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Joined: Mar 02, 2006 Posts: 384 Location: Bedford, England
Posted: Mon Mar 20, 2006 12:55 pm Post subject:
Quote:
How do you know this?
I presumed that this was how they worked too, but when I did some reading about it and a number of people say that the speed is measured by doppler because the position jumps around too much to get an accurate speed reading
Plus, it won't be doppler from a single satellite used to determine the speed, it will be averaged from all the ones the unit has a lock on. But you make agood point, if you have a GPS unit that doesnt lock you to a road, you'll see that your position jumps about a fair bit. Without WAAS, maybe by 20 to 30 metres at a time. even with WAAS it will vary a fair bit, certainly enough to make speed differences vary dramatically from one moment to the next. Using doppler would eliminate most of that. So, take your dopplered speed from say 6 satellites, and 'averaging' it is going to give you much more accuracy.
Tw
The jumping around on speed is resolved in several ways, on the "simple" satnavs that we use. First, by "jumping to nearest road" on the map, we can eliminate any lateral discrepancyand need only to consider time/distance along the road. Next, we can apply a smoothing algorithm to the position reports to "average out the positions". Finally, depending on the satnav, we can decide how often to update the speed display. The longer the time between speed updates, the less % discrepancy in the reading. More accurate satnavs can afford to update more often. My Tom Tom Go 700 updates speed every second. Older models update every 2 seconds.
To finally resolve the situation, I have asked Tom Tom and Trimble technical to say how speed is determined. I'll let you know the replies.
Joined: Feb 07, 2006 Posts: 616 Location: Midlothian
Posted: Mon Mar 20, 2006 1:04 pm Post subject:
As a Traffic Cop, I have to agree with Tom Davison. All Police radar systems including Gatso, use the doppler effect to calculate speed. Depending on whether the frequency is increasing or decreasing, this would indicate in what direction the target is travelling. The most accurate measurement using doppler is when a target is travelling directly towards or away from the transmitter. The more the angle away from directly towards or directly away from the radar, the more inaccurate the speed reading would be. And yes, a vehicle travelling at 100mph across the front of a Gatso will not be recorded.
Quite an interesting thread this one. _________________ Tommo...
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Mon Mar 20, 2006 1:41 pm Post subject:
TomDavison wrote:
in practice, you are almost never driving directly towards or away from a satellite. This would only occur if the satellite appeared on the horizon, directly in front or behind you. If the satellite is anywhere else, doppler speed calculations would under-read.
Yes, but if you knew the exact location of the satellite and your direction of travel then you could calculate the angle of your movement relative to the satellite and divide the doppler speed by the cosine of the angle of your movement relative to the satellite to get the true speed.
As you say, this wouldn't work well when your direction of travel was at right angles to the satellite, but you will always have between 3 and 12 sats to use to calculate the speed from so you can take multiple readings and use them to determine your speed.
As Tumbleweed observes:
"if you have a GPS unit that doesnt lock you to a road, you'll see that your position jumps about a fair bit."
With lock on road switched off, the position can jump by several meters between samples. So if the unit took a measurement from the last known position to the current position one second later and tried to calculate the speed from these, it could contain an error of several meters/second.
To get the claimed accuracy of .1 meters/second, the GPS would have to measure the distance to within 10 cm between two points spaced 1 second apart. Even if it averaged the position over 2 or 3 seconds, the position error would surely be substantially larger than the .1 m/s accuracy they are able to claim. _________________ Gone fishing!
Joined: Mar 02, 2006 Posts: 384 Location: Bedford, England
Posted: Mon Mar 20, 2006 1:51 pm Post subject: Re: Which speedometer is most accurate?
tonetheteach wrote:
Greetings-
I have recently bought a TomTom One and when my wife and I went out for a drive today I noticed that at the bottom of the TomTom screen it showed the speed at which we were travelling. However, when comparing this with the speedometer of our Ford Focus it was noticeable that the car registered a speed of three or 4 mph greater than that shown on the TomTom screen.
Which, if either, is likely to be the most accurate? I hope someone can help with this as it will put a stop to a little bit of disagreement!
Best wishes --tonetheteach.
We became a bit sidetracked on the original post, so I'll go back to that and answer the question of which is the more accurate system.
Let's start with the basic premise that all vehicle systems that record speed by calculating the rotation of the transmission are inherantly inaccurate. The main reason for this is that tyres wear. A new tyre with 8mm of tread will have a circumference which is 5.45cm more than a tyre which is bald. A tyre which is 18 " across has a circumference of roughly 5 feet. So it goes round roughly 1000 times a minute at 60mph. Therefore over that minute, the distance covered could be roughly 50 metres different for the same speed reading, depending on tyre condition.
