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Radar Wins Speed Test Against GPS


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Radar Wins Speed Test Against GPS

 

A Sonoma County, Calif., judge has ruled a speeding case supported by an officer's radar cannot be thrown out. The speeder's GPS system allegedly recorded a speed contradictory to the radar reading, but the judge ruled radar more reliable.

 

Roger Rude, a retired Sonoma County sheriff's lieutenant, brought the case to fight a ticket his stepson Shaun Malone received for going 62 mph in a 45 mph zone. Rude had installed a GPS system in the car to track his stepson's speeds. Rude alleged Malone never was speeding based on the GPS tracker.

 

The court ruled against the GPS data, holding that radar data is more reliable.

 

JB Harper, Radar Systems Engineer for Decatur Electronics, the manufacturer of the radar that clocked Malone, said radar is a time-tested speed assessment tool.

 

"Decatur radar has been catching speeders for more than 50 years," he said. "Radar reads a speed at the speed of light rather than calculating geographic and time differences between two separate readings as is done in a GPS system."

 

Officials with the Petaluma Police Department, which issued the ticket, agreed. Petaluma Police Capt. Dave Sears said GPS is a valuable tool but is not as accurate for tracking speed as radar.

 

For more information about Decatur, visit www.DecaturRadar.com.

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..."Decatur radar has been catching speeders for more than 50 years," he said. "Radar reads a speed at the speed of light rather than calculating geographic and time differences between two separate readings as is done in a GPS system."

 

Officials with the Petaluma Police Department, which issued the ticket, agreed. Petaluma Police Capt. Dave Sears said GPS is a valuable tool but is not as accurate for tracking speed as radar.

 

For more information about Decatur, visit www.DecaturRadar.com.

It would be nice if Radar read speed. Alas, just like the GPS it has to use information gathered to calculate a speed. The method of calculation is different, and subject to it's own error. Overall I'd trust radar for a calc over GPS but I'd trust GPS over my own Odometer. GPS is accurate enough to catch a miscalibrated or screwed up radar. So are mileposts and stopwatches for that matter.

 

It's odd that radar would be rulled as more reliable in a way that appeas to negate other technologies. Maybe it is, but in any one case it can be wrong. That's why they do get checked and calibrated. Oh well.

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I can't dispute that generally speaking Doppler radar is more accurate than GPS at determining vehicle speeds, in my opinion the point I believe the judge was making here is that an accepted technology used by law enforcement to enforce the law has legal precedent over other technologies.

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Radar Wins Speed Test Against GPS

 

A Sonoma County, Calif., judge has ruled a speeding case supported by an officer's radar cannot be thrown out. The speeder's GPS system allegedly recorded a speed contradictory to the radar reading, but the judge ruled radar more reliable.

 

Roger Rude, a retired Sonoma County sheriff's lieutenant, brought the case to fight a ticket his stepson Shaun Malone received for going 62 mph in a 45 mph zone. Rude had installed a GPS system in the car to track his stepson's speeds. Rude alleged Malone never was speeding based on the GPS tracker.

 

The court ruled against the GPS data, holding that radar data is more reliable.

 

JB Harper, Radar Systems Engineer for Decatur Electronics, the manufacturer of the radar that clocked Malone, said radar is a time-tested speed assessment tool.

 

"Decatur radar has been catching speeders for more than 50 years," he said. "Radar reads a speed at the speed of light rather than calculating geographic and time differences between two separate readings as is done in a GPS system."

 

Officials with the Petaluma Police Department, which issued the ticket, agreed. Petaluma Police Capt. Dave Sears said GPS is a valuable tool but is not as accurate for tracking speed as radar.

 

For more information about Decatur, visit www.DecaturRadar.com.

I can't say I'm surprised, but it does suck. From what I've heard, the most common problem with radar is misidentification (pulling over the wrong car for speeding). The GPS evidence would seem to provide at least reasonable doubt that the car was correctly identified as the speeder.

 

I commute through that area, and my wife just got a speeding ticket there on Monday (grumble punk motorcycle cop :huh: ). I had my GPS with us, but obviously it wouldn't have helped (she actually was speeding but just for an instant).

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Judging from the news report in the link, the judge just ruled that the case could not be thrown out. If there was a trial, the defendant would have the opportunity to bring in his own expert(s) and explain how the GPS system works and attempt to explain how the radar could have been wrong in this case and why the GPSr was right. Given that such a trial would be far more expensive than the fine or driving school, though, I suspect that this is the end of the matter (unless someone wants to spend money on making a point)

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If it has to do with cops and courts and traffic violations...the main thing is "follow the money"! It's ALL about fines. Of course the judge will rule in favor of the cops and radar, because that generates the money.

