Flutterby™! : GPS vs radar in court 2008-01-10 18:20:15.381575+00

GPS vs radar in court

2008-01-10 18:20:15.381575+00 by Dan Lyke 16 comments

Interesting: Parents put GPS based speed tracking device in teen's car. Teen gets speeding ticket based on radar. Tracking device indicates teen was following speed limit. Both sides fighting hard.

But others, including Petaluma police, question whether the time and expense is worth it for what amounts to an infraction.

If this is the police attitude in the town we're moving to, it sounds like I'm going to have to get involved in politics. "Who cares about the truth, just pay the fine" is not an acceptable attitude from law enforcement.

The parents have a web site in which they're pimping the device they installed, the cynic in me wonders if this isn't just a good way to make teens want the tracking device, on the other hand with some of the abuses I've heard local teens describe about the police in various Marin towns, if I were a teen I think I'd jump all over the availability of another authority tracking my driving.

I had read about this earlier and have had several discussions with police officers online about this. One of them, a lieutenant and traffic accident investigator for the Hamilton County Sheriff's Office agreed with me that GPS report should have provided reasonable doubt of the accuracy of the RADAR gun. I don't know why that wasn't the case.

I also question whether the time and expense is worth it in this case but not from the perspective of the Rude family, from the perspective of the city of Petaluma.

It's all about money, and speeding tickets (or illegal substances arrests) for yuppie teenagers aren't about safety, behavior modification or punishment for being a danger to society. It is about revenue, job preservation and revenue.

Larry, I not only think it isn't worth it from the perspective of the city of Petaluma, I think that, unless there's some really damning physical evidence on the relative accuracy of the two technologies, the Petaluma police department pursuing this is reinforcing a "all cops are corrupt" stereotype that'll lead to more distrust on the part of the teenagers.

I try to be open minded about such things, but it makes me instantly more willing to trust the teenager over the police officer, and that's not something that's good for law enforcement or the communities they serve.

What bothered me most about this was that the accuracy of the time the GPS data was gathered was questioned extensively by the court commissioner but from what I gathered the RADAR data was accepted without question. The accuracy of the speed measurement of the GPS was never questioned. Even if there was some question as to when the speed data was gathered by the GPS it still gives reasonable doubt as to the measurement from the RADAR. With reasonable doubt there should be an acquittal.

I'm not sure how traffic court is run in California or what the difference is between a court commissioner and a judge but I would think that this would give the traffic courts more of a black eye than the police.

I used to use two tuning forks to make sure my radar was calibrated every day at the beginning of my shift. One for 35mph and one for 80mph. I wonder if Petaluma has a similar policy. I also wonder how often the gps device was checked for calibration.

JT, how often did you find your radar out of sync with the tuning forks? I've not found any calibration instructions for my GPS. I know there are some instruments I use that do not require any calibration.

My impression with radar is that it's not calibration, it's all the spare signal stuff going on. With radar it's way easy to get bounces off the car into oncoming traffic, or all sorts of other ghosting or artifacts that don't show up on a single speed display. Heck, anyone who's ever tried to figure out their actual speed from those "your speed" devices (that are either trailered around and parked randomly, at least in several towns in Marin, or are fixed mounted in some high accident areas), knows that depending on what oncoming traffic or other cars are doing your speed can easily jump up and down by 20MPH.

With GPS there are ways to get inaccuracies in speed, but they'd show up obviously in other ways in the data, lots of jitter in the track, things like that, and there's enough other information in that stream that you can look at it for information.

I too would find it amazing if any modern radio devices that are deployed that widely needed tuning. Despite what the new-agers say, crystals don't drift all that much.

There's tons of inaccuracy in GPS. I can go into them if you want. The way GPS is "calibrated" is to collect packets from the satellites in a stationary position for a long time and then compare these packets to packets received by another logging GPS nearby with a known, fixed location (e.g., a Differential GPS base-station). GPS basically sucks in terms of accuracy on all counts.

