EODSarge
Well-known member
For what it's worth- here's my take on explaining how radar and lidar work. I'm not an electronics guru; while I have a ham radio license, I've only got a hobbyist's understanding of electronics and radio theory. I'm approaching this from the law enforcement angle as a more practical, rather than theoretical, explanation. I'm certified in my state as a general law enforcement instructor and hold specialty instructor certificates in, among other things, speed detection devices to include radar and lidar; and I've taught a fair percentage of the law enforcement in my corner of the state as well as certified others as instructors. This post is condensed, in fact, from my lesson plans for both. The first post will cover radar.
Some in law enforcement may become agitated at seeing another cop explaining to the public how speed detection works. Really, there's nothing here you can't get from other websites or NHTSA itself. There's no "top secrets" being given away.
RADAR
Is radar reliable and accurate? If by that you're asking "is that little black box really measuring vehicle speeds" then yes, it is; very accurate. As you'll find in this presentation, however, radar requires a trained operator to interpret what the radar is seeing. If the operator is poorly trained or just plain lazy, he will misinterpret what the radar is telling him.
Sometimes law enforcement is its own worst enemy. Every agency's got at least one crusty old veteran who's been looking at the world through cynical glasses for the last ten years. In this case, back in the early 80s, when radar was being used by more and more agencies, a Dade county newspaper sent a reporter to ride with one of these salty LEOs in the course of doing a story on radar and how it worked. The cop was, no doubt, not very happy at the idea of riding a reporter around all day and explaining radar to them. They had picked a spot on the side of the road to run radar and the cop flipped his unit on. It showed him a reading of 28 mph, even though there was nothing in front of them but a palm tree. Now, the cop knew exactly why this was- the radar unit was picking up any reflections it could, even very weak ones; in this case, the fan blades of his air conditioning. If a vehicle popped over the hill, the return signal from it would be very strong and the radar would ignore the weak fan blade signal and show the car's speed. But, when the reporter asked why the radar was showing 28 mph, the cop said "See that palm tree? It's doing 28 mph." Of course, the headlines the next day said "Radar clocks tree at 28 mph" and planted the seed in the minds of the public, that carries through to this day, that radar is prone to errors and unreliability.
RADAR is an acronym for RAdio Detection And Ranging. The principles behind radar were discovered in the 1900s and 1920s when researchers experimenting with radio transmissions across the Potomac River noticed that ships passing between the transmitter and receiver reflected the radio signals back at the transmitter.
This is because radio waves are electromagnetic waves- just like AM and FM radio signals, microwaves, light, and X-rays. Electromagnetic waves can be reflected off of objects; refracted through them- like sticking a pencil in an aquarium; the pencil appears broken because the water bends the light waves more than the air- or absorbed. A green t-shirt is green because the material of the shirt absorbs most of the colors of light but reflects the green light at the viewer. All electromagnetic waves share three properties that are connected by a simple equation.
1) Speed- all electromagnetic waves travel at the speed of light, or 186,000 miles per second.
2) Frequency- if we could see these waves traveling through the air, they would look like ocean waves:
If we counted the number of waves that passed us in one second, that would be the wave's frequency. Frequency is measured in cycles per second, or Hertz (Hz).
3) Wavelength- If we measured the distance from the peak of one wave to the peak of the next, or the start of a peak to the end of the valley, this distance would be the wavelength.
The formula that relates these three things together is frequency x wavelength = speed of light; or FxW=c. Since the speed of light is a constant, if the frequency goes up, the wavelength must get shorter, and vice versa. You can see in this illustration that as we go from 1 Hz to 2 Hz to 4 Hz, the distance between peaks gets shorter.
There are three bands used for police radar: X band, at 10.525 GHz (gigahertz, or billions of waves per second); K band, at 24.15 GHz; and Ka band, 33.4-36 GHz. X band requires a larger antenna and more power than K band, has a shorter range, and is more affected by rain attenuation. K band improves on X band, but is still a single frequency and easily detected- radar detectors only have to listen to one frequency and sound an alarm. Ka band is actually a range of frequencies, meaning that any detector would have to scan all of them. As technology improves, however, detectors that can do this get cheaper and simpler to build.
