RADAR instruments and the people who operate them are being challenged in court.  One of the best-known challenges to RADAR occurred in Dade County, Florida early in 1979.  It resulted in the rejection of RADAR evidence in 80 pending speeding cases.  Other attacks on RADAR will undoubtedly be made in the future.  Does this mean that RADAR instruments are simply no good?

Quite to the contrary: unbiased, scientific tests have consistently shown that the RADAR instruments used in traffic enforcement are reliable tools when properly installed and operated by skilled and knowledgeable operators.
The lack of proper operator training has been at the root of almost all the successful challenges to RADAR.  The Dade County incident is a good case in point.  Contrary to widespread belief, the Florida challenges did not prove that RADAR will “detect” 85-mph trees, 28-mph houses, or cars traveling much faster than they actually were. What it did show was that if certain basic operating procedures are violated those kinds of absurd speed measurements can appear to have been made.  There is a logical and obvious explanation for each of the speed measurements that were cited in Dade County.  Each of these absurdities is discussed and explained further in this document.

Speed enforcement based on RADAR is not difficult to learn, but is complex enough that shortcuts in training can result in less than effective performance.  The courts are aware of this, and many are now demanding evidence that RADAR operators have had sufficient training and experience.

HISTORY OF SPEED REGULATION
Various types of legislation to control speed have been introduced throughout our country’s history.  The primary purpose of this speed regulation had been to make traffic movement more efficient with minimum danger to people and property.
According to Joseph Nathan’s “Famous Firsts”, the first traffic law in America was passed on June 12, 1652, by New Amsterdam (now New York).  It prohibited the riding or driving of horses at a gallop within the city limits.  Hartford, Connecticut, lays claim to the distinction of having the first automobile speed regulation.  This law was enacted in 1901 and limited automobile speeds to 12 mph in the country and 8 mph within city limits.  As the number of automobiles increased, so did the number of laws governing their use.  This volume of statutes and ordinances was based, in part, on the assumption that no one should drive a vehicle at a speed greater than is reasonable and prudent under existing conditions.  This assumption became known as the “basic speed law”.

Enforcing basic speed law involves procedures different from enforcing speed limits.  Under the basic speed law, it must be shown that the violator’s speed was unreasonable or imprudent given the existing conditions.  This is not easy, since any basic speed law includes such ambiguous terms as:

REASONABLE – What is reasonable?
PRUDENT – Just what is a prudent speed?
UNDER EXISTING CONDITIONS – This term can refer to the condition of the road, the condition of the vehicle, or the condition of the driver.

Loosely translated, “prima facia” means “at first glance or in the absence of further proof.”  Prima Facie limits are those stated as specific rate and posted on the highway, e.g. “Speed limit 35”.  However the basic speed law is the one that has to be enforced and adjucated.  In other words, a speed limit is posted to tell the motorist what is considered a reasonable speed for that area.  If the motorist exceeds this speed, the motorist is said to have violated the basic speed law “prima facie”.
However, speed in excess of the prima facie limit is only an indication that the speed was unreasonable and imprudent.  The accused is entitled to produce evidence in court to show that the speed was reasonable and prudent for the conditions and circumstances at the time in question.  A court or jury provides the final decision.

“Absolute” speed limits are based on laws that simply prohibit driving faster than a specified speed, no matter what “the existing conditions.”  This school of thought insists that the basic speed law alone leaves too much room for individual interpretation by motorists-many of whom aren’t reliable enough to make correct decisions as to reasonable speeds.  It is also maintained that prima facie limits are practically unenforceable, since questions arise in almost every case as to the rate of speed in relation to the environmental conditions and what a reasonable speed really is for those conditions.  Driving in excess of that absolute limit, regardless of conditions, is a violation.  The only proof required is that the motorist exceeded the limit; circumstances and conditions have no bearing on the driver’s guilt or innocence.

In the early versions of the Uniform Vehicle Code, prima facie limits were recommended, and a majority of States adopted prima facie speed provisions.  Meanwhile, the absolute type of law fell into disfavor.  In the 1950’s more and more States began to adopt absolute limits and abandon the prima facie approach.  In fact, the 1956 Uniform Vehicle Code was revised to provide absolute maximum limits and all mention of prima facie was eliminated.

Current systems of speed control acknowledge that the speed control system must permit motorists to reach their destinations as rapidly as possible while giving all due consideration to safety, reason, and prudence.  Rapid movement of vehicular traffic is essential to efficient highway transportation.

DRIVER IDENTIFICATION

There are two aspects to driver identification.  First, the officer must be able to quickly identify the driver of the vehicle at the time of the initial stop and second, identify the same driver in court at a later time.

After making the initial stop, the officer should make an immediate visual identification of the driver.  Other vehicle occupants may attempt to change places with the driver in an effort to confuse the investigation.  An alert officer can counter these activities by initially noting driver characteristics such as clothing colors, hats, beards, or other distinguishing characteristics that can be observed at a quick glance.  When the officer has completed this first identification of the driver, more specific details should be gathered that would aid the officer in identifying the suspect in court.

BASIC PRINCIPLES OF RADAR SPEED MEASUREMENT

The word “R A D A R” is an abbreviation of the phrase RAdio Detection And Ranging.  This acronym implies that all RADAR’s are capable of finding a target and calculating its distance.  The acronym, as defined, does not exactly fit the description of police traffic RADAR.  Police traffic RADAR’s can provide a speed reading on a detected target, but they do not ordinarily measure the range to the target.

Actually, the inventors of RADAR did not make a mistake in their acronym.  The concept of “ranging” is correct for about 90 percent of RADAR in use today.  It is police traffic RADAR that is in the 10 percent of RADAR that provides no range information in the case of most devices.

It is important to recognize that many types of RADARs exist.  Some are complex, while others, like the police units, are simpler.  Even though there are many variations and different features among types and families of RADARs, the underlying principle remains the same: Radio-frequency energy is generated by a transmitter; an antenna forms the energy into a beam; and the beam is transmitted into space.  When the energy, or signal, strikes an object, a small amount is reflected back to the antenna.  From the antenna, the reflected signal is sent to the receiver, where, if the signal is strong enough, it is detected.  This is how the RADAR operator learns that a target is present in the beam.

The way that the receiver processes the energy reflected from the target determines what information will be available to the operator.  It the RADAR is to compute range to the target, timing circuits in the set will time the round-trip travel period of the signal-starting at the time the signal is transmitted and ending when the receiver detects the reflected signal.  Timing circuits are made possible by the fact that radio energy always travels at 186,000 miles per second, the speed of light.  The speed of radio energy is, therefore, a constant in all computations performed in any RADAR set.

Police traffic RADAR uses another characteristic of radio energy to measure speed.  A radio signal’s frequency (waves per second) is changed when the signal is reflected from a target that is moving at a speed different from that of the RADAR set.  This change or shift in frequency is known as the Doppler shift.

