e02b018.pdf

GENERAL INTEREST

Everything you always wanted to know about

Speed Cameras

but were afraid to ask…

By: B. Bouchez

Feared by many motorists (let those who have never driven too fast cast

the first stone), the speed camera, usually containing a radar, is an

electronic device whose workings are still a mystery to most electronics

hobbyists. In this article we take the covers of these curious devices and

describe their operation in detail, also considering a few ideas to guard

against their actions.

them will be reflected back to the

transmitter, where an aerial can

detect them.

In the original systems the start of

the horizontal sweep in the cathode-

ray tube was synchronised with the

transmission of the signal burst and

the return signal received by the aer-

ial was fed to the vertical deﬂection

ampliﬁer. When the aerial received a

return signal (the waves reflected

back by an object) a spike could be

seen further to the right of the tube,

the position depending on the dis-

tance of the object (see Figure 1 ).

This system could not only be

used to detect the presence of every

object (aeroplane) that was in its

ﬁeld of ‘view’, but it could also deter-

mine the distance accurately

between the aerial and the object,

since the propagation speed of the

waves and the rate of horizontal

deflection (time base) were known.

This is how R.A.D.A.R. came about,

an acronym that soon turned into an

everyday word.

Subsequently the radar was

improved to cover all of the sur-

rounding area rather than just a

small section directly in front of the

aerial. This was done by mounting

(Source: Applied Concepts, Inc. / Stalker Radar)

A little bit of history

Many, many years ago, the investigations car-

ried out on electromagnetic radiation in the

thirties revealed that RF waves appeared to

be reflected back from some objects. This

property became more discernible as the fre-

quency was increased (above 100 MHz). It

wasn’t until the end of the thirties that SHF (f

>1GHz) waves could be generated easily,

thanks to the introduction of the magnetron

(which is still used in modern

microwave ovens).

The first practical application of

this system was for Radio Detection

And Ranging, better known as its

acronym RADAR. The operation of

radar is based on the transmission of

a series of SHF waves at regular

intervals. When these waves hit an

object of sufficient mass then part of

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11/2002

GENERAL INTEREST

the aerial assembly on top of a

turntable (the cathode-ray tubes

were now given deflections in two

directions).

TX/RX antenna

controlled

SHF

oscillator

switch

Introducing the Doppler

effect

The device just described is not

capable of determining the speed of

the detected object; this was limited

to measuring the movement of the

echo on the screen, which gave a

rather inaccurate result.

As an example, consider a car

that makes a sound with a ﬁxed fre-

quency (a car that is driven with

fixed revs for example). When you

are in the car you won’t notice any

variation in the frequency of the

engine sound.

If however you stand at the side

of the road and listen to the car

when it drives past under identical

conditions you will notice that the

frequency of the engine sound

increases as the car comes nearer

and then decreases as the car trav-

els past you. (This effect is also

noticeable in F1 Grand Prix races

when the cars roar past the camera.)

This phenomenon works the other

way round as well: when you drive

your car past somebody who is

shouting on the pavement, you

should notice that the frequency of

the shouting increases as you go

towards the person, and then

decreases as you move away.

When the distance between the

sound source and the receiver

remains constant then the frequency

of the received signal won’t vary

either.

The Doppler effect (named after

the physicist who discovered it) is

nothing more than that described by

the formula in Figure 2 :

return

signal

amplifier

frequency

generator

vertical

amplifier

transmitted

burst

synchronised

sawtooth

generator

received

burst

horizontal

amplifier

020165 - 11

Figure 1: Principle of operation of ﬁrst generation radars (1940).

nal in air (300,000 km/s for radio

waves, 340 m/s for sound

waves).

From this we can deduce that

sending a fixed frequency signal

towards the car and then measuring

the frequency of the returning signal

will give you the speed of a car.

This is the principle used for

radars in speed cameras, although

they have little in common with the

systems described in the ﬁrst part of

this article.

It should be mentioned that the

sensitivity of the radar increases as

the angle between the beam and the

path of the vehicle decreases. For

this reason the aerials of speed cam-

eras are positioned parallel to the

roads rather than across them! This

is also the reason why few types of

radar can work along bends, since

the angle between the beam and the

vehicle continually changes, creating

errors in the measurements.

sure the speed of vehicles, we’ll take a look

at the commercial applications that are found

at the side of the road.

The basis of every speed camera (let’s call

it a radar) is a SHF generator, which can

transmit a beam in a speciﬁc direction. From

the previous section we have learnt that the

sensitivity of the device is directly propor-

tional to the frequency of the beam. The exact

frequency used depends on the manufacturer,

but is generally between 2 GHz and 15 GHz.

