CINCINNATI MICROWAVE ENGINEERING REPORT
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INTRODUCTION
====================================================================
This report explains in simple language the principles and
limitations of traffic radar. It also outlines documented errors that
can cause radar to give incorrect readings. ( It is estimated that as
many as 30% of radar tickets are in error.) Every motorist should
know these facts.
PART I - HOW RADAR WORKS
====================================================================
There are many kinds of radar. Military radar, weather radar,
and air-traffic control radar fall into one category. Traffic radar
is quite different and falls into its own category.
The first category of radar is highly sophisticated
equipment. The popular concept of a radar antenna - a dish mounted on
a tower, rotating to sweep the horizon - applies to this category.
The rotating dish produces the familiar radar sweep you see on TV
weather reports. These sophisticated radars sweep with a modulated
beam to measure speed of objects, distance to those objects, and
general shap of those objects as an aid to identification by ground
crew. Such radars cost hundreds of thousands of dollars.
Traffic radar, by contrast, is very simple. It must be simple
because it can be no larger than will fit on the dashboard of a
mid-size cruiser. And it must be simple becauseit can cost no more
than the low bid of a municipality's procurement process. Sometimes
this is $400 or less.
The necessary simplicity means that traffic radar has
important limitations.
Traffic radar does not sweep. Instead it uses a stationary
beam, much like a searchlight, and it shines down the road,
either forward or backward, but not both ways at once. This means the
area under surveillanceby traffic radar is quite limited.
Traffic radar does not use a modulated beam. It uses a
constant beam. This means that, if there are multiple moving objects
within range, traffic radar is unable to distinguish between them.
Traffic radar does not have a radar screen. It has only a
single digital readout. So, at any given time, the maximum amount of
information traffic radar can provide is one number. This is a very
significant limitation, and we'll talk more about it later.
TRAFFIC RADAR: THE BASIC IDEA
Traffic radar's constant beam acts very much like a
searchlight, both in shape and in its method of operation. That's
because microwaves behave much like light waves. They travel in
straight lines and they are easily reflected. Metallic objects like
cars, trucks, guard rails and overpasses make the most effective
reflectors, sending glints of microwaves around in unpredictable
directions, just like glints of light.
Unlike light, however, you can't see the radar beam because
microwaves are invisible. But they are very easily received by a
radio tuned to microwave frequency. And, in fact, such a radio
connected to a compact antenna forms the basis of all radars.
Traffic radar works by shining its microwave searchlight down
the road. When you come in range, the beam bounces off your car, and
the radar antenna looks for the reflections.
HOW TRAFFIC RADAR MEASURES YOUR SPEED
Traffic radar calculates speed from the reflections it
receives. It uses a phenomenon of physics known as the DOPPLER
PRINCIPLE. We've all heard how the Doppler principle works with sound
waves. The classic example is heard along railroad tracks. As the
train approaches, you hear the sound at a fixed pitch. The instant
the train passes and begins to move away, you hear a lower pitch. The
train itself is making the same sound both coming and going, but to a
stationary listener, the speed of the train adds to the pitch of its
sound as it approaches, and subtracts as it departs. This change from
true pitch is called the Doppler shift, and the magnitude of the
change depends only upon the speed of the train.
Traffic radar applies this Doppler principle to microwaves.
It compares the shifted frequency of the reflection to the original
frequency of the beam it sent out, and from the difference it
calculates speed, which it then displays on its digital readout.
That's all there is to traffic radar.
HOW FAR AWAY CAN RADAR CLOCK YOU?
Going back to the searchlight analogy, we all know there's a
limit to the effectiveness of any spotlight. The more powerful it is,
the farther it reaches. The same applies to radar. Since power costs
money, traffic radar is far less powerful than military radar.
Traffic radar's low power means that it has limited range.
It's a fact of microwave life that the strenght of the beam
diminishes with distance. The farther it has to travel, the less
energy it'll have when it gets there. For example, the radar operator
may spray your car with microwaves while you're still a mile away.
But the reflection signal has to travel that same mile back to the
radar before it's of any value. If it's so weak when it gets back
that the radar's electronics can't read it, then no speed computation
is possible. You're out of range.
Radar range depends upon two things: power of the radar and
the reflectivity of the target. The amount of power is determined by
the radar's designer: to the motorist, it's not a variable.
Reflectivity of the target, however, varies with each vehicle and is
therefore of great importance to each motorist.
For highway vehicles, radar reflectivity is mostly a matter
of size and shape. The smaller the vehicle, the smaller its
reflection, and therefore the shorter the range. Some cars are out of
range on some radars until they drive within 500 feet of the antenna.
On the other hand, flat surfaces perpendicular to the beam make
excellent reflectors. The same radar that may be blind to a small car
500 feet away can see a semi a mile and a half away. From this you
should conclude that the principle of radar is quite easily
understood, but the details of its operating behavior are hard to
predict with accuracy.
