Using a GPS for WOF purposes - Speedometer?

[mctshirt]

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.

I beg to differ - most (if not all) are now fitted with the Sirfstar III chip that dramatically improves the usability of GPS's. Cloud, walls, indoors, tree cover, etc interference is pretty much a thing of the past (although they still aren't magic). The best bit is a new GPS with the Sirfstar chip is not more expensive. Using the basic Garmin Etrex as an example Garmin simply fitted the Sirfstar chip, renamed the model to Etrex H and left it at the same price. In the past I would have agreed with you Hoon but the hardware has caught up enough to the theory. In my mind a GPS is a serious alternative to the traditional mechanical device which arguably suffers more inaccuracy than a GPS now.

http://en.wikipedia.org/wiki/SiRFstar_III

[Waihou Thumper]

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?

Just has to work, register up to 50kph, etc...according to the blurb I posted on the first post.

[Waihou Thumper]

Cloud, walls, indoors, tree cover, etc interference is pretty much a thing of the past (although they still aren't magic). The best bit is a new GPS with the Sirfstar chip is not more expensive. Using the basic Garmin Etrex as an example Garmin simply fitted the Sirfstar chip, renamed the model to Etrex H and left it at the same price. In the past I would have agreed with you Hoon but the hardware has caught up enough to the theory. In my mind a GPS is a serious alternative to the traditional mechanical device which arguably suffers more inaccuracy than a GPS now.

I totally agree. I have a Garmin and have accuracy down to +/- 2M most days and at its worse it is +/-4M, as for losing satellites, never have.
At night, clear days it is at its best I guess because of the lack of clouds etc, but it never loses the signals, just drops one or two satellites. I can have 9 or 10 at any one time down this part of the world. The more you have the more accurate it is due to the Triangulation theory, but in the case of 8 or 9 it is Octangulation or something....

[Jantar]

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.

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Sorry, but it is true. The causes you mention are the ones that cause satelites to either come into or out of view. Today's technology means that atmospherics are no longer an issue, and 99% of New Zealands roads are not close enough to large buildings to cause satelites to be hidden.

[Hoon]

I beg to differ - most (if not all) are now fitted with the Sirfstar III chip that dramatically improves the usability of GPS's. Cloud, walls, indoors, tree cover, etc interference is pretty much a thing of the past (although they still aren't magic).

I use a Garmin 60Cx which has that chip and yes agree that it way improved over my old GPS12XL. The point I was making is that these conditions are sources of 'reduced accuracy' and in extreme cases can cause 'loss of signal'. Yes the newer chipset greatly reduces this but doesn't eliminate it so as a motorcyclist relying on a GPS for speed calculation you need to be aware that your speed reading will be less accurate in these situations.

Edit: Thanks for putting me onto that chip. Found an interesting article here comparing the different chipsets.

Sorry, but it is true. The causes you mention are the ones that cause satelites to either come into or out of view. Today's technology means that atmospherics are no longer an issue, and 99% of New Zealands roads are not close enough to large buildings to cause satelites to be hidden.

I'm still not convinced, do you have any references to back this 'error is always the same' claim up??. Other than your assumption that no one ever ride past a tall building, most of us have seen the effects of these fluctuations in error when stopped at traffic lights and the GPS showing your position dancing around the intersection. Even now with my GPS sitting on my desk at +-7m I still end up with a spider web track log 10m wide after a few mins . If you're saying that GPS error is always constant then why does this happen?

[Jantar]

..... Even now with my GPS sitting on my desk at +-7m I still end up with a spider web track log 10m wide after a few mins . If you're saying that GPS error is always constant then why does this happen?

Inside a building with a roof that is opaque to radio signals? With limited direct line to satelites and plenty of interference patterns?
Try out in the open and see if it still happens.