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How does radar and the Doppler system work?
Radar, like sonar and
seismology, uses a man-made pulse of radio energy to map
distance based on the length of time it takes the pulse
to return from the source. Radar (short for "Radio
Detection And Ranging"), which can
be airborne or space borne, has greatly changed the way
we see the land and ocean surfaces. Radar is based on
the principle of sending very long wavelength radiation
(called microwaves) from an antenna, and then detecting
that energy after it bounces off a remote target. The
wavelength of the microwave, its polarization (vertical
or horizontal orientation) and strength can be
controlled at the source and measured when it returns.
Many common land-cover types and materials affect the
polarity and strength of the radar return differently,
which helps in their identification.
The concept of using
radio waves to detect objects goes back as far as 1902,
but the practical system we know as radar began in the
late 1930s. British inventors, aided by research from
other countries, developed a rudimentary warning system
which could detect objects moving towards the coastline
of England. The system used high-frequency radio waves
to detect German planes and calculate their distance.
This purpose lead to the acronym RADAR, short for RAdio
Detection And Ranging.
The principle behind radar may sound confusing at first,
but a simple experiment can demonstrate the basics.
Imagine you are facing the side of a mountain somewhere
in the distance. You have a very accurate stopwatch and
'super hearing' to help you.
Now hold the stopwatch in one hand and start timing as
you scream as loudly as you can towards the mountain.
Stop timing when you hear the first echo of your voice.
You have now become a basic radar unit. Since you know
how fast sound travels, you can calculate the distance
between you and the mountain by using the elapsed time
on the stopwatch.
Radar works on many of the same principles demonstrated
in this experiment. Instead of one person screaming, a
powerful radio beam is sent out at a specific frequency.
When this burst of radio energy strikes a solid object,
at least part of that sound wave will be reflected back
to the transmitter. This signal may not be very loud,
but a sensitive electronic receiver can amplify the
sound, much like your 'super hearing'. The transmitter
and receiver on a basic radar unit are usually mounted
close together, much like your mouth and ears.
By calculating the
speed of the radio waves and the time it takes for the
signal to bounce off the object and hit the receiver, a
radar operator can gauge the distance between himself
and the object. Moving the transmitter to different
points allows the radar operator to receive multiple
returns. All of these individual reflections are
combined to estimate the size of the object or objects
being struck.
Radar technology has improved considerably since the
days of World War II, but the underlying principles are
still the same. Calculations about an object's speed and
direction are made from the results of transmitter and
receiver data. When a radar antenna is seen spinning in
place, it is sending out thousands of signals and
receiving them just as quickly.
The radio frequencies
on modern radar systems are now largely in the microwave
range, unlike the shortwave radio frequencies used by
the British inventors. Radar jammers use matching
frequencies to confuse the receivers looking for
authentic ones. Microwave frequencies are much more
difficult to jam.
How does a Speed Camera
or Radar Gun work?
 To
understand how radar detectors work, you first have to know what
they're detecting. The concept of measuring vehicle speed with radar
is very simple. A basic speed gun is just a radio transmitter and
receiver combined into one unit. A radio transmitter is a device
that oscillates an electrical current so the voltage goes up and
down at a certain frequency. This electricity generates
electromagnetic energy, and when the current is oscillated, the
energy travels through the air as an electromagnetic wave. A
transmitter also has an amplifier that increases the intensity of
the electromagnetic energy and an antenna that broadcasts it into
the air. Below are two of the early Radar Guns used in the 1950s -
1970s.
A radio receiver is just the reverse of the transmitter:
It picks up electromagnetic waves with an antenna and converts them
back into an electrical current. At its heart, this is all radio is
-- the transmission of electromagnetic waves through space (see How
Radio Works to learn more).
Radar is the use of radio waves to detect and monitor
various objects. The simplest function of radar is to tell you how
far away an object is. To do this, the radar device emits a
concentrated radio wave and listens for any echo. If there is an
object in the path of the radio wave, it will reflect some of the
electromagnetic energy, and the radio wave will bounce back to the
radar device. Radio waves move through the air at a constant speed
(the speed of light), so the radar device can calculate how far away
the object is based on how long it takes the radio signal to return.
Radar can also be used to measure the speed of an
object, due to a phenomenon called Doppler shift. Like sound waves,
radio waves have a certain frequency, the number of oscillations per
unit of time. When the radar gun and the car are both standing
still, the echo will have the same wave frequency as the original
signal. Each part of the signal is reflected when it reaches the
car, mirroring the original signal exactly.
 But
when the car is moving, each part of the radio signal is reflected
at a different point in space, which changes the wave pattern. When
the car is moving away from the radar gun, the second segment of the
signal has to travel a greater distance to reach the car than the
first segment of the signal. As you can see in the diagram below,
this has the effect of "stretching out" the wave, or lowering its
frequency. If the car is moving toward the radar gun, the second
segment of the wave travels a shorter distance than the first
segment before being reflected. As a result, the peaks and valleys
of the wave get squeezed together: The frequency increases.
Based on how much the frequency changes, a radar gun can
calculate how quickly a car is moving toward it or away from it. If
the radar gun is used inside a moving police car, its own movement
must also be factored in. For example, if the police car is going 50
miles per hour and the gun detects that the target is moving away at
20 miles per hour, the target must be driving at 70 miles per hour.
If the radar gun determines that the target is not moving toward or
away from the police car, than the target is driving at exactly 50
miles per hour.
