Let’s talk about police radar for a moment. One would hope that highway patrol radar/laser guns are foolproof, with zero margin of error, right? Well, the thing is, even police radar gives false alerts, so it’s the work of the operator to discern the signals.
Before we look at what causes these errors and how to prevent them from occurring, it is important to mention that radar doesn’t generate “false” warnings per se. When the device displays a reading, you can bet that it has identified a signal. But, as stated, the radar can spawn “error” readings, and if the operator fails to notice it, then he/she may issue a ticket wrongly.
The biggest question then becomes; what causes radar errors and most importantly what can an operator do to ensure his/her device’s accuracy?
What follows is a rundown of the most common radar errors as per a manual provided by the Texas Department of Public Safety.
Incorrect Placement of the Antenna Error
This is the most common radar error. You see, a radar/laser gun projects a beam in a straight line. In effect, this means that the beam doesn’t bend. So, when you mount the antenna incorrectly, there’s every probability that the radar will deviate from the target car. In simpler terms, the device may seem to be identifying an oncoming car when in reality it is picking out a different vehicle in the background.
The Car Interference Error
This error occurs when an operator is trying to identify a speeding car in traffic primarily because moving radar is more susceptible to false alerts than stationary radar. For starters, moving radar works by calculating the speed difference between the patrol car and target vehicle. Thus, an interruption with this calculation will give a false reading regarding the acceleration of the target car.
According to the Texas Department of Public Safety, a radar operator should be keener if the difference between the patrol car and the target vehicle is anywhere between five and ten mph. In essence, this means that when the radar reads 71 mph, the target car could be moving at a speed of 61 mph.
The Look-Past Error
The positioning of the antenna may be correct, but the operator should be wary of what is known as the look-past error. This error occurs when radar “looks past” a small reflection in the foreground and picks out a larger reflection in the background. Nonetheless, this error is likely to happen if the operator doesn’t have sufficient training.
Thus, as an operator, never assume that the reflected signal identified by your radar’s antenna belongs to the nearest car to the antenna. It is very possible, as a result of the traffic conditions at present that the closest car to your antenna isn’t necessarily the one returning the strongest signal.
A case study carried out in October 1979 showed a Kustom Signals KR11 radar identifying a Ford 9000 Semi that was 7600 feet away while “looking past” a small sedan until it came to a distance of 1200 feet from the antenna. The Texas instructors advise that an operator targets a vehicle half a mile down the road to reduce this error.
The Double Bounce Error
Any radar operator ought to know that there are other sources of signals such as automatic doors and microwaves. He or she should, therefore, be in a position to differentiate between bad bounce and an ordinary reflection. Also, large trucks cause double reflection thereby increasing the probability of the radar beam bouncing off multiple moving vehicles at a go, thereby increasing the likelihood of false warnings.
The Cosine Error
This error is more or less similar to the interference error, but this time without moving traffic. It occurs when a stationary object close to the road such as a sign or building has a stronger reflection than a flat pavement. Naturally, the radar will use the reflection to estimate the speed of the target vehicle.
The thing is, if the stationary object were in a straight line with the highway patrol radar, then the speed estimate would have been more accurate. But, due to the location of the reflection, the highway patrol’s assessment of the target vehicle speed will be lower.
The disparity is as a result of the cosine angle between the target vehicle and the radar, which further explains the error’s name.
The Radio Interference Error
Latest police radar comes with UHF radio which may cause false readings. The Texas instructors, therefore, suggest that an operator should avoid making radio transmissions when targeting vehicles for over speeding.
The Fan-interference Error
This error occurs when you mount the antenna inside a patrol car as the device tends to identify the pulse caused by fan motors of items such as defrosters, heaters or air conditioners. Nonetheless, these signals die out as the target vehicles come into range.
A police radar/gun can indeed help determine the speed at which a target vehicle is traveling. However, even as this happens, the job to identify an over speeding car solely lies with the highway patrol officer more so because the radar is susceptive to errors.