We also need to understand the Doppler Principle to understand radar. The Doppler Principle, described by Austrian physicist Johann Doppler, explains how relative motion between two objects causes a signal's frequency to change. Relative motion is simply the motion between two objects. If you're sitting in a train car moving at 50mph down the tracks, there's no relative motion between you and the passenger sitting in front of you. But there is motion between you and the telephone poles outside the train. If you've ever been sitting at a railroad crossing and heard a train sound its horn as it approached, you've heard the horn sound high pitched as it got closer and low pitched as it passed and receded. To the conductor, though, the horn sounds like it's the same pitch, because there's no relative motion between him and the horn.
Imagine several people spaced 2 feet apart in a single file line, all walking forward towards a brick wall. Each person represents the peak of a wave. As each person hits the wall, they immediately turn around and head back. The next person hits the wall, turns around and head back, etc. They're all still 2 feet apart on the return trip. Now, if the wall is moving towards them, the first person hits the wall and turns around, the wall moves forwards slightly, then the next person hits it and turns around. On the return trip, they are all now closer together. Their wavelength got smaller, which means their frequency got higher. If the wall is moving away from them, the first person hits the wall and turns around, the wall moves away, and now the next person has further to go before hitting the wall and turning around. The distance between them on the return trip is greater, meaning their wavelength got longer and their frequency lower.
Radar sends radio energy out at a certain frequency and listens for reflections. If those reflections come back at the same frequency, the radar knows that object isn't moving. If they come back at a higher frequency, it knows the object is headed towards the radar; and away from the radar if they return at a lower frequency. The radar measures speed by measuring just how much higher or lower that return signal is. For example, with K band radar, a shift of 72 Hz represents a speed of 1 mph.
The radar energy leaves the antenna in a cone, whose angle varies from 9 to 18 degrees depending on band. Because of this, it's impossible to focus the radar energy on a single car or lane of traffic. The antenna also "leaks" some energy to the sides, called "sidelobes", that are smaller and weaker than the main cone; which contains 80% of the signal's energy.
The radio wave that the radar sends out will travel forever, spreading out and gradually growing weaker, unless it is reflected, refracted, or absorbed by some object in it's path. Under normal conditions, the useful range is around 500 yards; although under certain conditions you can get readings at a mile away. And, everything in it's path will reflect some of the signal- trees, bushes, cars, birds, even the ground.
Another thing to understand is that radar only sees that portion of the signal that is directed straight back at the antenna. If the vehicle is headed straight for the radar, it will show us the vehicle's true speed. However, the greater the angle of the antenna to the vehicle, the lower the speed the radar will show. For example, if you're walking up a flight of stairs whose steps are one foot tall and one foot deep (making a 45 degree angle- just go with it), your net motion is diagonally up the stairs, but we can break that motion into a vertical part and a horizontal part. If you're zooming up the stairs at 50mph, you're also going 25mph up and 25mph horizontally. If the radar is placed so it's shooting horizontally, it will only see that motion coming straight at it, or horizontally; and will only display 25 mph. The same thing happens with a radar unit that's at an angle to the road- the car is doing 50mph, but only 25mph of that motion is straight at the radar.
This is called the cosine effect. In stationary radar, where the radar unit is sitting still, it will always be in the favor of the motorist; because the greater the angle, the less the read speed. It doesn't become an issue- more than tenths of a mile per hour- until the angle exceeds 10 degrees. So, as long as the radar is pointed straight down the road, and is no more than 10 feet off the road for every 100 feet down the road, the reading is good.
So, we've got the radar sending out a beam of energy at one frequency, and receiving reflections at varying frequencies from everything out there. How does the radar know which one to pay attention to? First off, it ignores anything that comes back at the same frequency, as these items aren't moving. That still leaves cars, birds, A/C fans, etc. The radar will show us the speed of the strongest signal it receives. If it's getting readings from a bird, a flapping flag, the A/C fan, and a car; the car will be the strongest signal and that's what the radar will show. Without a car, it might show us the fan blades- but the operator can clearly see that there's no car there. If there's several cars, the operator knows that the car that throws back the strongest signal is going to be the one that the radar will show him. So how does he know which one this is?
