed, radar relies on a principle called the “doppler shift.” If you’ve ever heard an emergency vehicle with sirens drive past you and the sound changes as they pass you (it’s higher pitched initially and then the pitch drops after they pass you), you’ve experienced the doppler effect. The sound waves are compressed when the vehicle is driving towards you and then get stretched as they drive past you.
Police radar relies on the same principle. It shoots radar forward, and the radar waves get compressed (if the target vehicle is getting closer) or stretched out (if the target is moving away), and the radar gun measures this change in frequency and converts it to a speed.
If you’d like to see the math behind doppler shift to speed calculations, watch my video explaining how police radar works. Otherwise you’ve got the general idea so keep reading. 🙂
Can radar track multiple vehicles?
Now police radar guns aren’t limited to tracking one vehicle at a time. They can actually track and display the speeds of multiple vehicles simultaneously.
- They can track vehicles moving towards the radar gun or away from it.
- They can track vehicles ahead of the police officer (using the radar gun’s front antenna) or behind the police officer (using the radar gun’s rear antenna).
- They can track two vehicles moving together in the same direction, but at different speeds, one slower and a second faster.
Older radar guns could only track the strongest radar signal, typically the closest and/or largest vehicle that returns the strongest reflection. What if there’s a smaller target that’s farther away and presents a weaker return signal? Well most new radar guns have what’s called “fastest mode” capabilities where they can track both the strongest and the fastest signals simultaneously.
Take a look at the photo below. You’ll see the radar gun not only tracking vehicles in both directions, but you’ll also see it tracking faster vehicles (red arrows) farther away.
If you’d like to learn a little bit more about the science and math behind fastest mode operation, watch my video on how fastest mode radar works.
What are X band, K band, & Ka band?
If you’ve ever looked at radar detectors, you’ve probably noticed that they advertise being able to detect X band, K band, and Ka band radar, but what does that mean?
In short, police radar guns are typically designed to operate within several frequency ranges. You’ve got X band (~10.5 GHz), K band (~24 GHz) and Ka band (~33-36 GHz). Older radar guns used lower frequency antennas like X band, but as technology improved and things got smaller, radar gun manufacturers started switching to K band and then to Ka band, primarily because they allow for smaller antennas which takes up less room within a police officer’s already crowded vehicle.
What do those bands mean on my radar detector?
Ka band is the most common band of radar in use around the US today. Fortunately there’s not many other devices around us that also use Ka band so when your radar detector alerts to Ka, it’s almost always a police officer.
K band is regularly used around the country too. The annoying thing is that there’s lots of other things that use K band too including speed signs, automatic door openers, nearby vehicles with radar-based collision avoidance systems, and so on, so you’ll want a modern radar detector that’s good at filtering out the many new false (aka non-police) alerts you’ll see on K band. Learn about the best radar detectors here.
X band has been mostly phased out of the US. You’ll see it in use actively in OH, NJ, and in a few remaining rural areas around the country, but in most of the country you can safely just disable X band and get rid of all the false alerts from door openers that you’ll see. Awesome.
How accurate are police radar guns?
Because police radar guns are designed to be used to issue speeding tickets and the speeds they display need to be correct, they need to be accurate, within a certain tolerance. Radar guns are generally accurate within +/- 1 mph or +/- 2 kph. So they won’t always be exactly spot on, but often will be or at worst, extremely close.
Take a look at the photo below of two different radar guns being used simultaneously to track vehicles up ahead. You’ll notice both radar guns are displaying two speeds, the strongest (closest) target speed is on the left and the fastest target speed is on the right. You’ll notice that the strongest speeds don’t match exactly while the fastest speeds do. This may seem weird, but because radar guns only need to be accurate to +/- 1 mph, this is completely normal and okay.
How do police officers verify radar gun accuracy?