 

You could set up Olympics-grade timing systems with an accuracy of 1/100 mph...and they'd STILL rule in favor of whatever generates a fine payment.

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Everyone who geocaches knows the gps does not give you a very accurate position. It can be off thirty feet. What mystifies me is how people think that you can somehow take the distance between two inaccurate points divide by time and then magically come with am accurate number for speed. The radar is infinitely better than a gps for instantaneous speed. The gps can average speed if the points are far enough apart. 60mph = 88 feet/second so clearly you have to average for much much more than a second at 60 mph.

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Everyone who geocaches knows the gps does not give you a very accurate position. It can be off thirty feet. What mystifies me is how people think that you can somehow take the distance between two inaccurate points divide by time and then magically come with am accurate number for speed. The radar is infinitely better than a gps for instantaneous speed. The gps can average speed if the points are far enough apart. 60mph = 88 feet/second so clearly you have to average for much much more than a second at 60 mph.

Except that's not how a GPS determines speed. Like RADAR, the GPS also calculates the signal's Doppler shift. Sounds like the judge didn't have the full story.

 

"GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.

 

Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on."

 

- NAVSTAR GPS User Equipment Introduction document Section 3.7

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Doppler radar units calculate Doppler shift based on a radio signal projected and received from the device's antenna, whereas a GPS uses satellite X Y coordinates. The accuracy of the Doppler shift speed calculations of a consumer grade GPS are limited to the coordinate error/accuracy range of the GPS unit. I don't mean to imply a speeding ticket given by a policeman is always warranted or that a GPS speed is always incorrect but, as has already been said, the system is swayed in favor of enforcement.

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These are the specs for a Garmin 60CSx:

GPS accuracy:

Position: <10 meters, typical

Velocity: .05 meter/sec steady state

Now if my calculations are correct the velicity is very accurate (0.180km/h or 0.11mph). I'd like to see a radar be that accurate!

 

This accuracy is given when you are "steady" so not moving ! :P:laughing:

 

Off course no, but it means the speed has to be steady, so if you are in an acceleration/deceleration phase it will not be that accurate !

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Except that's not how a GPS determines speed. Like RADAR, the GPS also calculates the signal's Doppler shift. Sounds like the judge didn't have the full story.
Do you have a reference? I have suspected that newer gps's use some new form of speed measurement, but could never verify this. My old Garmin never showed zero speed.
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Does not sound like the defendant called in experts on the GPS system to support his position. Then the judge would only have information on the RADAR system. Their for he ruled in favor of the RADAR system (I am being very general here) because he knows what that system does. This sounds like it was not even a trial just a hearing to determine if it is worth it or not. So lets see what happens at trial (if their is one).

 

The RADAR system tracks the Doppler change from the speed of the target. It generates the speed from the freq. offset of the return pulse. (big thing is what vehicle is it tracking?)

 

Many GPSr give speed accuracy as .1 MPH. But this is updated at a 1/sec rate.

 

On another note:

 

I had a friend coming back from a family get together when he got stop via SP for speeding. The trooper said he was doing 93 MPH in a 65. My friend told the trooper that he was doing 70 MPH and not 93 MPH but the Semi passing him was really fast and most likely doing the 93 MPH. Trooper told him to take it to court. He did. Later in the District Justice He told what happen and said he also had a GPSr w/accuracy of .1 MPH saying he was doing 70 MPH and he would take and pay for a 70 MPH ticket. That he was being passed by a semi doing 90+ MPH when the trooper came after him. He then told them that the RADAR picks the biggest target and that was the semi. The DJ asked him if he was an expert in RADAR. He said no but he has a friend who is and is their. His friend at the time works on fixing RADAR systems. My friend also produced witness that their was a semi moving at a very hi rate of speed. The DJ then dismissed the the information provided said he was not going 93MPH but a semi was and that, that was what the RADAR had picked up on. The DJ did order the trooper to write a warning for 70 MPH.

 

So sometimes it does work the way it is suppose to.

 

kf4oox - Paul

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And just remember that a RADAR will display the speed that you are going 'directly' towards that RADAR. Some have a correction for the angle of the road vs the line of sight of the RADAR gun. But you have to trust that the cop entered it right and estimated the angle right...

 

GPS's is just plain crappy with speed and also with the odometer.

But just think of it, your GPS takes a reading a second, each of those readings has some godawful precision. The speed is calculated as the distance difference in that one seconds. You would think this has more of an effect at higher speeds but it's just the opposite. Just look at how fast you walk, at a steady pace you bounce around a bit, but on the road it's pretty steady. That's because, at high speeds lots of GPS's don't calculate where you are based on info from the satellites, it's based on the prediction of your motion. Have you never seen your arrow fly off the road at a slight turn?