He knows that the size, shape, and material the car is made of will affect how well it reflects radar. A larger object will be more reflective; a flat surface will reflect better than a slanted one; and steel will reflect better than fiberglass. (Don't get your hopes up, Corvette owners; there's still a big lump of metal called the engine that reflects radar very well) Motorcycles are usually very curved, small in cross section, with small engines and have small radar signatures- but they're still detectable. If the operator sees a semi and a motorcycle coming towards him, he can be pretty sure that the speed he sees came from the semi.
The distances of the vehicles can have an effect, as well. A semi 1000 feet away, because it's so much larger, may reflect more than a small car 500 feet away. But, when the car is 250 feet away and the truck 750 feet, the car is being hit with more energy than the truck and might now reflect a stronger signal. The operator can see the speeds "jump" between the truck and the car and hear them in a speaker as a high-pitched doppler tone. The higher the pitch, the faster the speed.
If all else is equal, and the unit receives two equally strong signals, only then will speed be a factor; and the unit would show the faster of the two. The operator understands all this, and determines by looking at the traffic which vehicle the speed reading is coming from.
So, what about moving radar, where the vehicle in which the unit is mounted is moving as well? It's actually pretty simple- the unit will measure the closing speed between it and the target vehicle. If the target is moving at 50mph, and the patrol vehicle is moving at 50mph, the closing speed that the unit will see is 100mph. It uses a very simple equation- Target Speed = closing speed minus patrol speed, or TS=CS-PS. So how does the unit figure how fast it's doing, or the patrol speed? Remember that the unit is receiving reflections from everything out there, including the pavement. It looks at all these speeds and figures the lowest speed it sees will be the motion between it and a stationary object, and uses this as the patrol speed. So it knows it's going 50mph, and the closing speed is 100mph, and plugs it into the equation and gets a target speed of 50mph.
But wait... remember the cosine effect? What if the antenna is pointed at an angle to the road? Because of the cosine effect, it will read the road speed as too low. If you plug this artificially low speed into the equation, the target speed ends up being higher. For example, the patrol car is actually going 50mph, but because of the cosine effect, the unit thinks it's doing 30mph. The closing speed is still 100mph. 100-30=70, so the unit displays 70mph, when the car is actually going 50mph.
How can the operator avoid this? By using a tracking history. First, he makes a visual estimate of the target's speed without ever touching the radar. He's trained to do this within +/- 5mph. So he sees several cars coming at him, picks one that's speeding, and estimates its speed at 70mph. He presses the transmit button and the unit sends out a signal. If the strongest signal is, say, a semi doing 50mph, he will hear a 50mph doppler tone and see 50 in the target speed window. He knows that he's clocking something other than the speeding car. If, on the other hand, he gets a 70mph doppler tone and 70 displayed in the target window, he's got a good clock on the car he targeted. The doppler tone will be pure and clear, no static or distortion; this tells him he has a good signal with no interference. If he's running moving mode, he now checks the "Patrol Speed" window. If it matches his speedometer, he knows cosine effect is not a factor. Finally, remember those sidelobes we mentioned? When the vehicle passes his car and passes through the sidelobe, he will hear the doppler get scratchy and drop off and watch the vehicle's speed drop off in the target window. Then the radar will switch to the next strongest signal. In this way, he's positively identified the vehicle from which he got the speed reading. That visual estimate, by the way, is so important that it is the primary piece of evidence in a radar case- the radar reading itself is secondary and supportive. That means that even if the radar reading was thrown out for some reason, the driver could still be convicted on the officer's visual estimate. A tracking history must be obtained for every radar ticket.
Some radar units will clock speeds of vehicles traveling in the same direction. Instead of an oncoming closing speed, they are now measuring a separation speed (if the target vehicle is faster and pulling away) or a closing speed (if the target vehicle is slower and being gained on). If the target is faster, the operator hits "target faster". This tells the radar to use the formula TS=Patrol Speed + Separation Speed. If the car is pulling away at 10mph and the patrol vehicle is doing 60mph, 60+10=70. If the target is going slower than the patrol car, the operator hits "target slower" and the unit uses TS=PS-CS. If the patrol car is doing 80 and gaining on the target at 10mph, 80-10=70. However, if the operator presses the wrong button and uses the wrong formula, the target speed will be wrong. To prevent this, he adds another part to the tracking history- target speed discrimination test. He does this by either speeding up or slowing down and watching the target speed. If he's got the right formula, he will notice his patrol speed rise or fall, but the target speed will stay the same. If not, the target speed will rise or fall as well.
