Radar Unit vs GPS: Which Speed is More Accurate?

That's interesting. My reaction is go with the Stalker. It's within sight of the vehicle. And certified.

From Garmin:

"Garmin® GPS receivers are accurate to within 15 meters on average."

From Wiki:

"One of the most significant error sources is the GPS receiver's clock." I'll leave verification of the facts up to you. Under ideal conditions GPS can get you close.

Not to mention that the signal travels up to the satellite and back to earth.

105 MPH, Your Honor.

Thanks. You have given me 2 things to research, the GPS receiver's clock and the Garmin site to see how they rate their unit's speed reading accuracy.

Excuse my manners. Welcome to CR4. Thanks for registering. We're a diverse lot with a healthy cross section of intelligence and experience.

You should stick around, we need legal opinions sometimes.

I give legal opinions freely without any formal training at all, so at least you know the "lingo".

Good Luck in your quest for truth and justice. ^{_{105 mph!}}

"Not to mention that the signal travels up to the satellite and back to earth."

Unless I'm badly mistaken, GPS receivers are just that: receivers. They send no signals to the satellites. The only signals they send are over wires to their own display or to connected computers. There are probably some GPS devices that connect to computers via one or another form of radio signal such as Bluetooth, but such transmissions would have no effect on the precision of position measurements or other values based on the position measurements, like velocity.

Even with the elimination of that possible source of error, the radar gun remains the more accurate system.

Then how do the satellites know Which receiver is where? There must be some form of information request or tracking signal from the receiver for the satellites to respond to. Some type of electronic identifier. (like beep find me, beep find me, beep find me, you are here, no here, no here.) After all, this is still some form of signal triangulation.

The satellites DON'T know anything whatsoever about the receivers.

If you get your strongest signal from KOA Denver by pointing your antenna directly East, and you get your strongest signal from KOB Albuquerque by pointing your antenna directly South, then you must be somewhere near Wolcott, Colorado. Neither KOA nor KOB have any idea whether you were listening to their signals or not. That is a very simple example of signal triangulation.

Hopefully someone who knows a lot more than I about the GPS signals will jump in and clarify this, but the satellites constantly transmit information related to time and the satellite's location. It is up to the GPS receiver to acquire information from several satellites, and use that information to calculate position. Once it calculates two positions, it can then use the elapsed time to calculate a velocity. No signals need be sent from the receiver to the satellites.

GA. I am amazed at how many posters to this thread think that their anecdotal tales would constitute good evidence of anything. For instance, it is conceivable that at time t₁ you could be about to enter a hairpin turn, and at time t₂ you had recently exited the same turn. You might have traveled say 500m road distance in a minute, but the two positions might be only 50m apart. This would compute to a speed of only 0.1 times your real average speed. And that's assuming the two locations were accurately determined in the first place, which we know is not necessarily true.

Even if the GPS were 0.9999 accurate, for any two points on a curve, it would under-report your speed. It is distressing that some engineering thread responders can be so gullible. But then we get the over-unity stuff as well.

Sounds like those SV's and the receiver are as busy as a one armed paper hanger. With present day technology the whole operation isn't too daunting a task. Understanding the entire process is.

We know that both devices are capable of presenting precise data, both are equally capable of fulfilling the equation "garbage in = garbage out". I think that an expert witness could make a valid case for either one being the more accurate, given all the standard disclaimers.

I do hope the study and application of law relative to DNA was a lot more conclusive than what we are seeing here.

I hate to be the one to throw in the monkey wrench but that is not exactly true. The unit is capable of that accuracy but there are several significant variables to consider. I had the same discussion with an officer who said that I was going over sixty in a residential zone, but what he actually tracked was the turning velocity of a large truck behind me. I asked him if he really wanted to go to court with a weapons radar/ Electronic warfare tech after I got a statement from the driver who was behind me. Law enforcement likes to publish best possible specs but if you dig into the information that spec is only under very well defined conditions. By the way the speed on the garmin is determined by rate of change of position so the instantaneous position is not really relevant.

The real issue is that the court is going to accept the radar gun before anything else and you are already guilty without expert witness to dispute the radar speed. You are going to pay the price either way.

Sorry, but I have to disagree.

The manufacturer's specification for this particular model is an accuracy of 0.05 m/sec, or roughly +/- 0.1 mile/hour. I'm sure that this is under perfect conditions, but I'm also pretty confident the radar gun's stated accuracy is under ideal conditions as well.