Frequency is usually measured in cycles per second.  A cycle is the same as a wave.  Scientists and engineers often use the term hertz (Hz).  One Hz equals one cycle per second, which is the same as waves per second.  Because the speed of RADAR wave is constant at 186,000-mps, wavelength and frequency have an inverse relationship.  As the number of RADAR waves transmitted each second (frequency) increases, the length of the waves (wavelength) must decrease.  The reverse is also true.

Theoretically, if a RADAR were to transmit only one wave per second, the length of that wave would have to be 186,000 miles.  Conversely, a RADAR transmitting 186,000 waves per second would produce a wavelength of one mile.  It is obvious then that any given RADAR frequency must be associated with a specific wavelength.

POLICE TRAFFIC RADAR ASSIGNED FREQUENCIES

Police traffic RADAR devices operate in the microwave frequency band; they transmit billions of waves per second.  The wavelength involved is therefore very short (hence microwave).  Almost all police traffic RADAR is operated on one of three Federal Communications Commission (FCC) assigned frequencies.

Due to the early popularity of police RADAR, older units operate within the so-called X-band, at a frequency of approximately 10.525 billion waves per second, or 10.525 gigahertz.  This RADAR signal has a wavelength of approximately three centimeters or about 1-1/5 inches.  However as technology advances, so does the police traffic radar unit.  Many newer models are operating on a frequency of 24.15 billion waves per second or 24.15 gigahertz.  This is called K-band and the wavelength is approximately 1-1/4 centimeters or about half an inch.

In either case, the frequency times the wavelength always equals the speed of light.  This relationship exists for all radio signals and is fundamental to understanding how the Doppler Principle is used to obtain a valid speed measurement.

 
Common Band Actual Frequency Metric Frequency Wave Length Signal Speed
X 10,525,000,000 10.525 GHz 2.84cm 186,000 MPS
K 24,150,000,000 24.150 GHz 1.23cm 186,000 MPS
Ka 34,250,000,000 34.250 GHz 0.87cm 186,000 MPS

THE RADAR BEAM

The radio wave energy transmitted by police traffic RADAR is concentrated into a cone-shaped “beam.”  Most of the energy that is transmitted remains in the central core of the beam.  The concentration of energy drops off quickly as one gets farther away from or off to the side of the main beam.

Once transmitted, the length of the beam is infinite unless it is reflected, absorbed, or refracted by some object in its path.  The typical objects from which the beam is reflected are made of metal, concrete, or stone.  Grass, dirt, and leaves largely absorb the beam, with little energy being reflected back to the antenna.

The term refraction refers to the radio waves that may pass completely through some substance and continue on infinitely.  As they do, though, their direction or velocity may be changed slightly.  Almost all forms of glass and many plastics will refract the RADAR beam.

The range, or maximum distance, at which the RADAR can interpret a reflected signal, is dependent on the sensitivity of the antenna receiver.  In other words, the RADAR antenna will not respond to every signal it receives.  It can only respond to those signals that are strong enough to be recognized.

If a RADAR beam’s operational range could be seen, it would have the appearance of an elongated cigar.  While this cigar shape is not the entire transmission of RADAR energy, it does represent that area of the beam from which usable reflections back to the antenna can normally be achieved.  Most police traffic RADAR now in use is capable of receiving and displaying reflected signals from targets of well over a mile.

Located close to the antenna are much smaller cone-shaped beams.  These beams, or side lobes, are a by-product of the RADAR antenna and are so reduced in power that they normally don’t affect RADAR operation.

Beam width will vary from manufacturer to manufacturer and from model to model.  The National Institute of Standards and Technology, in a laboratory environment, found beam widths to vary from 11.5 to 24.2 degrees.  As one can tell from the description, lane selection capability is virtually nonexistent with current RADAR.

The initial angle of the emitted RADAR beam will determine the relative beam width.  This initial angle may vary from 11 degrees to 18 degrees depending on the manufacturer.  For example, a beam emitted at an 18-degree angle will be approximately 80 feet wide at a distance of 250 from the source; 160 feet wide at a distance of 500 feet; and 320 feet wide at a distance of 1,000 feet.  Even with a device that emits a beam with relatively narrow angle of 11.5 degrees, the beam would be approximately 50 feet at a distance of 250 feet; 100 feet wide at 500 feet; and around 200 feet wide at a distance of 1,000 feet.

This makes it impossible for RADAR to select or focus in on one particular traffic vehicle at any significant distance.  It is vital that the operator understand that simply pointing the antenna at a specific target vehicle will not necessarily result in a speed reading from only that vehicle when other vehicles are within the RADAR’s operational range.  Other criteria must be used to determine which vehicle’s speed the RADAR is displaying.

BAND WIDTH AT 500 FEET (152 METERS)
X 157 FEET (47.86 METERS)
K 104 FEET (31.70 METERS)
Ka 79 FEET (24.08 METERS)
 

STATIONARY RADAR ANGULAR (COSINE) EFFECT
If a target vehicle is moving directly toward or away from the RADAR, the relative motion as measured by the RADAR should be equal to the target vehicle’s true speed.  Very often, however, this is not the case.  For safety reasons a stationary RADAR is set up a short distance from the traveled portion of the road.  Thus, cars traveling along the roadway will not be heading directly toward or away from the stationary RADAR-in other words, some angle between the car’s direction of travel and the RADAR’s position is created.

When a target vehicle’s direction of travel creates a significant angle with the position of the stationary RADAR, the relative speed will be less than the true speed.  Since the change in the signal’s frequency is based on the relative speed, the RADAR speed measurement may be less than the car’s true speed.  This is known as the angular or cosine effect.

The difference between the measured and true speeds depends upon the angle between the object’s motion and the RADAR’s position; the larger the angle, the lower the measured speed.  This effect always works to the motorist’s advantage when the RADAR is stationary.

Loosely speaking, the angular effect is not significant as long as the angle itself remains small.  Table 1 indicates how a stationary RADAR speed measurement can differ from true speed as a function of angle.  As can be seen in this table, the angular effect does not become a factor until the angle reaches about 10.  When a target vehicle passes by at a 90-degree angle, the RADAR is unable to perceive any of the vehicle’s speed because at this angle the target is getting neither closer or farther away from the RADAR.

It is important that the operator point the moving RADAR’s antenna as straight as possible into the patrol vehicle’s direction of travel.  The operator can obtain an alignment very close to 0 degrees by “eyeballing” the antenna in relation to the patrol vehicle.

