GPS Used As Defence In Radar Speeding Case

James Thigpen writes "There is an article over at Ars Technica about an accused speeder contesting his speeding ticket based on his car's built-in GPS system's records. According to the article his car says he was going slower than the radar gun clocked him at. Contesting a ticket based on GPS data has never before been tested in court."

used in Taiwan

[xldyniac]

GPS data was actually used recently in taiwan to prove a man's innocence. A truck driver A went into an accident with a motorcyclist B. A stayed and helped B up, and even paid cash. B said he's fine, so A drove off, only later to recevie a notice that B has filed a hit and run case against him. The court found A not guilty since the gps data showed that A stayed at the site for more than 15 mins.

Radar and GPS were in check in California

[CaptainAx]

When we were on vacation in CA, we were stopped for speeding on highway 299 and had the GPS running. I told it to stop tracking the rest of the trip so I can get the data later. When I looked at it, it was dead on what the officer clocked us at so I think this person has a good case.

Testing in UK court case and GPS won

[Anonymous Coward]

This was tested in a uk court case and the ticket was cancelled

http://news.bbc.co.uk/1/hi/england/wear/7033353.stm [bbc.co.uk]

Are you serious?

[HouseArrest420]

Contesting a ticket based on GPS data has never before been tested in court."

Yes it has...read up. The success rate, though, is the same as the rest of the cases. The majority of whom only get off because the cop that pulled them over never show up in court.

Speed = Distance / Time

[imstanny]

From my understanding, and the contention of the officer, the GPS logs average speed. Which means that during a short period of time, the defendant could have greatly exceeded the speed limit (and was clocked by the officer at that time), while the average speed was far lower than that. In which case, both the cop and the defendant are correct, and the cop is till valid in giving the ticket...

Re:Speed = Distance / Time

[ls -la]

Two things:

1. What is the time the GPS device averages over? On the devices I've seen it updates about every second. Unless you have a REALLY nice car you're not going to go from 65 to 90 and back down for long enough to average 65 over that kind of time.

2. At least one state (MA) and perhaps others have laws that require your AVERAGE speed over some distance (I believe MA is 1/4 mi) to be over the limit for a speeding ticket.

Re:Speed = Distance / Time

[DJGreg]

First, when the GPS unit itself calculates the speed, it records your instantaneous velocity, not an average. It calculates this using the doppler shift present in the GPS signals picked up by the unit, not from how far the unit has travelled.

Second, even the cheapest GPS units I've seen update at least once per second.

Third, the delay, or time offsets of the arrivals of signals from the GPS sattelites are exactly how a GPS unit calculates it's velocity and position. Once a GPS reciever has got a "lock" on its location (usually within first minute or five from startup), the position and velocity calculations it records are for the exact moment they are reported, not for some time in the past.

This link [hamradio.si] discusses alot of the theory behind how GPS works.

Re:This could only be the first step

[smallfries]

It reminded me of this case [bbc.co.uk] from earlier this month. The inventor in the story was testing his new device when he was clocked by a speed camera. In the court case his GPS logs were used as evidence that he was 12mph slower than the speed gun recorded. He may have had a motive for pursuing it through to a court case as he is starting a company to comercialise the device.

Re:This could only be the first step

[Anonymous Coward]

Handheld radar guns have a proven track record of reading 2 consecutive pulses reflecting off two different objects as a completely random speed. If that random within about 20% of what speed you're doing, the cop won't question it, and assume it was an accurate reading.

Re:Video Evidence

[praksys]

If you destroy evidence of a crime you can be busted for obstruction of justice. IIRC the prosecution doesn't even need to prove that a crime was committed - just that you destroyed stuff that you thought might be used as evidence.

Re:This could only be the first step

[Anonymous Coward]

This case is special though because this is a kid that had a monitoring system so that his parents could track him. It was designed to not be able to be hacked by someone driving the car. His dad is an ex-sheriff too and has the same story.

This is an old story by the way. I can't remember when I first read it but it must have been months ago.

Re:GPS more accurate than radar?

[canajin56]

There was a CNN report a while back, they tried out these police radar guns. Clocked a tree right in front of them going 17 MPH. So sounds like your GPS is way more accurate than the infallible radar gun used to convict ;)

GPS and such.