Police officers have been catching speeders this way for
more than 50 years. Recently, many police departments have added a
new sort of speed detector, one that uses light instead of radio
waves. In the next section, we'll see how these cutting edge devices
work.
More recently police started using newer laser guns, but
they soon proved too unreliable and had limitations and were
influences by heat, headlights and other conditions. The basic
element in a laser speed gun, also called a lidar gun (for light
detection and ranging), is concentrated light.
The lidar gun clocks the time it takes a burst of
infrared light to reach a car, bounce off and return back to the
starting point. By multiplying this time by the speed of light, the
lidar system determines how far away the object is. Unlike
traditional police radar, lidar does not measure change in wave
frequency. Instead, it sends out many infrared laser bursts in a
short period of time to collect multiple distances. By comparing
these different distance samples, the system can calculate how fast
the car is moving. These guns may take several hundred samples in
less than half a second, so they are extremely accurate.
Police may use handheld lidar systems, just like
conventional radar guns, but in many areas, the lidar system is
completely automated. The gun shines the laser beam at an angle
across the road and registers the speed of any car that passes by
(the system makes a mathematical adjustment to account for the angle
of view).
 When
a speeding car is detected, the system triggers a small camera,
which takes a picture of the car's license plate and the driver's
face. Since the automated system has collected all of the evidence
the police need, the central office simply issues a ticket and sends
it to the speeder in the mail.
We have seen how police use traditional radar as well as
new laser technology to catch drivers speeding. As it turns out,
conventional radar is relatively easy to detect. The simplest radar
detector is just a basic radio receiver, something like the one you
use to pick up FM and AM radio stations.
The air is full of radio signals -- they're used for
everything from television broadcasts to garage door openers -- so
for a receiver to be at all useful, it must pick up only signals in
a certain range. The receiver in a radio is designed to pick up
signals in the AM and FM frequency spectrum, whereas the receiver in
a radar detector is tuned to the frequency range used by police
radar guns. Periodically, the frequency range used by the police is
expanded, and speedsters everywhere have to invest in new detection
equipment.
A basic radar detector won't do you much good if the
police officer drives up behind you and turns on the radar gun. The
detector will alert you, but by that time, the officer already has
all the information he or she needs. In many cases, however,
detectors pick up the signal before the speeding car can be tracked.
Police often leave their radar guns turned on for a long period of
time, instead of activating them after sneaking up behind a car.
Radar guns have a cone or dish-shaped antenna that
concentrates the radio signal, but the electromagnetic wave quickly
spreads out over a wide area. The radar gun is configured so that it
only monitors the speed of a particular target, not everything in
the vicinity, so chances are a detector will pick up the radio
signal well before the radar gun recognizes the car.
Of course, with this sort of detector, you're relying
mostly on the luck of the draw -- if the police officer decides to
target you before any other car, you're caught. Modern detectors
offer much more extensive protection for speeders, as we'll see in
the next section.
A convention Radar Detector picks up Police Speed Guns
using a simple radio receiver and is a completely passive device: It
simply recognizes the presence of radar. More sophisticated
detectors actually take an active role in eluding the police. In
addition to the basic receiver, these devices have their own radio
transmitter, which emits a jamming signal. Essentially, the signal
replicates the original signal from the police radar gun, but mixes
it with additional radio noise. With this information added, the
radar receiver gets a confusing echo signal, and the police can't
make an accurate speed reading.
Modern detectors may also include a light-sensitive
panel that detects the beams from lidar guns. These devices are more
difficult to evade than traditional radar because the beam is much
more focused and it doesn't carry well over long distances. By the
time a detector recognizes the presence of the laser beam, the car
is most likely in the beam's sights already. Some speeders try to
get around these systems by reducing the reflectivity of their car.
A black surface reduces reflectivity because it absorbs more light.
Drivers can also get special plastic covers that reduce the
reflectivity of license plates. These measures reduce the effective
range of the lidar system, but not the range of the driver's
detector. With this extra time, a speeder might be able to slow
down.
 Multanova
is a company based in Switzerland who specialise in traffic
enforcement systems. They have many products to catch out the unwary
motorist, but the ones you ought to know about are the Multanova
6F-2 and 9F - both radar-based systems that can be installed in just
about any manner you can imagine, including pole-mounted,
box-mounted, mobile, built-in to a vehicle and many others. These
people are creative, to say the least. Multanova's products work
with off-the-shelf DRS-3 radar units in the Ka band radar - 34.36GHz
and are mostly used as mobile traps that can be set up on highway
bridges. The 6F-2 systems take 4 or 5 speed readings before taking
the photo, and can take up to 3 photos a second using a fast
motordrive camera system. The 9F systems extend this capability to
being able to determine the vehicle types as either trucks or
passenger cars. Both units can be configured either as a front- or
rear-facing trap. They are small, and from the front, look like
miniature rocket launchers. They are shielded to prevent external
interference, and because of the highly targeted radar beam (emitted
via a parabolic reflector to allow focussing), are almost entirely
undetectable by cheap radar detectors, therefore a quality and
proven detector is required to find the Multanova. Click here to see how the
Multanova works. The Multanova camera is
the lowest power (output) Phone Speed Camera available and is
therefore the most difficult camera to detector. Only the very best
and most sensitive Radar Detectors give adequate warning for
Multanova cameras. The Multa-Nova is used extensively in Western
Australia and is the ideal "revenue generating camera" as it can
take more photographs per hour than all over devices.
* Source: How Things Work.
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