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How do speed control radars work ?
Actually, there are 2 types of radars :
radars directly measuring the speed of vehicles
with the Doppler effect,
and radars deducting the speed of a vehicle
from a succession of distance measurements.
Most of our roadside radars
work according to the principle of the Doppler effect.
These radars, as those in airplanes,
emit a microwave radiation.
A microwave radiation is a wave
with a frequency that the eye cannot perceive.
This wave is partially reflected by the vehicle,
and comes back into the radar.
This return of radiation is analysed by a sensor:
if the frequency is different from the emitted radiation,
it means that the vehicle was moving.
This change of frequency
is caused by the Doppler effect.
What is the Doppler effect?
When a wave is emitted or reflected by a moving object,
its frequency is modified
according to the speed of the object;
actually, to a proportional quantity to its speed.
This is true for every wave.
For example:
if a boat is sailing facing waves,
the pitching frequency
(the rocking backwards and forwards) increases,
as the waves are coming faster.
On the contrary,
if it is sailing in the same direction than the waves,
its frequency decreases.
It even becomes null if the boat is sailing
at the same speed than the waves (as would do a surfer).
Another example: the sound.
When a Formula 1 car gets closer
to the video-camera microphone,
the sound emitted is more high-pitched
than when it is moving away.
(the sound frequency is higher).
And so it is for an ambulance siren.
And so it is for the light or the waves.
Let's get back to the radar,
the difference of frequency measurement
between the radiation emitted and the radiation reflected
gives us the speed of the vehicle.
But there is another way to measure speed:
laser binoculars.
Here, it is no longer a radar, but a laser,
that emits an infra-red light,
that the eye cannot perceive.
As for radars,
a small part of the radiation
is reflected by the vehicle,
and retrieved by a sensor within the binoculars.
But this time,
we measure in how much time
the light has gone back and forth
between the laser and the vehicle.
We do know the speed of light (about 300 000 km/s).
Knowing the speed and the time
required to do the path,
we deduct the distance
between the laser and the vehicle.
By repeating several times the measure of the distance
on a regular basis,
we are able to know the speed of the vehicle.
The measure of the distance
is done several hundreds of times
in a few tenth of seconds.
These principles of measurements we mentioned earlier
to measure the distance and the speed
are actually applied in a wide range of fields,
such as astrophysics or medicine for example.
Production: Unisciel/ University of Lille 1
Conception/Production: Maxime Beaugeois, Damien Deltombe and Daniel Hennequin
Editing/Special effects: Damien Deltombe
Music: Sébastien Ride, « Thunder Chacha » (SR Music)
Presentation: Maxime and Nina Beaugeois
Graphic design/Credits animation: Michaël Mensier.