[Anonymous Coward]

I use a GPS reciever. Depending the software package I am using I will get various speeds. Normally it is pretty good, but as it has to calculate speed and approximate position based on the data it is recieving. When the unit has road informations it will try to map the data (which is close, but not really close, depending on conditions) to the road. Depending on polling interval changes in direction can have a huge affect (set your interval to 30 seconds or a minute and make a turn or anything that is not linear).

I have a portable unit and software that tracks and logs speed. I do not recall offhand what the polling interval is though. I think at best I can get every few seconds (it may be less--it is average speed and delayed, so it will be how fast I was just going.) If you have a car that can accelerate very quickly try going from a dead stop to high speed as fast as possible. It will not mirror your speedometer. It will follow it. It is taking average speeds after you have begun moving. So when your speed is 30mph it is still calculating using 0+30/time. Fluctuations in speed cause pretty big changes. Again with the vehicle with a transmission designed for quick acceleration: accelerate as quickly as possible to 100mph and then bring the vehicle to a complete stop (disengage ABS if present). The numbers will not be anywhere near the actual speeds. Decrease your polling by 5 seconds and repeat. Do that a few times (it probably is not good for the car.) Better still compare these number with those from RADAR and LIDAR systems.

The gun is only calculating based on really one thing (doppler effect.) Angles and shit can affect numbers, but really it is measuring the speed. The polling interval is still much, much smaller. If it is lidar it could be 1/1000 of a second.

Either way we are looking at an average speed, but the interval during which it is calculated would vary and have a huge effect on the numbers.

This is all before we take into account the security of the data. So, yeah, maybe he did and maybe he did not. But GPS is not as amazing as people think. I guess RADAR is not either. But do not just be hating because you is worried about the man putting you down.

VORAD units have been used this way

[Animats]

Eaton VORAD units, which are a phased-array anti-collision radar for trucks, have been used to provide evidence in favor of the truck driver. [etrucker.com] The VORAD units track individual car-sized targets, and provide range, range rate, and azimuth. Range and range rate are quite good; azimuth isn't that accurate. The control unit keeps track of recent events ten minutes before a collision, and also has speed info available. The latest versions can interface with GPS and other vehicle systems. This allows detailed accident reconstruction.

It's most useful where an accident resulted when someone drove in front of a truck. [etrucker.com] The VORAD record shows not just what the VORAD-equipped vehicle was doing, but what the other vehicles were doing.

Actually, I know 2 ppl who have contested; lost

[WindBourne]

They had GPS and contested. In Wyoming in one case, and Utah in the other. In both cases, the judge sided with the law. What is needed to prove this is something that is IRREFUTABLE. Right now, the judge assumes that radar is always correct (even when it shows a dead corpse beside a road doing 100 MPH). Want to prove it? Then have a motion camera.

Your Mileage May Vary

[PPH]

Don't depend on GPS (or any other) evidence being allowed into court that contradicts the officer's testimony. Some courts may allow GPS data, others may not.

In most jurisdictions, such traffic cases are considered civil and the standards for evidence are different than those of criminal cases or what you may see on 'Law & Order'. The judge is free to weight the officers evidence more highly than yours and presume it to be correct unless you can show overwhelmingly that it is not. Sort of like being guilty until proven innocent.

Furthermore, courts have quite a bit of latitude to allow or deny the admissibility of data as evidence. For example: Radar is quite accurate (it reads the speed of an object quite close to its actual speed) but not very selective (it might be reading the speed of something else, or interpret some RF noise as speed). Take the boilerplate testimony that an officer reads about 'calibrating the gun with a tuning fork' and all the b.s. about standards traceability. None of this is necessary, as the most common source of errors are due to poor selectivity. But it sure sounds great in court.

In fact, calibrating a radar gun with a tuning fork is a good demonstration of its susceptibility to AM noise. An ideal radar gun should only measure frequency shift due to the Doppler effect and reject the sort of modulation that a tuning fork creates. After all, the instantaneous velocity of its tines is dependant on its amplitude and the average velocity is zero (unless you throw it). But no court would hear such an argument, as it would undermine their entire traffic enforcement/revenue collection program. And, as a civil case, they are not required to consider it.

Re:Speed = Distance / Time

[Dun Malg]

From my understanding, and the contention of the officer, the GPS logs average speed.

Your understanding and the contention of the officer are correct, but misleading. It's based on a failure to understand how short an interval that average is. GPS units report speed at one second intervals, which is how often the NMEA standard interface updates. Therefore, the speed reading they give is the average speed for that one second interval. This is not meaningful in the context of a 17MPH discrepancy, though, as it's highly doubtful that one could have a large enough swing in velocity over one second to hit 62MPH while still averaging only 45. That would require a minimum delta-V of 34MPH over half a second, which comes out to around 3 G's! I say show me the skid marks and I'll believe it.