Actual GPS readings vs. true speed studies indicate that such GPS velocity accuracy is realistic, also shown here.

So, if you go by device specifications, the GPS wins hands down, with 10X the accuracy of the radar.

Under real world conditions, GPS shows accuracy comparable to the radar guns specifications (specifications tend to be stated for ideal circumstances).

Both radar and GPS are prone to errors. It is impossible to determine which device had the most error at the time of the measurement.

So to answer the OP's original questions:

The GPS is more accurate under ideal conditions. It is also not prone to operator error, which the radar gun is. However, GPS does have it's own sources of potential error.

As to which is more reliable (accurate), there are just too many variables to know for sure.

With my Garmin I have never found the large discrepancy that the OP posted.

How many times do you get pulled over?

I haven't been pulled over in 25 years!

Previous to that once about every 7 years.

(Yeah, chicken shit driver that tries to obey the posted limits! Saves $ and lives!)

While I would agree with the radar unit being the more accurate, there are some caveats:

1. Was the calibration on the radar unit still valid at the time of the infraction?

2. Was the officer properly calibrated (training up to date) at the time?

I mention this because a coworker (~30 years ago - he was a radar technician) beat a ticket that way. Not only that, but his lawyer who took the case pro bono (provided that he had his facts right), and negated quite a few other cases he was representing - including his own.

My advise (usually not worth reading), is to say OK, you have just proven/admitted that you were going 96.4 mph (still an offence), without setting a precedent that GPS is better than radar.

Rereading the original post: "A duly calibrated 'Applied Concepts Dual Stalker' radar unit, used by an experienced Officer with no apparent outside interference."

Seems that the jig is up. ;-)

Why is this even an issue? What would be the difference in a fine between (let's say the speed limit was 70 mph) 26.4 or 35 MPH over the speed limit?

Would it make that much difference?

Anyway, I'm with the radar for accuracy, being a much more "local" system of measurement.

Some states make it a felony for speeds of 100 mph and beyond. So it can be a BIG deal to the offender.

In my experience GPS indicated speed is very accurate when tracking 8 or more satellites. Not so much around tall buildings, cliffs or in a tunnell. Also the max speed on the display can be reset in a second by the operator.

In my experience, my GPS receiver indicated speed always tracks within 1 mph of my car speedometer. And I have used the same GPS receiver in four of my vehicles, plus at least one, maybe more, rentals.

I don't know the update rate, but if I step on the gas or brake, the GPS indicated speed tracks with the speedometer.

I would say that if you were traveling at the higher speed long enough to be locked in by that radar gun, your GPS receiver should definitely have indicated that same speed, plus so should have your speedometer. If both your speedometer and GPS receiver indicate the same speed, that is a slam dunk that the radar is the outlier, and therefore questionable.

As others have pointed out, the GPS accuracy depends on signal strength and number of visible satellites. Somehow I can't imagine you were doing either 90 or 100 mph while tall buildings were blocking your view of the sky. If that were indeed the case, you would be an idiot and deserving of whatever they throw at you.

The real issue here is that even assuming your GPS and speedometer were both reading the speed you claim, how do you make that case in court?

emc_c

You should read the original post more carefully.

"I don't know the update rate, but if I step on the gas or brake, the GPS indicated speed tracks with the speedometer."

I've had the same result. When my speedometer reads 70, GPS says 72; speedometer 35, GPS 36. (I attribute to tire size and age of speedometer.) No deviation, even by +/-1 mph.

Dangerous Bill

I don't think anyone can make a case that the gps is more accurate than the hand held device. I do think that the driver is captive to the instruments in his vehicle.

The op states that a felony is not involved in this case, only the fine. So nothing of real value is involved except the principle. If the op had said that an experienced officer had observed the vehicle speed and estimated that it was X mph, I personally would have found that statement to be more objective than quoting what some digital readout had indicated to the same officer. Neither electronic device is infallible.

Unless this is a German autobahn, the driver admits his guilt, why would you split hairs over the numbers involved. To his knowledge he was making X mph based on two instruments of questionable accuracy that seemed to agree. The officer probably only had the one instrument. I doubt if he had much empirical knowledge about vehichles traveling 105 mph, most folks don't get to see much of that on the local highways.

While it's not stated in the original post what the difference in the fines would be, it is logical to assume that it must be significant.

If it was as petty as you suggest, then I don't think the question would have ever been posted.

