An inventor caught by kent police for apparently doing over 40 in a 30 zone presented evidence from a GPS product he was testing which showed his true speed to be 29.7mph.

Kent police have now suspended use of speed guns pending investigations.

All given GPS locations have a level of inaccuracy due to various issues - most recievers will show this somewhere.

Since this inaccuracy can change by a wide margin very quickly this makes snap distance calculations a problem. Since speed is worked out using distance and time this will affect any speed readings.

I have seen this on a handheld unit which would alter the speed by upto several mph whilst a) standing still and b) moving at a steady pace.

GPS is a location system, and should not be relied upon for accurate speeds at a given point - only an average over some time.

The comment from the officer about having a satillite signal possibly causing a delay demonstrates his lack of understanding of how a GPS works.

The 'delay' is actually what the GPS uses to measure it positioning - effectiving triangulating itself based on the known position of the satellites and the delay in signal. As such, the position and time recorded are accurate to within 3-30 meters and 1-5 nanoseconds respectively. However the resolution of the signal is once a second, so speed is calculated over one second intervals.

In a nutshell, the GPS is dead accurate.

Radar (and laser based) speed guns have issues with aiming and the electronics picking up the fastest thing in their entire field of view, which in the past has been shown can even be a fan in the officer’s car when improperly aimed.

GPS has the issue that if any one of the signals to the minimum of four satellites required for valid positioning drops out, the GPS does a combination of using its internal clock against the remaining signals plus the last valid velocity and direction readings to create a “pseudo present reading” to extrapolate the position ( and from that, speed & direction) until it reacquires that minimum of four satellite signals again to truly calculate valid position information.

Clearly whats more important in this issue is intent to speed, as the officer indicated he may have been infringing for 3 seconds, but clearly there was no intent as he knew there was a logger attached to the vehicle. the logger clearly shows that this person did not intend to speed on his journey and clearly did not speed through-out his journey.

what type of vehicle was it? it has been proven that flat surfaces reflect speeding devices and can infact inclued closing speeds of vehicles infront (and even bridges of windscreens) or behind, This has happened in the UK where (because of our two photo requirement - fixed time calibration) it was proved that a driver was doing 15mpg in a 30mph zone but the vertical rear window reflected the closing vehicles speed also, so the camera fired.

For allegedly doing 36 in a 30 zone by a laser wielding copper. Thing is, i fought it because I had my Tom Tom running at the time and both warns me of the camera position (they were lasering right next to a fixed camera...) and displays my speed in RED if I exceed the limit PLUS I'd been done in that exact spot 2 years before and was thus quite careful.

Needless to say the magistrate wasn't having any of it and I got fined an amazing £232 + 3 points - presumably for not submitting to the unjust tyranny of the fixed penalty.

This whole speeding ticket situation is on the verge of collapse with in-car GPS speed monitoring.

ANPR is next, of course...

If you have physical access to the gps device, then its completely possible to edit the data via a pc link. From the police's point of view it's probably not robust enough for a defense, unless there is an external, controlled logger somewhere the driver cant get at the data - an external company for example.

I read about that story too. If enough speeding tickets get reversed in this country I can see the speed guns being a thing of the past, but what can we look forward too? More gatso's?

It is a well know fact that GPS is not a reliable and accurate way of measuring speed. Especially when it's cloudy.

Can't believe the courts are even considering this...

If you are ever caught speeding, please do take it to court.

It took my case 9 months to be heard in court, after several letters to Thames Valley Police highlighting their mistake and repeated requests from Thames Valley Police to own up and pay £60 plus 3 points on my licence.

Thankfully they saw the light and admitted fault in court. I was only entitled to expenses I had receipts for, which was the £3 car parking ticket I had to pay to attend court that day: I did not engage a solicitor even though I received plenty of legal advice (for free!) from the guys at www.safespeed.org.

I kept the £3 cheque from TVP and will frame it one day!

Details of my case (and many others) are on www.safespeed.org 's forum.

