Technology Problems

http://users.cyberbeach.net/~mtrenout/errors.html accessed on 4 March 2003

Radar is a wonderful technology. It allows tens of thousands of planes to fly around the world every day without hitting each other. It also makes for some pretty cool pictures on the 5 o”clock news. Keeping that in mind a with cars and stereos, not all radar equipment is created equally. Police radar is definitely not in the same league as the radar in a modern jet fighter. The typical police radar units cost roughly around $2,000.00 as opposed to $1.1 million for the fighter”s radar.

What”s the Difference?

Radar works on the Doppler principal of frequency shift. The same way a train whistle changes pitch as it approaches then passes by you. By measuring the change in pitch the speed of the object can be measured. To cut costs, police radar uses comparatively inexpensive components and its operation is kept as simple as possible. To that end police radar share the following characteristics:
1- Low power emitter to keep the operator and the general public safe from microwave emissions.
2- Displays only the strongest reflected signal.
3- Relies on the operator”s skill to determine the speed and error correction.
4- None of the units in service follow a government standard for reliability or performance. i.e. Your taking the word of the manufacturer, that the unit performs as specified.

Since the emitter uses low power it stands to reason, that the reflected radar energy that returns to the gun is even lower. As a result in order for the gun to do anything with the returned signal, it must be amplified and processed, to register on the display. The problem is that the electronics aren”t very well filtered and radar occupies a very narrow spectrum in the RF band. As a result erroneous readouts can and are caused by such things as remote garage door openers, cellular phones, CB radios and even the heater/ventilation fan in the patrol car. Anyone that owns a radar detector can tell you how easy they are to set off if the filter is not turned on. The radar unit is just as susceptible as a detector, since they are listening for the exactly same thing. If you have a false reading with a detector, you get a signal from the unit. If you get the same error with a gun, you get a ticket. Aircraft radar can plot the position of a multitude of other aircraft. It can do that by modulating the signal to track one aircraft with radar energy at a certain frequency.

Police radar can not discriminate between targets. It will only read the strongest return signal it gets. The operator can only assume that the vehicle in the line of sight, is traveling at the speed displayed. If the vehicle has a low radar signature the reflection can come from a vehicle behind it or a building. One characteristic of radar energy is its ability to be reflected. Since its so easily reflected, to be used accurately there should be nothing in the way of the beam, but the vehicle in question. If not the you will get reflections and ghosts floating around. To get an idea of this principal, throw a superball as hard as you can in an enclosed room and see how many times the same ball passes by you!

There are a gaggle of reasons that can result in erroneous readings. Apart from the technology, operator error can seriously affect the accuracy of the unit. Without fail they all have to do with incorrect positioning of the radar beam in relation to the target. A complete listing of these errors can be found at the National Motorists Association”s web site. There is a link to their site on the links page. For a stationary photo radar van one of the most important of these errors is the cosine error. The Gastonomer units used in Ontario, had its beam offset at a 22 degree angle. The unit would correct the returned signal to take into account the frequency shift caused by the 22 deg angle. If the vehicle is not parked exactly parallel to the target vehicles it will affect the accuracy of the unit since the unit blindly assumes that the angle is always 22 degrees.

Lets look at how a simple 10 degree shift in alignment can affect the reading, either way. We will assume that the vehicle in question is traveling at 100km/h, the cosine of 22 degrees is .9272 and the photo radar unit will add 7.28 km/h to get the correct reading for speed.
1) 100 X .9272 = 92.72km/h + 7.28km/h = Readout is 100km/h
If the van is parked with the front of the vehicle parked 10 degrees away from the direction of the road, the angle would increase to 32 degrees. The cosine of 32 degrees is .8480
2) 100 X .8480 = 84.80km/h + 7.28km/h = Readout is 92.72km/h
You get a break and your speed is lowered by 7.92km/h
Now the opposite will happen if the front of the vehicle is pointing towards the road at a 10 degrees angle. The cosine of 12 degrees is .9781.
3) 100 X .9781 = 97.81km/h + 7.28km/h = Readout is 105.09km/h
Your vehicle has 5.09km/h added to the actual speed and if they are operating at zero tolerance, you get a ticket.