CINCINNATI MICROWAVE ENGINEERING REPORT *** TRAFFIC RADAR AND YOU *** This report explains in simple language the principles and limitations of traffic radar. It also outlines documented errors that can cause radar to give incorrect readings. It is estimated that as many as 30% of radar tickets are in error. Every motor ist should know these facts. PART ONE: HOW RADAR WORKS There are many kinds of radar. Military radar, weather radar, and airtraffic control radar fall into one category. Traffic radar is quite different and falls into its own category. The first category of radar is highly sophisticated equipment. The popular concept of a radar antenna, a dish mounted on a tower, rotating to sweep the horizon, applies to this category. The rotating dish produces the familiar radar sweep you see on TV weather reports. These sophisticated radars sweep with a mod ulated beam to measure speed of objects, distance to those obects and general shape of those objects as an aid to identification by a ground crew. Such radars cost hundreds of thousands of dollars. Traffic radar, by contrast, is very simple. It must be simple because it can be no larger than what will fit on the dashboard of a midsized cruiserAnd it must be simple because it can cost no more than the low bid of a municipality's procurement process. Sometimes this is $400 or less. This necessary simplicity means that traffic radar has important limitations. Traffic radar does not sweep. Instead it uses a stationary beam, much like a searchlight, and it shines down the road, either forward or backward, but not both ways at once. This means the area under surveilance by traffic radar is quite limited. Traffic radar does not use a modulated beam. It uses a constant beam. This means that, if there are multiple moving objects within range, traffic radar is unable to distinguish between them. Traffic radar does not have a radar screen. It has only a single digital readout. So, at any given time, the maximum amount of information traffic radar can provide is ONE NUMBER! This is a very significant limitation, and we'll talk more about it later. Really. TRAFFIC RADAR: THE BASIC IDEA Traffic radar's constant beam acts very much like a search light, both in its shape and in its method of operation. That's because microwaves behave much like light waves. They travel in straight lines and they are easily reflected. Metallic objects like cars, trucks, guard rails, and overpasses make the most effective reflectors, sending glints of microwaves around in unpredictable directions, just like glints of light. Unlike light, however, you can't see the radar beam because microwaves are invisible. But they are very easily received by a radio tuned to microwave frequency. And, in fact, such a radio connected to a compact antenna forms the basis of all traffic radars. Traffic radar works by shining its microwave searchlight down the road. When you come in range, the beam bounces off your car, and the radar antenna looks for the reflections. HOW TRAFFIC RADAR MEASURES YOUR SPEED Traffic radar calculates speed from the reflections it receives. It uses a phenomenon of physics known as the Doppler principle. We've all heard how the Doppler principle works with sound waves. The classic example is heard along railroad tracks. As the train approaches, you hear the sound at a fixed pitch. The instant the train passes and begins to move away you hear a lower pitch. The train itself is making the same sound both coming and going, but to a stationary listener, the speed of the train adds to the pitch of its sound as it approaches, and subtracts as it departs. This change from true pitch is called the Doppler Shift, and the magnitude of the change depends only upon the speed of the train. Traffic radar applies this Doppler principle to microwaves. It compares the shifted frequency of the reflection to the original frequency of the beam it sent out, and from the difference it calculates speed, which it then displays on its digital readout. That's all there is to traffic radar. HOW FAR AWAY CAN RADAR CLOCK YOU? Going back to the searchlight analogy, we all know there's a limit to the effectiveness of any spotlight. The more powerful it is, the farther it reaches. The same applies to radar. Since power costs money, traffic radar is far less powerful than mili tary radar. Traffic radar's low power means that it has limited range. It's a fact of microwave life that the strength of the beam diminishes with distance. The farther it as to travel, the less energy it'll have when it gets there. For example, the radar operator may spray your car with microwaves while you're still a mile away. But the reflected signal has to travel that same mile back to the radar before it's of any value. If it's so weak when it gets back that the radar's electronics can't read it, then no speed computation is possible. You're out of range. Radar range depends upon two things: power of the radar and the reflectivity of the target. The amount of power is determined by the radar's designer. To the mororist, it's not a variable. Reflectivity of the target, however, varies with each vehicle and is therefore of great importance to each motorist. For highway vehicles, radar reflectivity is mostly a matter of size and shape. The smaller the vehicle, the smaller its reflection, and therefore the shorter the range. Some cars are out of range on some radars until they within 500 feet of the antenna. On the other hand, big, flat surfaces perpendicular to the beam make excellent reflectors. The same radar that may be blind to a small car 500 feet away can see s semi a mile and a half away. From this, you should conclude that the principle of radar is quite easily understood, but the details of its operating behav ior are hard to predict with accuracy. PART 2 TYPES OF TRAFFIC RADAR All traffic radar operates according to the technical principles outlind in Part One. But just as carpenters use different tools for different jobs, so are there different types of traffic radar with different capabilities. Here are the categories of equip ment: 1. Stationary radar- Virtually all radar can be used in the stationary mode. This simply means that the radar setup operates from a fixed position. Stationary radar can be either mounted to a car, or it can be hand held the socalled 'radar gun'). 