Laws vary state to state, but, for example, in Virginia it's a fine of up to $2,500 and mandatory jail time once you cross 100 mph. They are the most strict that I know of when it comes to speeding.

Do we know yet whether this incident occured in the US before we start quessing which State has jusisdiction?

Looks like the OP has left the building.

People making all sorts of assumptions on motif now.

No we don't, but I can only assume that there is a battle between >100 mph versus <100 mph, there probably is more to it than just a proportionally higher fine because the delta between the two numbers is simply not that great.

Here is another key word,

"Guest"

At stake, besides justice is only the amount of the fine.

I guess that statement is open to interpretation. You are right about Virginia, Over 75 is considered reckless driving.

This is a first for me and quite enjoyable. Thanks to all of you for your input. The technical information and suggestions were/are most helpful, and even your gut reactions and other observations are of interest. As amusing as some of your exchanges were I didn't mean to start a mini firestorm (don't be too hard on them lyn) and I should have been clearer at the outset. As Hero succinctly stated and others surmised, a finding of a couple of mph either way can make a huge difference at these speeds. In California over 100 mph has a minimum fine of $776 versus $445 for 100 mph, no traffic school and 2 points on the record,(which is the same as for reckless driving and a definite insurance jump); a possible 30 day suspension. A second conviction in 3 years has a much >fine and mandatory suspension by the DMV. As to some of the other points: Besides the fact that I believe that everyone receive and hopefully feel they had a fair hearing, even if they lose, as noted by KilowattO, this Radar v. GPS case is important to local law enforcement and will be somewhat of a precedent, so the reasoning has to be as sound as possible. Finally, although I never assume that anyone is perjuring themselves, I want to ask Ace if he is sure that the display can be easily altered? Thanks again.

I can't speak for the specific GPS system employed in the car, but on mine I can go into specific screen and reset all the trip data gathered to date.

What your next course of action should be is to look online or get the paper copy of the specific operator manual for the GPS system in the car. That manual will tell you exactly how the user can reset the "trip data", which includes the trip odometer, average speed, and maximum speed.

From there you can make a case for or against the probability of the operator performing the reset function to confound the reading of the radar gun.

This is still an uphill battle, but if opposing counsel does not pick apart your argument you may still win the day. GPS operation is still a mystery to most people and that may play in your favor. However, you sound like an astute lawyer and I am sure this is not the only arrow in your quiver.

Good luck with your case. It sounds like it will be interesting.

My Garmin Nuvii routinely reads faster than my Speedometer by about 2 mph in a 2009 GMC F350, cruising in open country.

However, I have noticed on a level curve with cruise control set and engaged for 70mph the speedometer reads steady, but the Garmin will read 1 or 2 mph slower. I always figured it was the GPS measuring cords on the curve that were shorter than the path travelled.

About 35 years ago as a passenger in a Porsche, the driver got a ticket for about 15mph over in a 30mph zone. I was certain he was not speeding. As we sat in the cruiser with the Porsche ticking over behind us the radar was quite content to read 10 to 15 mph without another vehicle in site! To this day I think the driver got a bum rap.

I wish to advise that a Lidar unit is affected by many physical and weather phenomen and I have be the target of a zealous officer of the law. I was following another vehicle closely at the legal distance behind. I was clocked at 79 Kms/hr than the the mates car in front whose speed was supposedly clocked at 61 Kms/hr in a 60 Kms/hr zone. We had traveled in convoy for more than 80 kilometers and it did not ring true that such a speed variation could be for two vehicles that were traveling as though there was a tow rope between us. It depends on the officer behind the gun and I doubt there was a 2 second delay between us that the officer could have set and re-targeted my car. All I can say, is check the copious literature on just how in-accurate speed guns are away from the perfect conditions required to correctly calculate speeds. A famous court in California was shown that a 'Palm' tree in the court building was in fact doing 23mph.

Not to worry your Honor - we do this for fun.

Pigs in mud and engineers in arguments, as they say.

I work with high accuracy and high reliability GPS certified for flight and get asked regularly why someone's dash board model acquires satellites and provides position so much more quickly than my rather expensive ones.

Because there are no consequences for your dash board model being wrong.

So just a quick couple of technical details:

A. The radar gun is both more reliable, more accurate, and certified.

B. So as not to be a Luddite, if they wish to present contradictory evidence from on-board equipment, ask if they are willing to have the ABS downloaded. Also not certified, but an interesting third source of speed data and I would guess lots more accurate than the GPS. And ABS data HAS been used in evidence before.

