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Showing the radar flashilght like beam

Radar works by shining it's beam down the road. When/if you come into the beam, it bounces off of you and the antenna looks for those reflections.


Errors Radar Makes

  • Antenna Positioning Error:

Showing that the radar beam does not go around corners


shows that the radar beam does not go up and down hills

Radar travels in a straight line; it can't bend around curves or hills. The operator may think he's seeing the speed of the vehicle on the road in front of him, but in reality is seeing the reading caused by the vehicle on a different road nearby.


  • Look-Past Error:


shows that the radar beam reflects the largest object

This occurs when the radar reads the reflection from a larger vehicle behind the one being targeted. Poorly-trained operators assume this cannot happen, when in fact it can.

It is a widely- held misconception that the reflected signal is always coming from the target vehicle. Common vehicles reflect radar in vastly different ways. A 1999 test by Car and Driver magazine found that a radar gun locked on to a semi at 7,600 feet but couldn't see a small sedan until it was less than 1,200 feet away.


  • Vehicle Interference Error:

As was mentioned, moving radar has to do all the functions of stationary radar, plus several additional functions that apply to it alone. Vehicle interference error happens when moving radar is used in traffic. It calculates target speed by subtracting patrol speed from the closing speed of the target. Anything that produces a low evaluation of patrol speed will automatically result in an incorrectly high target speed reading.

Shows hat a large object will give a false speed radar reading


What's happening in the diagram above is the 50 mph patrol speed plus the 50 mph target speed equals 100 mph. But instead of subtracting the correct patrol speed, the radar would subtract the speed at which it's closing on the truck (50 - 30 = 20 mph) so it would display an incorrect reading of 80 mph for the target car.


  • Cosine Error:

Cosine error comes in two fashions:

For stationary radar, (and laser because it's stationary by design) cosine error always works in the driver's favor. If the radar gun is at a 20 angle to the road and you're traveling 50 mph, then 50 mph multiplied by the cosine of 20 (.939) equals 46.9 mph. Since radar guns truncate the display by ignoring any number to the right of the decimal, the display would read 46 mph.

For moving radar, the story is similar to vehicle interference error. A stationary object near the road, such as a sign or parked/disabled vehicle makes a better radar reflector than the road surface. The radar unit uses these reflections as the basis for it's determination of patrol car speed. It does not get the patrol car speed reading from the car's speedometer.

If that reflector was placed directly in front of the patrol car, the patrol car speed would be very close to the actual speed. However, the further the reflecting object is located from the centerline of the radar beam, the lower the estimate of patrol car speed. (This is a simple trigonometry problem relating to the cosine of the angle between the target and the ground reflector.) Since cosine error always makes patrol speed lower than actual speed, it always raises the target's displayed speed.

Example: An officer is moving down the road and sees a car which he estimates is traveling at 50 mph. Let's say the car is actually going 50. The radar is receiving a reflection from a car that's 25 to the right. The cosine of 25 is .9063. Multiply actual patrol speed (45) by that to get the apparent patrol speed of 40 mph (45 x .9063 = 40.7 mph). The radar's electronics then adds the 5 mph difference to the target car's speed reading. Therefore, a target speed of 55 mph is five mph higher than actual.


  • Double-Bounce Error:

Since the microwaves from a radar unit can't be seen, the operator merely matches the number on his display with what appears to be the fastest moving vehicle. This isn't always the right thing to do since microwaves bounce all over. Big vehicles like trucks reflect quite well, so it's possible for the radar signal to bounce off the back of one semi-trailer and hit another vehicle next to the patrol car. The combination of the truck speed, the patrol car speed, and the other vehicle's speed all contribute to this error.

Illustrates the large object speed trap radar problem


In this example, the patrol car's 50 mph speed is added to the 60 mph speed of truck 2. (total=110) Subtracting the patrol speed from the 30 mph speed of truck 1 produces a difference of 20. The combined speed of 110 mph minus the 20 mph difference will be displayed as a target speed of 90 mph.


  • Reflection Error:

A reflected beam

As mentioned above, microwaves can be easily reflected, even by the patrol car's rearview mirror(s). If the radar unit is hung on the rear door glass, parts of the radar beam may hit and be reflected backwards by the outside rearview mirror.


