Police Radar FAQ

By Sgt. Earl Faubion, Oklahoma City Police Department (1972 - 1999)

During my twenty-seven year police career I was asked many questions about police radar so in 1993 I began writing this document in order to avoid repeatedly answering the most common ones. Since the principles of speed radar operation are generally the same for all radars I decided to write the questions and answers in a generic format that should cover most radars in existence. This document has undergone several revisions since then and is still periodically reviewed and updated however having been out of the business so long means for questions on modern radars you should address any questions elsewhere. I can only address the scientific principles behind radar speed measurement as it pertains to radars made in 1999 and earlier. 

In 1981 I was certified by Oklahoma’s Council on Law Enforcement Education and Training (CLEET) as a Radar Instructor and I maintained that certificate until my retirement in September 1999 from the Oklahoma City Police Department. Please note that since 1999 I haven't been active in the radar field so I may not be familiar with technology that has been introduced since then.  This document is for informational purposes only and is not intended for use in any court of law. The author’s experience is in the United States.

Several people have asked me if I intend to write a book or pamphlet about police radar tactics or about the types of questions one can ask an officer in court. The answer is no.  While I could probably do that, there are already books on the market and information on the Internet that cover this topic.  The reason I make this document available to the public is to answer several commonly asked questions regarding police radar.  A generic document like this one can’t begin to address issues regarding an officer’s or a department’s tactics or procedures so those questions are avoided.  When they are touched upon the reader must remember that policies can vary widely from one jurisdiction to another so the information found here may not always be applicable. I’m often asked questions like is it legal for officers to hide behind bushes or park on the shoulder at night with their lights off. The generic answer to those two questions is yes, but this might not be true for all agencies so any question along those lines is best directed toward a local official or attorney. This document focuses on the radar instrument itself rather than on operational procedures and is for informational purposes only.

How does police radar work?

It transmits a radio signal in the microwave band. Upon striking an object some of that signal is reflected back toward the radar and, if that object is moving, a Doppler shift occurs in the radio frequency.  The radar then calculates that change in frequency and converts it into either miles per hour or kilometers per hour.

What is Doppler shift?

It's a scientific principle that an audio, radio or light wave will increase or decrease in frequency upon movement of the source.  In the case of police radar the transmitted signal’s frequency will increase if the target is moving toward the radar transmitter and decrease if it is moving away. It is this difference in frequency that is predictable and is used to calculate the relative speed of the target. An audio Doppler shift can easily be detected when a moving object such as car or aircraft passes near you, the sound seems to decrease in pitch as it passes by. This is because the source is moving relative to your position.  The Doppler principle is also widely used in weather radars, radio-telescopes and sonars to measure the speed of various targets.

What does "relative speed" mean?

This is the speed between the radar source and the target. If the target is moving directly toward or away from the radar transmitter then the relative speed and the true (actual) speed will be the same.  If the target is moving at an angle toward or away from the source as is often the case with police radar, then the relative speed indicated on the radar will be less than the true speed of the vehicle. Example: a police car sitting on the side of the road using radar. Traffic will not run into the police car so the traffic is traveling at an angle relative to the radar and it will read a speed slightly less than the target’s true speed. If this angle is less than about 11 degrees the difference between the true and recorded speed is very slight and of little consequence, hence this method if quite effective. At angles greater than 11 degrees the indicated speed on radar can be significantly less than the vehicle's true speed.

Is an FCC license required to operate radar?

The department's radio license also covers radar and individual officers are not required to carry a copy of that license.

Does the officer have to be certified to operate radar?

Most law enforcement agencies have some kind of certification process that must be completed before an officer is allowed to use radar. This may be mandated by law or departmental policy.  Certification standards may vary from one jurisdiction to another.  Federal guidelines spell out certain recommended minimum standards for radar operators but these are only recommendations, not requirements. A motorist may ask if an officer possesses a certificate but it is probably rare that an agency will require the officer to present it in the field.

How much does the officer have to know about how the radar works?

The officer needs to know how to setup, test, operate, and interpret the readings from a radar. He also needs to understand the basic principles of how the radar works. He does NOT need to know how to build or repair the unit nor does he have to understand all the electronic circuitry any more than the average homeowner needs to know how to build or repair a television set in order to use it.

How accurate is radar?

