back to article California teen offers GPS challenge to speeding rap

A California teenager is contesting a speeding ticket which claims he was doing 62mph in a 45mph zone, since a GPS system fitted to his Toyota Celica appears to show he was actually within the limit. Shaun Malone, 17, was caught on 4 July by a Petaluma police officer using a radar gun, AP reports. The lad had in the past …


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  1. Ian Michael Gumby Silver badge

    Fun with GPS... How accurate is it?

    A lot of incorrect facts and misinformation about GPS.

    Now I'm no expert, but I've spent quite a bit of time working on geospatial data. ;-)

    First, accuracy. What do you mean by accuracy? Plotting a car's position to within 1.5 meters? That's accurate enough for most application. Plotting a position on the ground for surveying? (accuracy to within centimeters.) Thats a different beast.

    The accuracy of your position will depend on the equipment you have, the time of day, and how many satellite signals you're receiving. Are you also referencing a base station? ... Survey accuracy is still a bit pricey.

    Clinton did away with the signal degradation many moons ago. You want a really accurate idea of where you are, you need to set up a base station, and take readings over time. The units should compensate for the shits and give you a very accurate reading. (Shifts can and will occur due to time of day and weather conditions.)

    Once you have an accurate base station, then getting a position relative to that should be trivial. (GPS signals + Base Station signal)

    The bottom line is that you can get very accurate readings from GPS if you know what you are doing.

    The accuracy of your portable units is roughly within 1.5 meters. Trying to determine speed from relative changes in position is accurate enough to give you a rough idea of how fast you are traveling but its not going to be as accurate as a radar gun shot from a mounted (stable) platform.

  2. Michael Miller

    It's all for the greater good.....

    <everybody now!> The greater good

  3. Clive Galway

    @ Cor - GPS not altitude aware?


    Maybe some satnav software or hardware discard the altitude component, but the GPS system is very much dependant on altitude as much as latitude and longitude.

    Otherwise it wouldn't work you numpty. It's TRIANGULATION.

    OK, so vertical accuracy is normally 1.5x as inaccurate as horizontal, but you can't do GPS with only 2D calculations, it *has* to be 3D.


  4. Anonymous Coward
    Anonymous Coward

    Military Signal Interference


    "End of US military induced SI (Signal Interference).

    F5 your info dude."

    You are being a little harsh there, and more than a little inaccurate.

    I think you meant SA (Selective Availability) which was indeed turned off for good in 2000.

    Peter D'Hoye may have been refering to PPS (Precise Positioning System) and SPS (Standard Positioning System) modes.

    In order to use PPS you need crypto keys which are only available to certain countries and organizations (the UK armed forces being one).

    PPS/SPS has nothing to do with SA.

    Incidently, prior to 2000, you could predict a US attack somewhere by the fact that your GPS (SPS) fix had gone sketchy.

    (From someone who can remember Navstar...)

  5. Anonymous Coward


    Everyone is talking about a 10%-15%-20% error in speed, but the police are claiming that he was going 28% over the limit. So, if boy and dad can show that GPS is less than 28% off in its average, that should suffice.

    (Indicating that the report is innacurate throws out the one important piece of state evidence.)

    It is technically possibly for the son to have sped up dramatically, been clocked, then slowed back down, but statistically that seems less likely than the possibility that the radar gun erred.

    [A while ago, I decided to buy a radar detector. In the areas where detectors are illegal, they have a system (called VG2) to find people using detectors. So, the radar detector companies engineered a system to show if the cops are pointing a VG2 at you.

    The upshot of all that is that I have a radar detector that detects VG2, which itself is a radar detector-detector. I am daily engaged in electronic warfare with my own government.]

  6. cor

    3d v. 2d

    @Clive Galway

    "Otherwise it wouldn't work you numpty. It's TRIANGULATION."

    Euhh, dude 'triangulation' has nothing to do with 3d positioning. It can be done with cellphone signal masts, with stars, with lighthouses....

    The altitude reading is so inaccurate that Garmin includes a pressure-sensitive altimeter for mountaineering. Numpty urself... :)

  7. cor

    Military Signal Interference II

    @ Anon

    I cannot deny this point. I am standing 'in a state of correction'.

    Btw, I didn't mean to sound harsh on you Peter, just fired off comment too quickly.