  • Radio Interference:

Police and CB radios can force a false reading on a traffic radar unit. Radio transmissions from within the patrol car cause false readings known as ghosting. For a proper radar reading, the officer must not transmit on any radios for the duration of the encounter.


  • Fan Interference:

The patrol car's heater fan can alter the reading of the radar unit, since the radar tends to pick up the pulses of the heater or air conditioner fan. With non-moving radar, this interference will disappear when a target vehicle comes into range. Mobile radar though, can be affected anyhow because the fan motor sometimes overpowers the signals reflected off of the roadway to determine patrol car speed. Thus, the speed reading for the patrol car will indicate what's actually being produced by the heater fan, not the car. Since calculation of target speed means subtracting patrol speed from target speed, if the fan reading is less than the patrol car's actual speed, the target car's reading will be erroneously high. The only correction to this is to turn off the fan. On some cars that can't be done.  The real one




Before you begin. Nothing on this page will help in your defense. If you get to technical the Judge will disallow the questions because they go to the realm of expert testimony. There are questions based on some of the principals of radar in the Tipmra however they are not as technical as the explanations here. This page is for those wanting to know more about the basics underlying speed radar. It is not necessary for you to understand any of this. If you are a glutton for punishment and want more then the definitive web site for radar and laser devices is:

Current Approved Radar Devices
Current approved Laser Devices    


Simplified for the Layman

Picture the radar's beam as very much like a searchlight. It spreads out from the source and is easily reflected, a feature that light shares with microwave radiation. Metal objects (cars, signs, bridges) make excellent radar reflectors, sending microwaves around at odd angles. However, microwaves are invisible. They are not invisible to a radio frequency receiver though, and radar is nothing more than a radio transmitter and receiver designed for those frequencies. Radar works on the "Doppler shift" principle. An example of this happens at railroad crossings. When you're stopped at the crossing, the approaching train's whistle changes pitch after it passes you. It goes from high to low. That's an example of a Doppler shift. A radar unit measures the frequency between the sent and reflected (received) signal. Since the frequency of the sent signal is known, it makes it's determinations based on that. If the reflected frequency is higher, the target is moving closer. If the reflected frequency is lower, it's moving away.


Radar is much like a flashlight; there's a limit to how much area the flashlight can illuminate. Naturally, the greater the power of the flashlight, the more light it puts out. The same holds true for radar units.

Traffic radar's low power limits its ability to detect vehicles far away. As with flashlights, the farther the microwaves have to travel to the target to be reflected, the farther the return trip is and the weaker the signal is. If an officer zaps you with radar while you're still a mile away, that radar signal has to travel two miles since it needs to return to the radar gun to be of any value. If the signal is weak, no speed is recorded. You're out of range.



Radar's effectiveness depends on two things: the power of the transmitted signal, and the reflectivity of the target. The amount of power is determined by the radar designers, who must keep in mind municipal budgets (more power = higher cost) and the typical police vehicle's electrical capacity. As far as you're concerned while you're on the road, neither of these two are variables. However, the reflectivity of the target is a variable.

For vehicles, radar reflectivity is mostly an issue of size and shape. The larger you are, the easier it is for radar to pick up and bounce off of you. A typical over-the-road semi is a wonderful radar reflector; It’s huge and the surfaces are primarily flat. However, a car is smaller and the sheet metal is generally not flat. This reduces the "visibility" of the car to the radar signals.

Types Of Police Radar

All traffic radar operates on the same principles we've outlined previously. However, there are different kinds of radars, just like there are different size flashlight batteries. Each has a particular use:

Stationary radar is the archetypical "radar gun" that everyone talks about although mobile radars can also be used in the stationary mode. Use of these radars is popular with motorcycle police, where two-piece radars are more convenient. Hand-held radars are also used to detect how fast a pitcher is throwing a baseball. However, due to the health problems associated with microwave radiation, some states (such as Connecticut) have outlawed some types of radar to protect the health of the officers operating them. This is very similar to the x-ray technician at your hospital. The few x-rays you get that day won't hurt you because you only come in once or twice a year, but the x-ray technician uses the device many times a day. That's why they leave the room.