The older radar gubs have to be calibrated every 6 months. A friend of mine, who actually manufactured these, said this was the exception, rather than the norm.

The laser guns are more acurate, unless hand help. movement of a mm or 2 at the gun, at an object 2 miles away (which many UK forces do), means by the time it hits the vehicle,it is moving several metres. Not so acurate now !

Didn't he get off because the plod in question had retired so it didn;t even reach court.

I'd have to agree with Tim Epstein above.

Was recently given a ride in a friend's new BMW M3, which came fitted with a GPS thingummajig - which was telling us the names of the roads we were turning into as we were still turning, which implies that the delay ain't all that big...

Are we to assume that Paris Hilton didn't have a similar get-out-of-jail-free card BTW?

..is it take in to effect changing altitude? For example, Consider two roads parallel to each other but one of them has an incline. If two cars travelled at the same speed along both roads would their GPS readings be the same?

Just a thought!

Speedometer.

Who needs new tech when drivers just need to stick to the limits?

I think that doing "15mpg in a 30mph zone" should indeed be a crime - what were they driving, a huge lorry?

You should always contest speeding tickets - that way they will hopefully make sure of their facts before even thinking of prosecuting. If they can't be sure who was driving or the equipment is not in calibration or is misbehaving or they haven't been properly trained - they should not prosecute.

@Tim Epstein:

If speed is calculated by dividing distance and time, it can never be more accurate than the accuracy of either. The time is accurate to a few nanoseconds, and therefore clearly not the problem.

The distance is the problem. If a location by GPS can be accurate to with 3-30 meters, the distance between two points is accurate to 6-60m. So in a worst case scenario, to determine the speed with a 1% accuracy, the best thing your TomTom can do is give you the average speed over the last 6 km! Even at an accuracy of 3 m, you still need 600 m to get that 1% accuracy.

Conclusion: GPS is not 'dead accurate' and clearly proves nothing in this case.

GPS can be wildly inaccurate. Problems with satellite pickup, moving between satellites all sorts of things can throw it out. It takes a lot of careful analysis to actually validate a GPS report. In the sport of Sailboat racing "GPS peaks" are notorious. In one on line competition recently a potential winner was demonstrated to have GPS results so far out that some of the plots were on dry land half a mile away from the lake he was sailing on!

Simple, buy a bike. no licence plate on the front and a pic from the back cant incriminate you. ;)

The point is that the driver _was_ sticking to the limit, but the speed gun said he was speeding. With just the speedometer you have no proof that you weren't speeding, so you get the fine and the points despite being innocent.

All well and good until you do get busted because of a dodgy speed camera/gun and have no way of proving you were within the legal limit.

However accurate the technology if it is maintained and used by people there is the potential for error and that should be a factor that is considered during prosecution.

However, it is not perceived to be in the immediate interests of the police, courts, government or suppliers of speed-gun/camera devices to admit the potential for error or to consider it during prosecution. The fact that the pursuit of justice ~should~ be the prime interest of all these groups falls by the wayside in deference to expediency, reduction of costs and (in the case of the treasury and manufacturers) income.

If you read the article closely it states that "it sends a signal every 30 seconds that records his whereabouts and travel speed", I assume the position is from gps and the speed from the speedometer.

So minor inaccuracies in the position won't affect the accuracy of the recorded speed.

"You should always contest speeding tickets - that way they will hopefully make sure of their facts before even thinking of prosecuting"

Well, when I got my 3 points the copper showed me the recorded speed on the gun, and sadly, it was correct :o) D'oh

Surely if they allow a GPS unit (which I'm sure the manufacturers probably don't guarantee for accurate speed calculations) as a defence for speeding, they should also be allowed to use the same GPS unit to prosecute?

Hedley, you do know that by law all speedometers are innacurate, so the only way to know your true speed is GPS.

Also as has been proven in a UK Court, Radar guns are not 100% accurate, so perhaps your smugness should be reigned in a bit.

ps. there are two types of people in this world, people who have broken the speed limit, and liars. ;)

you will need a GPS with speed signal from the car since those without are NOT accurate enough...

