GPS SPEED INDICATION

Dirk_digbee

just a quick question, how accurate are they at detecting the speed of a car? If i remember correctly my gps was fairly accurate in my old feista, but i bought a 04 audi a3 and the gps was way off with the speed. I apologise if this topic was discussed before, it's really hard 2search on a mobile. Cheers, D.

coylemj

The GPS calculates your speed based on how far you have travelled between two location fixes, the accuracy of the speed it displays is determined by (1) how often it calculates your location and (2) how accurate the fix is.

If you're driving in a straight line and the GPS is calculating your position once or more per second then the speed should be accurate. In the car make sure you position the unit where it can see the greatest amount of sky, the more satellites it can see the better. If you are using a typical car unit then the best place is as far forward on the centre of the windscreen as possible so it can see some of the satellites which are directly overhead and slightly behind you.

Dirk_digbee

coylemj,
Thanks for your reply.

I thought I’d give ye a bit more info.

The GPS in question is a Tomtom One V3, I always have it in the centre of the windscreen alright and I keep it updated with tomtom often, so it gets a fix quite quickly. I was giving me a reading of about 10-15Kmph under the speed indicated by the car.

Obviously I don’t expect it to be 100% accurate, 100% of the time, but I’m just wondering why is it only acting up now?

Someone told me the other day that the windows in modern cars can sometimes distort or interfere with the signal because of the coatings used to reflect sunlight.

Now, I’m not so sure about this because if there was interference then surely the unit would have given me wonky directions??

Sorry if I’m going on about this, it’s just that if the GPS is right, then I’ve just bought a car with a dodgy speedo.

coylemj

If it's a standard make like Garmin or Tom Tom then you should be getting an accurate reading, it might make the odd mistake but typically they're very accurate.

VAG (Volkswagen Audi Group) car speedometers can be adjusted if the reading is too high, you can either tell the garage the next time your getting it serviced or find someone who has a VAGCOM kit, they will be able to do it on the fly.

Dirk_digbee

Coylemj,
Apologies for the caps lock earlier and thanks for your help. I brought the Tom Tom out in my old car earlier for a bit of a test. I just had the GPS in my lap and the speed reading wasn’t at all accurate so I pulled in and mounted it on the windscreen as normal and the reading was bang on.

Maybe there was some truth to what I was told about the coating or composition of the windscreen distorting the signal in the new car. Although I think I will get the Speedo checked because I’m a little bit paranoid now!

Krusty_Clown

Hi Dirk_digbee, having a coating on your windscreen will not distort the data received by the GPS. What it will do is typically make it more difficult for a GPS receiver to connect to the range of satellites orbiting the earth. The less satellites your GPS receiver is actively 'listening' to, the less accuracy your GPS receiver will have. At the end of the day though, having a connection to 3 or 4 satellites will give you a pretty accurate representation of your straight line speed.

As CoyleMj (oops, sorry for the caps) alluded to, GPS receivers are better at representing your speed, in a straight line, at a fixed speed. Because they do not record constantly (instead recording data-points at regular intervals), they are not as good at correctly representing your speed when accelerating (or braking) or travelling around corners.

So, if you are travelling in a straight line, and at a constant speed, and your GPS receiver is receiving data from a number of satellites (does the TomTom have a satellite display screen?), then it is far more likely that your car speedo is inaccurate. This is not uncommon. Many manufacturers allegedly have their speedometers run intentionally fast, to avoid being sued my motorists for speeding offences, or something similar.

Slippin Jimmy

I have the Tomtom 730 and the speed is never right on it. It is always a little off. If im doing 60 for example it will say I am doing about 50. So when I use it and am speeding it will indicate that I am, but even when I am speed which I don't do often might I add, I am almost doing twice what I should be for the alarm to sound.

coylemj

The GPS chips are programmed not to give a fix unless they have a satisfactory number of satellites so if the unit says that you are at a specific point then you're either there or not far away. If you are moving then even if the unit doesn't know your exact location, it will probably be out by the same amount and in the same direction for multiple readings, e.g. it might show that you are 2m west of where you really are but as long as the error is the same over multiple calculations then the speed reading will be accurate.