Mobile radar is a more complex radar setup where the officer is able to check the speeds of drivers while his patrol car is moving (or not moving). The theory of operation is just the same as all other radar; a microwave beam bounces off a car and the returning frequency is measured to determine the target's speed. However, this beam has two purposes: to find the speed of the target vehicles, and to find the speed of the patrol car. The strongest reflection is assumed to be the nearby terrain, signs, bridges, etc. and is used to calculate patrol car speed. The second strongest reflection is assumed to be traffic, and an internal calculator compares patrol speed with target speed to produce a final target speed reading, provided there are no errors. The radar unit does not get the patrol car speed from the car's speedometer.

Instant-on radar is not really a kind of radar, but an operating procedure. Most of today's units can be operated in the instant-on mode, either while stationary or moving. It exists only to defeat radar detectors.

The instant-on radar speed trap is just like the normal trap, except that the radar unit isn't transmitting until the operator pushes a button. The system is on and warmed up but it is not transmitting. The operator sits and waits for a target to be in range. When a vehicle appears and the officer zaps it with instant on radar, its speed is recorded and a citation is issued if necessary.

Photo radar is a form of stationary radar hooked up to a computer and a still camera. The radar is transmitting continuously, and the officer sets the computer to a particular trip speed, usually just a few miles per hour over the posted limit. Any motorist exceeding that trip speed will have their front or rear license plate photographed. The picture normally has a time and date stamp on it in addition to the speed it recorded. The film is retrieved at the end of the day and sent to the photo radar rental company for developing. These companies normally provide the renters with the services necessary to find the registered owner's address and mail them the picture along with the citation.

Photo radar is experiencing legal challenges in the United States because most state laws hold the driver accountable, not the registered owner. This is as it should be. In order to make photo radar legal, these laws must be changed. Wisconsin and New Jersey have banned photo radar. Photo radar can be effectively challenged by applying the rulings in MUNICIPALITY OF ANCHORAGE, v. Clyde BAXLEY (included with The Tipmra)

Laser is not at all like radar, so calling it "laser radar" is not correct. Laser (or lidar as it's sometimes called, Light Detection And Ranging) uses invisible infrared light sent out in pulses. It does not use radio waves or the Doppler shift principle like radar does. It works based on the time-of-flight of a pulse of laser light. The operator aims at your license plate, as an aiming point nothing more. (some sites tell you to use a non reflective coating on your plates to make the light not reflect. This is pure unadulterated hog wash prorogated by the truly uninformed) The computer inside the laser gun measures the time between sent and reflected pulses. It sends out over 400 pulses per second, and an average of those pulses is computed and displayed as speed. Since time is constant and distance is known (laser guns also calculate distance, something police radar can't do), the only variable is your speed. 95 % of jurisdictions in Canada and the USA have not taken Judicial notice that Laser is acceptable for evidence in a speeding conviction. However the nice cops and courts don't tell you this as they are taking your money and convicting you. ADMISSIBILITY OF MOTOR VEHICLE SPEED READINGS PRODUCED BY the LTI MARKSMAN 20-20 LASER SPEED DETECTION SYSTEM.( in the tipmra ) will make the court, and officer Parker sit up and take notice.


Target Acquisition

Traffic radar's biggest problem is in the way it displays information - by a digital display. An officer can point the antenna down a road and it will cover vehicles ranging in size from a motorcycle to a tractor-trailer. All the radar unit will display is a number; it can't tell which one is moving fastest.

So then, how does the operator know which one is causing the reading? In truth, the operator often does not know for sure. He must guess, and may assume the vehicle in the left (passing) lane is the speeder. Because traffic radar is made to work in a car with it's space/power limitations and must be affordable enough for municipalities to buy them, it has to be a simple device. With a constant beam (as opposed to the modulated beam of military and air-traffic control radars) it can't distinguish between targets that are in range. Expect the officer at trial to testify that there was no other target then you and nothing between him and you. He will swear the readings were from your car and the court will believe him.

To make up for the lack of a modulated beam and a display screen, the radar manufacturers simply program their electronics to ignore all but the strongest reflections. It's up to the operator to decide which of the moving vehicles is producing the reflection. If there's only one vehicle on the road, it is likely that's the cause of the reflection, but keep in mind that microwaves can also bounce off of trees, trash blowing across the road, or be interfered with by electricity (thunderstorms, power lines) or signs wavering in the wind.