It is true that the mathematical potential for angular effect causing an improper target reading is not likely until there is about 10 degrees present.  However, the operator should not deliberately misalign the antenna of the moving radar because: IT MAY HARM THE OPERATOR’S CREDIBILITY IN COURT.  Because few RADAR antennas are provided with mounting brackets with degree markings on them, it is difficult for the operator to testify the antenna was aligned only 1, 2 or 9 degrees off center.  (Where RADAR units possess antenna brackets with such markings, testimony probably would have to be given showing that the brackets had been properly installed.) On the other hand, everyone is familiar with the term “straight ahead.”  The burden on the operator to disprove the existence of a low patrol speed angular effect is much less if it is concerned only with pointing the antenna straight ahead.  Even a defense argument alleging the RADAR could be a few degrees off can be refuted because a few degrees has no appreciable effect on the RADAR target reading.

It should be stressed that, with proper antenna alignment, the angular effect on moving RADAR does not often produce speed measurements that lead to high target speed-reading.  Most often the angular affect will produce low readings.  The point is that the angular effect can work EITHER way when MOVING RADAR is involved.  The possibility that the angular effect may produce a low patrol speed measurement and give a higher-than-true target speed is of most concern.

Even properly operated, the RADAR can perceive a less-than-true patrol speed when certain unavoidable conditions exist.  The operator must have some way of recognizing these conditions so that the resulting improper target speed-reading can be disregarded.  This is why all moving RADAR units now on the market have both a target speed display and a patrol car speed display window.  Vitally important to the operation of moving RADAR is the cost monitoring and comparison of the patrol car calibrated speedometer with the patrol speed displayed on the RADAR.

THE RADAR “DECISION” PROCESS

A considerable effort has been made to explain how police traffic RADAR works in relation to the Doppler Principle.  When multiple targets are present in the RADAR beam, additional factors must be considered.

A RADAR beam may be only a few inches wide at the antenna but several hundred feet wide at its maximum operational range.  The antenna may receive reflected signals from many vehicles.  Most RADAR now in use in this country are designed to display the strongest of the multiple signals available.

The RADAR unit’s operation is affected by three factors:  the reflective capability of the various targets; their position in relation to each other and the RADAR; and, occasionally, the targets actual speed.

REFLECTIVE CAPABILITY – Most of the vehicles on a given road will be of different sizes.  A large truck will obviously have a larger reflective area than a smaller passenger vehicle will.  Thus, a truck can create a stronger reflected signal than a passenger vehicle.  By the same token, a passenger car can have a stronger reflected signal than a motorcycle.

The shape and physical makeup of a target vehicle will also affect its reflective capability.  Low profile, streamlined vehicles have less surface area to reflect a RADAR signal than vehicles of the same relative size that are not streamlined.  Vehicles containing a large amount of plastic materials or those made of fiberglass are generally less reflective than those of metal.  Streamlined vehicles, or those made of fiberglass, will reflect a RADAR signal.  However, the distance at which the RADAR displays a reading for such vehicles will be reduced.

POSITION – The position of a target vehicle relative to other vehicles and the RADAR antenna is important in regard to which vehicle’s speed the RADAR unit will display.  Normally, the closer a vehicle is to the antenna, the stronger the reflected signal.  In other words, the closer a vehicle is to the antenna, the larger the portions of the cone-shape beam it occupies.  If vehicles of comparable size are in question, the target vehicles closest to the antenna will most often be the one displayed by the RADAR unit.

SPEED – The speed of a target vehicle is the last factor affecting how a RADAR unit will operate.  How much a target vehicle’s actual speed will affect the RADAR unit’s “decision” depends on the make and model of RADAR being used.  Generally speaking, speed is usually the least dominant of the three primary factors.

When multiple targets of unequal size are present, either reflective capability of position will most often be the determining factor.  IT is vital that the operators understand that reflective capability and position are completely different.  With this understanding, the operator will be better able to tell which factor is governing any particular multiple-target situation.  To illustrate this, an explanation of what actually happens to the radio energy wave after leaving the antenna is in order.

If a slice of the cone-shaped RADAR beam could be observed 250 feet from the antenna, your would find that almost all of the energy originally transmitted is still there.  However, instead of being contained in a circle a few inches in diameter, as originally transmitted, the energy would be dispersed over a circle approximately 70 feet in diameter.  If this distance were now doubled to 500 feet, the energy would be spread over an area four times as large as at 250 feet.  If the distance is again doubled to 1,000 feet, the area covered would be four times as large as at 500 feet, but 16 times as large as at 250 feet.

It is apparent the farther a target vehicle is from the RADAR unit, the lesser the amount of energy available to be reflected back to the antenna.  This relationship between energy and the distance from its source is called the inverse square rule.

To understand the impact of reflective capability, imagine a car 500 feet away and approaching the RADAR unit.  At the same instant, a truck is also approaching the RADAR unit 1,000 feet away.  Given these relative positions, there is four times less RADAR energy per square foot to be reflected from the truck at 1,00 feet than there is to be reflected from the car at 500 feet.  However if the truck has five times the reflective surface of the car, its reflected signal will probably be stronger.  In this case reflective capability would probably determine which vehicle’s speed is displayed by the RADAR unit.

To illustrate the impact of position, it is necessary to advance the positions of these imaginary vehicles to where the car is 250 feet from the antenna and the truck is 750 feet away.  At 250 feet the car now has four times as much RADAR energy being reflected from each square foot of surface area as it did at 500 feet.  The truck also has more reflected RADAR energy per square foot at 750 feet as it did at 1,000 feet, but proportionally hasn’t increased nearly as much as the car.  In this case, position would probably determine which vehicle’s speed the RADAR unit would display.

It should be noted that with each of the examples cited, the reflective capability of the vehicles’ size, shape, and composition remained the same.  The position of the vehicles to the RADAR was the only factor that changed.

Under certain circumstances, RADAR devices can select which vehicle to display on the basis of speed.  The most common instance involved multiple target vehicles of comparable size on an expressway.  If a similar-sized vehicle at a significantly greater speed is overtaking a vehicle approaching the RADAR unit, the faster vehicle’s speed may be displayed.  This normally will not occur, however, until the faster vehicle is reasonable near the lead vehicle.  The RADAR unit is less likely to be speed-selective on two-lane roadways because the front vehicle is likely to block the radio waves from striking the following vehicles.  Operators should be aware that with most RADAR units currently in use, the individual speeds of approaching target vehicles do not normally determine which vehicle will be displayed.

The possible combinations of these factors: reflective capability, position, and speed are infinite.  The interpretive process that results in valid target identification is generally easy for the trained operator because a RADAR reading is only one part of the evidence the operator has to have to establish a speed violation.

THE ROLE OF SUPPORTIVE EVIDENCE –TRACKING HISTORY

Several elements are involved in the valid identification of a target vehicle.  Together these elements comprise what is referred to as a complete “tracking history” and are listed below.

VISUAL ESTIMATION OF TARGET SPEED – This is the most critical element.  Testimony must substantiate that the vehicle in question was observed to be speeding.  An officer’s ability to estimate speeds is established separately from the RADAR evidence.  The officer should be able to testify that a target vehicle was traveling faster than the speed limit even if no RADAR or similar device was used.

