The following is an explanation of Police
Radar. It explains Police radar and it's
operation and theory - from the h2g2 web site
ran by the BBC.
You're driving
down the highway, and you're passed by a little
red sports car. Two miles down the road, you see
blue lights on a car parked just behind the red
car. Have you wondered how the officer knew how
fast they were going? How does the police radar
work? Is there anything that I can do to avoid
police radar?
Doppler
Radar
Police radar
is a doppler radar. It measures speed by looking
for a red shift or blue shift in light, similar
to the way astronomers measure the velocity and
and distance of stars.
The radar
antenna emits a beam of light in the radio
frequency range. The light bounces off of the
target, and then returns to the police radar
antenna. The velocity of the target will change
the frequency of the radar signal. That change
in frequency is interpreted by the radar unit
and shown to the officer as the target's speed.
Radar Case
In order for
the officer to make a speeding case he needs to
establish the following:
-
Jurisdiction
- Date and
time of the offence
- Roadway
on which the offence occurred
- Posted
Speed
- Identify
the vehicle and the operator
- Tracking
History
- Radar
Reading
It's often
helpful for the officer to include other
information such as weather and traffic
conditions, and any statements made by the
violator.
The officer
testimony will typically be something like this:
On 30
December 2001 at about 8:27pm, I was
operating stationary radar on Highway 1 near
Main Street, in the city of Centre, Georgia.
The area is posted as 45 mile per hour zone.
I noticed a red Saturn SL1 traveling east on
Highway 1 at a high rate of speed. I
activated my radar. It gave a high-pitched
clear tone, and it indicated a speed of 62
miles per hour. I stopped the Saturn and
made contact with the driver, Ms. Blank.
Some
jurisdictions may require additional
information, such as the calibration information
on the radar, the officer's certification to
operate the radar, information establishing why
the violator's speed was unsafe, etc.
Tracking
The most
important part of a radar case is a tracking
history. The radar unit will display a number,
and that's all. It doesn't tell the officer
which vehicle it is, or if there's even a
vehicle there. The officer has to track the
vehicle to make sure that his observations match
what the radar is showing him. Otherwise, the
officer might stop the wrong vehicle or a common
radar error might give an incorrect speed. In
some jurisdictions, the officer has to visually
estimate the violator's speed within 5 miles per
hour.
The radar beam
is a cone. It doesn't pick out individual
vehicles. It can't even pick out individual
lanes. The radar shows a speed based on three
factors:
-
Reflectivity
- Position
- Speed
This is
generally referred to as biggest, closest,
fastest. The radar usually picks up target that
is the largest in its view. Therefore, it might
pick up a motorcycle that was very close to it
before a tractor-trailer a mile down the road.
Many times the radar will display different
speeds of different vehicles that are close
together. The officer has to determine if he's
getting a good reading, and if so which
vehicles' speed is being displayed.
This isn't as
hard as it might sound. Radars are equipped with
a speaker which give a tone reflecting the
doppler signal its receiving. If it's a clear
high pitched tone, then it's getting a good
solid reading from a vehicle. It will give a low
raspy tone if it's not getting a clear signal.
This happens when there's something in between
the radar and the target or when the vehicle is
entering or leaving the beam.
Once you have
a solid tone, you look at how the traffic is
moving. If there is a clump of vehicles that is
moving at 65mph then a vehicle overtakes them at
a high rate of speed, and the radar shows 85mph,
it's easy to figure out who was going that fast.
Alternatively, if a group of vehicles is
traveling together in a clump, where no one is
overtaking or falling behind, all the vehicles
in that clump will be at about the same speed.
Some radars
have a fastest vehicle button that will display
the fastest vehicle in its cone. This is very
useful for when there are large targets such as
tractor-trailers in between the radar and a fast
moving small vehicle.
Modes of Radar
Stationary
Stationary
radar is radar at its simplest. The officer sits
on the side of the road, and watches traffic.
When he observes a vehicle moving at high speed,
he activates the radar. The radar goes through
its basic decision factors (Reflectivity,
Position, and Speed) then it displays that
speed. The radar will give a tone. If the tone
is clear and the displayed speed matches the
officer observations, the officer can make the
stop.
Moving
Moving radar
is very similar stationary radar, but it's
looking for two different speeds. The radar
looks for the largest object in its field, and
it assumes that this is the passing background.
Then it looks for the second most significant
object that it assumes is the target. The radar
actually measures the closing speed or
separation speed between the target and the
patrol vehicle. The radar's counting unit will
then use the following formulas.
Target Speed
(TS) = Closing Speed (CS) - Patrol Speed (PS)
or
Target Speed (TS) = Separation Speed (CS) -
Patrol Speed (PS)
The radar unit
will then display two speeds. It will show the
target speed and the patrol speed. The officer
must compare the patrol speed displayed on the
radar with that displayed on the car's
speedometer. This is an essential element of the
radar case. The radar speed will be more
accurate, but there are certain errors that this
will detect. The speeds must be consistent.
Same
Direction
Same direction
radar was developed when engineers were
examining the shadowing error. Same direction
radar uses very different logic than moving or
stationary radar. It also requires more a
complicated tracking history.
Basically, it
figures out the patrol speed. Then it looks for
the bounced reflection off of the other vehicle
and measures the relative speed between them.
This makes things more complicated because the
officer must decide to activate the radar, let
the radar know if the officer or the target is
moving faster.