    Will get coat now.

    Time for Friday beers anyhoe.

  8. RRRoamer

    Obvious most El Reg reads don't know dick about GPS..

    There are so many errors and out and out BS in the comments above that I just want to laugh!

    "Doppler Shift"

    "signal every 6 seconds"


    I ran the GPS master station in Buenos Aires, Argentina for a while back in the early 90's. Before I was sent there, they stuffed a LOT of GPS theory down my throat. So I DO have a pretty good understanding on how all this works and USED to have a VERY good understanding. Fifteen years has taken the edge off, so...

    I have NO intention of saying HOW GPS works. That information is out there. If you want to know, look it up. Just stay away from sites were ANYONE can post because you will generally GET anyone posting...

    So, a few issues;

    Doppler: No. It's measuring a TIME delta between the code the sat is sending out and the code the receiver is playing. There is an offset in position between the receiver and the sat and that difference is a time offset. How much time tells you how far away the satellite is. Because of this, every GPS receiver on the planet has to have a very precise clock.

    Differential GPS: Most inaccuracies in the signal are caused by signal propagation through the atmosphere. The trick is that ALL receivers in the same general area (say 30 mile radius or so) have pretty much the EXACT same errors in there signal. DGPS works with a GPS receiver at a fixed land base that you KNOW where it's antenna is. Then it sends out a radio signal with the current errors in the signal to all the DGPS receivers in the local area, so they can calculate a much more accurate position fix. "You are 5.12 meters lat, -3.43 meters long and 2.87 meters height from where your signal says you are"

    Accuracy vs. precision: I always want to cry when I see how few people actual understand the difference between these two terms. Accuracy reflects how close you are to the "true" measurement. Precision reflects how repeatable your measurement is (no matter HOW accurate or inaccurate!). GPS tends to have VERY "accurate" VELOCITY measurements (not speed, but the vector velocity) because the very act of measuring velocity with GPS is a differential measurement, so the raw ACCURACY of your time base (and the signal errors as well) actually gets subtracted out of the measurement. Position still depends on the accuracy of the time base, so it tends to be much less "accurate" than velocity for any given condition/equipment.

    So in very short: The raw ACCURACY of velocity of this guys GPS system is WAY more accurate than the police officers radar gun. Even if the data DOES show the kid was driving 2 meters over the edge of a cliff instead of on the road!

  9. Michael C

    Commercial vehicle tracking systems should not be confused with GPS

    I worked for a company in SC back in 2001 that designed a fleet tracking system based on GPS. Common vehicle and handheld GPS systems use a 4 point triangulation system to determine approxamately every 1 second the specific location (including altitude) of the device. (FYI, someone mentioned that 1 sattelite periodically goes out of servidce: first this is logged, second only 3 sattelites are needed for triangulation, the 4th is a built in redunndncy, and third, this very rarely happens for more than fractions of a second) In the US, GPS are accurate to 10 feet or closer (GPS itself is actually accurate to 0.5 inches, but the minitary limited civillian access to the more accurate signal, they haven't since Clinton was in office and new GPS can be accurate to as little as 2 feet).

    Comercial fleet systems monitor not only the GPS signal itself, but also the speedometer. In fact, good systems not only use speed info, but direction information from a digital compass contantly calibrated by the GPS location. The GPS information is actually made more accurate since the computer can access speed and direction information in real time to better calculate an exact position from the sattelites.

    We used to sell these system in 2001 for about $1200 per vehicle. The comany I stopped working for them in 2003) still sells them, but for about $500 now. They also have options to monitor fuel status, gas caps, door locks, engine use, and more. Many also record audio in a 30 minute loop that can be saved should an accident occur. They record information contantly in 1 second intervals, and report via a cellular pack every 30-60 seconds back to a central tracking office (or via wireless when the vehicle comes back to the depot if they don't opt for the cellular system). If this kid was using one of these advanced systems, it's certainly accurate enough to prove that there is no way, given the make/model of his vehicle, that he could have been speeding.