AUDIO TRACKING – The audio feature common on many police traffic RADARs allows the operator to hear the incoming Doppler signal.  A stable target signal will result in a single pure, clear audio tone.  The higher the pitch of the signal, the faster the speed of the target producing the signal.  With experience, an operator can correlate this pitch with actual speeds.  Interference that could affect the RADAR is heard as static or buzzing and is not consistent with the pure, clear Doppler return from valid target vehicles.

TARGET SPEED DISPLAY – The target speed displayed by the RADAR must correspond reasonable with the visual and audio estimations.  Each of the three must reinforce the other.  If any of them is incompatible, the reading must be disregarded.
With stationary RADAR, these three elements would be enough to constitute a valid tracking history.  One additional element is required for moving RADAR

PATROL SPEED VERIFICATION (MOVING RADAR ONLY) – Current moving RADARs, as previously noted, possess not only a target-vehicle, speed display window but also a patrol-car, speed display window.  The patrol speed indicated on the RADAR must correspond with the reading on the patrol vehicle’s speedometer, which must be certified as well.  This verification ensures that the RADAR computation of the target speed is based on a valid patrol car speed.  This additional element has been mandated by case law for moving RADAR and is considered essential for a valid moving radar case.

A tracking history must be obtained for each RADAR-based enforcement action.  Whenever RADAR speed measurements are conducted, two points must be kept in mind:

1) The RADAR-displayed speed measurements are only one part of the evidence and cannot be the sole basis for any enforcement action.
2) In order to be valid and admissible, the RADAR speed measurement must be obtained in strict compliance with all applicable case law.

These two points have significant implications for the manner in which RADAR operations are conducted.  Related to the first is the officer’s need for good judgement and experience in making visual estimates of a vehicle’s speed.  Almost everyone can estimate speeds visually; it would be impossible to drive a motor vehicle or walk across a busy street without some understanding of how fast traffic is moving.  Because observing traffic is a major part of their job, traffic law enforcement officers in particular can and do become very good at estimating speeds.

NEVER BASE A DECISION ON INSTANT RADAR MEASUREMENT! Instead, watch the speed measurement and listen to the audio output for at least a few seconds to make sure that the signal received is from a real, identifiable vehicle.
The biggest impediment to obtaining a valid tracking history is the locking feature that some RADAR instruments have.  This feature allows the operator to press a button or pull a trigger and cause whatever speed measurement is appearing at that instant to freeze on the display.  Many of these RADARs have an automatic locking feature that causes this freeze to occur as soon as some specified seed is exceeded.  The idea behind the locking feature is to preserve the evidence.  Whatever the idea’s merits might have been, locking the speed measurement can prevent the operator from correlating changes seen or heard in the target vehicle’s speed with the speed on the RADAR display.  If a good, complete tracking history has been obtained, the RADAR speed reading may be manually locked – if that is compatible with your agency’s procedures.  However the automatic locking feature and/or auto alert should NEVER be used for enforcement purposes.

When multiple targets are present, it is often preferable to continue observing the target speed measurement, paying close attention to what happens when the suspect vehicle passes out of the beam.  The display might suddenly either blank out or abruptly change to another speed.  If that happens, the implication is that the speed measurement obtained was from the target vehicle.

On the other hand, the display might hold steady after the suspect vehicle passed out of the beam.  The implication in that case would be that the RADAR was displaying some other vehicle’s speed.  The Doppler audio feature present on many units is also very useful in this respect.  If there is no change in the Doppler sound, you can infer that the suspect vehicle was not being displayed.

The burden of proof is obviously less if the operator can show that there were no other vehicles between the RADAR and the target vehicle.  Remember, however, that other vehicles farther away can produce a stronger reflected signal.  Again, good visual and auditory observations validate the speed-reading and complete the tracking history.

EFFECT OF TERRAIN ON TARGET IDENTIFICATION

Road terrain may affect the RADAR unit’s ability to process and display target vehicle readings.  The best areas for RADAR operation are straight and level roadways.  When traffic RADAR is operated on hilly or curved roadways, you must take their effect on the RADAR into account.  Police traffic RADAR units are designed to function on a “line of sight” basis and will seldom display a vehicle behind a hill or around a curve.

Hilly terrain creates the worst problems in target identification.  For example, if the patrol vehicle is positioned on the crest of a hill, with the RADAR antenna focused straight ahead rather than down, the RADAR beam may “overshoot” the approaching lead vehicle and display the speed for the vehicle behind it.  A dip in the roadway may also affect the RADAR’s ability to display the lead vehicle.  In this case, the roadway itself my shield most of the reflective surface of the lead vehicle and again cause the RADAR to pick up a following vehicle.  When roadway terrain problems exist, you must exercise discretion in using RADAR, tracking the vehicle long enough to be certain of target identification.

OPERATIONAL RANGE CONTROL

Some RADAR instruments have a control that permits the adjustment of operational range.  The range control allows an adjustment to the RADAR instrument’s sensitivity to reflected signals, and can be used to reduce target identification problems.  It must be stressed that the RADAR transmissions remain steady and unaffected by this range control.  This control only affects the RADAR’s ability to receive and process a signal.

Thus, a low sensitivity setting means that the RADAR will only perceive fairly strong signals- the RADAR will not “see” a vehicle until it is fairly close.  A high sensitivity setting means that the instrument will perceive even fairly weak signals from vehicles that are quite far away.  Atmosphere and other environmental conditions can affect the RADAR’s sensitivity to target vehicle signals.

For moving RADAR, the sensitivity setting must be significantly higher, because both vehicles are moving, and the distance between the patrol car and the target vehicle changes very rapidly.  This means that moving RADAR must be more sensitive to targets at longer ranges than stationary RADAR to achieve a proper tracking history.  Most range control units are designed so that the average automobile will be displayed when it is in the selected range.  Small vehicles may not reflect a signal strong enough to be displayed until they are close to the transmitter.  Larger vehicles, of course, may be displayed even though the vehicles are farther away.

In the past, operators often attempted to control RADAR sensitivity by tilting the antenna up or down.  This tilting is not recommended, since it may cause or worsen interference.  If your RADAR unit has no range control, keep the antenna pointed straight ahead and stay alert – do not tamper with the antenna.

One final point should be mentioned: adjusting the beam’s range control will have absolutely no effect on RADAR DETECTORS.  You can’t outwit a RADAR detector by turning down your range setting, because the power in the beam remains constant regardless of the range control setting.

FACTORS AFFECTING RADAR OPERATION

It is sometimes alleged that police traffic RADARs often display “false” target readings.  In fact, certain factors CAN affect RADAR devices.  Many of these factors can be avoided, provided you operate the RADAR unit properly.  Some are unavoidable, the result of natural causes.  All of them should be recognizable to the trained experienced operator.