C. In general...GPS is very good at locating you on the planet - everything after that depends on the machine that uses that data. It is actually quite difficult and expensive to make a good overland navigator that is moment-by-moment accurate. And they are not available for $499 at Best Buy.

But as others have said - glad to have you around!

You can 'splain to the rest of us the difference between law and justice ; )

Had the gps been reset the mileage would have read .1 miles. I do not know of being able to reset the max speed without resetting all the data.

There are requirements for calibration, certification and recertification of radar guns. There are no such requirements for onboard GPS units. The GPS could have been malfunctioning, its update rate might be to slow to see the brief peak to over 100 mph that the radar caught, etc.

Radar is admissible. Onboard GPS should not be, particularly because the onboard GPS is under the law violator's control: who can know how it might have been tampered with, and who knows if the maximum speed recorded by it was recorded at the time of the speeding offense? Who knows if the GPS was even turned on at the instant of the speeding offense? The maximum speed recorded by the GPS has no validity.

At very least, the evidence would have to be a strip chart plotted from GPS data, with each (second-by-second) data point date and time stamped. Very few GPS units have the capability to retain such data. But even then this would have to be a certified, calibrated GPS unit.

The guy was doing 105. No reasonable doubt that he was not.

The gun, just because the PIG* was holding it!

* Pride Integrity and Guts ...

Hi there, Guest,

Boy, that acronym takes me back about 25 years! Now I'll spend the rest of the day wondering why I ever left law enforcement, albeit it was only a small university department. I could've been retired and on a beach somewhere by now...

Logan

Sorry, all, forgot to mark this as 'off topic' and I don't see that capability in the Edit mode. Mea Culpa...

This is a strange one. Th "right" ruling here seems an ambiguous statement. The last paragraph about the stalker seems a dead dead giveaway of a done deal. Why then bother about the accuracy? You have already decided that the officer is right, therefor his evidence is above question.

If you are a real legal person and you practice "fairness and apply benefit of the doubt principals" then by all means use the supplier's (Garmin) evidence rather than a forum which is by and large hearsay anyway.

Some interesting posts here today, but another thought comes to mind. GPS should be a lot more accurate, (if good weather prevails) because strictly speaking you have only one moving party, the car. Contrary to that you have two moving parties in radar, the car and of course the radar gun. Irrespective of how well trained the operator is, he still has to aim and "contact" a particular area on a car that is moving toeward him ( or away), decreasing or increasing, the distance.Thus he is moving continuasly to maintain his aim. Nobody will ever,ever know wether the officer did not have a slight shake on any givrn moment in his aiming. Even the units on tripods are handled. Only the vertical plane may be still then.

"Because strictly speaking you have only one moving party, the car."

No, no, no! GPS is fraught with errors as the constellation of satellites is always moving and positions are not always optimal, ionospheric conditions are changing, which does impact signal path, and issues with what is called multipath.

As I posted earlier, GPS has an accuracy, at best, of 15 meters for a car's GPS unit.

I personally find the velocity component pretty good, but it's not dependable for some of the reasons I cited.

I have some knowledge in the field because I worked on systems for aircraft and "precision approach" autopilot landing systems using GPS as the primary positioning source for lat/lon and vertical height. The integrity of these systems are far beyond what you find in the car and still lack the absolute accuracy that you can get with either Doppler radar or laser radar speed guns.

However, I am not saying GPS is not good since it can successfully land an aircraft, its velocity component is not going to reach the precision a radar gun can achieve.

Personal observation & 1-time "calibration" with police laser: GPS is spot-on (Garmin Nuvi 200). Personal opinion is that GPS gives me an excellent indication of speed in open areas, good enough that I'll know when the traffic cops are doing something wrong.

Having said that, I think that only the type of GPS that keeps a tracking log over small time intervals will suffice for a court of law, as then you can see speed over distances, in the exact area you were caught. It allows you to state your speed at a place and time. This can be analysed and verified by an expert, and could probably cast enough doubt on the radar system - if the expert does indeed agree! However, only the upper models allows this, and their default storage interval may be too low to be of use.

Radar/laser is difficult to challenge: operator is usually believed, as he is certified & radar device too. But certification does not automatically mean that 1) the operator is using the device correctly or 2) that the device is still functioning correctly (I'm also certified to drive a car, yet I can make mistakes.)