If there is more than one vehicle, the operator must choose. Is the reading caused by the closest one to the patrol car or the biggest one? It could be both, depending on the situation.

An officer who believes in justice will not write a ticket until he is absolutely sure that the reading was caused by a particular vehicle. A less-skilled officer might think he has the right vehicle and be wrong. Finally, an officer who needs to meet the monthly ticket production standard (quota) may simply assign the number on the display to the "profile" car; a red Corvette or a black 300ZX.

Due to it's cost and physical constraints, traffic radar is as not as infallible as you're told it is. In the late 1970's, a Florida TV station reported that police radar clocked a tree at 37 mph and a house at 28. Were those readings wrong? Not at all. The radar was seeing something. But what? In this example, an officer's handheld radio (which was transmitting at the time) caused the erroneous readings. Wind-blown tree leaves, a patrol car heater fan, lightning, or signs wavering in the wind can all affect the radar's display.

In the late Seventies, the National Highway Traffic Safety Administration tasked the National Bureau of Standards (now the National Institute of Standards and Technology) to develop standards for the purpose of discovering its errors and developing standards to eliminate them. These standards were never published until 1994.

It is interesting to note that radar was first used for speed law enforcement back in the late 1940's yet U.S. government standards didn't exist for nearly 40 years. To avoid a repeat of that problem with laser, NIST took their radar standards and did little more than changed the word "radar" to "laser" in the text and released it as the definitive standard for police laser units. The radar standard calls for a level of immunity to transmitting radio interference, which is something radar is affected by. (see above). The laser standard also calls for this even though lasers are, by their very nature, immune to radio waves. Furthermore, since radar is not affected by light, the radar standards have no minimum requirement for light interference. But lasers are affected by light, and the laser standard makes no reference to light interference!


Radar Detectors And Jammers

A radar detector is no different than the radio in your car's dashboard. It merely "listens" to a band of frequencies like an AM/FM radio. Remember that a radar unit sends out a microwave beam and listens for the reflection. A radar detector is nothing more than the listening section of the radar gun.

A radar and laser jammer, is either reactive or active. The active types transmit a false return and are illegal in all States and have recently been banned by the FCC as well. Reactive jammers take the cops signal and add a chirp that confuses his radar or laser guns computer. He will not be able to read your speed. Only Rocky Mountain radar produces FCC legal Jammers and scramblers.


Can a detector find radar before you're in range? You bet. If you're on a straight and level highway headed right for the speed trap, the radar beam is reaching out for you. Remember though, that microwaves lose energy as the distance from the transmitter increases. To measure your speed, the beam has to have enough power to bounce off of you, return to the sender and be decoded by the radar's electronics. The return signal must have enough power to be of any use.

Say the radar gun can clock you when you're a mile away. That means it can detect a radar beam that has traveled two miles; one mile to you and one mile back. If your detector is as sensitive as that of the radar gun's receiver, you will be able to detect the trap two miles away, well before you're in range. It is not at all difficult to make a detector more sensitive than the radar gun.
But radar isn't usually used on the open road for that very reason. Traffic officers favor the ambush where they hide behind a bridge or over the crest of a hill. When you pop into view, you're already in range. So how can a detector work in this case?

Remember when we said radar is like the beam coming from a flashlight? On a foggy night you can see the beam even though it is not pointed at you. Microwaves act the same way. Because they are line-of-sight (like light), they can't bend and detect your speed around a corner or over hills. A detector can certainly pick them out though because microwaves get scattered by dust or fog and will shoot through a forest. While they are not reflected back to the radar gun, a detector will hear them.

All that's needed is a receiver as sensitive as that in the radar gun. However, unlike radar guns, a detector doesn't need to calculate speed, so it can pick out much weaker signals than the radar unit can and still make use of them.

Detectors are not infallible. If you believe one will prevent you from getting a ticket, you're in for a surprise. Radar units have had the "instant-on" capability for many years now, which is designed solely to defeat detectors. A radar gun is kept in the "standby" mode until the operator pushes the button. While in "standby" mode it is on and warmed up but not transmitting. So, a detector can't find it. Even the continuously-transmitting radar guns can be made into a form of "instant-on" radar by simply aiming the antenna away from the road until a speeder comes into range.