There are several factors that can affect radar readings. Note that the don't mean the radar is inaccurate- it's doing exactly what it was designed to do. And, so long as the operator has a good tracking history, these "errors" are not a factor.
Interference- can be electronic in nature (strong radio signals, airport radars, high tension power lines) or moving objects (A/C fan, signs wobbling in the wind, etc). The radar unit will see electronic interference and display "RFI" in the window, and will not provide a reading. Moving objects other than cars provide very, very weak signals and would only be observable in the absence of any traffic. In any case, if a speed were displayed, it would not match the operator's visual estimate and the doppler signal would be weak and scratchy.
Multi-path beam cancellation- This refers to "blind spots" where radio waves reflecting off of multiple flat surfaces- such as a tunnel or high-rise area- cancel each other out. In this case, the radar might display the speed of another vehicle briefly. The speed will not match the operator's visual estimate, and the effect is very brief and rarely occurs.
Panning effect- this occurs when a hand-held radar is rapidly swung back and forth, creating relative motion between it and another object. But... why would do this? Believe me, there's been many a boring 3am shift where I've tried this to try and get a speed reading off of a tree. I've never gotten it to work.
Scanning effect- this can happen when the antenna of a two-piece unit is pointed at its own counting unit. The microwave energy sets up interference in the counting unit and can cause a speed to display. However, the speed won't match the visual estimate and the doppler tone will be a horrid squeal. Avoid mounting antennas behind the counting unit, like on the cage.
Turn-on power surge- in the olden days, radars transmitted continuously whenever they were turned on. Older electronics took a few seconds to "warm up" when first switched on, and might display a false reading. As radar detectors became popular, operators of these older units would leave them off until they saw a car and then switch them on to transmit. No units today do this; they are constantly powered while in use, just not transmitting until the operator hits the transmit button, and are not subject to this power surge effect. This is commonly called "instant on" radar; but it should be noted that every radar in use today is an "instant on" radar; so the term is meaningless.
Mirror-switching effect- some handheld units would have a switch that would reverse the digits in the display so you could shoot over your shoulder and read the display in the rear-view mirror. Problem came when you forgot to switch it back and "18mph" was displayed as "81mph". However, I think just about anyone can tell the difference between a car going 18 and one going 81. Regardless, no unit sold today has this "feature".
Shadowing- this is one effect that is frequently seen, especially on multi-lane interstates with semi traffic. If the patrol car is going more than 20mph faster than a semi or other very large vehicle, and gaining on it, the radar may see this lower speed and think it's the patrol speed. This will throw the speed difference on the target vehicle. However, again, the speed won't match the visual estimate and the patrol speed won't match the speedometer. No valid tracking history means no ticket.
Batching- This can happen with older units that couldn't keep up with rapid changes in patrol speed. If the patrol car suddenly accelerated or braked, the unit couldn't keep up. However, again, visual estimate won't match and patrol speed won't match the speedometer; not to mention that units today are a lot faster and the only vehicle that could hope to induce batching is a motorcycle (other than a Hardley).
Jamming- Active jammers send out false "reflections", trying to fool the radar. They sometimes work, but are illegal. In any case, as an operator, you'll probably never run across one or realize it if you have. Even if you do pull someone over with a big box labeled "This Is An Active Jammer", it's an FCC violation, not a state law. You could try to push an obstruction charge, but good luck with your judge. Passive jammers don't work- period They are the modern day equivalent of snake oil. "Stealth" bras don't do much either- if it was that easy to "stealth" a vehicle from radar, the F117 wouldn't cost a billion dollars.
And that's the basics of radar. There are other legal considerations for officers, but they vary from state to state and the only ones I can talk about with any knowledge are Georgia's. Stay tuned for the next installment, Lidar, soon or when hell freezes over. *pours another margarita*
Some in law enforcement may become agitated at seeing another cop explaining to the public how speed detection works. Really, there's nothing here you can't get from other websites or NHTSA itself. There's no "top secrets" being given away.
