There are more and more speed controls with radar on the streets and highways of our country. Although it can be argued whether speed in absolute value causes many accidents, it is very true that it greatly aggravates the consequences of one. And in many cases the problem is an inadequate speed, something that unfortunately is not properly educated. On a highway with perfect visibility, exceeding the legal 120 km / h can be totally safe. But in the same place, at night and with heavy rain, it will be a serious risk. To control excesses and, let's not fool ourselves, fill the thirsty coffers of the Administrations, the radars that fine. Because there are also radars that do not fine:they inform and warn if one arrives too fast to a dangerous curve, for example. How do they work and what types are there?
Doppler effect
All radars rely on the "Doppler effect" for their operation. The name may sound strange to you but the effect will be very familiar: when you hear something it moves, it does not sound the same when it approaches as when it moves away . A train, an airplane, a car… when in motion they generate an “apparent” change in frequency in relation to our listening point, which is fixed. When an ambulance approaches, its siren sounds higher, and once it moves away from us it sounds more serious. It is because it travels at a small but appreciable fraction of the speed of sound (about 1,200 km / h).
Light is also emitted in the form of waves, and it also suffers from this same effect. Approaching light is deflected into the blue spectrum (shorter or “compressed” wavelength). When an object moves away, instead, it tends to red (longer wavelength). But appreciating it with light is more complicated, as it travels much faster than the objects we see on a daily basis. The percentage of variation is therefore less, and it is difficult to see it: astronomers observe it and thus know if a distant star is moving in our direction or the other way around.
Traditional radars
"Traditional" radars that have been in use for decades use the Doppler effect with high-frequency radio waves . In Spain the most common band is called Ka, between 26 and 40 Ghz. It is a frequency that is used in communications with satellites, and it is also used by some close-range military radars (planes or tanks). In traffic radars, the wave is sent towards the area where vehicles are passing. When it bounces off one, it arrives with the frequency altered by the movement. The calculation comparing that frequency with the original gives the speed at which that vehicle is moving very precisely.
This system is used in warning radars (those that do not fine and show our speed on a panel). It is also used in fixed and mobile police radars of all kinds until very recently. Both by the DGT, Ertzaina and Mossos de Escuadra, as well as by the municipal police of the town halls. Remember that each road has a person in charge and that entity is the one that decides whether to use radars, and whether or not to fine.
All these radars, being constantly emitting waves, are easy to detect . Carrying a device to do so has been illegal for a few years and exposes you to a fine, but it is possible. The detector receives the waves that bounce off other vehicles that circulate in front of us, and warns us of the presence of radar. There are even devices that generate waves that "cover" those of the radar: they are electronic countermeasures that are even more prohibited than detectors.
Laser Radars
A new generation of radars are lasers, of which there are two types. One, called LIDAR for "Light Detection And Ranging" (detection and measurement by light). These work like the previous ones, due to the Doppler effect. The difference is that instead of radio waves they use a laser beam of infrared (invisible) light . They also emit several pulses in very narrow beams: at 500 meters (very far) the precision is 2.5 meters, the width of a lane. And in addition to all that with current technology they are cheaper than traditional radars. In return, they must be used fixed or with a tripod, and they are the ones that lately are bought by municipalities for example.
A new type of laser radar is used by Pegasus helicopters. As it is not fixed (obviously) it needs very sensitive electronics that precisely define the position and speed of the helicopter. And the laser beam is capable of measuring up to 1 kilometer away, from a height of up to 300 meters above the ground. It measures the exact distance between the helicopter and the vehicle, not its speed like the others (it is a range finder). He does this every three seconds to calculate the speed, and repeats several times to get an average.
The other type of laser "radar" is not a radar itself but another way of measuring speed. They are two beams perpendicular to the road separated about 40 centimeters from each other. When the vehicle crosses one, it starts a timer until it crosses the next one, and its speed is calculated. They are the ones that are seen on one side of the road, with their side openings facing it (not forward) and with two small windows. A camera located on a bridge or nearby furniture will be the one that will take the photo of the detected vehicle, for the corresponding fine.
Section radars
Lasers are undetectable: they only emit when they are measuring. If we have a device that detects an infrared laser (they exist), it will warn us too late. And it is that we will have already measured and, where appropriate, photographed or fined. In the second case (laser perpendicular to the track) there is no way to detect it either. What is possible is to have a GPS notify us of its installation. In addition to GPS warning devices (not detectors), keeping an eye on suspicious parked cars is the only way to detect radars.
To avoid the fine, the only completely safe way is not to drive over the limit, logically. Recently, “section radars” have also started to work, although they are not radars as such. A camera detects the vehicle's license plate when entering a controlled section, and another camera when exiting after several kilometers. That information goes to a data center where speed is calculated (distance divided by time). If it is higher than the legal one, the fine is set in motion. It is possible, during the section, to exceed the maximum speed. If afterwards we circulate below it for the same time, we will not be fined if the average does not pass the limit. Something similar exists in some countries (like Japan) on the highways: if we take too little time between tolls, we will be fined for speeding.