2. Moving radar- This is a more complex system designed to allow the operator to check speed of patrol traffic from a moving patrol car. The principles involved are the same as for station- ary radar, a microwave beam looks forward, monitoring the stron- gest reflection, which in this case is the oncomming terrain. Simultaneously the beam monitors the SECOND stronget reflection, which it assumes to be traffic. An internal calculator then sub- racts the terrain speed (same as patrol car speed) from the clos- ing speed of the patrol car and the target vehicle. The result, if there are no errors, should be the road speed of the target vehicle. Like stationary radar, moving radar displays its con- clusion as a single number on the digital readout. 3. Instant on (or "pulse") radar- This is a method of operation rather than a type of radar equipment. Most traffic radar can be operated in the instant on mode. It can be used either station- ary or moving. The sole purpose of instant on radar is to defeat radar detectors. The instant on radar trap is set up just like an ordinary radar trap. The only difference is that the radar doesn't transmit until the operator pushes a button. So there is no radar signal for a radar detector to find. Then when the target is within speed measuring range, the operator triggers the beam. Hence the term "instant-on." Instant-on radar locks onto target speed within a fraction of a second after it is triggered. 4. Photo radar- This is a type of stationary radar designed to operate unmanned. It takes a photo of any car it thinks is speeding. The owner is identified from the license number and a ticket is mailed to the owner. This type of radar is common in Europe but is still an experiment in the United States. In early 1988, it was operating in two locations, pending court challenges. The locations were Paradise Valley, Arizona, and Pasadena, California. Photo radar is expected to face legal difficulty in the United States because American laws hold the driver responsible for moving violations, not the owner. And it's almost impossible to prove, at a later date, who was blasting down the road in your car on any given day. PART 3 WHY RADAR GETS INCORRECT READINGS The greatest weakness of traffic radar is the way in which it gives its information. It has only one readout and it displays only one number. You can point the radar antenna down a two-lane road with traffic going in both directions. Vehicles can range in size from economy cars to semi trucks. And all the radar will show is one number. Where does it get its one number? And how can the operator know which of the coming and going vehicles is responsible? The truth is, often the operator cannot know for sure. He has to guess. This is the most serious of all of traffic radar's limita tions. Because it's made to a low-bid price, it must necces sarily be a relatively simple device. Without the modulated beam of military radar, it cannot distinguish between targets within range. Without the radar screen, it cannot identify for the operator which target it's reading. At least the low power of traffic radar is an asset here in that it limits surveillance to line of sight. GUESSING THE OFFENDER To make up for the lack of a modulated beam and the absence of a radar screen, traffic radar makers use the cheapest imaginable substitute. They simply program the electronics to disregard all but the strongest reflection in the case of stationary radar, all but the TWO stronget reflections in the case of moving radar. This is the method of all traffic radars. The number displayed on the readout comes from this simplification. It's up to the operator to decide which, if any, of the moving vehicles within range is producing the unseen reflection. If there is only one vehicle in range, probably that vehicle is responsible for the number, although it could be caused by an electrical interference or blowing trash or some other less obvious distraction to the microwaves. If there is more than one vehicle in range, it's up to the opera tor to decide which one is producing the reflection. Is it the closest one to the antenna, or is it the largest one in the pack? In truth, it could be either, depending upon a host of subtle ties. A skilled operator intent on justice wouldn't write a ticket unless he was absolutely sure. A less skilled operator might write the ticket thinking he had the right answer, and be wrong. A careless operator intent on filling his quota might see the number and single out a likely perpetrator, the red sports car, and be done with it. When you deal with humans, you take your chances. Because traffic radar is built to a low-cost tartet, it is not the infallible electronic instrument that it purports to be. And because operators have a tough time keeping track of invisible beams, traffic radar invites human error in vehicle identifica tion. NO GOVERNMENT STANDARDS While military radar is produced to exact government standards to assure accuracy, traffic radar is not subject to any government standards at all. In the late 1970s there was wide-spread publicity about radar errors including an 86-mph tree and a 28-mph house. The National Highway Traffic Safety Administration (NHTSA) assigned to the National Bureau of Standards the task of testing all brands