It is very accurate.  Radar manufacturers normally certify their units to plus or minus one mile per hour and most radars have some form of built in testing devices that verify the radar's accuracy. Radars usually come from the manufacturer with external testing devices such as tuning forks that the officer can use to verify accuracy.  Some radars do an automatic self test at periodic intervals.

Can the radar be tuned by the officer?

Not unless the officer possesses the appropriate FCC license permitting him to make those adjustments.  The regular street officer can't adjust the radar for accuracy but can verify the unit's accuracy via certain tests.  If the radar is found to be in error the unit should be repaired by a competent technician. The officer can make certain adjustments to the radar in the field but these adjustments do not alter the internal workings of the unit itself.  These adjustments will be for things like audio level, range sensitivity, etc.

What's the difference between radars with one and two antennas?

Many in-car radars are capable of utilizing two antennas however only one can be used at a time.  Future models may allow both to be used simultaneously.  With a dual setup the operator can select which antenna to use thereby making operation a little more versatile than with a single antenna.  Typically one antenna is mounted on the dash pointing forward and the second antenna is mounted on the rear deck pointing aft.

Can radar be jammed?

Yes, however jamming radar does not make a speeding car invisible to the officer and since visual verification is the first step in identifying a speeder a jammer is rarely of any value. Active jammers are illegal radio devices that subject the owner to penalties that are often worse than any they'd face for speeding. The so called "passive" jammer is advertised as being legal in most (not all) states because they transmit no signal however tests run with these devices have shown them to be largely ineffective. As with the active jammer, they do not affect an officer's vision.  The author once read in a car magazine the test results of a jammer and it was found that its effective range was only two feet.  As always, the best way to avoid receiving a speeding ticket is to drive at a legal speed.

Do police radars interfere with each other?

In extremely rare instances they can but it is difficult to find any two radars that transmit on precisely the same frequency, even two units of the same model. In tests it has been found that interference among like units is exceedingly rare, and when it does occur it is quite obvious and essentially renders the radar useless until the two radars distance themselves from each other.  The author has only encountered one case of this type of interference and it was very obvious what was happening.

Does tin foil in hubcaps, honking horns, dragging chains, etc. affect whether a vehicle can be clocked with radar?

No. The tin foil trick is useless and probably originated with the Allies' use of tin foil strips (it was called “window”) dropped from bombers over Europe in World War II to confuse enemy radar.  Police radar doesn't operate in the same manner as search radars in that it clocks a target that is already visible rather than being used to locate the target in the first place.  Horn honking has no effect on radar since the audio of the horn does not affect the frequency of the transmitted signal. At one time a rumor circulated claiming that dragging a chain from a vehicle would foil police radar. All horn honking or dragging chains will do for a speeder is make him a noisy speeder.

What about the use of "stealth" bras to prevent the radar from clocking a vehicle?

Stealth bras and some car waxes have been advertised as being able to reduce a target’s reflectivity and tests have shown they often do but the degree or percentage of that reduction is somewhat dubious at best. As a general rule, if the vehicle can be seen, it can be clocked.

What is a "batching" error?

Technically this and other "errors" are not true errors because the radar is really functioning properly. Typically what is meant when the term "error" is used in this document is that the reading displayed by the radar is not a valid vehicle speed. In the case of the batching error this was common to older moving radars when the police car was rapidly accelerating or decelerating. It could cause small errors on the target speed resulting from a slight lag time in the radar's processor in keeping up with the patrol speed. Newer radars have faster processors and aren't prone to this error.  This error also gave rise to the notion that if you had a radar detector and it went off you could rapidly decelerate by hitting your brakes and keep the radar from accurately reading your speed. This is an old wives tale that belongs in the same category as tin foil in the hubcaps.

What is a "scanning" error?

On older radars this error could occur when the radar antenna was "scanned", or moved side to side in a sweeping motion.  It can easily be induced with a handheld radar gun if the operator moves the unit rapidly from side to side.  On car mounted radars it can occasionally occur when the police unit is in a tight turn. Radar operators are taught not to take readings when the radar unit is scanning.  Newer radars are designed to reduce this error and will almost always refuse to read anything when this phenomenon occurs

What is a "panning" error?