Any way, why not just install a taco in that boy race box?

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

My (admittedly very old) Garmin records the maximum speed I've reached is 199 mph. Given I have a Landrover, I'm trying to remember exactly when it was I drove off a cliff...

"Who needs new tech when drivers just need to stick to the limits?"

When drivers stick to the limits and the police prosecute them wrongfully, we need all the tech help we can get to curb the advance of a Police State.

I'd agree with the raw figures, but the logic behind the calculation is flawed.

The reading does not suddenly jump straight from one extreme to the other (in my experience, probably only about 200 hours total of real/constant GPS use), and therefore the probability of it being that significantly out is minimal.

I've been playing with a home made satnav/speedo/etc using a GPS receiver on my motorbike, and I've never had it be far enough out to get a ticket when I didn't expect to. In fact, I think it's more accurate than my speedo most of the time.

The GPS receiver I have (overnight) varied by up to 500m, and that was within a walled building - if it were in the open I'd expect it to be a lot smaller range. That was also (IIRC) before the intentional inaccuracy was removed.

Final comment - the speed guns are known to be flawed and give flawed readings, the GPS is known to be normally pretty good. Sounds like "reasonable doubt" at the very least to me.

Speedometers over state the speed by up to 10%, so are possibly less accurate than GPS devices. But anyhow regardless of that, given the story about that inventor who PROVED that he was going CONSIDERABLY slower than he was accused of, there appears to be a chance that he was under the speed limit (either according to his speedometer or not).

If these types of camera's are not completely accurate then they should be banned. PERIOD. 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).

Why haven't all forces reviewed their policy on using these camera's? Why... because their inaccuracy is making them money. When you get the ticket, it tells you that if you contest it they might decide to make the punishment worse. Surely that's blackmail?! But it works, and most people give in for fear of greater punishment.

Camera Safety Partnership - pah! Jobsworths the lot of them. They need there to be an issue so they can all have jobs. No-one is independent. Speed is a contributing factor to the amount of impact in an accident, but in most cases is NOT the CAUSE. If speed = crash, then why don't most plane's always crash when they take off. The most common cause of an accident is bad driving (or driving error). To solve this, get more Police actually out there driving around, picking up people with bald tires, no lights, or driving like an idiot.

This comes from a frustrated driver who is fed up with being taken as a mug by a bunch of money grabbing morons. I know very few people who haven't had points on their license, and those that haven't I don't know many who haven't come close. And fact, I know of no-one who hasn't sped at at least one point in their life - shock, horror... anyone who says they haven't is either a complete liar or is one of those drivers who should be pulled over for going 35 in 60 zones (clearly not recognising the national speed limit sign).

</rant>

To achieve any degree of accuracy with a gps system, you need to apply DGPS, or 'Differential GPS' . This requires at least 2 gps recievers and also often employs terrestrial radio beacons too. A moving, low-end gps reciever is not capable of precision calculations.

Also a car travelling up an incline will always appear to travel faster than one on the flat that has the same landspeed. Conversly racing downhill will give a slower reading on a gps than is really the case. This is real hair splitting, I know, but just to prove that gps is a global *positioning* system, and the devices do not generally posses the computing power required to calculate speeds. This is why an OEM navigation system in a car also includes a hardwire connection to the vehicle's own speedo system. (makes it work in tunnels too). TomToms etc are just cheap 'compasses' with a map database.

Even my handheld eTrex (1998) shows an altitude of -8 m when I walk on the beach......

The GPS inaccuracy is mostly due to variance the system introduces unless you are military. So your calculation is totally off. Between two readings separated by one second, the signal will bump about around 10 meters. GPS devices know that so they do a lot of averaging before presenting the data.

My GPS in the car gives stable readings to the point that I trust it to have at least 0.5kmh accuracy.

The trouble is that my GPS and many other commercial car systems do not offer any logging that can be used in court as evidence.