RADAR
Is radar reliable and accurate? If by that you're asking "is that little black box really measuring vehicle speeds" then yes, it is; very accurate. As you'll find in this presentation, however, radar requires a trained operator to interpret what the radar is seeing. If the operator is poorly trained or just plain lazy, he will misinterpret what the radar is telling him.
Sometimes law enforcement is its own worst enemy. Every agency's got at least one crusty old veteran who's been looking at the world through cynical glasses for the last ten years. In this case, back in the early 80s, when radar was being used by more and more agencies, a Dade county newspaper sent a reporter to ride with one of these salty LEOs in the course of doing a story on radar and how it worked. The cop was, no doubt, not very happy at the idea of riding a reporter around all day and explaining radar to them. They had picked a spot on the side of the road to run radar and the cop flipped his unit on. It showed him a reading of 28 mph, even though there was nothing in front of them but a palm tree. Now, the cop knew exactly why this was- the radar unit was picking up any reflections it could, even very weak ones; in this case, the fan blades of his air conditioning. If a vehicle popped over the hill, the return signal from it would be very strong and the radar would ignore the weak fan blade signal and show the car's speed. But, when the reporter asked why the radar was showing 28 mph, the cop said "See that palm tree? It's doing 28 mph." Of course, the headlines the next day said "Radar clocks tree at 28 mph" and planted the seed in the minds of the public, that carries through to this day, that radar is prone to errors and unreliability.
RADAR is an acronym for RAdio Detection And Ranging. The principles behind radar were discovered in the 1900s and 1920s when researchers experimenting with radio transmissions across the Potomac River noticed that ships passing between the transmitter and receiver reflected the radio signals back at the transmitter.
This is because radio waves are electromagnetic waves- just like AM and FM radio signals, microwaves, light, and X-rays. Electromagnetic waves can be reflected off of objects; refracted through them- like sticking a pencil in an aquarium; the pencil appears broken because the water bends the light waves more than the air- or absorbed. A green t-shirt is green because the material of the shirt absorbs most of the colors of light but reflects the green light at the viewer. All electromagnetic waves share three properties that are connected by a simple equation.
1) Speed- all electromagnetic waves travel at the speed of light, or 186,000 miles per second.
2) Frequency- if we could see these waves traveling through the air, they would look like ocean waves:
If we counted the number of waves that passed us in one second, that would be the wave's frequency. Frequency is measured in cycles per second, or Hertz (Hz).
3) Wavelength- If we measured the distance from the peak of one wave to the peak of the next, or the start of a peak to the end of the valley, this distance would be the wavelength.
The formula that relates these three things together is frequency x wavelength = speed of light; or FxW=c. Since the speed of light is a constant, if the frequency goes up, the wavelength must get shorter, and vice versa. You can see in this illustration that as we go from 1 Hz to 2 Hz to 4 Hz, the distance between peaks gets shorter.
There are three bands used for police radar: X band, at 10.525 GHz (gigahertz, or billions of waves per second); K band, at 24.15 GHz; and Ka band, 33.4-36 GHz. X band requires a larger antenna and more power than K band, has a shorter range, and is more affected by rain attenuation. K band improves on X band, but is still a single frequency and easily detected- radar detectors only have to listen to one frequency and sound an alarm. Ka band is actually a range of frequencies, meaning that any detector would have to scan all of them. As technology improves, however, detectors that can do this get cheaper and simpler to build.
We also need to understand the Doppler Principle to understand radar. The Doppler Principle, described by Austrian physicist Johann Doppler, explains how relative motion between two objects causes a signal's frequency to change. Relative motion is simply the motion between two objects. If you're sitting in a train car moving at 50mph down the tracks, there's no relative motion between you and the passenger sitting in front of you. But there is motion between you and the telephone poles outside the train. If you've ever been sitting at a railroad crossing and heard a train sound its horn as it approached, you've heard the horn sound high pitched as it got closer and low pitched as it passed and receded. To the conductor, though, the horn sounds like it's the same pitch, because there's no relative motion between him and the horn.