Re:Speed = Distance / Time

[Paul Jakma]

First, when the GPS unit itself calculates the speed, it records your instantaneous velocity, not an average. It calculates this using the doppler shift present in the GPS signals picked up by the unit, not from how far the unit has travelled.

Let's go over some basics:

a) There is no such thing as "instantaneous velocity" - as velocity is a function of time.

Corrolary: You can /approach/ t=0, but the closer to "near-instantaneous velocity" you try to measure, the more accurate your measurements must be - alternatively the higher the margin of error will be.

And the problem with the radar/lasar guns is indeed that, because they try calculate "near-instantaneous velocity" they are very *very* susceptible to error, particularly at the ranges the police often try use them at (hundreds of metres).

b) Noticing a doppler shift in waves from a (relatively) stationary source would require that you have a non-zero velocity relative to the source (ie the distance between you and the source change). I'm reasonably sure this velocity would be immeasurable from a consumer car in a GPS over a short period of time and, further, that any measurable doppler would be due far more to the /satellite/ moving, not the car..

I.e. I havn't done the calculations (it's not just linear, cause any doppler will be induced by the curvature of the earth, not directly by the car's speed), but you're talking about measuring doppler due to /millimetres/ of movement (curvature of the earth), as a car moves perhaps a tennes of few metres. It's beyond believable we could measure that with any useful accuracy in a car.

So I call bullshit, unless you show me the numbers to prove otherwise.

Sorry but you are wrong

[laing]

GPS units compute your speed by computing the difference between your current position and your previous position divided by the time between samples. There's no other way to do it. Doppler is not involved.

The time between samples is what's important here. If it's only a few seconds then there's a good case for innocence. If on the other hand it's 30 seconds or a minute, the cop with the radar gun wins. BTW, it is the radar gun that uses doppler to measure speed.

--
This space for rent

Kalman Filter estimate

[kybred]

First, when the GPS unit itself calculates the speed, it records your instantaneous velocity, not an average. It calculates this using the doppler shift present in the GPS signals picked up by the unit, not from how far the unit has travelled.

No, the speed is neither an instantaneous speed nor a simple average. It's most likely the speed estimate that falls out of the Kalman Filter [unc.edu] used in the GPS receiver.

GPS receivers use Doppler to track the signal being received from the SVs; but that is the Doppler from the relative velocity between each of the SVs and the GPS receiver.

(I used to work on a military GPS system.)

SMD vs GPS

[RomulusNR]

SMDs measure your speed based on the reflection of light waves traveling straight lines through short distances through clear air. GPS measure your speed by calculating the difference between points derived as the average of the intersection of between 3 and 12 paraboloids determined by light waves traveling through the atmosphere, weather, and possibly reflecting off of buildings, trees, hills, and the ground divided by the update interval.

Like it or not, the radar gun is a more accurate speed measuring device than a GPS.

Re:Radar and GPS were in check in California

[tylernt]

It's highly unlikely this kid's GPS module system was using 3 or more satellites to triangulate his speed.

You're right, it was probably using at least 4 or 5. My $100 Garmin eTrex usually does. Maybe you've been asleep for the last 10 years, but even cheap GPS units have 12-channel parallel receivers these days.

AC for obvious reasons.

Yup, you obviously didn't want the karma hit for being a troll.

Re:Video Evidence

[The_Wilschon]

Speed limits are often based on the quality of the road (of the road surface, of the ability of drivers entering the road to see cars coming, and of the ability of drivers on the road to see hazards on the road ahead of them (reduced by curves, for instance)). The road surface quality degrades with time. Occasionally, studies are done in specific areas that demonstrate that the speed limit really is too high for the visibility of the road, both for drivers entering the road and drivers on the road. Other times, the population increases, and thus so does traffic. In each of these situations, reducing the speed limit is the appropriate immediate step. Further steps might include resurfacing the road, widening the road, etc.

Yes, speed limits are often arbitrary and designed to trap drivers. But claiming that speed limits are never related to safety is foolish, and claiming that speeding is not at all dangerous is also foolish. Higher speed increases both your reaction distance and the severity of any mistakes. Increasing either of these reduces safety.

I'm glad I don't have to share the road with you.