    Keep in mind, it's not instant accuracy that matters here. Even if the recording window was 30 seconds, based on 1 second interval data, all data before and after that window must be callibrated to estimate the begining and end positions of those windows (other wise it would look like the car was jumping from 70 to 30 and back again every 30 second, or that the car phased through space and jumped from point to point). Inside of a 30 second window, with his make/model of car, to do 65 in a 45, but have the software report he was doing 45, would mean he would have to, in less than 30 seconds, accelerate from 45 to more 65MPH, get clocked at 65 by the cop, then decelerate to 25 and run at that speed for the same amount of time he was at 65, then accelerate back to 45. This schnario, even if all the timing was humanly possible, is not possible given his vehicle type. It's even harder to believe since he would not have known when the window of time began and ended. Also, most commercial systems not only monitor overall speed, but also aceleration, breaking, lane changes, and more. Companies don't like their driver using passing lanes on public roads, nor do they like drivers stopping short as these usually indicate agressive driving or tailgating, and can be easily monitored. Even a simple GPS system would hold up to this scientific scrutiny in court. More so if the lawyer calls into question the speed gun itself, which is very easy to do. (i've gotten out of 3 tickets myself by simply asking to see the tuning fork and calibration chart and validate the serial number of the radar and fork match as regular radar must be tested and documneted inbetween each and every speed stop in most states. Do you have any idea how easy it is for a cop to loose the tuning fork, or forget to bring it to court?)

  10. Paul Young

    My Satnav never agrees with my speedo

    I have a TOMTOM in my car and the speed display NEVER displays

    what my speedo says.!!

    I Wouldn't trust my satnav....

    Also I used to have a Garmin Etrex thingy (Useful for storing points of interest)

    The speed display on that was wildly inaccurate until I sat on a ship for 2 weeks......

    Then the speed was VERY accurate, 15 blinking Knots 7 days each way.

    Good fun going in a straight line on the ocean for 7 days!

    At least the Bar was open for 2 hours a day

    Just my thoughts

  11. Guy

    @Daniel get a bike

    yep they might nort be able to do you from the front, but if you get caught from behind (ooh Err) as the owner of the vehicle you are now liable for the points. Saying it wasn't me is now no defence as you need to say who was on the bike. Refuse and the points are yours!!

  12. milan

    The Clue is in

    the story.

    Most GPS (Not Satnav) systems used for commercial vehicles are not simply satnavs. They record information directly from the vehicle and then upload this every 30 seconds or so. When uploaded it also gives a 'marker' for where the vehicle currently is when the upload takes place.

    I use to work for Navman Wireless who pretty much brought this kit to the UK quite some years ago. The speed information doesn't require multiple satellite triangulation, nor does it require accuracy down to a 1m area.

    For 90% of the systems available, the log is available via an online control panel for the users/supervisors, and therefore cannot be tampered with by anyone outside the provider.

  13. Anonymous Coward
    Anonymous Coward

    Dispelling myths

    Wow - so much misinformation on a single page! (A few have it right, though).

    So - to dispel various myths above:

    * GPS operates in 3-space, not 2-space. [Actually, for purists who may disagree, GPS operates in 4-space, but I think I'll leave that out of here]. Some cheaper units without barometers are poorer at resolving the vertical component, but most are not. Some may only use X and Y for calculations, but those should probably be tossed on a heap right now. With any decent GPS, going downhill/uphill will not affect the trueness of the reported speed to the extent suggested (i.e. that it just ignores the Z component).

    * Even many small handheld units will report velocity quite accurately, assuming they have a fairly clear view of the sky. With most units, if you have a clear view of much of the sky, it will detect a movement as small as a few centimeters, certainly a meter. Sure - there are some vagueries in the system that are unpredictable, and these can minorly affect precision. However, *in practice*, most GPS units with a clearish view of the sky are plenty precise enough to determine the difference between e.g. 26 and 30 (or even 27) mph averaged over just 1 or 2 seconds.

    * As someone did mention (using different words), there is a difference between "precise" and "accurate". Even if the GPS accuracy is off by say 15 metres at the time this does not mean its precision is off by the same amount - it is quite likely to be very much less than that, and *precision* is what is important for speed determination. Accuracy is irrelevant to precise (and accurate) speed determination.

    * Clouds hardly affect GPS at all. Trees can. Buildings can. Clouds - no.