The device can either have a SHF oscillator

based on a Gunn diode and a resonant cav-

ity, or a transistor oscillator followed by a

power amplifier. The power of these oscilla-

tors is not very high (usually less than 10

mW), but the effective power output is

increased through the use of a directional aer-

ial.

The receiver for the reflected signal is

often based on a Schottky diode, situated at

the focal point of the aerial (usually the same

aerial is used for transmission and reception),

which functions as a mixer of the transmitted

and reﬂected signals.

The output signal of the receiver is ampli-

ﬁed, conditioned by an analogue circuit and

then passed on to the measurement section,

which is nothing more than a frequency

counter.

From theory to practice!

Now that we’ve seen how the

Doppler effect can be used to mea-

f M = 2 vf E cos (α

/ c )

where

f M is the frequency of the received

signal.

v is the speed of the vehicle.

f E is the frequency of the transmitted

signal.

α is the angle between the transmit-

ter and the path along which the

vehicle travels.

c is the propagation speed of the sig-

direction of vehicle M

f E

Speed check

TX/RX

f M

020165 - 12

Figure 2: The Doppler effect.

11/2002

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The signal from the frequency counter

goes to a microprocessor that calculates the

speed and sends it to a display. It also checks

if the measured speed exceeds a preset value

and warns the police officers who are nearby

that an offender has just passed, or it acti-

vates a camera and ﬂashgun.

In short, the basic principles behind a high

frequency speed detector ( Figure 3 ) are not

very complex. (Note that we’re talking about

the principles, a practical implementation is

something else altogether, especially the high

frequency section.)

take measurements up to twenty

meters, the newer models with their

ultra-sensitive detectors are capable

of taking measurements over several

hundred meters, so well before they

can be seen from the car!

surement. Actual radar equipment

works on a random basis or is acti-

vated only when a vehicle comes

nearby.

Discrimination : when several vehi-

cles travelling at different speeds

encounter a radar beam, the result-

ing Doppler signal contains a mix-

ture of signals at different frequen-

cies. The majority of current devices

can’t separate these components

and reject the measurement as

faulty. There are however newer sys-

tems that contain a DSP (the author

has worked on one of these sys-

tems), which can measure the speed

of several cars simultaneously. So

now only those cars that happen to

be in the ‘shadow’ of others can

escape the speed cameras.

The long and short of it is that

speed cameras have become so

accurate and reliable (which can be

conﬁrmed by drivers who don’t keep

to the speed limit), that it has

become extremely difficult to evade

them!

Reaction time : just as with any other

equipment that use frequency coun-

ters, these speed cameras also

require a certain time to take a mea-

surement. Furthermore, most

devices now take several measure-

ments rapidly, making it possible to

reject any possibly erroneous mea-

surements. Older models required

about half a second to take a reliable

measurement. Current models react

within a tenth of a second, so any

motorist who ignores the speed limit

will have very little chance of avoid-

ing a fine after noticing a speed

camera. Sometimes the radar equip-

ment also contains a DSP (Digital

Signal Processor), which uses a spe-

cial algorithm with a very short pull-

in time, making extremely fast read-

ings possible.

How well does it work?

Now that we know how it all works we may

wonder how reliable the measurements made

by these devices are. Let’s keep one thing

clear: we have no intention of encouraging

any of our readers to break the speed limit or

behave irresponsibly. We just want to look at

the problem from a technical viewpoint to dis-

cover what the limits are of SHF speed cam-

eras. In this way we can distinguish between

proven facts and ‘rumours’ that are doing the

rounds, made by people with little knowledge

of electronics. Instead of straying into difficult

technical considerations we’ll just answer the

most common questions.

Continuous transmission : in contrast

to what you may have thought after

reading the theoretical part of this

article, a radar does not need to have

its oscillator functioning continu-

ously. It only needs to be active long

enough to stabilise and take a mea-

On the wrong side

of the law

Mankind, and especially homo auto-

mobilis, behaves in such a way that

when he comes across an obstacle

he will try everything to get round

Operation during rain or mist: in contrast to

widespread opinion, radar works perfectly

well during rain or mist (after all, radar is

used extensively to help with the landing of

aeroplanes in bad weather!). In general,

when it rains it comes down vertically (at

least it does here!), which is at right angles

to the radar beam, bringing about a Doppler

effect of zero (cos 90° = 0, so f M = 0). Heavy

rain that comes down at an angle due to

strong gusts of wind can affect the signal-to-

noise ratio of the receiver and prevent its cor-

rect operation. In this case the processor will

simply reject the measurements.

Since mist doesn’t move with respect to

the radar beam (or only very slowly) it will be

practically invisible to the receiver and the

measurements are completely unaffected.