There needs to be a better understanding on how position accuracy affect speed calculations. Most errors in position accuracy do NOT come into play with speed accuracy. Sounds totally illogical I know but it is true. This is because most position inaccuracies are caused by atmospheric conditions. Another by terrestrial interference. The ones caused by atmospheric conditions are a gentle drift and affect the position relatively slowly and affect all samples by similar amounts. This type of interference has virtually NO impact on speed accuracy.

If you are in a city or have other natural things around like hard mountains, you can suffer terrestrial interference. This tends to happen very quickly and cause a very rapid deviation in position. This one can cause a large spike in speed but tends to only last for a couple of refresh cycles.

Another issue with accuracy is actually satellite geometry. If the satellites the receiver is using are all close together in the sky the math gets much harder because the angles are so small. If the satellites are overhead as well as near the horizon the accuracy in position and speed will be much higher. If you find your speed jumping around more than usual check the DOP reading in your receiver. You will likely find it is high or jumping around itself. This would indicate a geometry issue with bad satellite position relative to where you were.

Newer GPS receivers have filtering for terrestrial interference. They will automatically remove any samples that are considered outliers. The slow drift is also correctable here in the states if the GPS receiver has WAAS (Wide Area Augmentation System) ability. This system enhances position accuracy as well as accuracy speed accuracy.

Expect a GPS to report your speed accurately within about 0.3mph under good satellite geometry and low terrestrial interference conditions. Plenty accurate to beat a traffic radar.

Anne van der Bom is someone else who doesn't understand GPS. Accuracy may be +/- 3-30 meters, but it doesn't wobble by that amount between samples. If it did then someone standing still would be timed at between 0 and 67MPH. In practice the worst case drift you will see is +/- a couple of MPH, which is better accuracy than most car spedometers, and averaged over 5 or 10 samples you can improve on that.

@Anne:

"If a location by GPS can be accurate to with 3-30 meters"

Yes, but that's an /absolute/ location accuracy - the differential accuracy is much much better, especially over consecutive time intervals.

I will leave it to a GPS boffin to say how much better ...

To be honest Im not sure I like the idea of of using GPS (or any other continuously recording telemetry device for that matter) to verify speed. If everyone is happy to start using this information to disprove static and hand-held speed guns it is not that difficult a step for the powers that be to say "fair enough, we will stop using speed cameras (General public Huraah!).... we will just use the telemetry directly form you car (General public D'oh!)...". How many speeding tickets do you all think you could automatically generate on your daily round trip to work???

Jon

As most gps units do not rely on position to calculate speed the accuracy (even discounting dodgy weather conditions etc) is usually +/- 0.5mph

A quick search for 'gps speedometer doppler' will explain.

@ Anne van der Bom

The inaccuracies specified with GPS receivers may be 3-30 metres (although that sounds horribly overexaggerated - 3-30 feet maybe), but that's not saying saying you might get one reading at point A, and the very next reading at 30 metres west from point A, and the next 30 metres east from point A. For starters it means 30 metres diameter, not radius. And for seconds, it means general environmental conditions can lead to inaccuracies of up to 30 metres in a worst case scenario.

So unless the conditions accurate reception are flicking every second between completely perfect, and utterly terrible, and back to completely perfect, you'll never see consecutive readings vary in accuracy to much degree.

So, for the purposes of working out speed, this (fairly consistent) inaccuracy doesn't matter. For example; if I'm starting at point A and the GPS thinks I'm 30m to the west of point A, then it's 30m out. If at the next reading precisely a second later I'm 100m up the road at point B, and the GPS thinks I'm 30m to the west of point B, then the distance it's measured me travel is still absolutely correct - 100m. Granted, it may think I've travelled 100m along a path 30m to the west of where I did, but that doesn't matter - it still knows that I've done 100m in one second, so 360km/h or 223mph.

Yes officer, I was flying at the time.