For maximum accuracy make sure that WAAS is enabled on your unit, this takes correction data from a geostationary satellite so that minor errrors caused by atmospheric conditions are allowed for. The European WAAS system is called EGNOS so if you see a setting on your unit mentioning WAAS or EGNOs, switch it on.

Krusty_Clown

coylemj wrote: »

The GPS chips are programmed not to give a fix unless they have a satisfactory number of satellites so if the unit says that you are at a specific point then you're either there or not far away.

Actually, they will, however, they will provide a fix with a lower degree of accuracy. The degree of accuracy will increase as they establish data streams from more satellites. At least all of my Garmins have, so far. (Quest, Zumo, Forerunner).

Can you establish a connection to an EGNOS system? I never have.

CivilEx

Dirk_digbee wrote: »

Coylemj,
Apologies for the caps lock earlier and thanks for your help. I brought the Tom Tom out in my old car earlier for a bit of a test. I just had the GPS in my lap and the speed reading wasn’t at all accurate so I pulled in and mounted it on the windscreen as normal and the reading was bang on.

Maybe there was some truth to what I was told about the coating or composition of the windscreen distorting the signal in the new car. Although I think I will get the Speedo checked because I’m a little bit paranoid now!

I have the same model TomTom as you and I found last year that when I got 4 new tyres on the car, the speedo no longer agreed with the GPS. They disagree by about 4 mph at a speed of 60 at the moment. I have more faith in the GPS speed though and no penalty points to date:D

coylemj

Krusty_Clown wrote: »

Can you establish a connection to an EGNOS system? I never have.

The current EGNOS satellites are AOR-E, (PRN 120, Garmin 33) and Artemis (PRN 124, Garmin 37), if you can see either of these then you have WAAS/EGNOS enabled. There is a third satellite used for EGNOS but it's over the Indian Ocean and Ireland is on the extreme edge of it's coverage area.

Slippin Jimmy

CivilEx wrote: »

I have the same model TomTom as you and I found last year that when I got 4 new tyres on the car, the speedo no longer agreed with the GPS. They disagree by about 4 mph at a speed of 60 at the moment. I have more faith in the GPS speed though and no penalty points to date:D

The reason for that is cause once you get a new set of tires on your car you need to get your speedometre changed.

Krusty_Clown

coylemj wrote: »

The current EGNOS satellites are AOR-E, (PRN 120, Garmin 33) and Artemis (PRN 124, Garmin 37), if you can see either of these then you have WAAS/EGNOS enabled. There is a third satellite used for EGNOS but it's over the Indian Ocean and Ireland is on the extreme edge of it's coverage area.

But you also need to be relatively close to a ground station, which will send/ transmit details of local error margin data (atmospheric pressure, etc.) to the satellites, to improve accuracy in your area, right?

Alun

Krusty_Clown wrote: »

But you also need to be relatively close to a ground station, which will send/ transmit details of local error margin data (atmospheric pressure, etc.) to the satellites, to improve accuracy in your area, right?

Not really. The clue is in the name WAAS i.e. WIDE Area Augmentation System, the correction data is actually valid for quite a large area, i.e. the area covered by the WAAS/EGNOS satellites you can receive.

There are equivalent systems (Differential GPS or DGPS) for performing much more local and precise corrections used by professional surveyors etc. to get down to cm's of accuracy that indeed rely on you being close to a reference station.

coylemj

The 'wide area' name refers to the area covered by the system but the data transmitted includes local correction data. In the case of Ireland there is a monitoring station at Cork airport so the EGNOS satellites are constantly transmitting information which includes correction data for this part of the world as well as other regions, including most of Europe and some parts of Africa.