PART II - TYPES OF TRAFFIC RADAR
====================================================================
All traffic radar operates according to the technical
principles outlined in Part I. But just as carpenters use different
tools for different jobs, so are there different types of traffic
radar with different capabilities. Here are the categories of
equipment:
1. STATIONARY RADAR - Virtually all radar can be used in a
stationary mode. This simply means that the radar setup operates from
a fixed position. Stationary radar can be either mounted to a car, or
it can be hand held ( the so-called "radar gun" ).
2. MOVING RADAR - This is more complex system designed to
allow the operator to check speed of patrol traffic from a moving
patrol car. The principles involved are the same as for stationary
radar; a microwave beam looks forward, monitoring the strongest
reflection, which in this case is oncoming terrain. Simultaneously
the beam monitors the second strongest reflection, which it assumes
to be traffic. An internal calculator then subtracts the terrain
speed ( same as patrol car speed ) from the closing speed of the
patrol car and the target vehical. The result - if there are no
errors - should be the road speed of the target vehical. Like
stationary radar, moving radar displays its conclusion as a single
number on a digital readout.
3. INSTANT-ON ( OR PULSE ) RADAR - This is a method of
operation rather than a type of radar equipment. Most traffic radar
can be operated in the instant-on mode. It can be used either
stationary or moving.
The sole purpose of instant-on radar is to defeat radar
detectors. The instant-on radar trap is set up just like anordinary
radar trap. The only difference is that the radar doesn't transmit
until the operator pushes a button. So there is no radar signal for a
radar detector to find.
Then when the target is within speed-measuring range, the
operator triggers the beam. Hence the term "instant-on". Instant-on
radar locks on to target speed within a fraction of a second after
it's triggered.
4. PHOTO RADAR - This is a type of stationary radar designed
to operate unmanned. It takes a photo of any car it thinks is
speeding. The owner is identified from the license number and a
ticket is mailed to the owner. This type of radar is common in Europe
but is still an experiment in the United States. In early 1988, it
was operated in two locations, pending court challenges. The
locations were Paradise Valley, Arizona, and Pasadena, California.
Photo radar is expected to face legal difficulty in the
United States because America laws hold the driver responsible for
moving violations, not the owner. And it's almost impossible to
prove, at a later date, who was driving.
PART III - WHY RADAR GETS WRONG READINGS
====================================================================
The greatest weakness of traffic radar is the way in which it
gives its information. It has only one readout and it displays only
one number. You can point the radar antenna down a two-lane road with
traffic going in both directions. Vehicals can range in size from
economy cars to semi trucks. And all the radar will show is one
number.
Where does it get its one number? And how can the operator
know which of the coming and going vehicles is responsible?
The truth is, offen the operator cannot know for sure. He has
to guess. This is the most serious of all traffic radar's limitation.
Because it's made to a low-bid price, it must necessarily be a
relatively simple device. Without the modulated beam of the military
radar, it cannot distinguish between targets within range. Without
the radar screen, it cannot identify for the operator which target
it's reading.
At least the low power of traffic radar is an asset here in
that it limits surveillance to line of sight.
GUESSING THE OFFENDER
To make up for the lack of a modulated beam and the absence
of a radar screen, traffic radar makers use the cheapest imaginable
substitute. They simply program the electronics to disregard all but
the strongest reflection in the case of stationary radar, all but the
two strongest reflections in the case of moving radar. This is the
method of all traffic radar. The number displayed on the readout
comes from simplification. It's up to the operator to decide which -
if any - of the moving vehicles within range is producing the unseen
reflection. If there is only one vehicle in range, probably that
vehicle is responsible for the number, although it could be caused
by an electrical interference or blowing trash or some other less
obvious distraction to the microwaves.
If there is more than one vehicle in range, it's up to the
operator to decide which one is producing the reflection. Is it the
closest one to the antenna, or is it the largest one in the pack? In
truth, it could be either, depending upon a host of subtleties.
A skilled operator intend on justice wouldn't write a ticket
unless he was absolutely sure. A less skilled operator might write
the ticket thinking he had the right answer, and be wrong. A careless
operator intent on filling his quota might see the number and single
out a likely operator - the red sports car - and be done with it.
When you deal with humans, you take your chances.
Because traffic radar is built to a low-cost target, it is
not the infallible electronic instrument that it purports to be. And
because operators have a tough time keeping track of invisible beams,
traffic radar invites human error in vehicle identification.
NO GOVERNMENT STANDARDS
While military radar is produced to exact government
standards to assure accuracy, traffic radar is not subject to any
government standards whatsoever.
In the late 1970's there was wide-spread puplicity about
radar errors including an 86-mph tree and a 28-mph house. The
National Highway Traffic Safety Administration ( NHTSA ) assigned to
the National Bureau of Standards the task of testing all brands of
traffic radar in use at the time, for the purpose of discovering the
source of these errors and proposing federal standardsto eliminate
them. In January 1981, these proposed standards were published in the
Federal Register. The newly-installed Reagan administration book took
no action on the proposal.
After three years of government inaction on the problem, the
International Assocition of Chiefs of Police ( IACP ) provided
non-government standards by which all traffic radar units could be
tested to assure accuracy: Volume I of the standards was published in
April, 1984, Volume II in June, 1984.