@Anne van der Bom

You would be correct if your assumption about how speed was averaged in the receiver was correct. However it is not correct. In your assumption you imply that the errors are cumulative over a long period. They are not.

GPS does use time and distance to calculate speed but it is done over 30 or fewer position fixes. With the average sample rate of 4hz the time to accumulate 30 samples is only a few seconds and not a whole trip. Most modern receivers use a variable number of previous samples to average into the displayed speed. When speed is changing rapidly fewer samples are used to make the reading more responsive. When you are traveling at a steady speed more samples are averaged together to give a more stable reading.

@Anne van der Bom

"If a location by GPS can be accurate to with 3-30 meters, the distance between two points is accurate to 6-60m'

Doesn't follow.

GPS may have an uncertainty of several metres in terms of absolute position, but if you move exactly 10m south it will tell you that you've moved exactly 10m south. It will still have the same uncertainty about your absolute position, but not your relative one. That's how differential GPS can be accurate to millimetres.

As for it's accuracy against RADAR, GPS measures distance travelled horizontally. If you're going up or down a hill you WILL be travelling faster along the road than a GPS shows, and a RADAR gun should accurately measure your road speed, which is what you get ticketed for. From what I remember there are a lot of hills around Petaluma...

30 km per hour is 8.3 meters per second (30,000m/3600 seconds)

GPS recievers get a signal every 6 seconds?? (I did not find a definitive source for this), so the reciever will go about 50 meters between signals. Assuming an error of 5 meters, there is a 10 % variance in speed calculated vs actual speed.

If GPS sends more often (say once a second), the calculated velocity is useless unless you average out the signals over time (5 meters of possible error in 8 meters of distance). If GPS sends a signal only once every 30 seconds, then a 5 meter error over 250 meters between signals means the speed calculated is accurate to 2% - you get the same if you average the speed over 30 seconds.

The problem is that the police are measuring instantaneous speed while the GPS is measuring average speed. So don't tell the judge this :-)

Sorry for not using Reg units, the conversion to such is left as an exercise for the bored reader :-)

Yes, a lorry in a traffic jam... im sure its on google somewhere.. (maybe13mph??)

Gatsos Specifically: At low speeds with flat backed vehicles passing traffic reflections can cause false readings, say you were doing 12mph stationary object reflections would be 24mph , and passing traffic at say 26 would be a reflection at 50mph. hence the camera is triggered.

However these are easy to appeal as the only evidence is the pair or photos with a calibrated timegap showing distance travelled, and hence prooving innocence.

The Police are supposed to issue tickets based on operator measurements but have been know to just issue tickets to all people photographed. (its cheaper!)

The 3-30 error in one point does not correlate to another 3-30 error added on between two points. They are orthogonal and the two (assuming identical error properties) add up to be the square root of 2 times the error of one. Or about 5-45m.

However, a laser speedo has errors in calibration and in that the measurement is a field average or max (usually the latter, since the device isn't all that useful to police if it's adding in the stationary trees).

The speedo in the car has errors too. Gearing isn't 100% accurate and the change in wheel size as pressure changes (the speedo is really measuring the speed of rotation) and of course the ammount of slippage is a large error margin there too.

So, the person says they weren't speeding. Well, they would.

The policeman says they were. Well, they would for very much the same reasons.

The laser says they did (but the police can change the logs) and it has errors.

The GPL says they didn't (but the owner can change the logs) and it has errors.

So we can't prove who's telling the truth.

But, since the policeman is not on trial, we aren't trying to prove him lying. So we must allow the putative speeder the benefit of the doubt.

@ Peter D'Hoye

"The GPS inaccuracy is mostly due to variance the system introduces unless you are military."

Em... May 1st 2000?

Ring any bells?

End of US military induced SI (Signal Interference).

F5 your info dude.

@ Anne van der Bom

You probably only need 10% accuracy in a case like this (unless the police are really anal), which at 3m GPS resolution is only 30m. Still, at 30m resolution, you'd need 600m, which is still definitely greater than most speed traps.