The next premise is that it would be a bad idea if your speedo told you that you were doing, say 70mph, while you were actually doing 75. If it said you were doing 70 while you were actually doing 65, it wouldn't get you into trouble. For this reason, construction and use regulations say that the speedo in your car must NEVER under-read. To allow for the discrepancies, they do allow it to over-read by up to 10%. So if you are indicating 30mph, your true speed can legally be anything between 27 and 30. If you're indicating 70mph, your true speed can legally be anything between 63 and 70.
Because of manufacturing tolerences, speedos working on mechanical principles are unlikely to be absolutely accurate, so manufacturers need to ensure that the variations all lie within the legally permitted limits. It seems that most, therefore aim at about a 4-5% over-read. Indicate 70, and you are doing a true 66-67. With very accurate speedos, they can aim to be closer to 70mph, but must always ensure it never under-reads.
Separate satnavs don't have this legal constraint and they don't suffer from tyre wear. The satnav manufacturers can therefore aim to be exact. So, as we said in earlier posts, under ideal conditions - straight and level road, steady speed - satnav should be exact, but can differ from your speedo by up to 10%
As you go round corners, or up/down hills, satnav will underread., and become accurate again once you straighten/level out. Accelerate or decelerate, and satnavs will lag behind, (can be WAY behind) becoming accurate again once you achieve a steady speed. During these manouevres, your vehicle speedo may well be more accurate.
I therefore regularly (every 3 months or so, or on tyre change) "calibrate" my vehicle speedo by checking it at different speeds, on the straight and level so I know the discrepancies. After that, I drive on my car speedo, allowing for the errors, since these errors don't vary with hills/corners/acceleration/deceleration.
In all of the above, I am talking about simple satnavs for vehicle use. The more complex and much more expensive, aviation satnavs can work in 3D and cope with climb/descent.
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Mon Mar 20, 2006 3:22 pm Post subject:
TomDavison wrote:
by "jumping to nearest road" on the map, we can eliminate any lateral discrepancyand need only to consider time/distance along the road.
Not true. The jumping to road is a feature of the mapping software not the GPS receiver.
999tommo, this is called the cosine effect. If you divide the measured speed by the cosine of the angle to the direction of travel of the vehicle then you will get the correct speed. Provided the angle is small, the error is not too significant. Gatsos so fire a radar at an angle to the road but they are calibrated to correct for the cosine error caused. _________________ Gone fishing!
Joined: Mar 02, 2006 Posts: 384 Location: Bedford, England
Posted: Mon Mar 20, 2006 4:23 pm Post subject:
Skippy wrote:
TomDavison wrote:
by "jumping to nearest road" on the map, we can eliminate any lateral discrepancyand need only to consider time/distance along the road.
Not true. The jumping to road is a feature of the mapping software not the GPS receiver.
Sorry, I didn't make myself clear. I was referring to the operation of a vehicle satnav, which of course includes both the GPS receiver and the mapping software. (at least in my Tom Tom Go 700 it does. Some satnavs do have separate GPS receiver, which simply sends positional data to a separate navigation module ) In practical terms, I'm not differentiating between the two. Perhaps I would be better saying that by "jumping to the nearest road", the in-car satnav can eliminate any lateral discrepancy etc etc.
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Mon Mar 20, 2006 8:00 pm Post subject:
TomDavison wrote:
Perhaps I would be better saying that by "jumping to the nearest road", the in-car satnav can eliminate any lateral discrepancy etc etc.
The thing you don't understand is that the mapping part of the sat nav does not calculate the speed - the GPS receiver does this completely independently of the mapping program. This is proven by the fact that the speed is just as accurate with or without maps available. _________________ Gone fishing!
Posted: Tue Mar 21, 2006 5:45 pm Post subject: In real life
This is a very fine conversation, and as an electronic engineer its great to see the technology being discussed.
Either way, one evening on an empty, flat and straight part of the M62, at a constant speed of 75mph (ford focus speedo) and 71mph (tom tom), with the speed being held by the cruise control, I was pulled over by some of humbersides finest and cautioned for doing "almost 75mph"
So as I think the conversation was started to settle an argument between a couple who noticed the variation, I think the important thing is to know that our friendly police officers know what it says on our speedos as we cruise past them, and take that into account when deciding to see if they want a chat or not!
Posted: Tue Mar 21, 2006 10:16 pm Post subject: Re: In real life
mooseman wrote:
This is a very fine conversation, and as an electronic engineer its great to see the technology being discussed.
Either way, one evening on an empty, flat and straight part of the M62, at a constant speed of 75mph (ford focus speedo) and 71mph (tom tom), with the speed being held by the cruise control, I was pulled over by some of humbersides finest and cautioned for doing "almost 75mph"
So as I think the conversation was started to settle an argument between a couple who noticed the variation, I think the important thing is to know that our friendly police officers know what it says on our speedos as we cruise past them, and take that into account when deciding to see if they want a chat or not!