In June 1986, the traffic radar manufacturers announced the
formation of their own trade association, saying that they would not
submit traffic radar units for IACP testing. Instead, the radar
makers would use their own standards.
The industry standards have not been published thus far. So,
in effect, there are no performance requirements for traffic radar,
and the claims of 86-mph trees and 28-mph houses have never been
refuted.
PART IV - HOW RADAR DETECTORS WORK
====================================================================
A radar detector is, in essence, a radio tuned to receive
traffic radar frequency.
The technology involved is straightforward. Remember that
traffic radar transmits a microwave beam out through an antenna,
then listens for the reflection of the transmission. A radar detector
amounts to the listening section of a radar unit.
The key question is, Can a radar detector find radar before
radar finds you? The answer is yes. Consider the worst-case scenario
for pure range. You're driving down a straight and level highway,
heading right for the radar trap. The beam is reaching out, feeling
for you. But remember that microwvaes lose energy as they cover
distance. And to measure your speed, the beam has to reach out to
you, be reflected back, and then arrive at the radar unit with enough
strenght to activate the calculating circuits.
Let's say you come in range when you're a half mile away. The
microwave beam traveled a half mile out to you and a half mile back
to the radar. That's a mile of total travel. If your detector has the
same sensitivity as the radar's listening section, you can find the
radar a half mile before it finds you.
It is not at all difficult to build a detector as sensitive
as the listening section of the traffic radar. In fact, the
electronic components used in traffic radar are standard industry
components available to anyone.
Of course, radar is not always used out on the open road
where a detector can get a good look at it. Radar operators favor the
ambush; they hide over a hill or around a curve, and when you pop
into view, you're already in range. So how does a detector work in
this case?
The classic analogy for the radar beam likens it to a
flashlight on a foggy night. The guy holding the flashlight can't see
you until the beam scores a direct hit. But you can see the beam
tracing its pattern through the fog long before it falls on you. You
can see it because the light reflects off moisture, dust and other
objects in its path, sending off small glints of light easily seen
from a distance.
Microwave beams behave like light beams. They operate on a
strict line-of-sight basis. You can't be "seen" over a hill or around
a curve or behind a semi. Radar has to wait for you to drive into
view. But a detector can pick up the scatter from the microwave beam
well before the beam itself hits you directly.
Again, a receiver as sensitive as the traffic radar's will be
sufficient. But unlike the radar, a detector doesn't need to
calculate speed. It just needs to know that radar waves are present.
So a detector can act on a weaker signal than that necessary for the
radar itself.
This is not to say that detectors are infallible. Radar
operators have one trick that's hard to defend against. This is radar
used in the "instant-on" mode. Here a hidden radar is kept in standby
condition as a target approaches. Its internal ciruits are warmed up
and ready, but it is not emitting a beam. So, naturally a detector
can't find it. Then when the target vehicle is well within range, the
operator switches the unit on and it quickly calculates speed.
A good radar detector protects against instan-on radar by
warning you when the operator zaps one of the cars up ahead. It has
to be sensitive enough to respond to weak signals. Moreover, it must
be reliable enough so you don't interpet these brief, weak signals as
random false alarms and disregard them. Of course, if there is no car
ahead, you're on your own.
A good radar detector, like any sophisticated electronic
equipment, requires careful engineering. You will have the best
chance of satisfaction if you buy from a reliable source. Pick a firm
that will refund your money if you're not satisfied. There is no
better guarantee.
COMMON OPERATOR ERRORS
====================================================================
The proposed Federal standards for traffic radar were never
implemented. However, in an attempt to reduce radar errors, several
local governments used the Federal research to develope better
training programs.
The Texas Department of Puplic 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 many
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 - Because moving radar tries to
do a more complicated job than stationary radar, it is subject to all
the errors of stationary radar, plus several additional errors that
apply to it alone.
"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 closing speed of the target. Therfore
anything that produces a low evaluation of patrol speed will
automatically result in a high reading of target speed.
Texas tells its radar operators that this... " situation
becomes more critical if the 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
machinary, or even a sign, makes a more efficent reflector than
horizontal pavement. Therefore the radar uses that reflection as a
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 the 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 the patrol speed seem smaller than it
actually is, it always acts to raise the reading of the target speed.
5. DOUBLE-BOUNCE ERROR - Microwavesare 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 efficent 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 distingguish direction.
7. ROAD-SIGN ERROR - The ready reflectability of microwaves
means that road signs are also be a 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 will override patrol car speed reflected from the roadway." This
error is particularly egregious because the fan speed will be
substituted for patrol speed in the moving radar's calculation of
target speed. Since this 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 safe guard for this error.
In conclusion, the Texas Department of Puplic Safety notes
" Radar cannot identify ( the ) speeding vehicle: ( the ) officer
must do that."
For further information on the Passport or Escort Radar
Detectors, Traffic Radar or an illustrated, color copy of this
report, contact:
CINCINNATI MICROWAVE, INC.
One Microwave Plaza
Cincinnati, Ohio 45249-9502
1-800-543-1608
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