Do radar guns actually pick out vehicles from ground clutter and find the speed that way, like military aircraft radar? I always thought when used from a moving patrol car the radar found the closing speed of the target car and then subtracted off the patrol car's velocity from its speedometer. If it's the latter, how does it possibly work when shot at the side of the target car? I had a cop get me today from a minimum of a 30º angle across an interstate. The speed registered by the radar gun should be significantly lower than the speed I was traveling, no? If so how does the officer write me up for an exact speed? It was either K or Ka, pretty sure on K but can't remember for sure.
How does police radar work at large angles?
- Thread starter Sukhoi
- Start date
Calculus my friend...
He tagged you at 30 degrees right? So that means its a 30-60-90 right triangle.
so x^2 + y^2 = s^2 where s is the hypotenuse.
If you know ds/dt and you know dy/dt (speed of the cop car), you can easily work out the speed of your car =). I remember in high school my calculus teacher told us a story like that. He told us he got tagged like that and he did some measurements to show that ds/dt will be faster than he's actually goign because it's at an angle, and he used that case in front of a judge.
Crap, I think you need a few more piece sof information because wehn you diffrentiate you do implicit differentiation, and so you need info like distance and what not, but that's not too hard to obtain.
drinkmorejava
Diamond Member
- Jun 24, 2004
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Using calc, if the gun is positioned 30 degrees off of the line of travel we can make a sample calculation.
Using 30 degrees, the officer will be 61(y) feet from the side of the road and the car 100(x) feet down the road. Say you are traveling at 35m/s in the x direction. Doing a little implicit differentiation, we get that the distance between you and officer is changing at 29.88 m/s. Taking a limit however, the further you get away, it will read closer to the actual speed. At 300ft, or approximately 1.7 seconds later, it would read 34.3m/s. Sparing you all the math, assuming a constant speed, by taking two samples at say, one second apart, the angle can be determined from the rate of change of the speeds, and thus, the actual speed can be determined.
Using 30 degrees, the officer will be 61(y) feet from the side of the road and the car 100(x) feet down the road. Say you are traveling at 35m/s in the x direction. Doing a little implicit differentiation, we get that the distance between you and officer is changing at 29.88 m/s. Taking a limit however, the further you get away, it will read closer to the actual speed. At 300ft, or approximately 1.7 seconds later, it would read 34.3m/s. Sparing you all the math, assuming a constant speed, by taking two samples at say, one second apart, the angle can be determined from the rate of change of the speeds, and thus, the actual speed can be determined.
I understand it can be figured out with trig and calc, but isn't the angle measurement still needed? Unless the cop has a protractor on his dash, or the radar gun physically swivels and has a built-in device to measure angle off the centerline of the cruiser, how can the cop figure out the angle?
drinkmorejava
Diamond Member
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I never even thought of multiple samples to determine the angle, but I think there is still something missing with that explanation. Unless the samples are taken very close together (milliseconds, and quite possible with radar probably), the angle will be changing rapidly as I approach the cruiser.
It's really nice that you guys know how to use trigonometry. However, you guys are wrong. Radar guns cannot determine direction. Because of this, the radar gun can only guess the maximum speed the object is going. If the car is going by at an angle, the speed you read will always be less.
http://en.wikipedia.org/wiki/Radar_gun
Everything I've found on the internet seems to confirm this. Apparently the answer, OP, is that they can't adjust for the angle.
http://en.wikipedia.org/wiki/Radar_gun
Everything I've found on the internet seems to confirm this. Apparently the answer, OP, is that they can't adjust for the angle.
:laugh: Based on what he wrote the ticket for, the cop totally guessed my speed. Explains why he tried to pace me for a little bit before he pulled me over, and then kept trying to get me to tell him how fast I was going.
Wow found a great site off Wiki - http://www.copradar.com/preview/content.html
Yup, I did some Googling myself and found that the angle (called the Cosine Effect) can't be corrected for.
The problem with this explanation is that it assumes constant speed. That assumption is not possible practically, therefore not true.
notposting
Diamond Member
- Jul 22, 2005
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I was all set to reply....but damn, that necro
I didn't even notice this time. Usually a bunch of old-timers who aren't here, half the thread posters being banned, and the distinct feeling I've read this 4 addresses ago clues me in.
edit: a bunch of defunct Fusetalk emoticon tags are a good clue too. :beer:
Pulsar
Diamond Member
- Mar 3, 2003
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This information is out of date. Newer radar guns contain a multi-axis gyro that is accurate to the 100th of a degree. That allows them to take shots at the car and get accurate speed measurements even when the gun is at an angle. I'm sure most places don't have them yet, but don't assume.
boomhower
Diamond Member
- Sep 13, 2007
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The wider the angle to the lower the speed read is. Max recommended angle in my state is 20 degrees. The error is always in favor of the motorist. There is only one situation where the radar can produce a higher speed than the motorist is doing, and it's a rare bird to say the least.
Radar cannot pick out a single car. It returns the strongest signal. So if a semi is right behind you it will read the semi over a sedan even if the sedan is going faster. Laser on the other hand can pick out a single car with great accuracy over long distances.
edit
And it is written as clocked. No estimate of what the actual speed was, motorist gets the benefit of the slower speed do to the angle. No requirement in my state nor most states to show the radar unit to the motorist.
Radar cannot pick out a single car. It returns the strongest signal. So if a semi is right behind you it will read the semi over a sedan even if the sedan is going faster. Laser on the other hand can pick out a single car with great accuracy over long distances.
edit
And it is written as clocked. No estimate of what the actual speed was, motorist gets the benefit of the slower speed do to the angle. No requirement in my state nor most states to show the radar unit to the motorist.
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DrPizza
Administrator Elite Member Goat Whisperer
I think the mathematics would be fairly simple to determine your speed, even at an angle. Listen to the train in this video: https://www.youtube.com/watch?v=O5rqMPdQMQ8 Notice that as it passes in front of the person, it doesn't go instantly from a high frequency to a low frequency, but rather, there's a transition from higher to lower frequency. When the train's velocity is perpendicular to the straight line distance to the train, the sound heard is the actual frequency. While you're considering determining the speed of the train from the difference in perceived frequency, you could also determine the speed if you know the distance to the tracks, and the rate of change of the frequency from high to low. With a radar gun, distance would be able to be determined fairly easily, using the signal when the return signal has the same frequency as the sent signal.
This is just a guess.
Another guess: reflections, and determine the speed from the signal with the greatest frequency shift. So, instead of the signal going straight from the radar gun to the car and back, any reflections resulting in a signal nearly parallel to the road are going to result in a doppler shift that indicates the speed of the vehicle.
And best guess: you're naive in thinking that he didn't have his radar on until you were right next to him. He might have gotten you 300 yards down the road. When you noticed him is not necessarily when he noticed you.
This is just a guess.
Another guess: reflections, and determine the speed from the signal with the greatest frequency shift. So, instead of the signal going straight from the radar gun to the car and back, any reflections resulting in a signal nearly parallel to the road are going to result in a doppler shift that indicates the speed of the vehicle.
And best guess: you're naive in thinking that he didn't have his radar on until you were right next to him. He might have gotten you 300 yards down the road. When you noticed him is not necessarily when he noticed you.
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IHateMyJob2004
Lifer
- Sep 29, 2004
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