    * Hand-held speed guns - including laser and radar - do NOT measure instantaneous speed in a markedly different way to GPS, as someone suggested. Instantaneous speed is all but a mathematical on-paper concept. All of them - including those using the Doppler effect - rely in concept on *sampling*; i.e. taking more than one measurement over a period of time. With Doppler, you still have to measure the frequency or wavelength of the returning wave, and you cannot do this instantaneously, only by looking at the wave for at least one wave cycle. This may seem like splitting hairs, but rather I am trying to point out that laser and radar similarly rely on high-precision timing systems that are also subject to inaccuracy; i.e. they are not perfect just because they are laser, etc.

    * Someone suggests the unit dropping from visibility of 4 to 3 satellites causes a large loss of precision. This is correct; however - most units will be typically seeing around 10 satellites most of the time (once "warmed up"), constantly dropping some and re-using others as location changes. The loss of precision dropping from 10 to 9 or even 8 satellites is negligible compared to going from 4 to 3.

    * The intentional random inaccuracy for non-military applications (SA = Selective Availability) was removed as one of Clinton's last-minute decrees before leaving office. It does not apply anymore. The US have said it will never apply again; whether that turns out to be true is moot.

    * GPS signals can be used to locate the receiver precisely-enough for these purposes at worst every 1 second, not the 6/30/5/other guesses above. GPS satellites transmit various signals; the one used primarily once other data has been established using other signals (the "warmup" period) transmits every millisecond.

  14. Michael

    Black Box

    Really, this is one case where having black boxes in cars would be nice. As long as there were no transmitters on it, of course -- we don't need the state tracking us. It should only be used after the fact.

    I know the same rules of beyond reasonable doubt don't apply to speeding tickets, but it seems to me that an officer could lie about the speed readout just as easily as I could. There is no hard proof that the gun said what the officer wrote down. So why do we assume that police officers' word is more trustworthy than ours?? They have as much incentive to lie (quotas, etc) as we do (not getting cited), and with all the stories of cops that drive drunk and beat their spouses, should they REALLY be up on a pedestal beyond reproach??

  15. Outcast

    [Nitpick mode ON]

    15mpg for a wagon is pretty good. DAMN good if its a LARGE lorry as earlier described.

    My wagon is a 560 Scania (V8)

    and gets about 8mpg average running with a reefer @ 44 tonnes gross.

    As for fitting tacos to cars....

    hmmm ...tasty. but surely the year new vehicles spend leagured in a field before distribution would put those tacos past their "best before" date ? Still, a fur encrusted taco sounds........ different..... shame about the smell.

    I think you mean Tacho(graph)

    And iirc... Cars fitted with tacho's ARE being trialled.

    I think the future world is going to be an UGLY restrictive place.

  16. Morely Dotes

    Stupid cop displays his lack of brainpower

    'Petaluma police Lt. John Edwards said "he could not discuss Shaun's case", but disputed Rude's accuracy claim. He countered: "GPS works on satellite signals, so you have a delay of some type. Is it a couple-second delay? A 30-second delay?".

    A thirty-second delay would indicate a satellite at a distance of roughly 4,464,000 km from the car in question. The Moon orbits Earth at a distance of 372,000 km, corresponding to a signal lag of 2.5 seconds.

    GPS satellites are in low Earth orbit ("LEO"), well under 1,000 km altitude. The signal delay - which is how GPS works, by comparing signal delay among 2 or more satellites - could never be more than 3 one-thousandths of a second. that's 0.003 seconds.

    Thus, this police lieutenant demonstrates that he has absolutely no grasp of how GPS - and, by extension, Radar - works. Furthermore, he doesn't know how distant the Moon is, nor does he appear to be qualified to supervise people who use Radar speed guns - or GPS tracking units. His best option would have been to keep shtum and not reveal his abysmal ignorance, but he seems to be too stupid to do even that.

    I'm sure NASA and European Space Agency *wish* they could put a cloud of satellites around Earth at a distance of 4 million km. Fact is, they can't, not without bankrupting the entire Western world.

  17. John Murgatroyd

    GPS speed reading

    GPS Speed:

    How accurate is it? How fast can I go? How HIGH can I go?

    GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.


    From the NAVSTAR GPS User Equipment Introduction document Section 3.7:

    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.

