RADAR (Radio Detection And Ranging) is a system which uses electromagnetic (specifically radio) waves to determine the range, altitude, direction, or speed of both moving and fixed objects.

A radio wave is a type of electromagnetic radiation.

 

Let's examine this phrase, but we'll start with the word Radiation: radiation comes from the word 'radiate' which means to travel outward in all directions from source. If you put your hand near a heat source you can feel the heat hit your hand, this is (infrared) radiation! Not all radiation is harmful like you see in the movies - only radiation that has a very small wavelength tends to be harmful to humans.

Now lets looks at the word Electromagnetic: this is a combination of the words electric and magnetic. If you think about it, 'something' MUST be radiated for radiation to happen. This 'something' is a 'packet of energy' that has both electric and magnetic components...you can visualise that 'packet of energy' how ever you want, but for the purpose of this page I suggest thinking of a continuous flow of string that exhibits wave like characteristics (i.e. 'bouncing' or 'oscillating' up and down) as it flies through the air.

The actual anatomy of a wave is a very detailed area of science that is not unanimously agreed upon. As discussing the anatomy of a wave will not help in your understanding of how these devices work we'll avoid that whole debate here.

So Electromagnetic Radiation is a wave of energy.

 

Waves can be different lengths, different heights and different frequencies. The length of a wave is the space (usually in meters) before the wave repeats itself, such as the space between two crests. The maximum size of a wavelength is the size of the universe itself, and the minimum size is in principle infinitely small as well.

The main thing we're interested in however, is the frequency of these wavelengths. Frequency is the number of wavelengths (waves) per second. To fit more waves per second the wavelength must get smaller (as the wave itself only travels at one speed). Therefore wavelength and frequency are inversely proportional to each other; as one goes up the other goes down and vice versa.

This diagram illustrates waves with different frequencies (and therefore different wavelengths):

 

 

A RADAR 'device' transmits pulses of radio waves which bounce off objects in their path. A small proportion of these transmitted waves are reflected back to the 'device'. These reflected waves will be 'damaged' by the collision. If The object is moving this 'damage' manifests itself as a change in frequency see below:

As the car is moving towards the radio source (GATSO), the reflected waves will be compressed. If the car was moving away from the source, the reflected waves will be spaced out. Ergo, the frequency will go up or down respectively.

As we (should!) know what frequency the wave was when we emitted it from the gun and we can measure the frequency of the returned wave we can calculate the difference.

The difference in frequency is directly proportional to the speed of the object.

 

It is more than possible that more than one signal will bounce around and return to the device. Police devices will always use strongest returned signal. This means if two cars of the same size are targeted the closer vehicle will return the strongest signal.

If however you're in a small car with a larger vehicle behind you (e.g. a lorry) the larger vehicle will return a stronger signal, even though it is further away. This means a lorry which is speeding behind you could return a reading which makes the officer believe you were speeding.

For this reason RADAR devices (fixed and handheld) are not recommended for picking out a single vehicle in a line of traffic - and this is supported in ACPO guidelines. This can be the basis for a legitimate defence against a speeding allegation.

Below is an image depicting the typical coverage for a handheld radar unit based on the size of the target and the strength of the returned signal:

If the above was a real scenario, the strongest returned signal would probably be the lorry. Were you caught beyond these ranges? Were other larger vehicles speeding past you?

 

Street signs, lamp posts, parked cars etc can act like mirrors, reflecting radar signals. Because these objects are stationary they *shouldn't* change the frequency of the wave. This means that if the signal does return to the device then the reflection shouldn't have affected the speed reading.

However, reflection by these objects can mean vehicles traveling outside the typical area of coverage may be detected:

Notice in the above picture how the frequency of the wave changes only after it hits the car.

It's important to remember that every time the signal is reflected it is also weakened. Therefore although this scenario is entirely possible, it is unlikely that the double reflected signal would be the strongest of all returned signals (remember, only the strongest signal is used).

Reflection of RADAR is a totally different problem to the reflection of LIDAR. A reflection in LIDAR increases the distance the wave travels and therefore gives a totally false speed reading, whereas a reflection in RADAR *should* not affect the speed reading at all.

 

RADAR devices are (almost!) never placed in the path of an oncoming vehicle for obvious safety reasons. This means the devices are often positioned at the side of the road and the waves are transmitted at an angle. This reduces the calculation of your speed.

Example:

The typical angle of a speed camera in relation to a car is around 30 degrees. If the true speed of the car was 60mph the speed camera would only measure 51.96mph: cos(30) = 0.866. So 60mph x 0.866 = 51.96mph.

However, the devices will compensate for the cosine factor as part of their overall speed calculation. For fixed units this is relatively easy as once installed they just need to be correctly calibrated. For handheld devices the cosine factor can really throw off the reading, especially when used from a bridge.

 

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