Police traffic RADAR, like any measurement instrument, has inherent and logical limitations.  When a false reading is displayed, the RADAR unit is not making an error – it has simply been subjected to conditions beyond its capabilities.  As a rule, though, it is more likely that the operator, rather than the RADAR, will make an error.  This can happen if the operator forces the device to operate beyond its limitations or fails to recognize when its limitations have been passed.

INTERFERENCE – Interference encompasses a wide range of natural and artificial phenomena.  For this manual, the term “interference” will refer to RADAR effects that happen unintentionally.  Purposeful attempts to subvert or otherwise affect RADAR will be discussed later.

Generally, interference can be attributed to two primary sources:

1) HARMONICS – The first source of interference, harmonics, refers to RADAR’s tendency to occasionally process the wrong radio frequency.  Harmonics may include radio energy released by airport RADAR, mercury vapor and neon lights, high-tension powerlines, high-output microwave transmission towers, and transmissions from CB and police radios.

2) MOVING OBJECTS – The second source of interference results because Doppler RADAR is designed to measure the relative motion of moving objects.  This can mean any moving object, not just motor vehicles.  The most common moving objects that may interfere with RADAR are vibrating or rotating signs near the roadway and fan blades moving either inside or outside the patrol car.

A RADAR antenna’s sensitivity or capability to receive and process a signal depends on the number and relative strengths of the reflected signals it is receiving.  Strong signals received from bona fide target vehicles will almost always override weaker interference.  Interference caused some of the more bizarre and highly publicized “inaccuracies” that surfaced in Dade County, Florida and other places where RADAR has been challenged.  The infamous “85-mph tree” is a case in point.
The news has been well circulated that a RADAR antenna was pointed at a Banyan tree (which was obviously was not moving) and that a reading of approximately 85 mph appeared on the display.  Not so widely reported was the fact that a CM radio transmitter located in the same vehicle as the RADAR had been turned on and at the instant the reading was made a reporter whistled into the microphone.  The RADAR picked up feedback from the CB, and that caused the 85-mph reading.  Ordinarily, RADAR will not pick up a CB signal, but when the CB and the RADAR are extremely close together (as in the same vehicle), interference can result.

Another well-reported incident from Dade County was the “28 mph house.” Here again, interference was the culprit.  The RADAR antenna was aimed at the house through the car’s front windshield.  Meanwhile, within the antenna’s range the window defroster had been turned on and its fan blades were spinning.  What the RADAR actually measured was the speed of the defroster fan’s blades.

Most interference will come from inside the patrol vehicle.  Radio transmissions from within the patrol car and, occasionally, faulty ignition wires may cause harmonic disturbances; air conditioning and heater fans may cause moving interference, sources like airport RADAR, high-tension powerlines, and high-output microwave transmission towers may create readings.
The operator must be careful to avoid known interference sources and hot make radio transmissions while actually tracking a vehicle on RADAR.  If the RADAR antenna is inside the patrol vehicle, dash-mounting the antenna and aligning it away from fan vents will minimize movement-related interference.

MULTI-PATH BEAM CANCELLATION EFFECT

Multi-path beam cancellation refers to the RADAR blind spots produced by occasional oddities in roadway terrain.  The RADAR operator may be monitoring the speed of an approaching vehicle when suddenly the speed displayed will disappear for a few seconds and then, just as suddenly, reappear.  All this happens while the vehicle remains constantly in sight.
Technically, multi-path beam cancellation results when a 180-degree phase inversion occurs between the direct path signal from the target vehicle and the signal from the reflected path.  The signals, in effect, cancel each other out as far as the RADAR is concerned.  During this brief period the RADAR display goes blank. However, the operator must be alert to the possibility that a vehicle behind the lead target vehicle may briefly display a speed on the RADAR when the lead vehicle’s signal has momentarily been cancelled.  Obtaining a good complete tracking history will minimize the problem of multi-path beam cancellation.

SCANNING EFFECT

A hand-held RADAR antenna that is swung swiftly or “scanned” past the side of a parked car, a brick wall, or some other stationary object is alleged to produce a speed measurement.  The idea behind this charge is if the antenna moves, the RADAR will register the relative motion.  This effect is extremely difficult to produce.  However, by not moving the RADAR antenna while making a speed measurement, you will eliminate any possibility of its contributing to a false reading.  In any case, swinging an antenna around does not help obtain a valid tracking history.

PANNING EFFECT

The panning effect can occur only with two piece RADAR units; instruments whose antennas and counting units are physically separate.  If the antenna is pointed at its own counting unit, a speed-reading may appear on the display because of electronic feedback between the two components.  When this is done, the Doppler audio becomes a very inconsistent squeal.

TURN-ON POWER SURGE EFFECT

Suddenly turning on the RADAR unit’s power may result in a speed-reading displayed because of the sudden surge of voltage to the unit.  Operators are said to do this with the idea of outwitting RADAR detectors. Many newer-model RADAR units have a “transmission-hold” switch that keeps the power turned on in the computer module but prevents the transmission of the RADAR signal until the target vehicle is within range.  These “anti-fuzzbuster” switches are considerably more effective then flipping the unit off and on-and they don’t constitute misoperation.  Allegations that using the transmission-hold switch may also cause a power surge affecting the speed display are spurious.  Extensive testing by the National Institute of Standards and Technology on various makes of RADAR indicates no support for this charge.

MIRROR SWITCHING EFFECT

Some hand held-type RADARs can have the numerals of the readout displayed backwards when a switch is thrown.  This lets the operator point the RADAR rearwards and read the numbers correctly through the rearview mirror.  The claim here is that the operator may forget to switch the display back again when the unit is pointed forward and mistake the reversed reading for a proper reading.  For example, a reversed speed reading of 18 could be read as 81.

FACTORS AFFECTING MOVING RADAR OPERATIONS

The operator should be aware that moving RADAR is susceptible to some special problems that do not affect stationary RADAR.  Like the moving RADAR angular effect, these can produce a lower-than-actual patrol speed measurement and thus a higher-then-actual target speed calculation.

PATROL SPEED SHADOW EFFECT

Remember that moving RADAR depends on two speed-readings, that of the target and that of the patrol.  A shadow effect may be caused if the beam that is supposed to determine the patrol car speed by tracking stationary terrain instead locks onto a large moving vehicle in front of the patrol car.  This large vehicle (usually a truck) must be close enough to the RADAR unit to effectively reflect a major portion of the normal beam.  If this occurs, the patrol car’s speed will be displayed as the difference in speeds between the patrol car and the truck rather than the patrol car and the stationary terrain.  If a target vehicle is approaching at this time, the RADAR could add the remainder of the patrol car’s speed to the target’s speed.  (TARGET SPEED = CLOSING SPEED – PATROL SPEED.) It should be noted that there must be a significant difference in speed between the truck and the patrol car to produce this effect.

Most RADAR units can be made to shadow, under the right conditions.  During normal moving-RADAR operations, this effect is largely unavoidable, but luckily it is reasonably rare; and most importantly, is extremely noticeable to the operator, provided he monitors his vehicles calibrated speedometer.