In order to verify the accuracy of the radar guns, police officers generally need to test their radar guns at the beginning and ending of every shift, though this can vary from place to place. Traditionally they do what’s called a tuning fork test where the officer strikes a tuning fork that vibrates at a known frequency, puts it in front of the radar gun antenna, and confirms that the radar gun displays on screen the speed that correlates with the tuning fork’s frequency.
Police officers typically need to have their radar guns calibrated periodically as well, say every year or two, depending on local regulations. When being calibrated, the guns are all tested to ensure they’re working properly. Sometimes police departments forget to have their radar guns calibrated on time and so a common way to fight a speeding ticket in court is to ask see the records of when the guns were last calibrated and see if it has been done recently. Consult local laws or ask a lawyer to find out how often police need to do it in your area.
So long as the radar gun is functioning properly, the speed displayed on the radar gun will be correct, with an accuracy between just +/- 1 mph or +/- 2 kph.
What are the different radar gun modes?
Police radar guns have several different modes of operation. They can either be always transmitting (constant on) or they can be temporarily turned off and on in a specific way when the officer sees a vehicle they want to target. Usually this is done intentionally to defeat radar detector by preventing drivers up ahead from getting advanced warning. Usually this isn’t necessary because so few drivers run radar detectors so running constant on is like shooting fish in a barrel, but some places do use these techniques. Let’s take a look at the different radar gun modes of operation and how they work.
“Constant on” (C/O) is the normal way of using radar. The officer simply leaves it on, transmitting continuously. In this mode, they can easily clock everyone driving by without having to mess around with their radar gun. They can sit back on the side of the road or transmit while driving and when a big number pops up on the screen, winner winner chicken dinner. Time to make a stop.
In an effort to thwart radar detector users, decades ago radar gun manufacturers added the ability to put the gun into “hold” meaning that the gun is turned on, but not transmitting. When it’s in hold, the radar gun isn’t able to display speeds, but radar detector users will no longer get advanced warning the way they would against constant on. When the officer sees a car up ahead that they want to measure the speed of, they take their radar gun out of hold and tell it to transmit, a technique known as “instant on” or I/O.
The trick to countering instant on is to to have 1) a very sensitive radar detector and 2) a rabbit up ahead of you. The idea is that a sensitive radar detector will pick up I/O as it’s being used on vehicles up ahead of you and so you’ll rely on traffic up ahead (such as a rabbit.. a driver up ahead that’s drawing fire while you sit back a little ways behind him) to alert you to the presence of I/O before you get into the danger zone.
Radar detector manufacturers sometimes advertise that their radar detectors can detect “the latest instant on guns,” but that’s just marketing mumbo jumbo. Every radar gun for decades has been able to do this. A radar detector manufacturer advertising this is the equivalent a TV manufacturer making a big deal about the fact that they have the latest and greatest new feature called a “mute button.” Wow… 😀
Every radar detector on the planet can detect instant on. When there’s radar present, your detector will alert. No big deal. 😉
Quick trigger (QT) is a variant of instant on. The idea is that a police officer will fire an instant on shot, but only long enough for a speed to pop up on their radar gun and then they put the gun back into hold. The goal is to get a target vehicle’s speed in a way that doesn’t alert the radar detector at all because it’s too fast for the radar detector to register.
Some radar guns like the Stalker DSR 2X and Decatur Genesis II even have built-in QT capabilities where they can fire 0.5 sec shots automatically to simplify the process for police officers wanting to avoid triggering radar detectors up ahead.
This worked back in the day when radar detectors were slower, but now that detectors are much faster at scanning for all the different possible frequencies, thanks to the advent of digital detectors with DSP or analog detectors with band segmentation that allow you to tell it to not waste time scanning unnecessary frequencies (band segmentation explained), detectors are now very fast at alerting to brief bursts of radar and so there’s no issue detecting QT when using a modern radar detector.
Speaking of automated QT, MPH Industries developed a unique feature for some of their radar guns called POP radar. The idea is that instead of shooting 0.5s shots which are detectable, they’d shoot significantly faster shots of radar at only 67ms (0.067s) or even 16ms (0.0016s). This is way too fast for any radar detector to alert to. However, there are a number of technical limitations with this.