Imagine several people spaced 2 feet apart in a single file line, all walking forward towards a brick wall. Each person represents the peak of a wave. As each person hits the wall, they immediately turn around and head back. The next person hits the wall, turns around and head back, etc. They're all still 2 feet apart on the return trip. Now, if the wall is moving towards them, the first person hits the wall and turns around, the wall moves forwards slightly, then the next person hits it and turns around. On the return trip, they are all now closer together. Their wavelength got smaller, which means their frequency got higher. If the wall is moving away from them, the first person hits the wall and turns around, the wall moves away, and now the next person has further to go before hitting the wall and turning around. The distance between them on the return trip is greater, meaning their wavelength got longer and their frequency lower.
Radar sends radio energy out at a certain frequency and listens for reflections. If those reflections come back at the same frequency, the radar knows that object isn't moving. If they come back at a higher frequency, it knows the object is headed towards the radar; and away from the radar if they return at a lower frequency. The radar measures speed by measuring just how much higher or lower that return signal is. For example, with K band radar, a shift of 72 Hz represents a speed of 1 mph.
The radar energy leaves the antenna in a cone, whose angle varies from 9 to 18 degrees depending on band. Because of this, it's impossible to focus the radar energy on a single car or lane of traffic. The antenna also "leaks" some energy to the sides, called "sidelobes", that are smaller and weaker than the main cone; which contains 80% of the signal's energy.
The radio wave that the radar sends out will travel forever, spreading out and gradually growing weaker, unless it is reflected, refracted, or absorbed by some object in it's path. Under normal conditions, the useful range is around 500 yards; although under certain conditions you can get readings at a mile away. And, everything in it's path will reflect some of the signal- trees, bushes, cars, birds, even the ground.
Another thing to understand is that radar only sees that portion of the signal that is directed straight back at the antenna. If the vehicle is headed straight for the radar, it will show us the vehicle's true speed. However, the greater the angle of the antenna to the vehicle, the lower the speed the radar will show. For example, if you're walking up a flight of stairs whose steps are one foot tall and one foot deep (making a 45 degree angle- just go with it), your net motion is diagonally up the stairs, but we can break that motion into a vertical part and a horizontal part. If you're zooming up the stairs at 50mph, you're also going 25mph up and 25mph horizontally. If the radar is placed so it's shooting horizontally, it will only see that motion coming straight at it, or horizontally; and will only display 25 mph. The same thing happens with a radar unit that's at an angle to the road- the car is doing 50mph, but only 25mph of that motion is straight at the radar.
This is called the cosine effect. In stationary radar, where the radar unit is sitting still, it will always be in the favor of the motorist; because the greater the angle, the less the read speed. It doesn't become an issue- more than tenths of a mile per hour- until the angle exceeds 10 degrees. So, as long as the radar is pointed straight down the road, and is no more than 10 feet off the road for every 100 feet down the road, the reading is good.
So, we've got the radar sending out a beam of energy at one frequency, and receiving reflections at varying frequencies from everything out there. How does the radar know which one to pay attention to? First off, it ignores anything that comes back at the same frequency, as these items aren't moving. That still leaves cars, birds, A/C fans, etc. The radar will show us the speed of the strongest signal it receives. If it's getting readings from a bird, a flapping flag, the A/C fan, and a car; the car will be the strongest signal and that's what the radar will show. Without a car, it might show us the fan blades- but the operator can clearly see that there's no car there. If there's several cars, the operator knows that the car that throws back the strongest signal is going to be the one that the radar will show him. So how does he know which one this is?
He knows that the size, shape, and material the car is made of will affect how well it reflects radar. A larger object will be more reflective; a flat surface will reflect better than a slanted one; and steel will reflect better than fiberglass. (Don't get your hopes up, Corvette owners; there's still a big lump of metal called the engine that reflects radar very well) Motorcycles are usually very curved, small in cross section, with small engines and have small radar signatures- but they're still detectable. If the operator sees a semi and a motorcycle coming towards him, he can be pretty sure that the speed he sees came from the semi.