There will be two glaring inconsistencies.  One, the target speed displayed will be in excess of the visual estimation of the target as perceived by the operator.  Two, when the operator checks the patrol car displayed speed after the false target reading is obtained, it will in no way correspond to the calibrated patrol car speedometer speed.  For example, if a patrol car is actually driving at 55 mph and has locked onto a truck doing 40, the RADAR may display a target vehicle supposedly doing 100 while the patrol car is supposedly doing 15.  The reading obtained should then be ignored as it lacks the necessary supportive evidence for complete tracking history.

BATCHING EFFECT

Another problem unique to moving RADAR is known as the batching effect.  This is caused by slight time lags in the moving RADAR’s sensing/computing cycle.  Like the angular effect on moving RADAR, the batching effect can lead to the display of erroneously low or high speeds.

The Batching effect may occur if the patrol car is rapidly changing its speed while the RADAR speed measurements are being made.  Some RADAR counting units may be unable to keep up with the rapidly changing speeds.  Instead of using the true speed of the patrol car to measure closing speed, the counting unit may use the speed that the patrol car was traveling a few fractions of a second earlier.  If the patrol car is rapidly accelerating, then its earlier speed was lower than its present true speed, and the target speed calculation may be higher than the target’s true speed.  If the patrol car is rapidly decelerating, then its speed a fraction of a second before was higher than its present speed, and the target speed calculations may be lower than the speed that the vehicle is actually traveling at.

Most RADAR units are fast enough to keep up with the significant speed changes, thus avoiding the batching effect, and/or blank out when such changes occur.  You can avoid even the possibility of an improper speed-reading due to the batching effect by maintaining a relatively steady speed when taking speed measurement.

JAMMING AND DETECTION OF POLICE RADAR
In recent years, some motorists have used increasingly sophisticated means to try to sidestep police traffic RADAR, offering rationales like, RADAR is unfair!” and “Maybe some people can’t, but I can drive safely at higher-than-allowable speeds.”  Whatever reasons are offered, they all sound foolish when compared to the documented evidence that excessive speed kills.

Jamming Devices
Purposeful attempts to create false or distorted RADAR signals are called “jamming”.  This is not yet a widespread problem (Jamming devices are illegal, and tend to be expensive and complicated), but various agencies are encountering it more and more.

The most effective jamming device is a radio transmitter that sends out a relatively strong signal with a frequency close to that of police traffic RADAR.  The RADAR receiver “sees” that signal rather than (or in addition to) the signal reflected from the speeding vehicle, with the result that the RADAR displays either a false speed or no speed at all.  The Federal Communications Commission (FCC) will not license a device whose purpose is to jam police RADAR.  Even using an already-licensed radio transmitter would violate Federal regulations if the use were to purposely jam police RADAR.

It is easy to tell when a RADAR jammer is being used.  When a jammer is being used close by, your Doppler audio should be inconsistent and uneven in tone.  The speeds displayed on a RADAR being jammed tend to fluctuate, even though you can see no obvious change in the speeds of approaching vehicles.  The best tip-off that a jammer is around occurs with the RADAR units that have transmission hold switched.  Some RADAR devices are capable, when on transmission hold, of receiving and processing incoming signals.  If the RADAR is displaying a speed and the audio is emitting a tone even when the RADAR is not transmitting a RADAR signal, a jammer is almost certainly being used nearby.

If you encounter a jamming device, your can contact the nearest FCC regional office for assistance.  In addition, some States and local governments have language in their statutes outlawing the use of jamming devices similar to that in Federal Law.
One will occasionally encounter or hear of other “techniques” for jamming police RADARs.  These usually range from the laughable to the ridiculous.  Among the more common “jammers” based on pseudo scientific superstitions, one will find:

ALUMINUM PAINT STRIPS OR METAL FOIL STRIPS ON THE OUTER SIDE OF A VIOLATORS VEHICLE.  (If anything, this only increases the vehicle’s ability to reflect the RADAR beam and makes it easier to measure its speed.)

HANGING CHAINS UNDER THE VEHICLE. (This might help keep static electricity from building up on the vehicle, but it certainly will NOT distort or reduce the RADAR waves that the vehicle reflects.)

HIDING SMALL METAL OBJECTS OR STRIPS OF METAL FOIL INSIDE A VEHICLE’S HUBCAPS. (The only thing this can do is create unpleasant rattling sounds;  the RADAR beam will not penetrate the hubcap.)

FURIOUS HONKING OF THE HORN. (The vibrating diaphragm of the horn could modulate a RADAR signal, but the horn is under the hood, these vibrations are not detected.)

LEGAL AND GENERAL OPERATIONAL CONSIDERATIONS

FOUNDATIONAL ELEMENTS AND REQUIREMENTS FOR INTRODUCTION OF SCIENTIFIC EVIDENCE

Evidence derived from complex mechanical devices is typically challenged by the defense as to its accuracy and reliability (Commonwealth v. Buxton). The burden then rests on the prosecution to demonstrate to the court that these devices are capable of performing their function accurately.  To do this, the prosecution must introduce testimony by recognized experts in that particular field.  Suck expert testimony is required every time a case involving a new principle comes to court.  The process of expert testimony is long and tedious, and often bogs down the judicial process.

The court can dispense with the need for expert testimony only if the scientific principle underlying the new device has been given judicial notice.  Judicial notice indicates that a particular fact or principle is so generally known as to be familiar to all reasonably well-informed persons.  When the courts feel that a particular principle is commonly understood and accepted, they will take judicial notice of it; thereafter, expert testimony is no longer needed.  This approach by the courts has in the past been applied to such (at the time) new principles as clocks, chronometers, motion pictures, x-rays, fingerprinting, and television.
Bear in mind that judicial notice extends only to the scientific accuracy of the principle upon which a particular device operates.  It does not extend to the accuracy or efficiency of any given device designed to employ that principle.  Judicial notice has also been taken of certain methods or techniques for determining the accuracy and reliability of a particular device.

Once the courts accept a certain scientific principle and take judicial notice of certain tests for accuracy of devices that employ the principle, it must still be established that the individuals who used the device were qualified to do so and that the specific device used was operating properly at the time in question.

You should also be aware that case law, i.e., fundamental court rulings, apply directly only in the jurisdiction where they were handed down.  However, a fundamental ruling in one State will often be offered as persuasive authority in another State’s court.

JUDICIAL NOTICE OF THE RADAR PRINCIPLE

Before June 1955, the soundness of the Doppler Principle was the central issue in virtually all court cases involving the admissibility of speed measurement evidence obtained by RADAR.  The issues of the reliability and accuracy of RADAR devices were subsidiary questions.  In case after case, the prosecution had to prove the Doppler Principle through the long, involved testimony of expert witnesses.