Measurement range : the distance from which

a radar can measure the speed of a vehicle

depends on two factors: the power of the SHF

oscillator and the sensitivity of the detector.

We already knew that the oscillator power is

generally low and that the use of a directional

aerial increased the transmitted power. The

biggest problem for the detector is the signal-

to-noise ratio, which doesn’t get any better

with Schottky diodes. In this section the sen-

sitivity can also be improved through the use

of aerials. Whilst the first radars could only

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11/2002

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it. Speed cameras are no exception

to this, and numerous boffins

(whether competent or not) have

contributed to the development of

counter measures.

Before we continue we would like

to make one thing perfectly clear: in

the majority of European countries

the possession, and even more so,

the use of equipment intended to

disrupt the operation of speed cam-

eras is illegal. Indifference in this

area is not recommended, since the

consequences can be dire (handing

in of the driver’s licence, conﬁscation

of the vehicle, prosecutions, etc.).

In spite of this, some drivers think

it is worth the risk and don’t shrink

back from using these devices.

Loosely speaking, there are two

types of ‘anti-radars`: jamming

devices and detectors.

The jamming devices are simply

small SHF oscillators, which are

used to send a ‘fake’ signal back to

the speed camera, causing the mea-

surement to fail and preventing the

logical analysis of the frequency.

Besides the fact that these devices

are relatively ineffective (in most

radars the circuits are less sensitive

to interference signals: the frequency

of the jamming signal therefore has

to be as close as possible to that of

the speed camera, and every device

has its own frequency), the elec-

tronic circuits in the radar can detect

such jamming signals and notify the

police. A jamming device is therefore

a sure-ﬁre way to get caught!

A detector on the other hand con-

sists of a simple SHF receiver, and

by definition these can’t be

detected. In the USA (where their

use is permitted) they are sold in

large quantities. On the Internet

they are readily available and in

some European countries they are

also freely available (their sale is

permitted but their use isn’t!). These

are usually relatively simple circuits

containing a microwave detector

and a comparator that drives an

alarm. In short, simplicity itself

(although their price seems to sug-

gest the opposite is true!).

TX/RX

parabolic antenna

SHF

oscillator

(2 – 10 GHz)

Receiver/mixer

command

processor

amplifier/

shaper

analyser

display

020165 - 13

Figure 3: Basic principle of a high-frequency speed camera.

oscillator of a speed camera is set to

a frequency that is outside the range

covered by the detector, or it uses an

optical laser, then you’re bound to

get caught.

The second problem is that in

order to detect something, there ﬁrst

has to be something to detect (obvi-

ous, isn’t it?). Older radar equipment

transmitted continuously, which

made the task simpler, but newer

models only transmit intermittently,

either randomly or in short bursts,

reducing the chance of detecting

these devices. Some models (such as

the Mesta 208 sold in France where

the author lives) are even more cun-

ning and only come into action when

a car comes within their range.

These ‘green bullets’, as they are

known because of their shape and

colour, have an optical detector on

top that literally sees the vehicles

coming.

As soon as there is some move-

ment in front of the device it springs

into action.

This brings us to the third prob-

lem: a radar detector will sense the

beam at that instant. But at the

same time the speed camera is

already doing its work. From this it

follows that in the time taken by the

driver (a typical reaction time for

people is about half a second) to

take appropriate action (to brake or

disrupt the measurement), the radar

will already have taken four or five

measurements.

The detection is made more diffi-

cult by the fact that very narrow beams are

used, making for a small ‘detection area’.

Some users of radar detectors have noticed

that the beam can also be detected when it

is reflected off other cars ahead and have

gladly made use of this property.

And now for the ﬁnal problem: most radar

equipment can take measurements of

approaching (from the front) or receding (from

the rear) vehicles. But the sensitivity of most

detectors is limited to just one direction. To

be prepared for any eventuality the vehicle

should therefore have a detector at both the

front and rear!

And ﬁnally...

As the saying goes, if you play with ﬁre you

may get burned. As we have seen in this arti-

cle, speed cameras have become reliable

instruments that are difficult to locate or

interfere with.

Knowing how to detect a SHF beam is one

thing; to disrupt a speed measurement is

another story. In short, although most in-car

radar detectors sold at the moment do work

and show the presence of speed cameras,

they are of little use because the speed cam-

era is likely to be triggered before the driver

can slow down.

Even though speed cameras are some-

times situated in places where they aren’t

justified (which has given rise to the name

‘euro-pump` on the continent), in our opin-

ion it shows more sense to respect the speed

limits (also out of respect for other road

users), rather than attempt to get round the

law at all costs.

It isn’t difficult to design a broad-

band detector that reacts to fre-

quencies between 2 and 10 GHz,

which is the range where most mod-

ern devices operate. However, if the

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