Operational Errors

http://www.phrack.com/show.php?p=38&a=6 accessed on 1 March 2003

Below is a list of common errors and how they occur. This is the part of the article that must be used in conjunction with the previous file in this series. You must attempt, while pleading your case, to tie in some of the following errors to the situation you found yourself in when you got your speeding ticket. See http://www.phrack.com/show.php?p=37&a=5 for details.

The Road Sign Error

Due to the reflectability of microwaves, road signs, buildings, billboards, large trees, and other stationary objects are a source of errors.

Radio Interference Error

According to the Texas Department of PublicSafety, “UHF frequencies broadcast today can force RADAR to read various numbers when transmitted within the area.” This type of interference could come from the radio within the patrol car, citizens band radio, or television stations.

The Look-Past Error

Even when the RADAR operator aims his gun properly, the RADAR is subject to this type of error. This is caused by the RADAR reflecting off of a larger surface area in the background rather than the smaller reflective surface in the foreground. Evidence of this the Look-Past Error was printed in the October 1979 issue of “Car and Driver.” The author measured the effectiveness of KR11 RADAR system against various vehicles. The author showed that the typical sedan did not show up on the RADAR until it was less that 1200 feet away, however, a Ford 9000 semi tractor trailer could be picked up at 7600 feet.

Fan Interference Error

When the antenna is mounted inside the patrol car, “RADAR will have the tendency to read the pulse of the fan motor (air conditioner, heater, defroster).” This is a statement provided by the Texas Department of Public Safety who conducted a study of RADAR guns in 1987. The Texas Department of Public Safety offered no safeguard for this error.

Beam Reflection Error

Since microwaves are so readily reflected, the Texas Department of Public Safety cautioned mounting the antenna within the patrol car. One instructor said, “It is possible that a reflective path can be set up through the rear view mirror that will produce RADAR readings on the vehicles behind the patrol car when the RADAR is aimed forward. And those vehicles can be either coming or going since traffic RADAR cannot distinguish between the direction.”

Double Bounce Error

Again, since microwaves are easily reflected, the operator must be aware of a “bad bounce” and an ordinary reflection. And, as stated before, since large objects are more efficient than smaller ones, microwaves are attracted to them more. So, in effect, you could have an initial RADAR bounce off of the target vehicle, then from the target vehicle to a house or a truck going the opposite direction, and finally back to the patrol car. This error will mathematically get larger the slower the target vehicle is moving.

The Cosine Error

This is a mathematical error that takes place when the RADAR gun attempts to calculate the trigonomic equation that is programmed intoit. The RADAR gun measures the angle at which the target enters a point and then exits a point (i.e. 25 degrees). The cosine of 25 is .9063. The RADARgun was designed to calculate the speed of the patrol car by multiplying thespeed of the patrol car (i.e. 50 mph) and the cosine of the angle (.9063) andit gets the false speed of the patrol vehicle as 45mph. Therefore, when yousubtract the patrol speed from the target speed (i.e. 50, the same as the patrol car) you get the false sense that the target vehicle is traveling 5mphfaster than the patrol car.

Radio/Microwave Interference can come in a variety of forms, both natural and man-made, but they have one thing in common – they produce a false or incorrect reading on the radar unit’s display. Common sources of electromagnetic interference include airport radar; microwave transmissions; transmissions of CB, ham, VHF/UHF, and cellular two-way radio/ telephones, including police and business radios; faulty sparkplug wires; mercury vapor and neon lights; high-tension power-lines; and high voltage power substations. The radio energy from these sources can overload or confuse the sensitive circuits in a radar gun.

Shadowing/Look-Past Error is a problem that occurs only with moving radar, and plagues all moving radar. The radar locks onto a large moving object in front of the patrol car instead of the passing terrain and computes the difference in speeds between the two vehicles as lower than the actual patrol speed. Consequently, the radar adds the remainder of the patrol speed to the target’s speed, producing an erroneously high reading. Even if the operator aims his antenna properly, radar is still subject to “look-past” error. This is caused by the 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 reflected target signal received by the radar antenna will always be that of the closest vehicle to the antenna. There are times, due to traffic conditions, that the closest vehicle is not returning the strongest signal.”

Evidence of the potential size of this error appeared in Car and Driver (October 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 7600 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.”

Batching is caused by time lags in the computing of speeds by some types of moving radar. If the patrol car rapidly accelerates or decelerates while measuring target speeds, the display can read higher or lower than the actual speed.