  18. Anonymous Coward
    Anonymous Coward

    May be irrelevant no matter how accurate

    When I was clocked by radar going far faster than I was in fact riding a motorcycle in Colorado in the '70's, I tried to explain to the court that the radar gun could be easily confused by many things, not the least of which were the rather small amount of metal in a 350 cc motorcycle and the multiple reflections from the cooling fins on an air-cooled engine. She silenced me by noting that as long as the officer testified he had calibrated the gun against a tuning fork that morning, state law accepted the radar gun reading as accurate. So much for know-it-all physics graduate students.

    Moral: the law and physics are two different things.

  19. The Mighty Spang

    hill accuracy

    I thought it would be way out on hills, but do the math. for a 1 in 4 hill it would only be about 3% out if only using 2d calculations.

  20. Anonymous Coward
    Anonymous Coward

    Speed camera inaccuracy

    It doesn't get much more topical............

  21. Scott

    GPS not accurate...

    I have a GPS unit for my bicycle and while it is fairly reliable and accurate most of the time, it too has had glitches....on one particular ride it recorded my max speed at 356mph...which is impressive considering my top speed before that was around 35mph going down hill...

    Still, it will be interesting to see how this pans out if it makes it to a hearing.

  22. kain preacher Silver badge

    @By Guy

    "as the owner of the vehicle you are now liable for the points. Saying it wasn't me is now no defence as you need to say who was on the bike. Refuse and the points are yours!!"

    not in the state of California. The state still has to prove its you.

  23. J


    "Euhh, dude 'triangulation' has nothing to do with 3d positioning."

    That's why 3D would be called 'pyramidation' instead, methinks.

  24. Rene
    Thumb Up

    GPS != Vehicle Tracking

    There are other ways to determine a vehicle's speed—electronically reading the vehicle's tachometer, but GPS receivers when properly installed and configured (and when the antenna is well-positioned) provide an accurate, consistent report of velocity and heading (for the numerous reasons stated above). At least when a receiver can't get a proper fix, and therefore reports an erroneous location/speeding/heading it will let you know (whether the UI indicates GPS validity is another thing).

    Some basic info:

    Greater detail:


  25. heystoopid

    Ha Ha

    As Nelson Muntz would say Ha Ha !

    Sadly in Oz back in the sixties the police had an infallible speed measuring device called an Amphometer which involved two tapes across the road a fixed distance apart and a timing signal generated by a the cars front wheels passing over the tapes. It was not rocket science to calculate the average speed using those two inputs it only problem was that motorist caught was required to witness the speed registered on the device for court certification requirements and a regular check to validate it's crystal controlled timing clock!

    Modern digital speed cameras do not suffer from that problem as the photographs contain all the pertinent information !

    Further it is not rocket science under certain circumstances failing to follow the manufacturers explicit and detailed instructions that come with hand held speed check devices to introduce vector arithmetic errors to the average speed of an object ! This can be very easily demonstrated by showing a fixed object like a large tree to be shown as breaking the speed limit !

    Hence the extensive use of fixed position speed cameras which are not without problems if they are not sighted correctly or have regular certification of accuracy certificates !

    In Oz one ground breaking case against fixed speed cameras which were not maintained after installation , involved an allegedly speeding clapped out thirty year old Datsun(Nissan as it is now called today) 1200cc four cylinder clocked at 158kmh . Testing showed on the flat the car's maximum speed was 117kmh and with the speedometer tyre combination it read 11 % faster then true speed thus at 100kmh indicated it was doing a mere 90kmh !

    As for errors most appear to be citing the original vector errors used on civilian not military GPS systems prior to the time the full set of satellites were not up and running and President Clinton ordering the Military to switch off the degrading error signal information sent to any limited civilian specified receiver ! What they fail to mention is that if the receiver sees signals from more than three satellites at any point in time the positional inaccuracy actually goes down significantly ! Oh well let the silliness continue !

  26. Anonymous Coward

    Asked an officer

    I just spoke to an officer in New York about this (was reading the article when he came into my shop, so just about 5 minutes ago.) He said that normally, in the case of just a GPS, they'll throw it out if it was low enough over the limit, just because the hassle and cost of trying to prosecute isn't worth the fine. If the person was going high enough over the speed limit (20+ MPH) then they'll still go after them, and normally win. If it's a GPS hooked up to a speedometer, then they'll throw it out almost every time no matter what.