Mooseman
Ford Focus
iPAQ
Tom Tom
Police Caution
IF they used their car speedo to do you,and didnt actually report you officially, then they were trying it on IMHO (IANAL), I presume their speed said 'almost 75' so they presumed yours did as well. To actually report you so you get fined they need , AIUI, specially calibrated equipment. Did they just 'tell you off' and tell you to be on your way?
Joined: 06/06/2003 10:17:21 Posts: 57 Location: United Kingdom
Posted: Wed Mar 22, 2006 3:17 am Post subject:
I know not the answer here, but Datron and Vbox are two companies/products that provide solutions for accurate speed measurement based upon GPS. Here`s some links to their very interesting websites which seem to shed some light on the matter. At worst they seem quite approachable for a direct answer, if asked:
IF they used their car speedo to do you,and didnt actually report you officially, then they were trying it on IMHO (IANAL), I presume their speed said 'almost 75' so they presumed yours did as well. To actually report you so you get fined they need , AIUI, specially calibrated equipment. Did they just 'tell you off' and tell you to be on your way?
Tw
Yes, I just got the telling off. I was wondering if I could bill them for having to brake, and then having to accelerate again?
I find the best approach is just to agree with the fine officers, and hope they will let you on your way! It was unusual to hear a focus described as a big powerful car during the telling off though!
IF they used their car speedo to do you,and didnt actually report you officially, then they were trying it on IMHO (IANAL), I presume their speed said 'almost 75' so they presumed yours did as well. To actually report you so you get fined they need , AIUI, specially calibrated equipment. Did they just 'tell you off' and tell you to be on your way?
Tw
Yes, I just got the telling off. I was wondering if I could bill them for having to brake, and then having to accelerate again?
I find the best approach is just to agree with the fine officers, and hope they will let you on your way! It was unusual to hear a focus described as a big powerful car during the telling off though!
MM
Got to agree, even though you could have said 'actually I was driving at exactly 70 as indcated by my GPS' they might then have taken a look around your car and booked you for something else.
ISTR reading about a guy who had an altercation with police but they couldnt arrest him as he hadnt atually done anything wrong, that evening a squad car with several officers arrived outside his house and went over his car with a fine toothcomb, and he was prosecuted for not having any windscreen washer fluid. Made the front page of the Times IIRC.
We are not sure what you mean by measuring doppler shift in satellite frequency. What the Go 700 does do is calculate your speed by dividing distance by time taken to cover this distance. This is all done with GPS accuracy hence the speedometer on your Go unit will be more accurate than the one on your car.
With best regards,
The TomTom Customer Support Team
I still haven't heard from Trimble, but I'll add that when I do. (If anyone thinks I'm faking this, I'm happy to forward the original e-mail.)
We are not sure what you mean by measuring doppler shift in satellite frequency. What the Go 700 does do is calculate your speed by dividing distance by time taken to cover this distance. This is all done with GPS accuracy hence the speedometer on your Go unit will be more accurate than the one on your car.
With best regards,
The TomTom Customer Support Team
I still haven't heard from Trimble, but I'll add that when I do. (If anyone thinks I'm faking this, I'm happy to forward the original e-mail.)
Regardless of whether their answer turns out to be correct or not, I would seriously question the ultimate validity of a technical answer provided by a general customer support team. Can you determine from the email that the question has been referred to a technical person (most probably not if my experience working within such companies is typical), or has the answer been provided off-the-cuff by the 18 year old know-it-all at the next desk to the person receiving your email (far more likely given the wording of the response) ? Note I have no fixed idea of what the actual answer might be here - that`s what we are trying to determine - but from my own experience I would place zero credence upon such answers from customer service teams. The poor oik would probably get his head ripped off if he stepped up to the design engineer`s desk !
I think a much better route would be to try and speak to one of the engineers who make the VBox at my link above, because this is their exact specialised subject - as opposed to the more diverse GPS application that is TomTom. It`s certainly a topic much more directly tackled on their website at least - increasing the chances of you getting an accurate answer (out of a customer service team). Not an easy task of course, as most design engineers don`t want to go near their customers, most of which go out of their way to be as awkward as possible (without always realising it). Nor do their employers wish for them to be diverted from their real tasks in hand, particularly for internet tittle-tattle such as this. ;)
Interesting if you can get to converse with an engineer though. The more I think about this, I wonder how such speed accuracy could be achieved (particularly at slower speeds) given the current ability of GPS to position someone. But I`m sure someone with good technical knowledge and *direct application experience* can be found out there ??
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