Figure I. The Electromagnetic Spectrum Image taken from ClumsyDuck.com. Radar guns typically use the frequencies highlighted above, in red.
The Doppler Effect
The Doppler effect is a scientific phenomenon that describes the change in wavelength and frequency of electromagnetic waves according to the relative motion of an observer. In the case of a radar gun, the Doppler effect is the change in frequency of microwaves due to relative motion of an object (e.g. vehicle, baseball, golf ball). In Figure II below, we can assume that the vehicle is emitting a certain frequency of microwaves. When the vehicle is stationary, the same frequency can be detected in front of the car as can be from behind. However, if the vehicle is moving (2nd car) relative to the observer the frequency increases in front of the vehicle and decreases behind the vehicle. The effect is exaggerated as the relative velocity increases (3rd car).
A basic radar gun transmits a predetermined frequency to the object in question.
The waves strike the object and reflect off of the surface. The reflected waves will then be traveling at a different frequency once they return to the radar gun (higher if the object is moving toward the gun, lower if moving away). The difference in the reflected and transmitted frequency is used to calculate the relative speed of the object.
Figure II. Doppler Effect
Common Features of a Basic Radar Gun
III. Basic Radar Gun From Sawicki,
Donald: Police Traffic Radar Handbook
From Sawicki, Donald: Police Traffic Radar Handbook
I. Moving vs. Stationary Modes on Radar Guns
Nearly all models of radar guns have a stationary radar mode, while some have both stationary and moving modes. Stationary radar is used when the radar gun is operated in a stationary position. Radar guns capable of a moving radar mode can be used while a police patrol vehicle is moving. In this mode, the radar gun calculates the speed of the patrol vehicle by measuring the passing ground speed and then uses this patrol vehicle speed to calculate the speed of the vehicle in question. Most of the time, moving radar is used to calculate the speed of opposing traffic but, depending on the model, can also calculate vehicles moving in the same direction as the patrol vehicle.
II. Doppler Audio
Doppler Audio is audible sound transmission that varies depending on multiple factors that helps police officers to determine the following:
· The strength of the returning signal.
When the returning signal is weak, and therefore a greater possibility of inaccuracy, the Doppler audio function returns a faint audible transmission. A strong returning signal results in a clear sound.
· Whether the radar is currently reading the fastest or slowest vehicle.
A higher pitched sound is transmitted when the radar is currently tracking the faster vehicle in the range of the radar gun whereas a lower pitch corresponds to the returning signal from the slower vehicle.
· The presence of interference from other electromagnetic sources.
In the presence of interference, the sound transmission will be garbled. The lack of clarity of the sound indicates that electromagnetic interference may be decreasing the accuracy of the speed reading.
Although some officers end up shutting off the Doppler audio due to its "irritating" noise, the use of the feature along with the interpretation of visual cues of the surroundings is vital to making certain that the readings are both accurate and correspond to the correct vehicle.
Possible Sources of Error with Radar Gun Use
Ideally, a radar gun would be used during a clear day, with only one object in relative motion, and with the transmitted waves being directly in line with the path of travel of the object. However, the likelihood that all of these conditions are met during common use is slim. Therefore, understanding how differing situations affect the reading is crucial to effective use.
I. The Doppler Effect and Cosine Error
One source of error in the reading of the radar gun is called the cosine error. This is created when the transmitted microwaves strike the object at an angle to the travel path. The greater the angle between the wave path and the direction of travel of the object, the greater the error (resulting in a slower reading than the actual speed). This seems obvious if the extreme case is considered, where the transmitted wave and the object path are perpendicular. In this case, the frequency change would be zero and therefore the relative speed in that perpendicular direction would read zero.
II. The Effect of Traffic on Radar Gun Use
A common circumstance that is faced when using a radar gun to assess the speed of vehicles is the presence of multiple vehicles. Careless use of a radar gun could register a speed for the wrong vehicle as the transmitted waves travel in a linear path. Many radar guns compensate for this error by displaying the speed of the strongest signal vehicle. With everything else equal, a larger vehicle will register a stronger signal than a smaller vehicle, and the same can be said of the proximity of the vehicles (closer would have a stronger return). Another way that the radar gun can help with this error is to show the speed of the fastest vehicle. In both cases, however, visual recognition of which vehicle is going fastest or will return the strongest signal is necessary to correctly use these features.
III. Atmospheric Conditions and Radar Gun Range
Atmospheric conditions have a different effect on radar gun use. Environmental conditions such as temperature, humidity, and fog do not affect the accuracy of the radar gun but instead affects the range of the radar gun. Depending on the model of radar gun, ranges can extend from a mile (under good conditions) to less than 30 meters (under poor conditions). To illustrate this point, many police radar guns have switched from the Ka band, a subset of the K band, due to the frequencies corresponding to the absorption frequencies of water. This correlation between the emitted and absorbed frequencies drastically decreased the radar gun range in rainy weather.
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