Using a GPS for WOF purposes - Speedometer?

[CookMySock]

As has been observed, the GPS speed can be quite laggy, and if you are prone to sudden speed bursts before corners etc, the gps "speedo" will be quite useless at those times.

Steve

[sinned]

As has been observed, the GPS speed can be quite laggy, and if you are prone to sudden speed bursts before corners etc, the gps "speedo" will be quite useless at those times.

Steve

Sometimes very laggy - you could get up to license losing speeds and back down again and it may still read 100. Good luck if you can get a wof with a gps as the speedo but don't rely on it too much.

[Jantar]

GPS speed should be accurate as it *usually* works off Doppler from the satellite signals, so no funnies with going round curves, up/down hills, etc....

Ummm . . . WHAT????

Unless I have somehow completely misunderstood how GPS works then I am pretty sure it does not use Doppler in any way.

As has already mentioned a GPS is unreliable. It is only as accurate as its signal strength and this is affected by rain, cloud cover, tunnels, canyons, high rise buildings and also the inside of a building such as the testing station.

Some do but only for correction. Basic position is calculated by the usual distance triangulation thing but is adjusted for the movement of the satellites and rotation of the earth.....

Ok, I've had quite a bit to do with GPS algorithims for navigation (aviation related) and for speed measurement.

Doppler is not used in any manner. It is triangulation for location, and time between locations for speed. Modern units are generally high sensitivity and are not as prone to signal loss due to forest cover, rain etc. GPS will not work in tunnels and depending on the location of satelites may lose accuracy when close to high rise buildings, canyon walls etc.

One of the things to remember when considering GPS accuracy is that if the error is 10m on a particular reading it will be the same 10m on the next and the next reading. ie 10m error to the west will stay 10m error to the west while on that particular set of satelites. Another satelite coming into view will generally increase the accuracy, while losing one will decrease the accuracy. Thus an accuracy indication of +/- 10 m doesn't mean that it may be +10 m on one reading and -10 m on the next.

Speed reading on GPS are extremely accurate, with the proviso that they are averaged over whatever refresh setting the unit is set for (usually 1 second), and there is often a slight delay in indiaction when speeds are changing rapidly.

[Jantar]

Sometimes very laggy - you could get up to license losing speeds and back down again and it may still read 100. Good luck if you can get a wof with a gps as the speedo but don't rely on it too much.

Many of the old mechanical speedos with a spinning disc dampened by a magnet, are also very laggy. Its just that we didn't notice the lag.

[mujambee]

Speed reading on GPS are extremely accurate, with the proviso that they are averaged over whatever refresh setting the unit is set for (usually 1 second), and there is often a slight delay in indiaction when speeds are changing rapidly.

GPS can be fairy accurate and get very fast refresh.

Commercialy available GPS units, on the other hand....

[sinned]

I am waiting to see when the Zumo 660 is available in NZ. It sounds like the ultimate GPS toy for the bike.

[BMWST?]

bike speedo for the win....even really cheap ones will have average speeds etc.And they can be calibrtated to be very accurate

[BM-GS]

Different GPS chipsets will use the same information to calculate the position, and knowing all the stuff they do about the last fix and the current one, they can work out what they need to however they like. The weightings applied to the results of the different calculations will vary. Doppler is known as it has to be calculated to receive the signals accurately. Once the fix is established (the constellation positions are known, etc) Doppler on the received signals can be used to calculate velocity, or as a sanity check on the distance/time calc.

This was mentioned by an engineer working for a chipset vendor, I think not just with the intention of confusing the opposition by waiting for the info to go to them via me... Admittedly, some chipsets work better than others (by which I mean different chipsets have different strengths - absolute accuracy vs power consumption vs multipath resistance vs price, etc).

It's still a minor miracle to me that the damned system works at all in consumer-bought units, the way people use it & it's expected to operate.