This is an operator induced error caused by pointing the radar antenna at the radar's processing unit which can cause wild and erratic readings.  It is much like keying the microphone of a radio transmitter near a receiver tuned to the same frequency which causes "feedback" or a loud screeching noise. If the radar is properly mounted this is not a problem. The term "panning" is sometimes used interchangeably with the above described "scanning" error.

What is a "shadowing" error?

It is a phenomenon that can occur when a moving radar is calculating a patrol speed inaccurately because it is interpreting a nearby vehicle moving in the same direction as the ground speed.  This is typical when approaching a large slower moving truck. The radar may briefly "mistake" the truck for the ground speed thereby giving a low patrol speed on the radar which in turn can cause a target speed of an opposing approaching vehicle to register too high. This error is very obvious to the watchful operator and not likely to be a problem. If the officer has the radar's audio turned up it will also be readily apparent by an abnormal audio presentation. This effect ends when the patrol car passes the truck or the two vehicles' speeds change enough to more closely match each other.  In some newer radars this phenomenon is actually harnessed and used to clock vehicles moving in the same direction.  See the question about same direction radar below.

What is a "ghosting" error?

This is a false reading caused by a manmade object and gets its name from the fact that a target may not be visible even though a reading is being displayed.  This can occur with the patrol car's blower motor being "clocked" as a small portion of the radar beam bends downward into the windshield vents and picks up the revolving blades on the fan. This is most noticeable when no vehicles are in range and the radar is transmitting. This problem is increasingly rare with today's solid state radar circuitry although it can still occur and is addressed in operator training classes. This is what happened in the infamous palm tree clocked by radar in Florida in the late 1970's. A reporter aimed the radar at a tree and keyed up a nearby radio causing the radar to give a false reading. It should be noted that if a reporter knew the reading was bogus then certainly a novice radar operator would too.

What is an "averaging" error?

In the past there were a few motorists who thought that radar could average the speeds of two different vehicles to produce an erroneous reading however this is not true.  What the radar may do is compare readings each few microseconds with other readings it has obtained over the past fraction of a second and compare them to see if they are consistent within the changes common among a moving vehicle and, if they are, display the calculated result.  Mathematically the calculated result may be a fraction to several decimal points but the display will then round or average them out for the sake of simplicity. In other words a display of 42 mph may actually be something like 42.36451.

What happens to the radar if the power supply is defective?

All modern radars are designed to cease functioning when the supplied voltage drops below a factory predetermined level.  Most have a low voltage indicator that will light up alerting the operator of a problem and no speed readings will be displayed until the voltage returns to a level that the radar needs to function properly.  Most radars operate off of a 12 volt power source from the patrol car or rechargeable batteries and some will interchangeably work on both.

Does a vehicle's shape and size affect its ability to be clocked with radar?

Yes.  It stands to reason that a smaller target will need to be closer to the radar in order to be clocked. An aerodynamically shaped vehicle will present a slightly smaller target for the radar and will likewise need to be closer to the radar before being clocked.

Does the officer have to show the radar reading to a motorist?

No. Some jurisdictions may have a policy to this effect however the author is not aware of any that do.  Most officers will show the radar to a motorist as a courtesy if asked in a polite manner and when the officer’s and the citizen's safety is not jeopardized by doing so. If the officer did not lock the speed reading on the radar there may be no display of the speed to be viewed anyway.

Why is locking the speed reading discouraged?

Target tracking is an important part of verifying that a particular reading belongs to the vehicle an officer is looking at and most radars, when locked, stop reading the target speed thus eliminating the tracking history.  Some newer radars provide an extra readout so the reading can be locked while continuing to track.  Having a tracking history is a suggested operating procedure and not a requirement so some officers may lock the reading once they are satisfied they have the correct vehicle, especially when no other vehicles are present.

What is Stationary Radar?

This is when the radar is operated from a stationary position.  Virtually all radars work in this mode and some will also work in the moving mode.

What is Moving Radar?

This is a radar that will clock vehicles while the patrol vehicle is moving. Typically, the targets are those approaching in the opposite lane of traffic but, depending on the radar model, can sometimes be used to clock vehicles going away from the patrol vehicle in the opposite lane or even moving in the same direction. These radars also "clock" the patrol vehicle by measuring the speed of the ground passing beneath the patrol car and interpreting this as the patrol speed. This speed is deducted from the closing speed of the target thereby giving its speed.

Can an officer write a speeding ticket without radar?