    I was hit once in one of our more ticket happy area (in the city I live in, you can drive 50 MPH in a 30 MPH and not get bothered, but other areas don't go over at all or you'll get nicked) for going 45 in a 30. I was accelerating to 45 because of the sign right in front of me that said 45, though, and was coming out of a 30. His argument was that the speed zone didn't start for another 100 feet or so. That ticket was thrown out by the District Attorney without a second thought. I avoid driving in that area whenever possible, because they don't even base the ticket on a gun. They'll just say they followed closely and based it on their speedometer. At the time he pulled me over, I had only gotten up to about 35. (And yes, my speedometer does work.)

  27. Andy Bright


    The only delay is the time it takes to display the information on your GPS unit or receive the information on your PC.

    To say his GPS speed recording was wrong, because it took a couple seconds before the data was shown on a video device, is like saying it took Linford Christie 14 years to run 100m, because I didn't watch him win the Olympic games till last week.

    The inaccuracy has to be in the measurement of the information, not the time it takes for someone to see it. If the time it took to measure your position between two points was significantly delayed then you might be on to something - but it can't be or the GPS navigation systems would be worthless.

    Your instructions would be so late you'd have driven past all the roads you wanted to turn down. Sorry but at 60mph you'd be 1/2 mile out of reckoning if the total delay was 30s. Even at 30mph you'd me over 400 yds further down the road than your GPS was displaying.

    I reckon he should win this - it seems his parents aren't the typical "my kid can do no wrong" bunch, and if his previous punishment (having the gps unit fitted to his car) actually proved on this occasion he was in the right, why shouldn't he use that to fight a bogus ticket.

  28. Tim Hogard

    More misinformation?

    The GPS system works by finding out how long it is between the sats and the receiver. It starts by syncing its time to as many sats as it can see and then figures out the speed of light delay and Doppler shift for the signal from each satellite. It then uses that info to readjust its internal clock and then readjusts all the other timing calculations. Once it has a good fix then it starts feeding in the receivers velocity and temperature change back into the calculations of the Kalman Filter. Deep inside the GPS chips is a time base that will be in sync with the sats to about 90 billions of a second or it won't get a fix. The result is that if it has a good lock on several satellites and has had time to stabilise the oscillations of position, time and speed then its position will be far better than any radar device that simply displays the highest Doppler shift it sees being reflected to it. Radar guns don't measure speed, they only measure the Doppler offset and show what the speed might be if there is no vibration involved. After all the tuning forks they use to calibrate them aren't moving at 100km/hr are they?

  29. Steve VanSlyck

    Kudos to all

    Anonymous Coward: Speding is a strict liability offense. Intent don't enter into it.

    Frank Bough: If you don't have an expert witness, you lose. The state, however, gets off scott free because *it* gets to "prove" reliability in a single state or district wide case and forever after the defense gets to prove itself innocent, or at least not guilty, while the state rakes in the cash.

    Filippo Negroni: Yes, take EVERY case to court. In my experience, in Ohio you'll get out of about 20% of them if you do.

    Hedley Phillips: BECAUSE THE LIMITS ARE WRITTEN TO PATRONISE THE LOCAL NEWS AND OTHER CHIKEN LITTLES, THAT'S WHY! Speed limits have nothing to do with the design of the road or the objective conditions. Your point, however, is well taken.

  30. Lickass McClippers

    RE: Intent to speed 2

    I concur, you should always contest a speeding ticket if there's the slightest doubt. If only because Plod will carry on booking people willy nilly if you and I don't contest, and Plod will continue to hand over your hard-earnt to Comrade Brown.

    My brother got caught by two seprate fixed cameras in South Wales. Said cameras were only 2 miles apart. He got caught doing 80mph+ on each one, and duly recieved two tickets. South Wales Police retracted the second ticket when our father (an ex-traffic policeman himself) pointed out to them it was a continuation of the same offense. Judging by the time between the two pictures, and the similarities in speed, he'd obviously not slowed down and they weren't entitled to bill him twice for the privilage.

    Always contest. How many people have been caught by those two same cameras and paid up twice..?? Nice little earner that one...

  31. Joe

    Gosh, what a serious debate!

    Am I the only one to guffaw out loud at the name "Roger Rude"? That's the best name I've ever heard!

    At last, I can become a porn actor, as I have found the perfect name!