A detector is still useful against instant-on radar. When radar hits a car in front of you, your detector will sense the microwaves and sound an alarm. For this reason, it is very important to believe all alarms. At highway speeds, you and the officer in the oncoming lane are approaching at 140 miles per hour. Distances shrink quickly at that speed, and you'll soon be in range.
A detector never gives false alarms. That may be hard to believe, but it's true. Every time it sounds an alarm, it is detecting a signal that it was designed to hear. However, it cannot determine the source of the signal since a 10.525 GHz microwave beam from a supermarket door opener is the same as a 10.525 GHz beam from a police radar unit. It's your job to identify the source.

Laser detectors do work, contrary to the belief among some. However, the laser beam is a lot harder to find because it is much smaller than a typical radar beam. It is imperative that you heed every alarm if the laser detector is going off. Although the police normally have your speed and distance information when the laser detector goes off, there is a chance the laser is being aimed at a vehicle in front of you. The invisible laser light will travel through car glass. Lasers are new to the highways, so you can be sure that laser detectors will improve.

Photo radar detectors also work, but because of the very low power of the radar, you better be smiling for the camera by the time your detector goes off, because it got you. You are normally able to see the photo radar vans long before your detector will.

Frequency bands: (USA, Canada, United Kingdom)

X band: 10.500 to 10.550 GHz (gigahertz, or 1,000,000,000 Hz)
K band: 24.050 to 24.250 GHz
Ka band: 33.4 to 36.0 GHz
Laser: 904 nanometers



A common question today centers around radar jammers. Are they legal? There are two answers.

  • "Passive" radar jammers do not transmit anything. Some make the claim of being "radar re-radiators," meaning they are radar reflectors. Well, so is your car! It doesn't matter if it has electronics inside, if they don't transmit, or if they claim to mix in some white noise  with the radar signal or do some 'phase-shifting' of the radar beam, they don't work. So they're legal. But why spend $200 on something that doesn't work? You can get a detector that actually works for less.

  • "Active" radar jammers do transmit. In the United States, in order to transmit anything in the radar bands, the transmitter must be type-accepted by the Federal Communications Commission, and the operator must be licensed. (In the case of police radar, the department's or municipality's license is good enough; individual cops don't need an FCC license.) These jammers are not FCC type-accepted because the FCC doesn't approve devices when the sole intent is to jam other transmitters. Notice too, that they don't include license applications with the jammer.

So, "active" jammers do work, but are illegal. By the way, many newer radar units can detect when they're being jammed, and an indicator lights up. Since the police are trained to visually estimate speed, it's easy for them to spot someone zooming by at 85 mph and think something's wrong when the radar display shows only 32 mph, or some equally erroneous speed. Even untrained people could do that. So, by using a jammer, you may be getting exactly the attention you had hoped to avoid. It may be possible for the officer to cite you for obstruction of justice, or interfering with a police officer.

Laser jammers do work, and are legal as far as the FCC is concerned. The FCC doesn't regulate devices that transmit above a certain frequency. If they did, they'd have to issue licenses for every light bulb and TV remote ever made. However, the same story about obviously erroneous speeds or interfering with an officer applies here as well. States may have laws banning laser jammers.


The VG-2 Radar Detector-Detector

For those that don't know, it is possible for police officers to know that you have a radar detector even if it is not visible. How can they know?

All modern radar detectors are simply super heterodyne radio receivers (like your AM/FM radio) that listen for radio frequencies in the bands where police radar units operate. Super heterodyne radios utilize a circuit known as a local oscillator (LO). The use of this circuit is a double-edged sword - it vastly increases the sensitivity of the radio to weak signals (that's good), but it also creates what are known as spurious emissions. That's bad.

Spurious emissions are unintended radio frequency (RF) signals generated inside of electronic equipment. All electronic equipment generates some amount of spurious emissions. If they leak out, and they almost always do, they can be detected. When super heterodyne radar detectors first became common in the early 1980's, they sounded an alarm for no apparent reason. It was eventually realized that our radar detectors were detecting the spurious emissions of other radar detectors! The radar detector manufacturers quickly developed methods of ignoring the spurious emissions from other detectors, but did nothing at the time to reduce the emissions themselves. [Note: Short bursts of Ka-band alerts are caused by the same phenomenon, even today.]