The distances of the vehicles can have an effect, as well. A semi 1000 feet away, because it's so much larger, may reflect more than a small car 500 feet away. But, when the car is 250 feet away and the truck 750 feet, the car is being hit with more energy than the truck and might now reflect a stronger signal. The operator can see the speeds "jump" between the truck and the car and hear them in a speaker as a high-pitched doppler tone. The higher the pitch, the faster the speed.
If all else is equal, and the unit receives two equally strong signals, only then will speed be a factor; and the unit would show the faster of the two. The operator understands all this, and determines by looking at the traffic which vehicle the speed reading is coming from.
So, what about moving radar, where the vehicle in which the unit is mounted is moving as well? It's actually pretty simple- the unit will measure the closing speed between it and the target vehicle. If the target is moving at 50mph, and the patrol vehicle is moving at 50mph, the closing speed that the unit will see is 100mph. It uses a very simple equation- Target Speed = closing speed minus patrol speed, or TS=CS-PS. So how does the unit figure how fast it's doing, or the patrol speed? Remember that the unit is receiving reflections from everything out there, including the pavement. It looks at all these speeds and figures the lowest speed it sees will be the motion between it and a stationary object, and uses this as the patrol speed. So it knows it's going 50mph, and the closing speed is 100mph, and plugs it into the equation and gets a target speed of 50mph.
But wait... remember the cosine effect? What if the antenna is pointed at an angle to the road? Because of the cosine effect, it will read the road speed as too low. If you plug this artificially low speed into the equation, the target speed ends up being higher. For example, the patrol car is actually going 50mph, but because of the cosine effect, the unit thinks it's doing 30mph. The closing speed is still 100mph. 100-30=70, so the unit displays 70mph, when the car is actually going 50mph.
How can the operator avoid this? By using a tracking history. First, he makes a visual estimate of the target's speed without ever touching the radar. He's trained to do this within +/- 5mph. So he sees several cars coming at him, picks one that's speeding, and estimates its speed at 70mph. He presses the transmit button and the unit sends out a signal. If the strongest signal is, say, a semi doing 50mph, he will hear a 50mph doppler tone and see 50 in the target speed window. He knows that he's clocking something other than the speeding car. If, on the other hand, he gets a 70mph doppler tone and 70 displayed in the target window, he's got a good clock on the car he targeted. The doppler tone will be pure and clear, no static or distortion; this tells him he has a good signal with no interference. If he's running moving mode, he now checks the "Patrol Speed" window. If it matches his speedometer, he knows cosine effect is not a factor. Finally, remember those sidelobes we mentioned? When the vehicle passes his car and passes through the sidelobe, he will hear the doppler get scratchy and drop off and watch the vehicle's speed drop off in the target window. Then the radar will switch to the next strongest signal. In this way, he's positively identified the vehicle from which he got the speed reading. That visual estimate, by the way, is so important that it is the primary piece of evidence in a radar case- the radar reading itself is secondary and supportive. That means that even if the radar reading was thrown out for some reason, the driver could still be convicted on the officer's visual estimate. A tracking history must be obtained for every radar ticket.
Some radar units will clock speeds of vehicles traveling in the same direction. Instead of an oncoming closing speed, they are now measuring a separation speed (if the target vehicle is faster and pulling away) or a closing speed (if the target vehicle is slower and being gained on). If the target is faster, the operator hits "target faster". This tells the radar to use the formula TS=Patrol Speed + Separation Speed. If the car is pulling away at 10mph and the patrol vehicle is doing 60mph, 60+10=70. If the target is going slower than the patrol car, the operator hits "target slower" and the unit uses TS=PS-CS. If the patrol car is doing 80 and gaining on the target at 10mph, 80-10=70. However, if the operator presses the wrong button and uses the wrong formula, the target speed will be wrong. To prevent this, he adds another part to the tracking history- target speed discrimination test. He does this by either speeding up or slowing down and watching the target speed. If he's got the right formula, he will notice his patrol speed rise or fall, but the target speed will stay the same. If not, the target speed will rise or fall as well.
There are several factors that can affect radar readings. Note that the don't mean the radar is inaccurate- it's doing exactly what it was designed to do. And, so long as the operator has a good tracking history, these "errors" are not a factor.
