Radar Gone Bad

In early 1979, a Miami television station showed viewers a radar gun clocking a palm tree at 86 mph and a house at 28 mph. In the first instance, the reading was caused by panning the radar antenna and in the second, the radar unit was measuring the fan motor in the patrol car. The TV report prompted a court case that brought radar errors national attention. A year later the National Bureau of Standards tested the six most popular police radar models, finding that all produced false speed readings in the presence of CB or police radios. Each of the two-piece units produced panning errors like the one that caught the Miami house apparently moving at 28 mph. All of the moving radar units were subject to "shadowing," causing some of the patrol car"s speed to be added to that of the target vehicle (Federal Register, Vol. 46, No. 5, Jan. 8, 1981).

When the International Association of Chiefs of Police tested 24 radar models in 1983 and "84, the results showed that nearly all of the units were affected by temperature variation, five failed accuracy tests, four had unacceptably wide beam widths and three tended to provide inaccurate readings due to nearby police or CB radios. Federal performance standards were proposed but never adopted during the Reagan administration. Instead, radar manufacturers promised to police their own ranks. From out perspective, things haven"t improved. Police radar is as error prone today as ever, particularly with the widespread use of radar in the instant-on mode. And the effectiveness of the manufacturers" self-policing policy came to light recently (3/89) when it was revealed that one radar maker sold thousands of units bearing fraudulent Federal Communications Commission certification. 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.

Technological Problems

Radar is a wonderful technology. It allows tens of thousands of planes to fly around the world every day without hitting each other. It also makes for some pretty cool pictures on the 5 o"clock news. Keeping that in mind a with cars and stereos, not all radar equipment is created equally. Police radar is definitely not in the same league as the radar in a modern jet fighter. The typical police radar units cost roughly around $2,000.00 as opposed to $1.1 million for the fighter"s radar.

What"s the Difference?

Radar works on the Doppler principal of frequency shift. The same way a train whistle changes pitch as it approaches then passes by you. By measuring the change in pitch the speed of the object can be measured. To cut costs, police radar uses comparatively inexpensive components and its operation is kept as simple as possible. To that end police radar share the following characteristics:
1- Low power emitter to keep the operator and the general public safe from microwave emissions.
2- Displays only the strongest reflected signal.
3- Relies on the operator"s skill to determine the speed and error correction.
4- None of the units in service follow a government standard for reliability or performance. i.e. Your taking the word of the manufacturer, that the unit performs as specified.

Since the emitter uses low power it stands to reason, that the reflected radar energy that returns to the gun is even lower. As a result in order for the gun to do anything with the returned signal, it must be amplified and processed, to register on the display. The problem is that the electronics aren"t very well filtered and radar occupies a very narrow spectrum in the RF band. As a result erroneous readouts can and are caused by such things as remote garage door openers, cellular phones, CB radios and even the heater/ventilation fan in the patrol car. Anyone that owns a radar detector can tell you how easy they are to set off if the filter is not turned on. The radar unit is just as susceptible as a detector, since they are listening for the exactly same thing. If you have a false reading with a detector, you get a signal from the unit. If you get the same error with a gun, you get a ticket. Aircraft radar can plot the position of a multitude of other aircraft. It can do that by modulating the signal to track one aircraft with radar energy at a certain frequency.

Police radar can not discriminate between targets. It will only read the strongest return signal it gets. The operator can only assume that the vehicle in the line of sight, is traveling at the speed displayed. If the vehicle has a low radar signature the reflection can come from a vehicle behind it or a building. One characteristic of radar energy is its ability to be reflected. Since its so easily reflected, to be used accurately there should be nothing in the way of the beam, but the vehicle in question. If not the you will get reflections and ghosts floating around. To get an idea of this principal, throw a superball as hard as you can in an enclosed room and see how many times the same ball passes by you!

There are a gaggle of reasons that can result in erroneous readings. Apart from the technology, operator error can seriously affect the accuracy of the unit. Without fail they all have to do with incorrect positioning of the radar beam in relation to the target. A complete listing of these errors can be found at the National Motorists Association"s web site. There is a link to their site on the links page. For a stationary photo radar van one of the most important of these errors is the cosine error. The Gastonomer units used in Ontario, had its beam offset at a 22 degree angle. The unit would correct the returned signal to take into account the frequency shift caused by the 22 deg angle. If the vehicle is not parked exactly parallel to the target vehicles it will affect the accuracy of the unit since the unit blindly assumes that the angle is always 22 degrees.

Lets look at how a simple 10 degree shift in alignment can affect the reading, either way. We will assume that the vehicle in question is traveling at 100km/h, the cosine of 22 degrees is .9272 and the photo radar unit will add 7.28 km/h to get the correct reading for speed.
1) 100 X .9272 = 92.72km/h + 7.28km/h = Readout is 100km/h
If the van is parked with the front of the vehicle parked 10 degrees away from the direction of the road, the angle would increase to 32 degrees. The cosine of 32 degrees is .8480
2) 100 X .8480 = 84.80km/h + 7.28km/h = Readout is 92.72km/h
You get a break and your speed is lowered by 7.92km/h
Now the opposite will happen if the front of the vehicle is pointing towards the road at a 10 degrees angle. The cosine of 12 degrees is .9781.
3) 100 X .9781 = 97.81km/h + 7.28km/h = Readout is 105.09km/h
Your vehicle has 5.09km/h added to the actual speed and if they are operating at zero tolerance,
you get a ticket.

No Standards

After grumbling from some circles, the U.S. National Highway Traffic Administration set up some guidelines for traffic radar units. The NMA has contacted me, to let me know that NHTSA finally did adopt minimum standards for traffic radar devices, but it wasn"t until 1994, about 15 years after it was first asked to. No word if Canada follows these same standards. In 1984 the International Association of Chiefs of Police published standards that manufacturers would have to submit too. In response the manufacturers formed a trade organization and refused to submit their products for testing and came up with their own standards, which they refuse to make public. These actions speak volumes on how manufacturers feel about the accuracy and the reliability of their products.

Profit Motive

Police organizations do not purchase photo radar units. The units are leased from the manufactures. In most cases the company is also responsible for maintenance of the units, developing the film and issuing the summons to the "offender". For this service. the manufacturers receive a kickback from the government in the form of a percentage of the money collected in fines. These payments range from 5 to 20% depending on whether the fines are processed by the manufacturers as well. As such there is very little incentive for the manufacturers to assure the accuracy of their photo radar units.

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