  32. Giles Jones Gold badge

    Radar guns are flawed

    Using radar for this purpose is flawed, you have to point the gun at the right section of the car, the guns need calibration regularly.

    Some guns are that bad that they register a speed of 10mph on a stationary object.

  33. Anonymous Coward

    Oh no, not AGAIN.... :-(

    > Everyone knows these are just money making scams by the police, so they probably don't really care about the accuracy unless the public begin to realise that it is a big CON or get ‘outed’ in public (like that inventor did).

    No, everyone does NOT "know" this... because (sigh...) it's bollocks.

    For the umpty-umpf time, the police get NOTHING from speeding fixed penalties, in fact they get NOTHING from ANY fixed penalties (they aren't fines, by the way, because you don't need to pay them if you ask for a Court hearing).


    NOT A PENNY... got that? When hypothecation was in force, local speed camera partnerships could deduct their capital and revenue costs before passing the balance to the Treasury... but every penny goes to the Treasury now.

    So the more that the police spend on speed enforcement, the more that it costs them.. OK? Can we assume that we won't read any more drivel from badly-informed f*ckwits about "money-making" by the police?

  34. Spider

    Finally a subject for me....

    ...and i wish it wasn't. I never thought I'd read so much rubbish about a subject from supposedly clued up individuals! If you are going to rant about GPS take the time to learn a little about it first..."it only updates every 6 secs", "unless you're military"," it doesn't work so well if it's cloudy"...FFS. The info is out there, but i guess it's more fun to just sound off.

    For the record SI has been switched off (although the DoD retain the right to switch it back on if they so choose), updates depend entirely on the system you use, and of course it's not perfect. But then again neither is the police with the Radar gun....

  35. Anonymous Coward
    Anonymous Coward

    Re: Doppler shift (there's no f in doppler shift, not in the GPS-velocity picture anyway)

    RRRoamer is right, the alleged doppler shift referred to by many people in this thread just shows how few people actually know when they're talking utter BS. Being mistaken doesn't make them idiots, it just makes them ill-informed. Being wrong *and still denying it* in the face of People Who Know may well make them idiots though, but this kind of discussion makes it hard for Joe Public to know who's got Clue.

    As the online NAVSTAR docs are offline at the moment I'm going to assume that what's happened here is some technical author along the way has "simplified" the picture for the benefit of the intended audience. Unfortunately that error seems to have transmogrified from oversimplification mistake all the way to urban myth. After all, everything you read on the Web is true, right (including this, obviously)?

    I cba explaining here how GPS works, or how I come to know, but would just like to say that the writeup at looks pretty decent to me, certainly much better than much of the doppler-related BS being spouted here.

  36. Anonymous Coward

    "those who speed, and those who lie"

    there's a difference between the occasional and often unintended minor breach (it's happened to me and many others), and those who regularly or seriously break the law because they're daft enough to believe that illegal speeds don't make them **OR OTHERS** less safe. But what can you expect from the stereotype Top Gear audience, the kind of clots that think leaving fog lights on all the time when it's illegal to do so doesn't make them **OR OTHERS** less safe on the roads (or more likely, just don't care about it because they're dim enough to think that, like blue windscreen jets and 2kW stereos with hilarious gold-plated fuses, it's kewl)...

    "Can we assume that we won't read any more drivel from badly-informed f*ckwits about "money-making" by the police?"

    Would be nice, wouldn't it? Sadly I suspect these f*ckwits only have room for one idea at a time and once the Sun/DailyMail/Clarkson have brainwashed them it takes a hell of a long time for real facts to get through again.

    Where's AManFromMars when you need some proper drivel?

    Subaru drivers of the world unite, you have nothing to lose but your licences.

  37. Anonymous Coward

    THE MONEY...

    The money goes direct to the treasury, and who decides how much money the police get ? The treasury - and if you think there's no connection between the two amounts you're as naive as the people you accuse of thinking the money "goes to the police". Of course it goes to the police, it goes to them via the treasury.

  38. Anonymous Coward
    Anonymous Coward

    Missing the point

    These comments seem to have gone wildly off track. Isn't the point here that that the GPS device fitted in the car in the original article "sends a signal every 30 seconds that records his whereabouts and travel speed". So there are data points every 30 seconds. What happens in between is anyone's guess. The chance of the contested speed reading being right on a data point seems less likely than more likely. I don't see how the recorded data can be used to contest a speed reading.

