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Problems and faults with police radar and police
speed guns.
 The
use of police traffic radar is so widespread that we naturally
assume thee technology is reliable.
Traffic Radar Reliability
The use of police traffic radar is so widespread that we naturally
assume the technology is reliable. After all, if there were
questions about radar's accuracy, would the courts process speeding
violations with such assembly-line efficiency?
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 Police 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 travelling,
limitations that compel manufacturers to build in certain electronic
compromises.
Police 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 travelling 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 power lines; 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.
SOURCE: Blind Trust.
Common errors encountered in operating
police radar guns
 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 operator must be familiar with
situations that can produce 'error' readings." If the operator does
not detect the error, a ticket will be wrongfully issued.
Here are the radar "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 positioned, it may
seem to clock an approaching car when, in fact, it's clocking
another car in the background.
2. Look-Past Error
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."
3. Vehicle Interference Error
"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 the closing
speed of the target. Therefore, anything that produces a low
evaluation of patrol speed will automatically result in a high speed
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 66-71. These borderline 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 directions.
7. Road-sign error
The ready reflectability of microwaves
means that road signs are also source of errors.
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 transmissions 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." 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.
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