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  1. #2
    Join Date
    Aug 2006
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    563

    Default Re: How is Radar Effected by the Police Driving Speed

    Some of radar's shortcomings are readily apparent. Beam Width is one. Think of a radar beam as a cone - narrow at the radar antenna and widening as it heads for the horizon. Even the narrowest of radar beams - 11 degrees - is 38 feet wide when 200 feet down the road and 57 feet wide at 300 feet away. Some radar units transmit a beam as wide as 24 degrees. By the time a radar beam is several hundred feet from a patrol car, the microwaves are blanketing an area as wide as an expressway.

    Now picture that expressway full of cars and trucks, and remember that traffic radar can't tell its operator which vehicle it is monitoring, or whether the target is approaching or traveling away from the police car. You quickly understand how great the potential is for misidentification.

    Let's throw in another twist or two. Even though police radar is based on the Doppler Principle, most units do not interpret the Doppler shift itself. Rather, they process the frequency of the signal and use its analog to represent target speeds. Known as phase-lock loop, or PPL, this processing can lock onto the wrong target, double or triple low speed readings, or produce "ghost" readings. Other types of common radar errors are:

    1.Radio or 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 powerlines; and high voltage power substations. The radio energy from these sources can overload or confuse the sensitive circuits in a radar gun

    2. Mechanical Interference is any moving object, other than the target vehicle, that can produce a false or incorrect radar reading. The most common sources are vibrating or rotating signs near the roadway; fan blades moving inside or outside the patrol car (air conditioner, heater, defroster or engine fan); another moving vehicle that reflects radar waves better than the target vehicle; and multiple targets in the main radar beam causing multiple reflections of nearly equal strength and making the display read, high, low, or completely blank.

    3. Multi-Path Beam Cancellation occurs when the radar signal returning directly to the radar gun from the target vehicle is canceled by a secondary reflected signal. This cancellation can occur while the target remains in plain view of the operator. The display may blank or suddenly switch to another vehicle beside or behind the original target until the cancellation ceases.

    4. Panning occurs when the radar beam accidentally sweeps past the counting/computing unit. This can happen only to a two-piece radar unit. The radio energy from the antenna portion overloads or confuses the counting/computing circuitry.

    5. Shadowing 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.


    6. 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.

    7. Stationary Cosine Error 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.

    8. Moving Cosine Errors 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.

    9. Multiple Bounce Errors sometimes happen when there are multiple moving targets within the main beam, causing several reflected 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.

    10. Terrain Error takes place when hilly or curved roadways affect radar's ability to process information. When the patrol car is at the crest of a hill, it is very easy for radar to overshoot the nearest vehicle and instead take a reading from a vehicle on the next hill. Because traffic radar is "direction blind," differences in reflectivity may cause instant-on readings to display the speed of a receding vehicle rather than of an approaching vehicle. So that vehicle "on the next hill" need not even be traveling the same direction as the supposed target vehicle.

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