Edit: Bugger. Let my posts in here serve as a warning never to let your fingers work without supervision.
Doppler would never work for user velocity, just as sanity for the position fix. How could a user's 50km/h or 60km/h do much to the speed of light and the speed of the satellites and the speed of the earth's rotation? This post is mostly OK, but it's in support of the premise that Doppler is used by GPS chipsets to calculate velocity, which is complete crap. And I said it. Publicly. ... Can I find a nice stone to crawl under? Hope the Edit feature keeps this post buried, where it belongs.

[mujambee]

A silly question that just came to mind: If the bike is stationary inside the WoF shop, how can they test that your GPS unit is accurate? No amount of roller speed will make that thing believe it has moved a bare mm.

[Waihou Thumper]

A silly question that just came to mind: If the bike is stationary inside the WoF shop, how can they test that your GPS unit is accurate? No amount of roller speed will make that thing believe it has moved a bare mm.

I will take it to my motorcycle shop, they test the speedo and brakes by riding the bike, simple way and the reliable way too

[Crisis management]

A silly question that just came to mind: If the bike is stationary inside the WoF shop, how can they test that your GPS unit is accurate? No amount of roller speed will make that thing believe it has moved a bare mm.

Since when do they get on the bike? All my wofs consists of a mechanical check and a chat about where the best places to ride are.....and thats at VTNZ station.

[Waihou Thumper]

Since when do they get on the bike? All my wofs consists of a mechanical check and a chat about where the best places to ride are.....and thats at VTNZ station.

My local here is good, he is quite relaxed, knows the bike and the predicament too...
I even called the VTNZ station in Hamilton and he said the idea and the reasoning is fine, couldn't see an issue with GPS.
All the issues with lag, cloud cover, bushes, tunnels etc....
I don't see this as a problem. I only want to pass a warrant, what goes on after that is immaterial really. It is the same as putting on a mates tyre to get a warrant or the spare and then changing it back to the bald one....Although you would be a bit of a plonker to do that on a bike...lol
The accuracy on my GPS is as good as +/- 2m, so that is good for the little unit. The lag, updating speed etc isn't an issue really, as someone suggested prior, the lag on a wheel is probably worse or similar. By the time the speedo registers the speed, travels up the wire to the unit, you have either changed speed, sped up or slowed for a corner.
Bike computer is fine, would consider that or take the one of my Husaberg if the WOF became an issue...

I will get it fixed, just will be a winter project and will need some patience tracing the loom, the points and where my voltage is leaking or shorting?
Bet it is sourced in a jiffy with the right tools. The work is the removal of the dash, the fairing, tank to expose the wiring harness and auxillaries.

[Hoon]

One of the things to remember when considering GPS accuracy is that if the error is 10m on a particular reading it will be the same 10m on the next and the next reading. .

No thats not true. You can't assume that an error will be identical between samples. The error range maybe the same but the value and direction will be different every time (though probably not by much in most cases but still different). Errors are due to a combination of factors from atmospheric to bouncing off buildings, not just satellite positioning.

Edit: Bugger. Let my posts in here serve as a warning never to let your fingers work without supervision.
Doppler would never work for user velocity, just as sanity for the position fix.
..... This post is mostly OK, but it's in support of the premise that Doppler is used by GPS chipsets to calculate velocity, which is complete crap.

I'm no GPS expert but Google disagrees with you...especially this guy.

How could a user's 50km/h or 60km/h do much to the speed of light and the speed of the satellites and the speed of the earth's rotation?

Timings errors account for +- 2m apparently. I guess when its all about making a GPS as accurate as possible every last bit counts.

[mujambee]

I will take it to my motorcycle shop, they test the speedo and brakes by riding the bike, simple way and the reliable way too

My fault. Here they put the bike on rollers to do all those checks. Just assumed it must be the same over there.

How can they tell if your speedo is accurate?

[Conquiztador]

Some do but only for correction. Basic position is calculated by the usual distance triangulation thing but is adjusted for the movement of the satellites and rotation of the earth.

To get the best accuracy these days they take everything possible into account including time dilation due to the satellites high speed and time slowing down (einsteins theory of relativity).

Bloody Einstein. He was soo wrong. I remember how the summers used to last forever when I was young. Now they just whizz by and its frikkin winter again! My hair is falling out faster each day and I am getting old at the speed of lightning!