Reprinted with permission from the brochure Blind Trust. Blind Trust is a publication of the non-profit organization RADAR (Radio Association Defending Airwave Rights, Inc.).

We tend to take the answer to this question on faith. That may be unfortunate, because radar makes mistakes. Lots of them. Some experts estimate that 10-20 percent of all radar-backed speeding tickets are issued in error; and in the case of radar that is operated from a moving police vehicle the number of bad tickets may be as high as 30 percent!

This Brochure {a print version of this info. is available from RADAR) is intended to familiarize the reader with some of the most common radar errors. Our Hope is that more people will realize that traffic radar is not infallible, and will challenge speeding tickets they know they don't deserve. The end result will be a greater effort by the radar industry to build better products, and by law enforcement to use this technology more responsibly.

Two Kinds of Radar

To understand how radar makes mistakes, it is first necessary to know how radar works. Basically, there are two kinds of radar - traffic radar, and rotating- antenna radar. The latter group includes weather, airport, military and other types of commercial radar. By contrast, police traffic radar uses a stationary single antenna that points in a single direction; does not transmit a modulated signal; and does not use a cathode ray screen to display information.

These three differences are extremely important.

All radar works by transmitting a microwave beam on a specific frequency. Targets that are struck by the beam reflect microwave energy to the antenna, a computer analyzes any changes in frequency and displays this information. Military-commercial types of radar use a sweeping , modulated beam which provides details about an objects' shape, speed, and direction for the operator. By contrast, the stationary beam and digital readout of police traffic radar yield only one piece of information: how fast a target is approaching or receding from the radar.

Police traffic radar doesn't tell its operator which object it is measuring or the direction that the object is traveling, limitations that compel manufacturers to build in certain electronic compromises.

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.