Arguing that GPS lends "reasonable doubt" is kind of like saying "well, we don't know that JFK actually died from the bullet to the head - maybe he had a stroke 1/2 a second earlier - that means Oswald was only guilty of man-slaugher or perhaps desecrating a corpse".

I'm pretty sure that radar guns used by law enforcement and the officers who use them are certfied as being able to collect legally valid information about the velocity of a vehicle. GPS devices are not certified as such. It's kind of like how surveyors are able to establish the legal boundaries of your property but if you walk outside with your GPS, you might think the line is in the wrong place. Even if you use new a differential GPS and establish your boundaries to 1/1000th of a millimeter, it's still not the legal line. To establish a legal line requires the use of the methods that have been certified for finding the line (and haven't changed in around 150 years) and the methods are applied by someone licensed to do so.

Yes, there are all sorts of inaccuracies on GPS, but most of them involve absolute positional accuracy far more than relative positional accuracy, and when they're averaged out to generate a speed over some moderately large distance (say, a quarter mile), I'd put those inaccuracies up against the issues of reflections, ghosting and target choice of radar any day.

Larry, Sorry, went down to LA for the weekend so I missed this one. My radar was never out of tune, it's a complicated transmitter and antenna inside of the radar guns that policemen and police departments have no access to. You can't calibrate your radar, but you check calibration every day before duty and at the end of duty to make sure it wasn't out of calibration. If the second calibration check fails, you notify the court and they invalidate all of your tickets for that day, even if it's 1 mph off of calibration.

Yeah, I'd expect +-5MPH out of GPS over a moderately short distance, heck, given what I've seen from trying to time measured miles on highway trips I'm pretty sure my car's speedometer isn't +-5MPH accurate (actually I think it usually reads about 3MPH high), but in playing with those road-side "your speed is" radar devices I've easily seen at least +- 20MPH (especially fun to tweak with 'em on a bicycle... 45MPH on a knobby tired mountain bike on the flat!).

And I also admit that part of this is trusting the judgement of the parent. Of the two rat boys

, if one of 'em got a questionable ticket I'd not question the facts and just say that he had it coming, but I'm pretty sure the other one has never intentionally violated any traffic laws, including ones that even I'd consider flaunting.

Oh, in the interests of full disclosure, I'd also add that I've had two moving violations in my life, in both cases I was busted fair and square, and in both cases I had it coming.

(Well, okay, one of 'em, a rolling right turn on a red light, I can try to rationalize a little bit, but I fessed up and said "yeah, you got me".)

Hmmm... By my calculation 45MPH and 30 seconds works out to 1,980 feet, and, yes, that's constant speed, completely ignoring acceleration. Interesting.

I wish the author of this article were a little more precise. Specifically, there's the statement paragraph that says that the distance between samples was 1980 feet and that "Malone would have had to have been traveling at an average speed faster than 45 mph".

1980 feet with 30 seconds between samples would mean that Malone would have been traveling an average speed *exactly* 45 MPH between samples, which would suggest a *maximum* speed faster than 45MPH.

It seems that there's a lot that was probably precise in the court testimony that the author of this article has, for whatever reason, decided to fuzz up and obfuscate.

There is no Freitas Road in Petaluma that I can find. If he was stopped at a light at Frates Road, and then clocked "400 feet west of South McDowell", then he was heading *away* from Infineon Raceway, not toward it.

It's also unclear that Andrew Martinez's written statement that the distance was 1,950 to 2,010 feet, further than one can travel at 45MPH in 30 seconds, would make 45MPH possible.

In fact, as I go back and read the article critically, I'm rather ticked off: This article removes information from the discussion rather than adding to it, and at the very least is dramatically misleading.

Derek Moore and the Press Democrat staff: How about trying again, this time with an article that's actually written with critical thought, and edited?

And Petaluma has prevailed, and I still wish we had a real newspaper so that we could figure out what was really going on in this case.