You see, by turning a radar gun on and off this quickly, you don’t give the gunn oscillator inside the antenna (the part that’s actually generating the radar signal) enough time to stabilize and transmit a consistent frequency. Without a stable signal frequency, you’ll have a difference between what the radar gun thinks it’s transmitting and what it’s actually transmitting, and so the radar gun will display incorrect speeds. Whoops! The farther away the target vehicle, the more likely the errors will be and the bigger the errors will be.
Because of this, and because a police officer needs to get a tracking history too when using the radar gun (seeing the car driving at speed for a period of time, confirmation with the sound the radar makes called the audio doppler, and so on), MPH says that POP is intended to be used only to preview the speed of a vehicle ahead and when the officer wants to issue a citation, they should switch back to using normal radar.
In practice, POP is almost never ever used. It’s not accurate and it’s not legal to issue tickets on its own. The officer needs to press a few buttons to go in and enable POP which makes it extra work for him to use. There are lots of problems with POP it’s recommended to disable POP detection on your radar detector so that your detector isn’t unnecessarily hyper-focused on detecting POP radar that isn’t used in the first place. By doing so, you’ll have better performance against traditional radar, plus you’ll avoid all the false POP alerts that your detector will otherwise give you. (Just because your detector alerts to POP radar when POP is enabled does not mean that POP is actually being used. POP falses are very common.)
So just disable POP…
How quickly can a police officer get my speed?
Speaking of radar guns operating quickly, what if it’s used in the way it’s supposed to be, such as if an officer is using instant on? By the time your radar detector goes off, are you able to brake in time to avoid him getting your speed?
Well radar guns typically can lock in a speed in about 0.3 sec. By the time your radar detector alerts, your brain processes what’s happening, your foot moves over to the brake, and your car starts losing momentum, your speed is already going to be locked into the police officer’s radar gun.
If you’d like to see this, let’s take a look at a best case scenarios. Here’s a Stalker DSR 2X radar gun shooting I/O. The radar detector user is even expecting the shot and already has their foot on the brakes. When their radar detector goes off, they immediately slam on the brakes. As you’ll see, you can’t outbrake I/O…
This is why it’s so important to have a rabbit up ahead and a sensitive radar detector (avoid the cheapy $50-100 detectors, for example) so that you can pick up I/O being used up ahead.
Sure it’s possible to avoid a ticket against I/O if you get lucky and the officer had his radar gun in the wrong mode (clocking the wrong direction of traffic), there’s no convenient turn-around spots for him, and so on, so sure you could avoid a ticket even if you get I/Owned, but in practice, there’s no way you’re going to outbrake the radar gun, even with carbon ceramic brakes on dry pavement. 😉
How far away can a radar gun get your speed?
What about distance? How far away can a police officer get your speed?
It really depends on the radar gun as well as terrain, but in ideal conditions, radar guns can get your speed a mile away or more. Larger vehicles like big semis present an even larger reflection and so they can be clocked farther away. Smaller vehicles like cars or even motorcycles present smaller reflections so they’ll need to be closer. As a general rule, if the radar gun has a clear line of sight view of the target, they’ll be able to get its speed.
The things that will prevent a radar gun from being able to lock in a speed are things like hills, curves, buildings, or trees. Radar guns do need direct line of sight to work (as opposed to radar detectors that can see around bends thanks to reflections) so anything in the way of the radar gun’s signal, including traffic and other vehicles, can prevent the radar gun from acquiring a target at a distance.
Dashmount vs. handheld vs. fixed radar?
Radar guns come in several different form factors.
Dashmount radar guns are very common. They typically include several components including the counting unit (the brains of the radar gun, plus the display that is typically attached to it), one or two antennas (for front or front and rear operation), and a handheld remote.