Alun

True, but the local correction data is calculated from the sum of all the data received from the complete network of WAAS or EGNOS ground stations and then recalculated and transmitted based on a 5x5 degree lat/lon grid, so is still quite coarse or 'wide area', if you like, and there's no real direct link to a particular ground station location any more. Whereas if we're talking about RTK DGPS, or Local Area Augmentation, systems, then the accuracy attainable is directly related to how close you are to the beacon you're receiving your corrections from.

Krusty_Clown

Alun, do you find your accuracy improved using WAAS?
Do you find that you're receiving data from the Egnos system?

Alun

Krusty_Clown wrote: »

Alun, do you find your accuracy improved using WAAS?

Yes, but I don't usually bother. When hiking, my Vista HCx gives me more than enough accuracy in open country (usually 3-5m) to not need it, and it wouldn't pick them up in tree cover on forest tracks anyway. I don't think my nuvi is configurable to use it.

Do you find that you're receiving data from the Egnos system?

Yes. I can't remember which ones I usually pick up when I have enabled it, but they are definitely EGNOS satellites.

coylemj

The EGNOS system appears to be funded based on the fact that it significantly improves vertical accuracy and so will eventually be used to guide aircraft when landing but for hikers and motorists it doesn't do much. As far as I kow it does make some difference to lateral accuracy but not so much as would matter.

BIG TRIPOD

Hi All,

SAT Nav and low end GIS gps units using EGNOS can at best in my experience achive no better than +/-1.5m in this country. But the main issue is the speed or frequency that the gps unit recalculates its position. In ideal situations with a min of 5 satellites most SAT Nav units will probably only recalculate its position every 5 seconds, (cm level professional gps works at 20 times a second) so the comparison of the 2 last "fixes" are used to calculate the speed of the car.

Different size wheels/tryes that are not calibrated to the speedo will give a greater error than correctly functioning gps. In several different cars/vans I have personaly tested highend gps at motorway speeds and achived fantastic results.

Hope this helps

coylemj

BIG TRIPOD wrote: »

Hi All,
In several different cars/vans I have personaly tested highend gps at motorway speeds and achived fantastic results.

Hope this helps

Could you explain what you mean by this? You tested GPS at motorway speeds and what happened? What made the results 'fantastic'?

BIG TRIPOD

Hi Coylemj

Connecting to a realtime gps corrections network provider via a gprs signal and recording cm accuracy positions while driving down a motorway at the legal limit was a fantastic experience 5 years ago. It was pushing the limits of the existing technologies and proving the quality of a now nationwide service.

While I understand this will not rock everybodies boat for the geek in me it was a thrill. At the time the technology was still in beta mode and 3G was still a dream....

coylemj

Sounds good, who's the nationwide provider? I think my Nokia E51 (which I use with a Bluetooth GPS receiver) can receive this type of data but not the Garmin units.

BIG TRIPOD

Hi Coylmj

The data is provided by the OSi to 2 resellers (see OSi.ie) in what is known as RTCM format that is then sold to the public by 2 of the larger surveying equipment suppliers.

It is expensive and really intended for professional applications.

If the device can accept differential corrections in either RTCM or CMR than you can avail of them, however for standard SAT NAV applications this is probalbly a little OTT.

As most SAT NAV works off the L1 wavelenght you will at best only achive circa 1m consistantly with the corrections and 600mm in the very best instance.

For a service that could cost over 1500€/annum it is difficult to justify it for typical non commerical SAT NAV applications.

coylemj

Agreed, an amateur like me could never justify the expense but I can see how the likes of surveyors would use it for increased accuracy.

mndgz

just on the above topic, what I have found is that the GPS (tried 4 or 5 different ones) show around 3 or 4 km/h less than the cars speedometer...
tested in different cars with different GPS receivers and the funny thing is, the roadworks signs on the side on the road that sometimes show your current speed always give an indication of 3 or 4km/h less as well...

anyone else noticed the same pattern?

wonder what reading would speed cameras show then...could it be the case that speedometers in cars are purposely a bit off..?