Stationary Cosine Effects is a problem which occurs when the radar unit is not taking its readings from vehicles that are directly ahead of or behind the police vehicle – there is an angle between the radar unit and its target, and it is the cosine of this angle (remember your high school trigonometry?) that determines the magnitude of the error. With stationary radar, the greater the angle between the radar and the roadway, the lower the indicated speed. This error does not become significant until the angle to the roadway exceeds 10 degrees. Fortunately, with stationary radar, the cosine error is in favor of the motorist.

Moving Cosine Effects can result in readings that are either higher or lower than the target’s actual speed. More often, the erroneous reading is not in the motorist’s favor. The moving radar can lock onto a large object to the side of the roadway instead of the ground and, due to the cosine error, compute a lower-than-actual speed for the patrol car. The remainder of the patrol speed is added to the target’s speed. Similarly, if the radar is not carefully aligned within 10 degrees of the patrol car’s direction of travel, it will compute a lower-than-actual patrol speed. Again, the remainder is added to the target speed. On the other hand, if moving radar is used to measure across a wide median, as on an interstate highway, the large angle can cause a lower-than-actual target speed to be displayed , assuming the patrol speed is correctly computed.

Multiple Bounce Effects sometimes happen when there are multiple moving targets within the main beam, causing several reflected signals of near-equal strengths but varying frequencies to arrive at the radar gun’s antenna. Depending on the gun, the display may switch from one speed-reading to another, it may show a combination of the reflections, or it may blank out. Overpasses on freeways are commonly the source of multiple bounce errors. The error observed most often is a double bounce, which causes the patrol car’s speed to be indicated in both the target display and the patrol display. Sometimes the overpass bounce will involve a large, slower-moving vehicle near that patrol car, causing the target speed to be displayed as the speed of the patrol plus that of the slower vehicle. In another multiple bounce case, the signal can be reflected more than once, by two moving objects, and the total Doppler shift will be displayed as a higher-than-actual speed.

Pulse Problems

The careful, well-trained operator can spot many of these radar errors when they occur. By constantly monitoring traffic speeds, he will notice the oddball reading or the onset of some other problem. This ongoing record of vehicle speeds and possible sources of interference is known as a traffic or tracking history. With no traffic or tracking history, it is difficult for the operator to know which vehicle produced a particular speed reading, or whether there is some sort of interference present. When radar is used in the instant-on mode – short bursts of one or two seconds duration – the potential is great for these errors being misinterpreted as actual speed readings. Particularly troublesome are problems involving target identification, and the first three types of errors mentioned above (radio or microwave interference, mechanical interference and multi-path cancellation). One Pennsylvania driver tells of following an Ohio Highway Patrol car on a rolling interstate highway. Each time the police cruiser neared the top of a hill, the trooper would flip on his radar (setting off the Pennsylvanian’s radar detector). After cresting the hill and taking a brief reading of whatever was on the other side, the officer would shut off his radar again. “This went on for several miles,” the driver explained, “until the police car did a U-turn in the median, presumably to go trolling in the opposite direction.” As we pointed out earlier, instant-on radar has only one purpose – to try to confound radar detector users. But the drawbacks are many. In addition to increasing the likelihood that radar errors will go unnoticed, instant-on radar also defeats one of radar’s most useful features. Studies have shown that highly visible police patrols are most effective when it comes to encouraging drivers to obey traffic laws; radar and radar detectors increase the “visibility” of a patrol car, slowing traffic over a wider area. But despite such evidence, instant-on radar is growing in popularity. Too many officers see radar detectors as a threat to their authority, rather than an ally in helping encourage motorists to obey the rules of the road. Antenna Positioning Effect when 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 positioned, it may seem to clock an approaching car when, in fact, it’s clocking another car in the background.

Beam-Reflection Effect occurs 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 directions.

Road-sign error is caused by the reflectability of microwaves means that road signs are also source of errors. However, in the case of moving radar, they say, “Sometimes a steady fan speed will override patrol car speed reflected from the roadway.” When this happens, the false speed-reading produced by the fan will be substituted for patrol speed in the moving radar’s calculation of target speed. Since the calculation 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 safeguard for this error. In conclusion, the Texas Department of Public Safety notes “Radar cannot identify (the) speeding vehicle: (the) officer must do that.”