Dashmount units are common inside of police cruisers because of how easy they are to operate while driving. They also make motorcycle versions with waterproof components.
Handheld radar guns also exist. These are more mobile, they can easily be aimed in different directions such as out the side window, and they’re also common for motorcycles because the officer can quickly and easily pull over and start clocking vehicles without worrying which direction their bike is pointed.
Some handheld radar guns even have removable handles so that you can slide them on to an optional dashmount and get both dashmount and handheld operation out of one radar gun.
There are also fixed photo radar systems which operate in tandem with a camera. The idea is when a speeding vehicle drives past, the camera takes a photo with the license plate and they mail the driver the speeding ticket in the mail.
Photo radar is less common in the US, but it is very common overseas and is starting to show up more and more here Stateside as well.
How do radar detectors help you avoid tickets?
Radar detectors are THE tool to help you avoid speeding tickets from radar guns.
The general idea with radar detectors it that they’re designed to alert you to the presence of police radar up ahead, before you’re close enough to where the radar gun can acquire your speed. Radar detectors can detect police radar much farther away than radar guns can clock traffic up ahead, so the odds are in your favor here. Radar detectors are designed to give you plenty of advanced warning so that you can double-check your speed accordingly and avoid a speeding ticket.
There’s a lot of great radar detectors out there and there’s also a lot of lousy ones. If you’d like help picking out a radar detector, take a look at my Radar Detector Buyer’s Guide.
What about police radar jammers?
What about instead of detecting radar, we wanted to jam the police officer’s radar gun and prevent him from getting our speed in the first place?
Well it turns out that radar jammers are highly illegal. We’re talking $50,000-$100,000 fines plus jail time. It’s a big deal and not something you want to mess around with.
Not only that, but radar jamming is technically challenging as well. It used to be doable with some of the older analog X and K band radar guns, but it’s much more difficult against the newer digital Ka band radar guns. There were a few companies that used to sell radar jammers years ago, but when radar guns became more sophisticated and the FCC started shutting down radar jammer companies, no company has bothered attempting to develop or sell radar guns anymore… for obvious reasons. 😉
The one “sort of” exception is Rocky Mountain Radar who advertises a jamming-like feature they call “scrambling.” It’s supposedly a passive form of jamming that wouldn’t violate the laws regarding active jammers, but the fact is that it doesn’t work at all. I’ve tested the two most recent RMR detectors against a variety of different police radar guns and they had zero impact on them. Read my review of the Rocky Mountain Radar Judge for more info. Rocky Mountain Radar are basically scam artists preying on the innocent.
When it comes to radar jamming, there are no radar jammers sold that are effective. For more information, read my complete article about police radar jammers.
What about police laser?
In addition to radar, police commonly use laser guns as well. These are completely different than radar guns. They function differently from the police officer’s standpoint, they have a number of advantages and limitations compared to radar, and they require completely different tools to combat as a driver.
Radar detectors provide little to no protection against laser. Because a laser beam is very pinpoint and doesn’t spray everywhere like a radar gun, a radar detector user will generally receive no advanced warning against laser and by the time the officer shoots your car, because the beam is so narrow and thin, if your radar detector with its built-in laser detector goes off at all (it often doesn’t, even when directly targeted), the officer already has your speed.
To combat laser, you need a good laser jammer. For more information about laser, read my review covering the best laser jammers.
To learn all about laser, read my follow-up article explaining how police laser works.
How to avoid speeding tickets?
So now that we know all about police radar guns, how do we avoid getting tickets from them? Here’s several basic, but key suggestions:
- Get a good radar detector
- Set up your radar detector to optimize both its performance and its false filtering capabilities
- Be wary of officers using instant on, especially hiding around a bend or beyond the crest of a hill
- Use a rabbit up ahead to draw fire
- Run Waze on your phone to alert you to police up ahead
By understanding a little bit about how radar guns work, you’ll have a better idea of how to protect yourself from them. 🙂
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