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Radar / Lidar - Accuracy In Rain&Snow
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PostPosted: Mon Jun 01, 2009 3:31 am 
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Curious could someone post manuals up for this?

Are these devices tested to read speeds accurately in rain or snow?

If one received a speeding ticket in rain or snow what kind of chance would they receive on fighting it?

I am sure these units are all properly tested, calibrated to be accurate with in X km/hr but does this apply for wet weather aswell?

I remember watching a video on youtube where an Australian traffic cop was driving in the rain showing that his mounted K band radar was getting really screwy and weird ratings when measuring his speed while driving.

Would police still do speed enforcement in rain and if So would they add some room for error?


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PostPosted: Mon Jun 01, 2009 9:08 am 
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Location: In YOUR rearview mirror!
RADAR / LIDAR are completely accurate in rain/snow.

The only difference is that the range is reduced, so the target vehicle is actually closer.

Some LIDAR have a weather mode for the operator to switch to in rain/snow.

WHY are people driving fast in those conditions anyway, they need to be hammered with tickets. They do not know where the grooves are in the road, where a tree line makes a barrier and wind forms ice.....next thing...WooHoo ....errrrrrrkkkkkkk.......BANG!

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Above is merely a suggestion/thought and in no way constitutes legal advice or views of my employer. www.OHTA.ca


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PostPosted: Mon Jun 01, 2009 12:07 pm 
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Bear, do you do sound effects for movies, cause dude i was like right there...... :D

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PostPosted: Mon Jun 01, 2009 12:42 pm 
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:lol: :lol:

Do we have a "too fast for conditions" charge? Just careless driving?

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PostPosted: Mon Jun 01, 2009 1:09 pm 
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In precipitation, radar signals are subject to signal attenuation - the simplest definition meaning it loses its range. Radar works by sending out a radio signal from the antenna. When it strikes an object, the portion of the signal that hits the object is reflected back to the antenna. Since the radar signal travels at a fixed speed (speed of light), it measures how long it took for the signal to go out and be returned, so it has the distance of the object. A speed-measuring radar uses Doppler principle - as the object moves closer, essentially the frequency of the return changes. The difference in frequency is measured and this gives a speed.

Now for a bit of somewhat irrelevant info:

Radar beams will bounce off anything, precipitation is one of them. As the precipitation gets heavier, more of the radar beam will be unable to penetrate the rain/snow/whatever and the effective range of the radar will be reduced. The size of the precipitation does matter - bigger rain drops will give a bigger return.

Weather radar actually measures how much of the beam is returned and at what distance to paint weather cells. When more of the signal is reflected back, it means more precipitation - therefore, stronger weather. The type of precipitation also matters. In terms of reflectivity, the strongest type of returns come from (in order of strongest to weakest) wet hail, rain, wet snow, dry hail, and then dry snow. So the radar will have a longer range in dry snow than in rain.

As for causing screwy things, if there was a substantial amount of wind and rain, the radar may pick up some Doppler shift of the rain drops, but that's about it. Weather Doppler radar is specifically built to look for it, but police radar isn't, and it doesn't have as much sensitivity. The only time I've known rainfall to cause weird things with radar equipment is if the rain got into the antenna, there was ice on the radome or the operator wasn't using the equipment properly.


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