Good luck deciding, your honour!

One point that has not been addressed is that the driver may have been convinced that he was doing 96mph. If this was in error and his real speed was 105mph then it could have been a calibration fault in his speedometer (if he was relying on that). If he was relying on the GPS then in my view he has no leg to stand on (if his speedometer was showing the right speed that is). In that case technically the only thing to do is to verify if the speedometer of the car is calibrated correctly. I don't know legally where the driver would stand if his speedometer would be found way out but I would think that he would have a good reason to sue the car manufacturer. I believe that in the past there have been cases where speedometers were not calibrated correctly by the manufacturer and were showing underspeed (i.e. 55mph when they were doing 60mph). The manufacturer was taken to court and failed his defence. Since then speedometers are always calibrated on the safe side (i.e. showing 60mph when they are really doing 55mph). Also, accurracy of the reading reduces at higher speeds which is why for example in the netherlands justice allows for 3kmh + 10% of speedlimit in favour of the speed offender.

For what it's worth I would believe the GPS over the radar gun, if you can be sure that the data has not been tampered with. As Jim pointed out

the speed on the garmin is determined by rate of change of position so the instantaneous position is not really relevant

There is a huge difference between resetting the data and editing the data. You should get someone who really understands the GPS unit in detail to assess the possibility of tampering and examine all the data in detail.

I would also treat is as an opportunity to get to play with the radar gun (I don't know what your "budget" is here): take it to a disused airfield or race track with an expert and some trusted colleagues and see how easy/difficult it is to get erroneous results by moving your aim up and down/side to side fractionally whilst taking a reading.

Dear friend!

the result of a GPS measuring is a statistical answer coming out from a lot of calculations. The result may be from diluted, reduced, accuracy because of weather and somthing like disturbances. A civil GPS result is never more precise then +- 15 meters, as long as you do not use differential senders, this will improve the results and make them for about +-5 meters precise. A military used GPS delivers informations with an accuracy +-50 millimeters.

In the opposite, the radar signal won´t be disturbed in accuracy by clouds, rain and something like.

So the measuring result of radar will the more precise.

Trying to think about the accuracy of the speed calculated by the GPS, and nothing else, it would seem that if the GPS was having issues with determining accurate position (the +/- 15m spec) then the speed over any time should fluctuate high and low and that the maximum calculated should always be higher then the actual maximum.

Example:

Sample 1 is accurate

Sample 2 is 5m closer to 1 then is should be = calculated speed is too low.

Sample 3 now reads 2m further away then is should = calculate speed is too high

So I don't see how the GPS calculated maximum speed would ever be less then the actual unless the car only obtained that maximum speed for a fraction of a second, (not likely) or, in this case, the data from the GPS was not the data for this particular speeding excursion.

I would like to interject a different line of thought to this whole process.

Having taken rides with my father in law many times in his cruiser. I came to the understanding of a couple of things.

1. the radar locks onto the largest fastest moving vehicle.

2. The radar can give an improper reading if the gun is a handheld device and is moved in a forward motion towards the vehicle being tagged.

The GPS is easily resettable no matter what type it is and nothing is date stamped.

Those being said I would like to interject that if the vehicle in question is later than a 2005 model, it can be taken to a dealership and a log can be drawn from it giving the highest miles per hour that the car has gone. Dealer use this to verify warranty conditions have not been infringed upon by the owners when making claims.

You may be able to easily settle this by simply having the vehicle inspected at the local dealership and find out what the maximum speed it has been driven to. It wont be hard evidence as to the specific date but it could give you an idea as to whether the vehicle has reached the 105 mph that is in question.

Ignoring the physical cosine error if you motion is not directly on line with the radar, the radar is more accurate. GPS time averages and is not nearly as accuatrate as radar (ignoring the angle error between you and the radar).

Prejudice notwithstanding and as much as I'd like to support the driver, I'm inclined to support the readings taken by the officer, I am making certain assumptions in so doing:

1. that the officer's patrol car or radar unit was itself stationary
2. that the ticketed driver was the only vehicle in the cone of radar emission.
3. that the speed measurement was made within the maximum range of the radar unit and for which reflected signal strength and calibration can still be trusted.

A a general rule, high frequency radar units are more precise and more responsive over the short echo ranging distance than changes in satellite signals used by the GPS unit to approximate speed and distance.

L.J.