In 1955, the Supreme Court of New Jersey finally took judicial notice of the principle behind Doppler RADAR.  The case in question, State v. Dantonio, proved a landmark decision.  In deciding this case, the court drew a parallel between RADAR meter readings and those registered on more well-known instrumentation, such as fingerprints, x-rays, cardiographs, etc., stating:
 

“The law does not hesitate to adopt scientific aids to the discovery of truth which have achieved such recognition…Since World War II members of the public have become generally aware of the widespread use of RADAR methods in detecting the presence of objects and their distance and speed…”

With this, the court affirmed that the RADAR concept was generally known and understood by all reasonably well-informed individuals: The court extended judicial notice.

Other States quickly followed suit.  The Supreme Court of Arkansas, in Everight v. City of Little Rock, followed the New Jersey court’s decision, saying:
 

“We are of the opinion that the usefulness of RADAR equipment for testing [the] speed of vehicles has now become so well established that the testimony of an expert to prove the reliability of RADAR in this respect is not necessary.  The courts will take judicial notice of such fact.  Of course, it will always be necessary to prove the accuracy of the particular equipment used in testing the speed involved in the case being tried.”

JUDICIAL NOTICE OF TESTS OF ACCURACY

The accuracy of a particular RADAR unit, as distinguished from the accuracy of the RADAR principle, is not a proper subject for judicial notice.  No court can accept every RADAR device as always being accurate.  The prosecution must prove that a particular device functioned properly at the time in question.

What the court may do is take judicial notice of certain methods or techniques for determining accuracy.  It can reasonably be assumed that if a particular device was checked for accuracy at various intervals and through accepted methods, that device’s readings would be accepted as accurate.  In a Virginia case, Royals v. Commonwealth, the court quoted, with approval, Dr. John M. Kopper, a recognized authority on electronics:
 

“It is important to check the meter for accuracy each time it is setup for use; if the meter is to be used at two sites in one morning then it should be checked at each site to avoid the contention that the meter was thrown out of adjustment during transit.  The meter should be checked before the beginning of the period of observation of a highway and at the end of the period.  In scientific work it is usual to assume that if a given instrument reads correctly at the beginning and end of a set of measurements, its readings during the interval were also correct.  The check can be made by having a car with a calibrated speedometer run through the zone of the meter twice, once at the speed limit for the zone and once at a speed 10 or 15 mph greater.  As the test car goes by the meter the driver can notify the operator of the meter what [the] speed is.  If the difference between the speedometer reading and the RADAR meter reading is more than 2 miles per hour, steps should be taken to see why this is the case and to remedy the matter.  Such a test naturally requires a periodic checking of the speedometer of the test car.  If such a procedure is carried out each time the RADAR meter is set up, the check measurements made with the automobile speedometer become supporting evidence.”

The court established guidelines for when RADAR equipment must be tested and the issue of the best method of testing remained.  The use of the tuning fork is efficient, convenient, and popular method of testing a RADAR device’s accuracy.  The use of the tuning fork as a reliable test of accuracy as established by the Supreme Court of Connecticut in State v. Tomanelli.  However, the court pointed out that the tuning fork’s own accuracy may be questioned.  In effect, the courts have recognized the tuning fork as an accurate testing device.  If no challenge is offered, the tuning fork’s accuracy may be assumed and therefore the accuracy of any RADAR device properly tested by that tuning fork.

VEHICLE IDENTIFICATION

As discussed earlier, certain elements of the speeding offense must be established for prosecution to be successful.  Beyond establishing the vehicle’s speed, the officer must also be able to prove that a particular speed law was violated; that the defendant was the driver of the vehicle at the time of the offense; and that the offense occurred on a public thoroughfare.  In cases where RADAR has been used to obtain the speed measurement, the officer must also be able to identify the violator’s vehicle.

Identifying a vehicle does not mean just saying that it was, for example, a yellow Mercedes.  In cases involving RADAR, vehicle identification refers to the operator’s ability to tell which vehicle’s speed registered on the instrument.
For example: An officer on RADAR patrol is monitoring a section of highway at a time of moderate traffic flow.  The officer discovers a speed violator and obtains a RADAR reading on that vehicle.  Naturally, the defense will maintain that the officer could not possibly have singled out the defendant’s vehicle from all the others on the road.  How then can the RADAR officer assume that the violator was properly identified?  In Honeycutt v. Commonwealth, the Kentucky Court of Appeals dealt with this problem:
 

“The RADAR device used in the instant case simply registered a speed reading on a speedometer dial.  It did not show a “blip” on a screen or by any other means undertake to show location or direction of a vehicle in its field.  The testimony of the policeman was that he had set up the instrument to cover northbound traffic on the four-lane, two-way street in question.  He said that he observed, in the rearview mirror of the cruiser, several vehicles approaching from the south.  One of them was passing the others.  The RADAR speed meter registered an unstable reading, with a to of 50 mph.  Directly, the reading stabilized at 50 mph and the observed in the mirror that one car had passed the others and was itself out in front of the others.  The fact that one car was by itself, away from the others, and closest to the RADAR unit, enabled the RADAR unit to make a sable reading of its speed.  The policeman pursued the car in question, in [his] cruiser, and caused it to stop.  The appellant was driving the car.

The appellant argues that there was insufficient evidence that his car was the one, which caused the RADAR unit to show a 50 mph reading’ that a southbound car in the other lane could have caused it.  In our opinion the reasonable import of the policeman’s testimony is that he observed the appellant’s car pass others at the same time the RADAR dial showed a fluctuating reading with a 50 mph maximum.  When the dial stabilized at 50 mph, the car was in front by itself, nearest to the unit.  The policeman’s estimate of its speed, by visual observation alone, was from 40 to 45 mph.  This evidence reasonably points to the appellant’s care as the offending vehicle, and we do not think that the evidence is reduced to worthlessness by the remote chance of coincidence that a southbound vehicle broke clear from a passing situation, at 50 mph, at the same moment that the appellants car got out in front of the northbound lanes.  Furthermore, the testimony indicates with reasonable certainty that a southbound car, when it entered the range field of the RADAR, would have been beyond the north bound cars and therefore would not have registered a stabilized reading.”

SPECIAL REQUIREMENTS OF MOVING RADAR

Moving RADAR presents special problems in vehicle identification because the speed of the patrol car itself enters the picture.  In effect, when moving RADAR is used the courts demand that the officer verify both the defendants’ vehicle speed and that of the patrol car at the same time of the violation.

In 1978, in the landmark case of State v. Hanson, the Wisconsin Court addressed several issues on the use of moving RADAR.  As with earlier case law, Hanson affirmed that:

1) The operator must have sufficient training and experience in the operation of moving RADAR.
2) The moving RADAR instrument must have been in proper working condition when the violation took place.
3) The moving RADAR device was used where road conditions would distort readings as little as possible.
4) The patrol car’s speed was verified.
5) The instrument’s accuracy was tested within a reasonable time before and after the arrest.