of traffic radar in use at the time, for the purpose of discovering the source of these errors and proposing federal standards to eliminate them. In January 1981, these proposed standards were published in the Federal Register. The newly-installed Reagan administration took no action on the proposal. After three years of government inaction on the problem, the Inter-national Association of Chiefs of Police (IACP) Provided non-government standards by which all traffic radar units could be tested to assure accuracy: Volume I of the standards was pub lished in April, 1984, Volume II in June, 1984. In June 1986, the traffic radar manufacturers announced the forma tion of their own trade association, saying that they would not submit traffic radar units for IACP testing. Instead, the radar makers WOULD USE THEIR OWN STANDARDS. These industry standards have not been published thus far. So, in effect, there are no performance requirements for traffic radar, and the claims of 86-mph trees and 28-mph houses have never been refuted. COMMON OPERATOR ERRORS The proposed Federal standards for traffic radar were never implemented. However, in an attempt to reduce radar errors, sev eral local governments used the Federal research to develop bet ter training programs. The Texas Department of Public Safety produced a comprehensive manual based on the Federal tests. It cautions operators, "...the radar does not generate 'false' readings. Anytime a reading appears, the radar has sensed a signal. The radar opera tor must be familiar with situations that can pro-duce 'error' readings." If the operator does not detect the error, a ticket will be wrongfully issued. Here are the "errors" detailed by the Texas manual: 1. ANTENNA POSITIONING ERROR- The radar beam travels in a straight line, neither bending around curves nor following the contour of hilly terrain. If the antenna is not properly posi tioned, it may seem to clock an approach-ing car when, in fact, it's clocking another car in the background. 2. LOOK-PAST ERROR- Even if the operator aims his antenna prop erly, radar is still subject to 'look-past' error. This is caused by radar looking past a small reflection in the foreground to read a larger reflection behind. This error is all the more insidious because poorly-trained operators assume it can't happen. Texas instructors warn, "It is a widely-held misconception that the re-flected target signal received by the radar antenna will always be that of the closest vehicle to the antenna. There are times, due to traffic con-ditions, that the closest vehicle is not returning the strongest signal." Evidence of the potential size of this error appeared in Car and Driver (Oct, 1979). The author measured the effective range of a Kustom Signals KR11 traffic radar against various vehicles. The typical small sedan did not show up on the radar until it was less than 1200 feet away from the antenna, but the same radar unit locked on to a Ford 9000 semi at 7,600 feet. This shows how common vehicles reflect microwaves differently. The Texas instructors confirm this problem with radar, saying "It is not unfair to say that the reading you register could be a larger, better target, three quarters of a mile down the road." 3. VEHICLE INTERFERENCE ERROR- Because moving radar tries to do a more com-plicated job than stationary radar, it is subject to all the errors of sta-tionary radar, plus several additional errors that apply to it alone. Vehicle interference error occurs when moving radar is used in traffic. For example, traffic ahead can confuse the radar's estimate of patrol speed. Moving radar calculates target speed by subtracting patrol speed from closing speed of the tartet. Therefore anything that produces a low evaluation of patrol speed will automatically result in a high reading of target speed. Texas tells its radar operators that this... "situation becomes more critical if difference in patrol speed and interference- vehicle speed is five to ten mph. A target vehicle moving 61 mph may be recorded at 66 to 71 mph. These border-line speeds are more difficult to detect with the eye." 4. COSINE ERROR- Cosine error produces a result similar to Interference error, except no moving traffic need be present. A stationary object adjacent to the road, such as a building, or road machinery, or even a sign, makes a more efficient reflector than horizontal pavement. Therefore the radar uses that reflection as the basis of patrol speed. If this reflector were positioned straight ahead on a collision path, the patrol speed estimate would be close enough. But the further the object is located off a direct line to the target, the lower will be the estimate of patrol speed. This is a simple trigonometry problem relating to the cosine of the angle between the target and the ground reflector, hence the name Cosine Error Since Cosine error always makes patrol speed seem smaller than it actually is, it always acts to raise the reading of target speed. 5. DOUBLE-BOUNCE ERROR- Microwaves are easily reflected. That's what makes radar possible. But the operator must be aware of the difference between an ordinary reflection and a bad bounce. Big objects such as trucks are very efficient reflectors, and it's possible for the radar beam to bounce off several moving trucks at once, always producing erroneous readings. 6. BEAM-REFLECTION ERROR- Because microwaves are so readily reflected, Texas instructors recommend caution, even in mounting the antenna within the patrol car. They say it's possible that a reflective path can be set up through the rearview mirror that will produce radar readings on vehicles behind the patrol car when the radar is aimed forward. And those vehicles behind can be either coming or going, since radar does not distinguish dir- ection. 