Yes.  Most courts have ruled that a trained officer can visually estimate the speed of a vehicle with some degree of reliability. Speed estimations are usually a part of radar operator training and some officers become quite skilled at this. There may be times when an officer sees what he knows to be a speeder traveling well over the posted limit but the radar is not positioned to obtain a reading, or maybe he has no radar at all. This does not prevent him from stopping the motorist and issuing a citation unless there is a specific local law or policy prohibiting it. Radar is not the only mechanical means of obtaining a vehicle's speed. An officer armed with a stopwatch can check vehicles traveling over a known distance quite reliably too although this rather old method is seldom used today.

What does "pacing" mean?

Pacing means the patrol car matches the suspected violator's speed. This is done with a calibrated speedometer or radar inside the patrol vehicle. When radar is used in this manner it is actually reading out the patrol vehicle's speed rather than the target speed and it is up to the officer to ensure that he is indeed matching the suspect's speed.  Some jurisdictions may specify a minimum distance over which the pace must take place before it will be admitted into evidence.

At what angle must a car approach the radar to be accurately clocked?

Police radar measures the relative speed of the target as it approaches or moves away from the radar (see the question on relative speed).  For angles of less than 11 degrees when the radar is in stationary mode the difference is negligible and the indicated speed will be either the same as the true speed or maybe one mph less. As the angle increases the speed reading drops off sharply until, at 90 degrees, it momentarily reads "zero" or no reading. For a moving radar that is properly aligned in line with the patrol vehicle's path of travel the effect is much the same. If the antenna is misaligned (aimed substantially away from the line of travel) while in moving mode it is possible for the target to register a higher than true speed if the radar is generating a lower speed for the patrol vehicle. These anglular effects are often called “cosine factor” and should be addressed during operator training.  See the question about scanning errors for related information.

Can other radio transmitters interfere with the radar?

Yes, in certain instances they can, but this is relatively rare.  This can vary from one radar to another and is dependent upon the frequency of the radar and the strength of the signal source.  Most modern radars have a built in "RF" (radio frequency) indicator which will automatically blank the radar’s readout screen during activation by a strong RF signal that exceeds a predetermined threshold level.  Due to their higher power outputs, TV and commercial radio stations can cause interference at close range. Cell towers, ham radio repeaters, personal cell phones, wireless Internet and other devices are relatively low powered and aren’t likely to cause interference, even at very close range. If by chance they should, the radar will temporarily go blank and readings will not be possible.  Radar manufacturers know their products are going to be used in a radio rich environment in a police vehicle so it stands to reason their radars will be specificailly designed to work in that type of environment. A rule of thumb is, if your cell phone will function wtihout interference, so will the radar.

Are there other sources that can interfere with radar?

Yes.  Nearby electrical lines, fluorescent lighting, even the patrol car's own heater blower motor can cause interference.  These problems are addressed in training and a good radar operator will immediately know when these problems are present. On some older radars these sources can cause fake readings on the radar while the newer radars are designed to eliminate most of these.  Nonetheless, the trained radar operator is taught to watch for these problems and isn't likely to misinterpret them.

What is the audio feature on a radar for?

Radar converts the Doppler shift of the reflected radio signal and displays it as a digital readout.  This Doppler shift is also converted to an audio signal that the operator can listen to as an aid in target identification. The higher the speed, the higher the pitch of the audio. Also, a fading audio would indicate a target that is moving away which is helpful when more than one target is in range.  A good operator will use the audio to assist in target indentification

How many targets can the radar clock at once?

Some radars can clock several targets simultaneously while others are limited to one at a time. Either way it is up to the operator to properly identify which target is being clocked before initiating a traffic stop.

Does the radar tell the officer which vehicle it is clocking?

No. Target identification is the ultimate responsibility of the radar operator Bear in mind this does not apply to LIDAR where the laser is aimed at a specific vehicle. With Lidar target identification is extremely accurate.

How does the officer know which vehicle he is clocking?

The officer is responsible for visually verifying the target vechicle.  Cases of mistaken identity are possible if the officer is poorly trained or inexperienced.  The more vehicles within the range of the radar, the more difficult target identification becomes.  Some new radars have a feature that tells the officer if the target is closing or going away.

What is the range of police radar?