 

As for the odometer, have you never noticed a 100 mile trip showing up as 92 miles. Sure your tires might be low by a couple pounds and that will have an effect. But when that same 100 mile trip shows up differently on the GPS odometer each time you drive it you know not to trust that a bit.

 

edited to add:

Now as to trusting a GPS, unless you surrender the GPS to the cop on the spot then you can finagle any of the data in the GPS before heading to court. Tracks are manipulatable with the right software and can be re-uploaded. They can't believe anything you bring in now can they?

Edited by trainlove
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GPS's is just plain crappy with speed and also with the odometer.

Did you read the post I made further up with the Garmin specs? GPS may be inaccurate for distance / odometer, however, they are very accurate for velocity.

 

Yes, its true that co-ordinate position is generally only good to 3m (10ft) and yes its also true that GPS calculates velocity by calculating the time between 2 co-ordinates. However, as I understand it, whilst the GPS points are inaccurate, the error in the co-ordinate position is very repeatable. This why GPSs are so good at measuring velocity. It doesn't matter that the GPS is 3m (10ft) off in its co-ordinates as long as the next point is off by the same amount in the same direction. This is how Garmin are able to claim 0.18km/h (0.11mph) speed accuracy.

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GPS's is just plain crappy with speed and also with the odometer.

Did you read the post I made further up with the Garmin specs? GPS may be inaccurate for distance / odometer, however, they are very accurate for velocity.

 

Yes, its true that co-ordinate position is generally only good to 3m (10ft) and yes its also true that GPS calculates velocity by calculating the time between 2 co-ordinates. However, as I understand it, whilst the GPS points are inaccurate, the error in the co-ordinate position is very repeatable. This why GPSs are so good at measuring velocity. It doesn't matter that the GPS is 3m (10ft) off in its co-ordinates as long as the next point is off by the same amount in the same direction. This is how Garmin are able to claim 0.18km/h (0.11mph) speed accuracy.

No, GPS calculates velocity from doppler shift, not from the difference between two points. If you don't believe me, I posted a few months ago with links providing evidence for this.

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"Radar reads a speed at the speed of light ....."

 

Did the technology behind radar magically change recently?

 

That caught me by surprise too, but I got to thinking that perhaps the officer was using a laser system. Perhaps they are simply using RADAR as a generic term... very badly though.

 

Weren't there some cases of rental companies placing fines on people for speeding in rental cars that had GPS speed tracking systems? I believe that would be a legal precedent of GPS use for speed in the law.

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"Radar reads a speed at the speed of light ....."

 

Did the technology behind radar magically change recently?

 

Weren't there some cases of rental companies placing fines on people for speeding in rental cars that had GPS speed tracking systems? I believe that would be a legal precedent of GPS use for speed in the law.

 

No, it wouldn't. What a private company does as a result of a signed contract would not set a legal precedent for a court proceeding.

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That caught me by surprise too, but I got to thinking that perhaps the officer was using a laser system. Perhaps they are simply using RADAR as a generic term... very badly though.

 

Weren't there some cases of rental companies placing fines on people for speeding in rental cars that had GPS speed tracking systems? I believe that would be a legal precedent of GPS use for speed in the law.

 

Radar travels at the speed of light.....

 

Radio "waves" and light "waves" are the same thing at different frequencies.....

 

http://en.wikipedia.org/wiki/Electromagnetic_radiation

Edited by Red90
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Everyone who geocaches knows the gps does not give you a very accurate position. It can be off thirty feet. What mystifies me is how people think that you can somehow take the distance between two inaccurate points divide by time and then magically come with am accurate number for speed. The radar is infinitely better than a gps for instantaneous speed. The gps can average speed if the points are far enough apart. 60mph = 88 feet/second so clearly you have to average for much much more than a second at 60 mph.

It's a function of the type of error. If it's high precision (systematic error - it's off 30' but always the same 30') the GPS can calculate speed better than it can show you your position. If it's random error then it's like you are thinking.

 

I don't know enough about the various GPS errors to say for sure, but it looks like someone else has said that GPS error tends towards systematic.

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...But just think of it, your GPS takes a reading a second, each of those readings has some godawful precision....

 

You are thinking accuracy.

Accuracy. If you are shooting at a target accuracy is hitting the bullseye on average but actually hitting the bullseye is optional. Your target can resemble a shotgun blast.

Precision. You may never hit the bullsye but you pegged every bullet in a 1" group.

 

With Precision you can be wrong about your location every time, but your GPS will give you great speed readings. That's because the measurment of distance traveled is both precice and accurate.

With accuracy, the faster you go the more accurate your speed reading gets. This is because the error matters less and less the more distance you travel between readings.

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