Nonethless, the point remains that even if GPS speed measurement could achieve that level of accuracy, the devices would need to be calibrated, certified and tamper-proof in order for their evidence to be admissable in court. Consumer/commercial navigation/tracking systems are of no use for that present.

Question. Why GPS? Why don't commercial vehicle owners just attach a datalogger to the speedo?

The comments up till now assume that GPS speed is measured by dividing the difference between two positions by the time difference. This is not usually how it works, because position measurements can be wrong, by a large margin.

What GPS receivers actually do, is measure the doppler shift in the satellite signal as the receiver moves relative to the satellites. Usually this is accurate to 0.1 m/s or 0.36 km/h or 16.68E-9 Ssx in El Reg units.

The upshot of this is that speed measurement is much more accurate than the accuracy of positions would lead you to conclude (this also answers Dave Baraham's question on the altitude differences, although the altitude component is less accurate).

If this system is being used in his defence, they should take all the historical records, plot them onto a map with speed limits and also use the data to convict him if he broke the speed limit at any time in the past.

This should only be used in defence if you are going to be bound by all the data collected.

I've got a Garmin c320, and once I have a location fix the speed is pretty darn accurate. I've never had a problem with overcast days or snow fall. I can accelerate and de-accelerate and see my speed realtime, and it always matches the speedometer (Go straight to map mode to see your speed. You can't see it if you have put in a destination).

...Also...to the person says they've never received a ticket because they use their speedometer and stick to the limit....I guess you've been lucky enough never to have been hit with an inaccurate gun, or been hit in POP mode which they aren't supposed to use due to inaccuracies.

The GPS is not checking location every 30 seconds, it is continually monitoring its own location. The device UPLOADS data every 30 seconds, which INCLUDES the average speed across that 30 second interval. You guys are assuming that the average speed is calculated using a location every 30 seconds, which could lead to inaccurate readings. However, the average recorded by the device is accurate since it is a continual reading of the position in fractions of a second.

Random inaccuracies every 30 seconds could lead to a 60m range difference from point A to B. But if the readings are every 1/10th of a second, then you have 300 readings from point A to B, and thus those inaccuracies average out to nothing. Because it's being monitored continually, there's no chance that the kid can speed up between points A and B without the device monitoring 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.

<everybody now!> The greater good

Codswallop!!

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.

Jeez

@cor

"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...)

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.]

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"

"SI"

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!

@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... :)

@ 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.

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?)

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

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!!

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.

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.

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??

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

My wagon is a 560 Scania (V8)

http://farm3.static.flickr.com/2168/1625734937_62a5108095_b.jpg

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.

'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.

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.

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.

HOW ACCURATE IS THE SPEED READING?

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.

http://gpsinformation.net/main/gpsspeed.htm

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.

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

http://www.motorcyclenews.com/MCN/News/newsresults/mcn/2007/October/october23-30/oct2607speedcamerainaccuracycouldget100soffspeedingfines/?&R=EPI-96644

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.

"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.

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

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

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: http://www.gpspolice.com/faq/

Greater detail: http://en.wikipedia.org/wiki/Global_Positioning_System

...Rene

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 !

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.)

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.

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?

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.

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...

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!

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.

> 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).

THE MONEY ALL GOES DIRECT TO THE TREASURY FROM THE COURT FIXED PENALTY OFFICE... it doesn't even go VIA the police.

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?

...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....

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 http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html looks pretty decent to me, certainly much better than much of the doppler-related BS being spouted here.

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.

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.

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.

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.

So:

-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...

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.

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.

en

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.

>The comment from the officer about having a satillite signal possibly causing a delay demonstrates his lack of understanding of how a GPS works.

Surely his choice of job, large cop butt and "donut" diet demonstrated that before he said anything?

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

No, because money making BY the police is precise what it is. All you are saying is that it isn't necessarily money making FOR the police.

Would it make the police better if they went around arresting and beating up black people because the treasury is racist rather than them? Of course not, so take some responsibility.

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!”

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.

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.