Sir, Whilst I agree that you have a difficult job, and I would not change rolls with you, I have seen the other side of the force here on Oz. Too many times people have been booked at speed over and above the actual capable speed the vehicle is capable of achieving. Ie: a vehicle was clocked at 138Km/hr when the car in every test under the judge forced the officer to trial in the impounded car and the very best he could achieve was 119km/hr in the test track. This made a mockery of the certified and supposedly duly calibrated speed gun and the correctly trained officer then also had to admit the gun could not have read the reading of 138km/hr. The vehicle had been impounded from the time of the alleged offence until the time the judge ordered the test be done with the co-operation of the vehicle owner. The gun was duly rechecked and found to be accurate. The weather conditions of the day may have been a limiting factor as were the road site conditions with rail lines across the roadway and locomotives on the side of the road, lights with 2 x 10" diameter quartz halogen tungstun ablaze (required by the department to be illuminated at all times the locomotive is running) large hording signage, severe patched road surface of differing texture, and the area is prone to mirage conditions over 300 metres.

The officer and meteorological department verified the day was at 43 degrees celcius (109.4F) with intermittent cloud cover just prior to a torrential thunderstorm that damaged the area just minutes later. There was also high levels of static in the atmosphere and the officer agreed that he had been 'zapped' when he placed his hand on the vehicle when issuing the infringement notice.

I don't know if this is conditions that you would operate under, but the officer was and is a known friend of mine and is quite believable as a diligent and honest person. He is very hesitant to be in charge of Lidar and Laser guns anymore and stated that the police force have a duty forced on them to achieve a certain number of tickets per month and the current affairs program have bought this to the public on a few occasions with the applicable government department denying it is a revenue raising monster of the government.

I have my own personal take on RADAR, Lidar, Laser and other types of speed entrapment machines, that none can be trusted under all setup conditions at all times outside the laboratory conditions.

I concur with Anonymous Hero as the most inclusive best GA, and too others were good adders.

I would think the sampling rate of the GPS (Garmin) would be key, certainly much greater (not to be confused with better) than the radar. P.s.: I used to calibrate radar guns.

By the way, the OP was not asking for opinions about excessive speed.

From the Garmin site itself:

"The software is provided on an "as is" basis without warranty of any kind -- either expressed or implied -- including, without limitation, any implied warranties of merchantability or fitness for a particular purpose. Garmin does not warrant the performance of the software or that the software will meet your requirements or operate error free."

The above is the warrenty statement of the GPS's Firmware. A pretty strong arguement against the GPS from the manufacturer itself. I will keep digging and post more as I find it to assist in this matter.

JavaHead

Man up and pay the fine. In the future drive like you have some sense.

Read the post. Better to remain silent and be thought of as a fool than to say something and remove all doubt.

When all is said any timing devise that must be accurate needs to be checked periodically against the national standard to determine its accuracy. I believe police speed radar guns must be re-certified periodically. I would suspect the same criteria should be applied to the GPS. Till the timing devise, whether GPS or radar, is certified, its readout must be considered anecdotal evidence.

GPS receivers do not have an internal clock, they use the satellite data for the clock.

No receiver calibration is ever required.

I am curious about the relation between the absolute position accuracy and the calculation of velocity. So far it appears it is assumed the speed calculation is determined for short periods of time and displacement. I have seen the error window shown +/- 15 to 50 meters, but the position reading does not "jump around" in that window.

Depending on the speed algorithm (lets assume it is not instantaneous between every reading), then the absolute position error could become negligible. Example: reading a shaft position with an absolute encoder that has a +/- 5% error. If I take 2 readings and use that to calculate instantaneous velocity I can have very large errors. However, if we take 1000 shaft revolutions only the first and last positon errors are accumulated, the other 1000 revolutions are absolute, so the percentage accuracy of an average speed or total shaft revolution count inproves with the higher counts.

The same thing woud occur with the GPS. The first position may be out +/- 15 meters, and the last position may be out +/- 15 meters, but if the total position change and elapsed time are large, the relative error diminishes.

In my humble opinion.

Speed detectors that are in use by police agencies today have been under the scrutiny of every trial lawyer from sea to shining sea. Any that could not stand that abuse have been removed from the market. There is an entire industry that has evolved to prove that someone is speeding. As good as GPS systems may be, they have not had to withstand the close inspection that the successful "radar guns" have. That may happen some day, but right now, there is just no support system for the GPS units.