Interference- can be electronic in nature (strong radio signals, airport radars, high tension power lines) or moving objects (A/C fan, signs wobbling in the wind, etc). The radar unit will see electronic interference and display "RFI" in the window, and will not provide a reading. Moving objects other than cars provide very, very weak signals and would only be observable in the absence of any traffic. In any case, if a speed were displayed, it would not match the operator's visual estimate and the doppler signal would be weak and scratchy.
Multi-path beam cancellation- This refers to "blind spots" where radio waves reflecting off of multiple flat surfaces- such as a tunnel or high-rise area- cancel each other out. In this case, the radar might display the speed of another vehicle briefly. The speed will not match the operator's visual estimate, and the effect is very brief and rarely occurs.
Panning effect- this occurs when a hand-held radar is rapidly swung back and forth, creating relative motion between it and another object. But... why would do this? Believe me, there's been many a boring 3am shift where I've tried this to try and get a speed reading off of a tree. I've never gotten it to work.
Scanning effect- this can happen when the antenna of a two-piece unit is pointed at its own counting unit. The microwave energy sets up interference in the counting unit and can cause a speed to display. However, the speed won't match the visual estimate and the doppler tone will be a horrid squeal. Avoid mounting antennas behind the counting unit, like on the cage.
Turn-on power surge- in the olden days, radars transmitted continuously whenever they were turned on. Older electronics took a few seconds to "warm up" when first switched on, and might display a false reading. As radar detectors became popular, operators of these older units would leave them off until they saw a car and then switch them on to transmit. No units today do this; they are constantly powered while in use, just not transmitting until the operator hits the transmit button, and are not subject to this power surge effect. This is commonly called "instant on" radar; but it should be noted that every radar in use today is an "instant on" radar; so the term is meaningless.
Mirror-switching effect- some handheld units would have a switch that would reverse the digits in the display so you could shoot over your shoulder and read the display in the rear-view mirror. Problem came when you forgot to switch it back and "18mph" was displayed as "81mph". However, I think just about anyone can tell the difference between a car going 18 and one going 81. Regardless, no unit sold today has this "feature".
Shadowing- this is one effect that is frequently seen, especially on multi-lane interstates with semi traffic. If the patrol car is going more than 20mph faster than a semi or other very large vehicle, and gaining on it, the radar may see this lower speed and think it's the patrol speed. This will throw the speed difference on the target vehicle. However, again, the speed won't match the visual estimate and the patrol speed won't match the speedometer. No valid tracking history means no ticket.
Batching- This can happen with older units that couldn't keep up with rapid changes in patrol speed. If the patrol car suddenly accelerated or braked, the unit couldn't keep up. However, again, visual estimate won't match and patrol speed won't match the speedometer; not to mention that units today are a lot faster and the only vehicle that could hope to induce batching is a motorcycle (other than a Hardley).
Jamming- Active jammers send out false "reflections", trying to fool the radar. They sometimes work, but are illegal. In any case, as an operator, you'll probably never run across one or realize it if you have. Even if you do pull someone over with a big box labeled "This Is An Active Jammer", it's an FCC violation, not a state law. You could try to push an obstruction charge, but good luck with your judge. Passive jammers don't work- period They are the modern day equivalent of snake oil. "Stealth" bras don't do much either- if it was that easy to "stealth" a vehicle from radar, the F117 wouldn't cost a billion dollars.
And that's the basics of radar. There are other legal considerations for officers, but they vary from state to state and the only ones I can talk about with any knowledge are Georgia's. Stay tuned for the next installment, Lidar, soon or when hell freezes over. *pours another margarita*
- come out ahead is that there are so few of us compared to the rest of the vehicles out there, and we're smaller than most of them, as well. And there are many police officers who do not take the time to use the "instant-on" capability of the police radar. No, they're not lazy; they get all the business they can handle without the extra steps of activating the radar every time they see a speeding vehicle. Beyond that, if the traffic is even "light" your chances of seeing a radar on your detector that was activated for another target are quite good and, even though the Sarge doesn't agree with me on this one, he hasn't explained to me how the police radar detects the patrol vehicle speed in moving mode without transmitting now and then. <_< 