This will vary depending on the model of radar, the volume of traffic present, the terrain, and the size of the targets being clocked. Some radars are capable of clocking vehicles over a mile away. The author has used older model radars that under the right conditions could track vehicles at almost two miles.

What is the minimum/maximum speed that a radar can read?

Again, this will depend on the radar model. The low end is usually 15 or 20 mph but some will go down to 1 mph.  The upper end is generally around 199 mph.

Do radar detectors really work?

Yes.  They are designed to alert when a radar signal is detected and in that regard most of them work quite well however the radar must be transmitting for the detector to do its job. If the radar is equipped with a "standby" or "hold" switch (and most are) the officer needn't transmit until he is sure of getting a reading before the speeding motorist can react. Some people call this "instant on" radar which is a misnomer but is a fair description of how it works. A radar detector won’t detect radar when it is in this standby or hold mode. Radar detectors are illegal in some states and Canadian provinces.

What is the "standby" or "hold" feature?

This feature enables the operator to turn the radar's transmitter on and off without turning off the radar itself. The radar is still on but no signal is transmitted when this switch is in the off position.  This switch is also used on radars that have more than one antenna (front and rear for example) to select between the two.  In the hands of a skilled operator this feature is especially effective against speeders who use radar detectors.

What about the risk of cancer to officers and the public?

While no one can totally discount the possibility of a cancer risk to the radar operator over an extended period of time, research has shown that the risk is minimal if not nonexistent. Police officers are taught not to let the radars transmit unnecessarily and to point them away from their bodies. The power output of most police radars is extremely low and the average officer will get more exposure in his own kitchen from his microwave oven than from the radar in his patrol car. The risk to the public from police radar emissions is virtually nonexistent.

What comes first, the radar reading or the visual sighting?

Training dictates that the officer first identify a potential speeder visually, then use radar to confirm or deny that suspicion. This is why jammers and other alleged cloaking devices are of little value.  There are times when a radar might pick up a vehicle before it becomes visible such as in foggy or dusty conditions. The author has seen instances where buildings, median barriers or fences would allow a radar to pick up a target out of sight around a corner or curve. This was especially common with older stationary radars in use in the 1970’s. Enforcement action was not possible on those vehicles until they had been visually identified and clocked again. 

Is the radar accurate at all speeds?

Yes.  The typical police radar is designed to be accurate for the entire range of speeds it is designed to display.

How does the officer go about checking the radar's accuracy?

The procedures may vary between models. Tuning forks are used to check the radar readings and most radars also have some form of internal calibration test. These tests are customarily performed at the beginning of a shift and sometimes more often. The tuning forks simulate the Doppler shift of a vehicle at a known speed. Most radars also have means of insuring that the digital display has no burned out elements which could cause a misreading of the numbers. In addition to these tests an officer can check the radar by driving and verifying the indicated speed against a certified speedometer or using it side by side with another radar to ensure they both read the same speed on the same target. Many newer radars also do periodic automatic self tests.

Does the radar have to be recalibrated periodically?

Not unless it is required by department policy or by law. Radars contain solid state electronics and have no higher failure rate than other similarly engineered electronic devices. Most radars will simply cease to function when something goes wrong. In those few instances where a radar continues to work when a malfunction exists the officer operator should recognize that a problem exists and take the unit out of service for repair or replacement.

Does the weather affect the radar?

Yes but usually not to the extent some people might think.  By the very nature of the radar's location inside a police unit they are built to withstand a great deal of shock and temperature and humidity variations.  Precipitation may reduce the effective range of the radar by causing the signal to scatter and break up more quickly than in clear air but the accuracy of any reading obtained will be unaffected.  Extremes of heat or cold might have a slight temporary effect on the accuracy of the tuning forks used to test the radar while the radar itself remains accurate.

What about laser radars (LIDAR)?

Whereas radar utilizes a radio beam and the Doppler principle, LIDAR utilizes a light beam (low power laser) and the time/distance principle. They are remarkably accurate and work with near pinpoint precision and normal radar detectors won't signal their presence. The author has little experience with LIDAR so this document does not address that form of speed detection.

What is same direction radar?

This is also sometimes referred to as "same lane" radar. Basically, it is a standard moving radar that in addition to having the usual features associated with moving radar, also possesses the ability to clock vehicles moving in the same direction as the police vehicle.  This type of radar is usually used only by experienced radar operators.


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