Radar Errors
There are
several things that will affect a police radar
unit. There's a famous example of a lawyer
aiming a radar at the courtroom wall and
clocking it at 19 miles per hour. Radars will
pick up interference from things other than
vehicles. Power lines and the patrol car's air
conditioner are the most common things that a
radar will register. This is why training and
experience is important. Officers will learn
where the power lines are, and how the radar
will react to them.
Interference
Police radar
uses part of the electromagnetic spectrum. They
can be influenced by any number of
electromagnetic and physical phenomena. For
instance, targeting radars on fighters use the
same frequencies. Air conditioning units in
patrol cars can create a reading (generally 32
mph). Some high power lines can also set off
radars (generally in the around either 92 mph or
101 mph).
Officers must
have a good tracking history in order to confirm
that his observations are matched by the speed
displayed by the radar. If an officer is
traveling along a road with a 35 mph limit, and
sees a vehicle traveling at around 50 mph, and
the radar displays 100 mph, he knows that the
result is bogus. An officer should know his beat
well enough that he's aware of the common
sources of interferences.
Some forms of
interference, such as the air conditioning
units, will disappear when the radar detects an
actual moving object. Its decision factors will
ignore any signal as weak from the air
conditioner unit.
Cosine
Error
Cosine error
is when the radar antenna is at an angle to the
target. Instead of coming straight towards the
antenna, the target is moving across the beam.
Some of the speed is lost.
Imagine that
an officer is sitting at the right angle in the
figure to the left. The target is moving at five
blocks per minute, but the since light travels
in a straight line, it's only measuring the
speed along line b. It looses one block per
minute of speed.
Basically,
this means that if an officer is sitting at an
angle to the flow of traffic, the speed
indicated will always be lower than the actual
speed of the target. In stationary mode, it's
always to the advantage of the violator.
In moving
mode, a cosine error can reduce the computed
speed of the patrol vehicle. So when the
counting unit computes the target speed with
CS-PS=TS, the target speed will be higher than
it should be. To counteract this, the officer
needs to check his speedometer against patrol
speed displayed by the radar.
Masking
Masking is a
rarely observed error where the radar antenna is
pointed at the counting unit (the part of the
radar that shows the speed).
Shadowing
Shadowing is
when an officer is behind another moving object.
Usually it will be something large like a
tractor-trailer. The radar will interpret the
tractor-trailer as the background instead of the
actual background. Therefore, when an officer is
running moving radar, he has to check the patrol
speed showed by the radar unit against his
speedometer. If they don't match then he may
have a shadowing error.
Batching
Batching is
when an officer is accelerating and activates
the radar. Most modern radars have internal
error checking that prevent this from being an
issue.
Scanning
Scanning is
when you swing a radar antenna across a
background. It's possible to get the radar to
show a speed this way, but it is hard.
Other Potential
Issues with Radar
Officer
Training
An officer
must be trained to operate the radar. It doesn't
take much to figure out how the radar works, but
it does take some training and experience. In
many states, the officer will have to be
licensed to operate the radar. It will be an
element of the case that the officer will make
in court. Asking the officer for this permit on
the side of the road is probably a waste of
time.
Two Officer
Teams
On some
occasions, officers will act in teams. One
officer will operate the speed detection
equipment, and another officer will issue
citations. This is particularly common when the
police use airplanes to find speeders.
In order to
obtain a conviction, the officer who identifies
the violation must be in court to identify the
violation. The officer who issues the citation
must come to court to identify the driver. The
officers must also be able to say how they were
to pass the information about the violation
between them.
Radar
Detectors
A radar
detector is just a radio receiver that flashes a
light and makes a noise whenever it receives a
signal in a certain frequency range. That's very
useful right? The answer is maybe.
Just as there
are numerous things that a radar picks up as
interference, there are a number of things that
will activate a radar detector. Furthermore,
most police radars are equipped with an instant
on feature. The officer will activate a radar
whenever he identifies a potential speeder.
Therefore, there may be no signal for the
detector to pick up until it's too late.
That's not to
say that radar detectors don't have value. If
you're traveling across level ground, then you
may pick up the radar signal when the officer
checks a driver in front of you.
Calibration
Radars should
be checked for accuracy occasionally. Under
Georgia law, the officer has to check it at the
beginning and end of each shift. The check for
accuracy consists of the following:
- A light
check. The officer presses a button on the
radar, and all the LED lights light up.
- An
internal circuit check, which is
accomplished by pressing a button on the
radar unit.
- Tuning
fork check. Tuning forks that are tuned to
vibrate at a certain frequency are put in
front of the radar antenna. The radar unit
will display a certain speed.
If the radar
doesn't perform within the manufacturer’s
specifications, it has to be removed from
service until the radar can be repaired.
Radars also
have to be calibrated by specially trained
technicians occasionally, usually once a year.
Other Methods of
Speed Detection
There are
other methods to detect speed. The most common
are LIDAR (Laser) and pacing.
One of the
most accurate and easy to use technologies is
Laser (LIDAR). A laser is similar to a radar,
but it is aimed like a rifle. The officer can
specify a particular vehicle whose speed the
officer wants to determine. The officer just
aims it, pulls the trigger, and the unit
displays the speed and distance to the target.
Some newer models also take a digital picture of
the target.
Officers can
also pace speeders using their speedometers. The
officer maintains a constant distance from the
violator. He watches his speed over a certain
distance. The violator is then cited with the
lowest speed that the officer observed. This
method depends on the accuracy of the officer's
speedometer. Officers must be able to testify
that the accuracy of their speedometer has been
checked or use a radar to confirm the officer's
speed when following the violator.