  39. Matthew

    My £0.02

    Pretty well everyone is familiar with the idea that the police allow a margin of error (10% + 1mph?) *because* of speedomoeter inaccuracies.

    The idea - proposed by some here - that a different GPS speed must be wrong is laughable! When cars are tested on the track, GPS telemetry is used precisely because the car's own recorder (and a policeman's hairdryer) aren't accurate enough. Read the reports of Richard Hammond's crash, with second by second GPS values and it is clear that this is far more accurate.


    -speedometers have an expected inaccuracy and the manufacturers have (and use) a wide leeway)

    -radar guns have been proven inaccurate when not used precisely as intended

    -GPS is finely calcuated for accuracy and relies upon well-understood mathematics and cannot be misled by operator error.

    Which would I put my trust in? GPS every time...

  40. Anonymous Coward

    Uninformed (rank) speculation...

    I do wish people would research some facts before opining on these things. Bloody place is turning into slashdot with all this uninformed bullshit completely speculative drivel... as bad as that tosser from the cops throwing in his uninformed opinion in the first place.

  41. elder norm

    Maybe we are all forgetting something

    Actually, isn't the goal here to all be safe??? I think that the cops and courts forget this when they get so filled with self importance that they start thinking of themselves as perfect.

    I received a ticket for speeding when it was the car beside me. The officer said it was me cause 40 seconds after taking the reading and driving past us, I was going faster than the other car. When I told him that the other car had passed me, then slammed on the brakes, my ticket was changed from speeding to ..... wait for it...... failure to obey a sign. What the f**K does that mean??/

    It means that he realized that I was right but since he started writing the ticket it was easier to blame me for something i did not do, than explain to his boss why he screwed up. Yea. I want this guy having a loaded gun. !!!!!

    Remember, cops are people too. People that can easily have a issues that spill over to their work.


  42. Charles Manning


    The thing with radar gubs etc is that unless they use some very accurate external reference they require calibration to be known good. Then they need to be operated well.

    GPS is in essence "self calibrating" since clock error is one of the things that GPS needs to calculate to keep tracking them garbage-cans-in-the-sky. If the GPS clock error is wrose than a few hundred ppm, then the GPS will not lock onto the signal and it won't get a position etc.

    Not all GPS receivers use Doppler, though the best do. They won't use doppler all the time and don't always provide up-to-date info particularly if there are trees etc.

    It is very easy for the young fellow to have got a false low reading on his GPS, as follows:

    1. Drive along at 35mph.

    2. Go under trees (or cover antenna with hand/metalic item etc).

    3. Accelerate to 50mph while under trees/hand over antenna.

    4. Brake like hell before leaving tree cover/removing hand.

    5. Leave tree covered area/remove hand at 35mph.

    Many GPS systems would likely show 35mph with no over-speed events.

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  45. Jon Double Nice

    So anyway...

    One day, Werner Heisenberg gets pulled over by a traffic cop. The officer walks up to the driver's side and says, “do you have any idea how fast you were going?”

    “No,” replied the scientist, “but I can tell you exactly where I am!”

  46. Andy Dent

    Cops clean up after speeding accidents!

    I used to work in a startup where the CEO was an ex-copper.

    Amongst his other harrowing tales was the description of the first accident scene he was called out to at the age of 19 - a "bucket job".

    When you've had to pick small pieces of a young driver off the road, you might develop a more intolerant view of speed.

  47. Beachhutman

    Not the point

    Really gets me irritated when the sunday schoolers come up with "If you're innocent you have nothing to fear" argument over speed cams, ID cards, DNA databases, and such stuff. They are oh so virtuous until the day THEY are the ones who get tagged by a cop wobbling his speed cam, or visited by the police and accused of being in a place they weren't, or denied their civil rights because the government data is wrong, or have a child DNA tagged because the cop THOUGHT he MIGHT have done something wrong.

    It isn't about how gruesome accidents are, or if you were speeding (an emotive nonsense word that actually means "faster than the police prefer") The fact is that the police are so often wrong with speed, victims, crimes, data, attitudes, that alas in UK we no longer trust them. That is tragic. Speeding fines are a symptom.


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