Receiver noise

The sensitivity of a radar receiver is determined by the unavoidable noise that appears at its input. At microwave radar frequencies, the noise that the receiver itself (i.e. usually generates limits detectability by the random motion of electrons at the input of the receiver) rather than by external noise that enters the receiver via the antenna. The radar engineer often employs a transistor amplifier as the first stage of the receiver even though lower noise can be obtained with more sophisticated devices. This is an example of the application of the basic engineering principle that the “best” performance that can be obtained might not necessarily be the solution that best meets the needs of the user. The receiver is designed to enhance the desired signals and to reduce the noise and other undesired signals that interfere with detection. The designer attempts to maximize the detectability of weak signals by using what radar engineers call a “matched filter,” which is a filter that maximizes the signal-to-noise ratio at the receiver output. The matched filter has a precise mathematical formulation that depends on the shape of the input signal and the character of the receiver noise. A suitable approximation to the matched filter for the ordinary pulse radar, however, is one whose bandwidth in hertz is the reciprocal of the pulse width in seconds.

Clutter

Echoes from land, sea, rain, snow, hail, birds, insects, auroras, and meteors are of interest to those who observe and study the environment, but they are a nuisance to those who want to detect and follow aircraft, ships, missiles, or other similar targets. Clutter echoes can seriously limit the capability of a radar system; thus a significant part of radar design is devoted to minimizing the effects of clutter without reducing the echoes from desired targets. The Doppler frequency shift is the usual means by which moving targets are distinguished from the clutter of stationary objects. Detection of targets in rain is less of a problem at the lower frequencies, since the radar echo from rain decreases rapidly with decreasing frequency and the average cross section of aircraft is relatively independent of frequency in the microwave region. Because raindrops are more or less spherical (symmetrical) and aircraft are asymmetrical, the use of circular polarization can enhance the detection of aircraft in rain. With circular polarization, the electric field rotates at the radar frequency. Because of this, the electromagnetic energy reflected by the rain and the aircraft will be affected differently, thereby making it easier to distinguish between the two. (In fair weather, most radars use linear polarization–i.e., the direction of the field is fixed.)

Atmospheric effects

As was mentioned, rain and other forms of precipitation can cause echo signals that mask the desired target echoes. There are other atmospheric phenomena that can affect radar performance as well. The decrease in density of the Earth’s atmosphere with increasing altitude causes radar waves to bend as they propagate through the atmosphere. This usually increases the detection range at low angles to a slight extent. The atmosphere can form “ducts” that trap and guide radar energy around the curvature of the Earth and allow detection at ranges beyond the normal horizon. Ducting over water is more likely to occur in tropical climates than in colder regions. Ducts can sometimes extend the range of airborne radar, but on other occasions they may cause the radar energy to be diverted and not illuminate regions below the ducts. These results in the formation of what are called radar holes in the coverage. Since it is not predictable or reliable, ducting can in some instances be more of a nuisance than help. Loss of radar energy, when propagation is through the clear atmosphere or rain, is usually insignificant for systems operating at microwave frequencies.

Interference

Signals from nearby radars and other transmitters can be strong enough to enter a radar receiver and produce spurious responses. Well-trained operators are not often deceived by interference, though they may find it a nuisance. Interference is not as easily ignored by automatic detection and tracking systems, however, and so some method is usually needed to recognize and remove interference pulses before they enter the automatic detector and tracker of a radar.

Electromagnetic Energy Emissions

Traffic radar systems operate within specific frequency bands assigned by the Federal Communications Commission [FCC]. The frequency bands (and their center frequencies) in use today are X-band (10.525 GHz, or 10,525,000,000 cycles per second), K-band (24.150 GHz), and Ka-band (33.700 GHz). Mobile radio, commercial television and FM radio broadcasting, and cellular telephones in the non-ionizing portion of the spectrum, which includes the bands, use these frequencies. Microwave energy in this end of the electromagnetic spectrum is referred to as non-ionizing radiation, because it cannot break the bonds of molecules. [3] The microwave transmitters used in most traffic radars produce total output powers in the range of 10 to 100 mill-watts, with 15 mill-watts being a fairly typical value. A mill-watt [mW] is one-thousandth of a watt. Other uses for the same types of microwave transmitters include automatic door openers and intrusion alarms, such as those often found in airports and shopping malls. The chart graphically shows the relative power outputs of some other common emission sources.