A) The officer must establish the time, place, and location of the RADAR measurement; the location of the offending vehicle when the violation took place; that the defendant was driving the vehicle; and that State law regarding the posting of speed limits and RADAR signs had been complied with.
B) The officer must state his qualifications and training.
C) The officer must establish that the RADAR device was operating normally.
D) The officer must establish that the RADAR device was tested for accuracy, both before and after its use, using a certified tuning fork or other accepted method.
E) The officer must accurately identify the vehicle.
F) The officer must have seen that the vehicle appeared to be speeding and estimated how fast.
G) The officer must have gotten a RADAR reading that agreed with the visual estimate of the target vehicle’s speed.
H) If a Doppler audio feature is present on the RADAR device, the officer is strongly encouraged to establish that the audio Doppler pitch emitted correlated with both the visual estimate and the RADAR reading.
I) If moving RADAR is used, the officer must testify that the patrol vehicle’s speed was verified at the time the speed measurement was obtained.

SAME-DIRECTION MOVING RADAR

Same direction moving radar was developed in 1989 and is designed to measure the speed of traffic that is moving in the same direction as the police vehicle.  Numerous changes have been made to conventional radar which are described below.

While operating same-direction RADAR on a limited access highway you are traveling at 55 mph and the approaching traffic is traveling between 55 to 60 mph. Their speed added to your speed gives a closing speed of 110-115 miles per hour.  You notice a vehicle entering the highway in front of you from an entrance ramp and it enters the same lane as you.  You also notice that the vehicle appears to be traveling at a much greater speed than the posted limit.  You then target the vehicle with your same-direction RADAR.  Your RADAR will ignore the approaching traffic, since the closing speed of 110-115 mph is far in excess of our limit of 15 mph above the 55-mph patrol speed.  Your RADAR is now free to clock the same-direction vehicle and it receives a measurement of 20 mph. (The speed at which the target vehicle is pulling away from your patrol vehicle.)  This speed becomes the value of C in the formula by which your RADAR calculates the speed T of the target vehicle.  T = P + C  T=55+20  T= 75 mph.  Remember that your RADAR needs to measure relative motion.  Two vehicles traveling in the same direction at the same speed would not have any relative motion.  In the above scenario, the target vehicle is traveling 20 mph faster than you are, and that the RADAR measures difference as relative motion.  The RADAR adds the 20-mph difference in speed C to your patrol speed P to attain the same-direction speed of the target vehicle.

Now with this comes a new problem.  In the above example, the target vehicle is traveling faster than you are.  Sometimes you may want a reading on a slower-moving vehicle ahead of you.  Since the RADAR works in absolute values and does not detect direction, it will display an inaccurate reading (on the high side) for the slower moving vehicle unless it is prevented from doing so.  This error would manifest itself when the RADAR would add C to P and calculate a high target speed T, instead of subtracting the speed difference of C from the patrol speed P.  This error will only occur when the operator fails to switch the RADAR from “Target Faster” to “Target Slower”.

EXTERNAL TESTS FOR ACCURACY

Over the years, a number of procedures have evolved to test the accuracy and calibration of police traffic RADAR.  Some of these methods are now mandated by case law.

INTERNAL CIRCUIT TEST – Testing typically begins with an internal circuit test.  These circuit tests vary from device to device and therefore will be discussed in subsequent sections. In essence, the internal circuit test checks the circuits inside the counting unit by means of either crystal(s) or internal electronic tuning fork(s).  It should be noted that the internal circuit test checks only the counting unit, not the antenna.  On most RADAR instruments, pressing a button and checking the speed display to verify that a particular number appears performs the internal circuit test.  In all cases, the internal test is passed only if the proper number appears EXACTLY.

LIGHT SEGMENT TEST – Many police traffic RADARs have a feature that allows the operator to make sure all the individual light segments on the RADAR speed display(s) are working.  A burned out light segment could cause the operator to make a mistaken speed-reading.  If a burned out segment is discovered, the RADAR unit should be taken out of service and repaired.

EXTERNAL TUNING FORK TEST – Next comes a test of the RADAR’s calibration.  The tuning fork used in this test should not be confused with those used for tuning musical instruments.  The RADAR tuning fork is specially calibrated for use with a RADAR device.  The external tuning fork test checks that ability of both antenna and counting unit to process and display a simulated target speed properly.

To use the fork, grasp its handle and strike one of the tines against a surface.  It is better to strike the fork against a surface that is reasonably firm but not as hard as the fork itself.   Tests by the National Institute of Standards and Technology have shown that even a badly chipped fork will (under most cases) continue to vibrate properly and give valid results.
Case law over the years has affirmed that using tuning forks is a valid way to check a RADAR unit’s accuracy.  Traditionally, only one tuning fork has been needed to check a stationary RADAR unit although local case law or departmental policy my dictate using two or more forks.

For moving RADAR, two forks are necessary because of the RADAR’s additional circuitry.  One tuning fork usually simulates a low speed, 30 to 50 mph.  The second tuning fork is usually a high-speed fork, simulating a speed of between 60 and 90 mph.

First striking the low speed tuning fork and holding it in front of the antenna checks moving RADAR.  This simulates a patrol car speed in the patrol speed display window.  A second, high-speed fork is then struck and also held before the antenna.  This second fork, presented in conjunction with the first fork, will simulate a target closing speed.  The speed displayed in the target display window should show the difference between the high and low speed forks, in other words a simulated moving target speed.  This process checks the RADAR unit’s ability to properly subtract the patrol car speed from the closing speed (remember Target Speed = Closing Speed – Patrol Car Speed).

RADAR tuning forks must not be mixed between X-Band, and K-Band RADARs.  An X-Band fork used on a K-Band RADAR (and vice versa) will not yield accurate results.

PATROL SPEED VERIFICATION TEST – The RADAR tests for accuracy discussed so far (internal circuit, light, and external tuning fork tests) apply to all traffic RADAR units, moving and stationary.  The final test, verification of the RADAR patrol speed readout against the patrol car calibrated speedometer, is required only for moving RADAR units.
This check is to establish that the moving RADAR unit is properly displaying the actual patrol car speed.  The operator accelerates to a steady speed and compares the RADAR’s patrol speed readout with the patrol car’s calibrated speedometer.  The speeds must correspond closely; if there is any noticeable deviation, the RADAR unit should not be used.

SUBSEQUENT TESTS FOR ACCURACY – It can only be assumed that the RADAR was working properly when a speed measurement was made if it can be proven that the RADAR was working properly both before and after the measurement was made.  An important procedural question is: How soon before and after a speed measurement must the RADAR’s accuracy be tested?  Each law enforcement agency determines its minimum requirements for re-testing the RADAR device as part of its own standard practices and in response to case law applicable to its own jurisdiction.

*** STAY TUNED FOR THE NEXT BOOKLET ABOUT “SPEED MEASUREMENT USING LIDAR” ***