7. ROAD SIGN ERROR- The ready reflectability of microwaves means that road signs are also a source of errors. As the patrol car approaches a sign along the road, the radar beam bounces off the sign to produce a reading on another car heading in the same direction as the patrol car. This phantom target can be either ahead or behind the patrol car. 8. RADIO INTERFERENCE ERROR- According to the Texas course, "UHF radio now in use can force radar to read various numbers when you transmit, or just key the mike. Citizens band radio transmissions from within the patrol vehicle can cause ghosting (false readings)." It recommends that no radio trans-missions be made while clocking target vehicles. 9. FAN INTERFERENCE ERROR- When the antenna is mounted inside the patrol car, the Texas course says, "Radar will have a tendency to read the pulse of the fan motor (air conditioner, heater, or defroster)." The instructors go on to say, however, that the fan reading will disappear when a target comes into range, and that the fan will not distort the speed reading of the target car. However, in the case of moving radar, they say, "Sometimes a steady fan speed will override patrol car speed reflected from the roadway." This error is particularly nasty, because the fan speed will be substituted for patrol speed in the moving radar's calculation of target speed. Since this cal-culation consists of subtracting patrol speed from closing speed, if the fan reading is less than patrol speed, then the speed displayed for the target will be incorrectly high. The Texas course offers no safe guard for this error. In conclusion, the Texas Department of Public Safety notes "Radar cannot identify the speeding vehicle, the officer must do that." PART 4 HOW RADAR DETECTORS WORK A radar detector is, in essence, a radio tuned to receive traffic radar frequency. The technology involved is straight-forward. Remember that traffic radar transmits a microwave beam out through an antenna, then listens for the reflection of that transmission. A radar detector amounts to the listening section of a radar unit. The key question is, can a radar detector find radar before radar finds you? The answer is yes. Consider the worst-case scenario for pure range. You're driving down a straight and level highway, heading right for the radar trap. The beam is reaching out, feeling for you. Buyt remember that micro-waves lose energy as they cover distance. And to measure your speed, the beam has to reach out to you, be reflected back, and then arrive at the radar unit with enough strength to activate the calculating circuits. Let's say you come in range when you're a half mile away. The microwave beam traveled a half mile out to you and a half mile back to the radar. That's a mile of total travel. If your detector has the same sensitivity as the radar's listening section, you can find the radar a half mile before it finds you It is not at all difficult to build a detector as sensitive as the listening section of a traffic radar. In fact, the electronic components used in traffic radar are standard industry components available to anyone. Of course, radar is not always used out on the open road where a de-tector can get a good look at it. Radar operators favor the ambush; they hide over a hill or around a curve, and when you pop into view, you're already in range. So how does a detector work in this case? The classic analogy for the radar beam likens it to a flashlight on a foggy night. The guy holding the flashlight can't see you until the beam scores a direct hit. But you can see the beam tracing its pattern through the fog long before it falls on you. You can see it because the light re-flects off moisture, dust, and other objects in its path, sending off small glints of light easily seen from a distance. Microwave beams behave like light beams. They operate on a strict line-of-sight basis. You can't be 'seen' over a hill or around a curve or behind a semi. Radar has to wait for you to drive into view. But a detector can pick up the scatter from the microwave beam well before the beam itself hits you directly. Again, a receiver as sensitive as the traffic radar's will be sufficient. But unlike the radar, a detector doesn't need to cal culate speed. It just needs to know that radar waves are present. So a detector can act on a weaker signal than that necessary for the radar itself. This is not to say that detectors are infallible. Radar operators have one trick that's hard to defend against. This is radar used in the 'instant-on' mode. Here a hidden radar is left in a standby condition as a target approaches. Its internal circuits are warmed up and ready, but it is not emitting a beam. So, naturally, a detector can't find it. Then when the target vehicle is well within range, the operator switches the unit on and it quickly calculates speed. A good radar detector protects against instant-on radar by warning you when the operator zaps one of the cars up ahead. It has to be sensitive enough to respond to weak signals. Moreover, it must be reliable enough so you don't interpret these brief, weak signals as random false alarms and dis-regard them. Of course, if there is no car ahead, you're on your own. A good radar detector, like any sophisticated electronic equipment, re-quires careful engineering. You will have the best chance of satisfaction if you buy from a reliable source. Pick a firm that will refund your money if you're not satisfied. There is no better guarantee. CINCINNATI MICROWAVE ONE MICROWAVE PLAZA CINCINNATI, OHIO 45249-9502 1-800-543-1608 A Personal Note... Cincinnati Microwave produces the Escort and Passport radar detectors. Available ONLY through them, the Escort is $245, the Passport is $295 dollars. I have read all the recent comparison and road tests. Passport and Escort consistantly out perform the competition.