                            HOW TRAFFIC RADAR WORKS

The first thing to understand about traffic radar is that it has almost
nothing in common with the military radar used for civil defense, or the
aviation radar used by air-traffic controllers, or the weather radar used by
the meteorologist on the ten o'clock news.  Those sophisticated radars can all
measure speed of objects, distance of those objects from the radar station,
and general shape of those objects to aid in identification by a ground crew.
Those radars also cost hundreds of thousands of dollars.

Traffic radar, on the other hand, can't be any larger than what will fit on
the dashboard of a mid-size cruiser, and it can't cost any more than the low
bid of your municipality's procurement process.  Sometimes this is $400 or
less.  So traffic radar is necessarily a device with limited capabilities.

THE CONSTRICTED VIEW

The first limitation restricts the amount of territory traffic radar can
cover.  Military-aviation-weather radars use a rotating beam to sweep a full
360 degrees around the antenna tower.  All targets within range are displayed
on a radar screen.

Traffic radar uses only a stationary beam, much like a searchlight, and it
shines down the road, either forward or backward, but not both ways at once.
So the area under surveillance by traffic radar is quite limited.

The second limitation of traffic radar concerns the amount of information it
can provide.  Traffic radar doesn't have a radar screen.  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.

LOOKING FOR TROUBLE

It's reasonable to think of a traffic radar beam as a searchlight because,
even though the beam is made up of microwaves, it behaves very much like a
light beam.  It travels in straight lines and is easily reflected.  Metallic
objects like cars, trucks, guard rails and over passes 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 this beam because microwaves are
invisible to the human eye.  But they are very easily seen by an electronic
eye, in this case a radio tuned to microwave frequency.  And, in fact, such a
radio connected to a compact antenna forms the basis of all traffic radars.

HERE'S LOOKING AT YOU, KID

Traffic radar works by shining its microwave searchlight down the road.  When
you come in range, the microwave beam bounces off your car, and the radar
antenna looks for the reflections.

How does radar get your speed from these microwave reflections? It used 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 what you
hear when you stand along the 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.  In fact, 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.

SCIENCE ON THE GO

Traffic radar applies this Doppler principle to microwaves.  The microwave
reflection from an approaching car will be shifted upward in frequency.  A
departing car will cause a downward shift.  The radar compares the shifted
frequency of the reflection to the original frequency of the beam it sent out,
and from the difference it calculated speed, which it then displays on the
digital readout.  Thats really all there is to traffic radar.

A CHEAP BULB IS A DIM BULB

Traffic radar is purposely kept simple.  Going back to the searchlight
analogy, we all know there's a limit to the effectiveness of any flashlight or
spotlight.  The more powerful it is, the farther you can see.  The same
applies to radar.  And since power cost money, whether you're speaking of
searchlights or radar a traffic radar will have far less power than expensive
military radar.  And traffic radar's range will be limited accordingly.  It's
a fact of microwave life that the strength of the beam diminishes with
distance (radio waves in general act this way too (ed)).  The father it has to
travel, the less energy it'll have when it gets there.  For example, the radar
operator may be able to spray your car with microwaves while you're still a
mile away.  But the reflected signal has to travel that same mile back to the
radar before it's of any value, and if it's so weak when it gets back that the
radar's electronics can't read it, then it will be unable to compute speed.
You're out of range.

THE NEARSIGHTED ELECTRONIC EYE

In real life, radar range depends upon two things: power of the radar and the
reflectivity of the target.  We've already talked about power.  That's built
into the radar and there's nothing you can do about it.  Reflectivity of the
target is definitely something you should be aware of, however.  Perhaps
you've heard of an Air Force project called the "stealth bomber." This is an
attempt to build an airplane with poor radar reflectivity so it won't bounce
back the microwave beam.  Such a bomber operating at its normal altitude would
be invisible to enemy radar.  Radar reflectivity is mostly a matter of size
and shape, at least for purposes of describing highway vehicles.  The smaller
the vehicle, the smaller its reflection, and therefore the shorter its range.
Some cars are out of range on some radars until they drive within 500 feet of
the antenna.  On the other hand big, 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 under the right
conditions.  So the principle of radar is quite easily understood, but the
quirks of its operation behavior are hard to predict with accuracy.

THE BOTTOM LINE

The one thing you can be absolutely sure of, however, is that traffic radar
can only monitor one target at a time.  It has one readout and it displays one
number.  Where does it get the number? If there are several cars and trucks in
the beam, as there surely would be on a roadway with even moderate traffic,
how can it give only one number? This, in fact, is the most serious of all of
traffic radar's limitations.  Because it's made to a low-bid price, it must
necessarily be a relatively simple device.  At least the low power of traffic
radar is an asset here in that it pretty well limits surveillance to line of
sight.  And the simplest way of narrowing the field further - down to the
mandatory single number - is to program the electronics to consider only the
strongest reflection.  That has been done in all traffic radars.

And it's this one simplification, more than any other factor, that causes
errors.  This one simplification introduces the human element.  An operator
must look at the one number and decide what or who is responsible for it.
Humans have a tough time keeping track of invisible beams.

BELIEVING WHAT CAN'T BE SEEN

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 strongest reflection.  Is it the closest one to the
antenna, or is the largest one in the pack? In truth, it could be either,
depending upon a host of subtleties.  A skilled operator intent 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 perpetrator the red sports car - and be done with it.
When you deal with humans you take your chances.

THE PLOT THICKENS

So far, this discussion has been confined to stationary radar, the kind
waiting for you in a trap over the hill or hidden behind a bridge abutment.  A
more complex system exists - usually called "moving radar" - which allows an
operator to check the speed of oncoming traffic while the patrol car is in
motion.  The priniciples involved are the same as for stationary radar: a
microwave beam looks forward, monitoring the strongest reflection, which in
this case is the oncoming terrain.  Simultaneously the beam monitors the
second strongest reflection, which it takes to be oncoming traffic.  An
internal calculator then subtracts the terrain speed (same as patrol car
speed) from the closing speed of the patrol car and the oncoming traffic.  The
result - again, displayed as a single number - should be the road speed of the
target vehicle.

A FALSE WITNESS

Should be, but frequently isn't.  Unquestionably, military engineers could
design a moving radar that would deliver accuracy every time.  But on traffic
control budgets, no one has figured a way to eliminate all of the mistakes.
For example, moving radar frequently underestimates patrol car speed because
of "cosine error" with the result that the speed of the target car is
substantially overestimated.  So Well knows is this and other errors that in
some states, like Wisconsin, the courts do not take "judicial notice" of
moving radar, which is to say that radar evidence alone is not considered
persuasive.  The point to remember is that radar is a very sophisticated
electronic concept, a system of proven effectiveness, but traffic radar has
limitations that can't be ignored.  The best way to defend yourself is to have
a good working knowledge of these limitations.