Because of the way that X and K band relate to each other in frequency, almost all of the radar detectors on the market used the same LO frequency. All it took was for someone to realize that it was possible to intentionally listen for this LO (rather than ignore it as the detector manufacturers did), and the idea for the VG-2 was created.

That's what a device called the VG-2 Interceptor does. It simply listens for a signal that is only likely to come from a radar detector that's turned on.

Or is it? As mentioned, all electronic devices produce some spurious emissions. Your car radio, your desktop computer, even a battery-operated AM radio produce some. It is unavoidable. Most of this stray RF energy is corralled by "shielding," or encasing the electronics causing the interference in a metal enclosure that's grounded to the frame. Shielding the receiver section of a radio is particular problem as the antenna connection goes almost directly to the LO circuit. Size, weight, economics, and sometimes heat are other issues engineers must take into account when designing the shielding for a device. Once your detector has been manufactured, shielding cannot be added to reduce it's visibility to the VG-2.

Further, amateur ("ham") radio operators that operate their radios in the 10 and 24 GHz bands can set off the VG-2 because either the frequency of their radio's super heterodyne receiver and/or their transmitters are in the band that the VG-2 listens to (11.4 to 11.6 GHz). Note that police radar operates in the 10 and 24 GHz bands too. These are licensed radio operators legally operating in the bands they share with other services. There have been cases of licensed hams being pulled over in Virginia because the VG-2 was set off by ham radios.

The VG-2 isn't very expensive. It is a very simple device with no anti-falsing circuitry or other electronic wizardry. It is capable of listening to only one band. It is not state-of-the-art in electronics.

The VG-2's antenna is somewhat unidirectional; that is, it "hears" best in the direction it is pointed, just like a regular radar detector. The reported range is 30 either side of center, but real-world experience shows that it responds to RF from all directions (again, just like an ordinary radar detector). It has a bar-graph display to indicate signal strength, which gives some indication of the location of the device it is hearing. If the meter peaks, then drops as you pass by, there’s a chance the detector in your car was causing the reading.

The VG-2 is most effective when it's antenna is aimed at right angles to the flow of traffic, since the area that it can scan is minimized, and it takes advantage of the scenario just described. Unlike radar detectors, the VG-2's beep rate is not variable, so the operator must constantly watch the signal strength meter to determine the proximity of a violator. (Try driving safely at the same time.) Since radar detectors vary wildly in their RF leakage, two vehicles may give identical readings on the VG-2's display at vastly different distances.

Two-lane highways are where it is most effective. Light traffic combined with little or no outside interference work to its advantage. City streets are not favorable to it due to the large number of vehicles close by, and because RF can reflect off of buildings. On interstates, it's ability to pick out vehicles with detectors is inversely proportional to the number of vehicles on the road. A low number of cars means a high probability of correct "hits." Conversely, while the officer is moving among a large amount of traffic, the VG-2 is less effective.

The VG-2 can report legal items. Here's a sampling:

  • Ham radios, as mentioned above.
  • Satellite links (radio and TV).
  •  Cell phones. Police radios.
  • Microwave burglar alarms.

Because microwaves bounce off of trees, overpasses, signs, etc. serious errors in identification can occur, especially if the VG-2 is used while moving.

Example: A trooper is out patrolling on an Interstate. You are in the same lane, approaching from behind in a car with a detector. Just as the officer approaches an overpass, its metal structure reflects your detector's RF leakage into the antenna of the VG-2 causing it to beep. As this happens, a tractor-trailer goes by in the other lane. (Keep in mind that detectors are illegal in big trucks.) The trooper could very easily pursue, stop and search that truck even though it had no detector. What would remain unanswered is the legality of such a stop when no detector was found. This is what happens when you put blind faith in technology and the manufacturer's claims.

Since the VG-2 came out, many radar detectors have been designed to reduce the leakage of spurious emissions, and/or to change the frequency of the LO to make the VG-2 ineffective. Of course, since the VG-2 uses a super heterodyne circuit that that leaks RF, a VG-2 detector could be developed, or perhaps a "smart" radar detector could combine that feature with the ability to go into a "sleep" mode and shut down for a few seconds when it detected a VG-2.

Measure - Countermeasure; the game continues.

In Today's World a Radar Jammer Radar Detector is essential to prevent tickets  Radar Detector/Jammer

Often Imitated but Never Duplicated