UK Stuff

[mistralol]

In the UK the speed camers are checked by hand (or at least are ment to). They do this by taking 2 photos from the camera at a set time interval. On the road by the camera there is meant to be a lot of little white marks which are big enough to see in the camera and are spaced at .25meter lengths apart. So now yo uknow how longits been and how far its traveled the rest is easy :)

Though i know this system is sometimes ignored by police int he uk ... eg lazy gone home early etc..

Also in the UK (at least n.ireland) the police are meant to run a test car though speed traps a few times documenting it and keeping a tape etc.. If they fail to produce a copy of the documentation on this to you at the scene if you ask for it then they dont have a case if oyu push it to court.

Another thought about safty on the road is around where i live they have started putting in speed bumps everywhere. Which is meant to slow the traffic down to improve safty. Slow it donw it does. It also annoys the crap out of people driving over bumps all the time and it makes the safty problem worse. Since all the traffic is now slower no gaps form in it so people can no longer cross the road when its busy whichout taking higher risks which is exactly the opposite they were traying to prevent.

Its amazing what the UK goverment can come up with.

Not Going To Work

[Anonymous Coward]

It's probably far too late to get a mod up on this, but I thought I'd add it to the knowledge base of the Internet in case somebody decides to Google it one day.

Simply put: This is not going to work.

The system is rigged against fighting speeding tickets. Even if you've got the money to pay for evidence collection, expert witnesses, and everything else -- BEFORE your trial -- you'll still lose. The justice system will protect the police from having even one ticket investigated, because it calls into question other tickets the officer may have written using the same or similar equipment; a very large expense. It just won't happen.

Here's a TRUE story, as related to me by my friend who drives commercial truck:

My friend was pulled over by a police officer for "speeding" and given a ticket for 75 in a 65. Only one small problem here: The area in Ohio where he received the ticket was absolutely flat, and his truck is GOVERNED at 68. Exceeding 68 miles per hour on a flat road is literally IMPOSSIBLE for his truck, so says the manufacturer of the engine and the manufacturer of the vehicle. Understandably, my friend was very upset at receiving such an obviously bogus ticket, and decided to fight it.

Nice thing for my friend, engines in big trucks have computers to track fuel usage, speed, etc. over time. Getting the data from the engine is a matter of taking it to the service center, hooking up a computer, and getting a printout. He obtained this printout and showed it to me; it's so simple my grandmother could easily see his truck hadn't gone over 68 at any point during the data recording. The dates were clearly marked; it showed on the day in question, the truck did not go anywhere near 75.

Armed with this and people willing to testify that the truck's governor was functional and the printout was accurate, he attempted to fight the ticket. He was informed that he would have to pay all of the trial costs up front ($10,000) before the trial began, and even if he won, he wouldn't be able to get reimbursed for this expense. So basically, it came down to a choice: Swallow pride and pay the $350 ticket, or pay $10,000 to prove he was in the right and get the ticket voided on the basis of the evidence.

Sadly, but wisely, my friend opted for the former. Proving his case was not worth the extra $9650 it would cost to do so.

Take note: Traffic court is rigged against regular people. If the highwaymen in blue try to rob you, just give up the money; losing your time, energy, and sanity over government sponsored theft will just victimize you more.

Re:Video Evidence

[psmears]

In Europe, none of what's described in the article would be happening because you have no right to contest a ticket over there (except maybe in England since their legal system and ours have a common basis, but maybe not even there).

In the UK you can most definitely contest a speeding ticket [bbc.co.uk]. In fact you can in France [sudouest.com] and Spain [spainlawyer.com] too. What makes you think the legal systems in Europe are so unjust?

(Note that the Spanish link mentions fees of €930. You don't have to pay that to contest a ticket, that's just what some lawyer wants to charge for assisting you... seems lawyers are the same the world over ;-)

Re:Video Evidence

[Ihlosi]

How do you figure that? In Europe, none of what's described in the article would be happening because you have no right to contest a ticket over there

Yikes. Who told you nonsense like that ?

Of course you can contest your speeding tickets over here. Heck, the appropriate advice is printed on the ticket you get by mail. However, since we've got a "loser pays" court system over here, you'd better be ready to cough up the court costs in addition to your fine when you lose. Also, police here are quite good at making sure that their speeding tickets are valid. So no ticketing someone for going 3 km/h over the limit and such crap - if you get a ticket, you were, after subtracting appropriate tolerances, _really_ going quite a bit above the limit.